EVALUATION OF EYE LENS DOSE TO WORKERS IN THE STEAM GENERATOR AT THE KOREAN OPTIMIZED POWER REACTOR 1000

EVALUATION OF EYE LENS DOSE TO WORKERS IN THE STEAM GENERATOR AT THE KOREAN OPTIMIZED POWER... Abstract ICRP (2011) revised the dose limit to the eye lens to 20 mSv/y based on a recent epidemiological study of radiation-induced cataracts. Maintenance of steam generators at nuclear power plants is one of the highest radiation-associated tasks within a non-uniform radiation field. This study aims to evaluate eye lens doses in the steam generators of the Korean OPR1000 design. The source term was characterized based on the CRUD-specific activity, and both the eye lens dose and organ dose were simulated using MCNP6 combined with an ICRP voxel phantom and a mesh phantom, respectively. The eye lens dose was determined to be 5.39E-02–9.43E-02 Sv/h, with a negligible effect by beta particles. As the effective dose was found to be 0.81–1.21 times the lens equivalent dose depending on the phantom angles, the former can be used to estimate the lens dose in the SG of the OPR1000 for radiation monitoring purposes. INTRODUCTION A recent epidemiological study of the radiation effect on the eye lens suggested that the associated cataract risk is far higher than that proposed in a previous study, which had been the basis for the ICRP 60 dose limit. As a result of the review, ICRP revised the dose-equivalent limit to the eye lens from 150 to 20 mSv/y in 2011(1). Extensive research thereafter has attempted to evaluate lens doses more accurately under various radiation conditions and geometric configurations of the source to the receptor so as to meet the new reduced dose limit. However, the current Korean regulatory framework for radiation protection is based on ICRP 60. The Korean nuclear regulatory body is performing a feasibility study to apply the new ICRP recommendations, including ICRP 103(2), which is expected to be codified in the National Nuclear Safety Act in the future. The current lens dose-equivalent limit for radiation workers (150 mSv/y) in Korea is 7.5 times higher than the effective dose limit of 20 mSv/y for an average of 5 years. Due to the fact that it is highly unlikely to expect a non-uniform radiation field where the dose rate in space is marginally different at the general access area in nuclear power plants, common understanding in Korea has prevailed between the regulatory body and utilities that the lens dose equivalent would be below the limit (150 mSv/y) if the effective dose is within its limit (20 mSv/y). Thus, it is a general practice at nuclear power plants regularly to measure workers’ effective doses and skin doses but not to evaluate the dose equivalent of the eye lens, except in special cases. The occupational radiation dose incurred at nuclear power plants amounts to ~34% of the total national radiation exposure in Korea as of 2015(3). The Korean optimized power reactor (OPR1000) is a 1000 MWe pressurized water reactor with two coolant loops developed based on System 80 by the Korea Hydro and Nuclear Power Co., Ltd. (KHNP). Currently, 12 OPR1000 systems are in operation in Korea. During normal operations and outages at nuclear power plants, their employees and contractors are generally exposed to various radiation sources, such as the reactor coolant and contaminated equipment and pipes. Figure 1 shows the average cumulative radiation exposure (CRE) at OPR1000s in 2016(4). It is necessary to investigate radiation tasks including the assembly and disassembly of the reactor where monitoring of the lens dose is required. As a first step in this study, an evaluation of the lens dose to the steam generator (SG) maintenance workers has been selected due to the high radiation level, the harsh working environment and the limited working time (a few minutes) allowed. Among the radiation tasks performed at nuclear power plants, SG-related maintenance is among those during which workers incurred high CRE levels at OPR1000 in 2016. In addition, the CRE associated with the maintenance of the SG primary side amounts to nearly 15% of the total CRE, as shown in Figure 1, as SGs become highly contaminated by activated corrosion deposits, i.e. Chalk River Unidentified Deposits (CRUD), on their surfaces during operation(5). If the regulatory limit for the eye lens were to be identical to the effective dose limit, it would be necessary to investigate whether the abovementioned practice of radiation protection at nuclear power plants remains valid or needs to be revised. In addition, guidance on evaluations of the lens dose equivalent must be established. However, very limited work has been carried out to evaluate the eye lens dose to people working in highly active SGs. In this regard, this study aims to characterize the radiation sources of the SGs of OPR1000s and to evaluate the eye lens dose to plant workers, particularly those involved in the maintenance of the SGs in preparation for the possible revision of the dose limit for the eye lens in Korea. In addition, the present study proposes a technically credible relationship between the effective dose or skin and lens equivalent doses. Figure 1. View largeDownload slide OPR CRE distribution by task. Figure 1. View largeDownload slide OPR CRE distribution by task. METHODOLOGY For an investigation and evaluation of the eye lens dose, the radioactive source term of the SG is one of the most important factors. However, it is generally not accessible for direct measurements of radioactive materials owing to the high dose rate (~tens of mSv/h) and the complexity of the geometry of the SG. In addition, the source term of the SG is known to vary depending on the plant operational history and the primary shutdown chemistry, among other factors(6). Thus, a conservative analysis based on the plant design report was conducted for radiation protection purposes to estimate the concentration of the CRUD deposited on the surface of the SG. The Monte Carlo N-Particle transport code (MCNP6) with a continuous energy spectrum and the latest nuclear cross-section library (END/B-VII.1) were used in this study. The ICRP 110 male voxel phantom was adopted in order to evaluate the eye lens dose equivalent and the effective dose(7). As a voxel of the male phantom is composed of a 2.137 × 2.137 × 8 mm3 cube, it is larger than very small organs such as the eye lens such that the associated dose calculation may be less accurate for weak beta particles(8). Therefore, the dose to the sensitive lens in the SG chamber was calculated for comparison purposes using a tetrahedral-mesh phantom developed based on ICRP 110 by Hanyang University under ICRP Committee 2(9). Figure 2 shows the eye model of the mesh phantom. For the dose calculation, the mesh phantom was directly implemented, without voxelization, in the MCNP6 code using the unstructured mesh geometry(10). Figure 2. View largeDownload slide Enlarged eye shape of the mesh phantom. Figure 2. View largeDownload slide Enlarged eye shape of the mesh phantom. SOURCE TERM The OPR1000 design has two SGs with cold and hot water chambers for each, in which maintenance tasks such as the opening and closing of manways are implemented. The maintenance of the SG primary side takes place mostly in water chambers with diameters of 2 m, where the whole body of the worker is exposed directly to the radiation source, as shown in Figures 3 and 4. The radiation source of the SG typically has CRUD deposited on its inner surface and is known to be highly dependent on the materials of the reactor coolant system, the chemical treatment practices of the reactor coolant, and the operational history(6). In addition, most of the radioactive sources present at nuclear power plants which cause external doses are contained in tanks, pipes and equipment generally made of steel, which beta particles cannot penetrate. However, beta particles from CRUD, such as 58Co, which is the main source used with SGs, can relatively easily reach the bodies of SG maintenance workers and present a dose to the eyes and skin(11). CRUD is deposited on the surfaces of the water chambers, dividers and tubes during the circulation of the reactor coolant, as sketched in Figure 4. On the other hand, it is very difficult directly to measure nuclides and the activity and distribution of the source inside the SG due to the high dose rates and the complex geometry. Therefore, it is a generally accepted practice for this source term to be estimated based on the equilibrium thickness of the CRUD and its specific activity. With regard to the SG tubes, their surface contamination levels are converted to the volume contamination for an efficient simulation assuming that the tubes are homogeneously distributed with water outside and air inside. The chamber and the divider are the surface source. Their activities are calculated as follows: Ai(Bq/cm2)=D(g−CRUD/cm2)×Si(Bq/g−CRUD), where Ai is activity of nuclide i per unit area; D (equilibrium thickness of CRUD): CRUD weight per unit area of the SG compartments (g-CRUD/cm2); Si (Specific activity of CRUD): activity of nuclide i per unit CRUD weight (Bq/g-CRUD)—Table 2; and D for the chamber and the divider is 1.0E-03g-CRUD/cm2, whereas D for the tubes is 1.0E-04g-CRUD/cm2. Figure 3. View largeDownload slide Schematic diagram of the SG. Figure 3. View largeDownload slide Schematic diagram of the SG. Figure 4. View largeDownload slide Geometric configuration of the SG sources and the phantom Figure 4. View largeDownload slide Geometric configuration of the SG sources and the phantom For the tubes, the surface activity was calculated according to the equation above. The total activity of the tubes was then calculated by multiplying its total surface area. The total activity was divided by the weight of the tubes, the air inside the tubes, and the water surrounding the tubes outside. For this study, the final safety analysis report of OPR1000 was used for the calculation of the SG source term, with the associated main parameters described in Tables 1 and 2(12). In comparison, a smear sample was taken from the cold leg manway of the SG of one of the OPRs and analyzed for nuclides using an HPGe detector system. Table 1. Main parameters for the estimation of the SG source term. Parts Dimension (mm) Material CRUD thickness (g/cm2) Chamber 2083(ID) Carbon steel 1.00E-03 Divider 2020 SA240 1.00E-03 U tubes (No. 8,214) 19.1(OD) 16.9(ID) Alloy 690TT 1.00E-04 Parts Dimension (mm) Material CRUD thickness (g/cm2) Chamber 2083(ID) Carbon steel 1.00E-03 Divider 2020 SA240 1.00E-03 U tubes (No. 8,214) 19.1(OD) 16.9(ID) Alloy 690TT 1.00E-04 Note: OD, outer diameter; ID, inner diameter. Table 1. Main parameters for the estimation of the SG source term. Parts Dimension (mm) Material CRUD thickness (g/cm2) Chamber 2083(ID) Carbon steel 1.00E-03 Divider 2020 SA240 1.00E-03 U tubes (No. 8,214) 19.1(OD) 16.9(ID) Alloy 690TT 1.00E-04 Parts Dimension (mm) Material CRUD thickness (g/cm2) Chamber 2083(ID) Carbon steel 1.00E-03 Divider 2020 SA240 1.00E-03 U tubes (No. 8,214) 19.1(OD) 16.9(ID) Alloy 690TT 1.00E-04 Note: OD, outer diameter; ID, inner diameter. Table 2. CRUD-specific activity of the SG. Nuclides CRUD activity (Bq/g, CRUD) 51Cr 7.92E09 54Mn 2.79E07 59Fe 4.49E07 8Co 2.53E09 60Co 6.57E07 95Zr 5.96E07 Nuclides CRUD activity (Bq/g, CRUD) 51Cr 7.92E09 54Mn 2.79E07 59Fe 4.49E07 8Co 2.53E09 60Co 6.57E07 95Zr 5.96E07 Table 2. CRUD-specific activity of the SG. Nuclides CRUD activity (Bq/g, CRUD) 51Cr 7.92E09 54Mn 2.79E07 59Fe 4.49E07 8Co 2.53E09 60Co 6.57E07 95Zr 5.96E07 Nuclides CRUD activity (Bq/g, CRUD) 51Cr 7.92E09 54Mn 2.79E07 59Fe 4.49E07 8Co 2.53E09 60Co 6.57E07 95Zr 5.96E07 DOSE SIMULATION MCNP6 was used to simulate the eye lens dose incurred by workers in the SG together with effective dose and the skin dose. The ICRP 110 male voxel phantom was incorporated in this study, as noted above. Although various postures of workers in the SG can be expected in actual situations, here, the phantom was placed in a standing position in the SG, as the posture of the ICRP 110 phantom cannot be changed and because phantoms for which the posture can be thus adjusted have not been fully developed(13). In addition to the lens dose, the skin dose and organ dose were evaluated at angles of 0, 90°, 180° and 270° in order to investigate the effect of the angular position of the phantom on the dose. The position of the phantom is shown in Figures 4 and 5. Once the source is estimated, the full energy spectra of each source location are input into MCNP6 for the dose simulation. In addition, the beta energy (bin) and yield of each source are separately input to investigate the contribution of the doses to the lens by beta and gamma energy. RADAR was used to convert the continuous energy spectrum of the beta sources into discrete energy bins for the MCNP input(14). The track length estimate of the energy deposition (F6:E tally) was used to determine the dose absorbed by the organs, and the maximum number of source particles for the dose simulation was set to 5E09. Furthermore, a variance-reduction technique, in this case source biasing, was applied such that the relative error associated with the voxel simulation would be less than the MCNP recommended value of 10%. Primary photons and secondary electrons were tracked down to levels of 1 and 14 keV, respectively, for the simulation. The lens dose was calculated by averaging the doses to the right lens and the left lens(2, 15). The organ equivalent dose for the male phantom was calculated based on ICRP 103 organ weighting factors and the ICRP 116 methodology for the calculation of the doses to red bone marrow and the bone surface(15). Most previous studies of eye lens doses focused on a few idealized geometries and mono-energetic gamma or beta sources in the medical area but not in nuclear power plants. There have been no in-depth studies of eye lens doses in non-uniform radiation fields incurred by equipment such as a SG at a nuclear power plant. Moreover, high levels of concern over the contribution of the dose to the lens by beta particles have been raised due to the short distance from the radioactive sources of the SG to the receptor. Thus, the doses to the eye lens by beta and gamma rays were separately calculated for an evaluation of their dose contributions and the special need to monitor beta radiation. In addition, the presence of any correlation of the skin dose equivalent or organ dose to the eye lens was investigated because the eye lens lies geometrically between the skin and the organs. Therefore, the skin and organ dose equivalents were evaluated along with the lens dose equivalent in this study. A voxel phantom is reported to be less accurate when used to simulate the eye lens equivalent due to the size of the voxel, which is found to be relatively large compared to the corresponding human anatomical size(8). Therefore, the dose to the sensitive lens was additionally evaluated using the head part of the mesh phantom in comparison with the voxel dose result. Figure 5. View largeDownload slide Angular position of the phantom in the SG. Figure 5. View largeDownload slide Angular position of the phantom in the SG. RESULTS AND DISCUSSION Source inventory of the SG The estimated radionuclide inventory is summarized in Table 3 for the main parts of the SG, including the water chambers, the divider and the tubes. It was used as the input of the radionuclide sources into the Monte Carlo dose simulation code. The amount of surface contamination on the chambers and dividers was calculated and found to be to be 1.06E07 Bq/cm2, and contamination of the tubes was 3.56E05 Bq/cm3, stemming from the assumption that the surface activity of the tubes was distributed throughout the volume of the tubes, air and water both inside and outside of them. The gamma and beta energy spectra for each source location are presented in Figures 6 and 7. The main nuclides of the SG were found to be 58Co and 51Cr. The average gamma and beta energy levels were analyzed and found to be 427 and 2.6 keV, respectively. Although there are a few models which can be used to estimate radionuclides on the surfaces of the primary side of reactors(16), they estimate only CRUD activities while excluding fission product activities, which can be neglected unless fuel defects arise. Furthermore, there is very limited research on measurements of actual inventories of SGs. Thus, as a comparison indicator, one of the water chambers at the OPR was smear-sampled and analyzed for radionuclides using an HPGe detector system because there was no way to access the other parts of the SG without waiting for dismantling during the plant’s decommissioning stage. The composition of the radionuclides from the actual smear test results was compared with the estimated inventory, as shown in Table 2. However, these values could only be qualitatively compared because smear tests do not give credible quantitative results pertaining to contamination, as the results strongly depend on the fraction of fixed and non-fixed contaminants on the surface and their collection efficiency(17). The two main nuclides for both the smear samples and the estimated source term were found to be 58Co and 51Cr. Table 3. Estimated radionuclide inventory of the SG. Nuclides Chamber (Bq/cm2) Divider (Bq/cm2) Tubes (Bq/g) Measurements (Bq/300 cm2) 51Cr 7.92E06 7.92E06 2.65E05 6.64E02 54Mn 2.79E04 2.79E04 9.27E02 7.04E02 59Fe 4.49E04 4.49E04 1.49E03 7.44E01 58Co 2.53E06 2.53E06 8.40E04 5.60E03 60Co 6.57E04 6.57E04 2.18E03 2.15E03 95Zr 5.96E04 5.96E04 1.98E03 4.16E03 Nuclides Chamber (Bq/cm2) Divider (Bq/cm2) Tubes (Bq/g) Measurements (Bq/300 cm2) 51Cr 7.92E06 7.92E06 2.65E05 6.64E02 54Mn 2.79E04 2.79E04 9.27E02 7.04E02 59Fe 4.49E04 4.49E04 1.49E03 7.44E01 58Co 2.53E06 2.53E06 8.40E04 5.60E03 60Co 6.57E04 6.57E04 2.18E03 2.15E03 95Zr 5.96E04 5.96E04 1.98E03 4.16E03 Table 3. Estimated radionuclide inventory of the SG. Nuclides Chamber (Bq/cm2) Divider (Bq/cm2) Tubes (Bq/g) Measurements (Bq/300 cm2) 51Cr 7.92E06 7.92E06 2.65E05 6.64E02 54Mn 2.79E04 2.79E04 9.27E02 7.04E02 59Fe 4.49E04 4.49E04 1.49E03 7.44E01 58Co 2.53E06 2.53E06 8.40E04 5.60E03 60Co 6.57E04 6.57E04 2.18E03 2.15E03 95Zr 5.96E04 5.96E04 1.98E03 4.16E03 Nuclides Chamber (Bq/cm2) Divider (Bq/cm2) Tubes (Bq/g) Measurements (Bq/300 cm2) 51Cr 7.92E06 7.92E06 2.65E05 6.64E02 54Mn 2.79E04 2.79E04 9.27E02 7.04E02 59Fe 4.49E04 4.49E04 1.49E03 7.44E01 58Co 2.53E06 2.53E06 8.40E04 5.60E03 60Co 6.57E04 6.57E04 2.18E03 2.15E03 95Zr 5.96E04 5.96E04 1.98E03 4.16E03 Figure 6. View largeDownload slide Energy spectrum of the SG chamber/divider. Figure 6. View largeDownload slide Energy spectrum of the SG chamber/divider. Figure 7. View largeDownload slide Energy spectrum of the SG tubes. Figure 7. View largeDownload slide Energy spectrum of the SG tubes. Simulated dose with the voxel phantom The dose to the eye lens of the ICRP male voxel phantom in the SG at OPR1000 was simulated. Both the organ dose and the skin dose equivalent were calculated for comparison. In addition, the highest dose rate in the SGs of OPR1000s measured using an Automess Teletector 6112 MH was plotted in order to investigate the validity of the dose simulation results as well as the SG source. The eye lens dose equivalent with the ICRP 110 voxel phantom The eye lens dose rate at an angle of 0° was determined to be 7.65E-02 Sv/h. As shown in Table 4, gamma rays from the SG were found to play a dominant role in the dose to the eye lens, whereas the dose effect by beta rays was minimal. This phenomenon could be explained by the low average energy of beta particles, i.e. 2.6 keV. According to Behrens and Dietze, the equivalent dose to a sensitive lens per photon fluence (pSv cm2) at 400 keV is 2.281, whereas the beta conversion factor at the average energy level of 2.6 keV was not presented but is <1.9E-03 (pSv cm2)(18). In consideration of this ratio, the dose contribution by beta rays is likely negligible. In addition, 83% of the lens dose was identified to have been caused by the water chamber and the divider. On the other hand, radiation originating from the tubes is greatly reduced owing to the self-shielding material of the tubes, alloy 690, as well as the water outside the tubes. Table 4. Simulated organ dose using the voxel phantom at an angle of 0°. Radiation Sources Lens (Sv/h) Skin (Sv/h) Organs (Sv/h) Gamma Chamber 2.37E-02 3.67E-02 2.35E-02 Divider 4.03E-02 4.61E-02 3.73E-02 Tubes 1.30E-02 1.05E-02 1.27E-02 Beta Chamber 8.60E-07 1.29E-04 1.43E-06 Divider 1.47E-06 5.58E-04 6.16E-06 Tubes 9.90E-10 4.68E-08 7.69E-10 Total 7.70E-02 9.36E-02 6.88E-02 Radiation Sources Lens (Sv/h) Skin (Sv/h) Organs (Sv/h) Gamma Chamber 2.37E-02 3.67E-02 2.35E-02 Divider 4.03E-02 4.61E-02 3.73E-02 Tubes 1.30E-02 1.05E-02 1.27E-02 Beta Chamber 8.60E-07 1.29E-04 1.43E-06 Divider 1.47E-06 5.58E-04 6.16E-06 Tubes 9.90E-10 4.68E-08 7.69E-10 Total 7.70E-02 9.36E-02 6.88E-02 Table 4. Simulated organ dose using the voxel phantom at an angle of 0°. Radiation Sources Lens (Sv/h) Skin (Sv/h) Organs (Sv/h) Gamma Chamber 2.37E-02 3.67E-02 2.35E-02 Divider 4.03E-02 4.61E-02 3.73E-02 Tubes 1.30E-02 1.05E-02 1.27E-02 Beta Chamber 8.60E-07 1.29E-04 1.43E-06 Divider 1.47E-06 5.58E-04 6.16E-06 Tubes 9.90E-10 4.68E-08 7.69E-10 Total 7.70E-02 9.36E-02 6.88E-02 Radiation Sources Lens (Sv/h) Skin (Sv/h) Organs (Sv/h) Gamma Chamber 2.37E-02 3.67E-02 2.35E-02 Divider 4.03E-02 4.61E-02 3.73E-02 Tubes 1.30E-02 1.05E-02 1.27E-02 Beta Chamber 8.60E-07 1.29E-04 1.43E-06 Divider 1.47E-06 5.58E-04 6.16E-06 Tubes 9.90E-10 4.68E-08 7.69E-10 Total 7.70E-02 9.36E-02 6.88E-02 Skin dose equivalent with the ICRP 110 voxel phantom The skin dose rate at an angle of 0° in the SG as shown in Table 4 was determined to be 9.36E-02 Sv/h. The contaminated chambers and the divider were found to contribute to the skin dose rate most prevalently, as in case of the eye dose rate mentioned above. The skin dose in this study was found to be greater than the lens dose by 22% at 0° and by maximum of 76% at 270°. This stems from the fact that the skin equivalent depth (7 mg/cm2) is less than the lens equivalent depth (300 mg/cm2). The ratio of the simulated skin to the lens dose was found to be 0.95–1.76 in comparison with 1.1 as determined by Behrens, seemingly due to difference in the source configuration and energy spectra(18). Organ dose with the ICRP 110 voxel phantom The organ dose rate at an angle of 0° was simulated and found to be 6.88E-02 Sv/h with an error of 0.4%. This was in relatively good agreement with the measured exposure rate, as shown in Figure 8, in consideration of the detector error of ± 10% and variation in the dose rate (at a factor of 3 at most) at measuring points. As indicated in Table 4, gamma rays also played a dominant role in the effective dose, whereas the beta ray contribution to the dose was found to be <1% due to the shielding effect of the skin covering the organs of the phantom and considering that the maximum range of beta particles with an average energy level of ~3 keV in water is ~2E-03 mm. Figure 8. View largeDownload slide Simulated angular dose rate inside the SG. Figure 8. View largeDownload slide Simulated angular dose rate inside the SG. Dose at the angular position of the phantom Considering how workers move in the SG and use different postures when performing their designated work tasks, the phantom was placed for the dose simulation at four different angles ranging from 0° to 270°. Table 5 and Figure 8 show the simulated dose rates for each angle. Lens doses at 0°, 90° and 180° were found to be identical within the simulated range of error, except for that at 270°, where the lens of the phantom is opposite to the divider. The skin doses at four angles were found to be greater than the lens dose and the effective dose, except at 90°. The lens dose rate was determined to be between 5.35E-02 and 9.39E-02 Sv/h depending on the phantom angle. The average dose rate in the SG chambers of OPR1000s was measured and found to be 46 mSv/h, with maximum and minimum values of 70 and 28 mSv/h, respectively, during the first to the fourth outages(4). As shown in Figure 8, the simulated doses at the four different angles were relatively well in the range of the exposure rate in consideration of the uncertainties involved in the simulation and measurements. This can also support the contention that the source inventory of the SG used in the study was technically reasonable. Table 5. Comparison of the lens, skin dose equivalent and effective dose at different angles. Angle Eye lens (Sv/h) Skin (Sv/h) Effective dose (Sv/h) 0° 7.70E-02 (10.1%) 9.40E-02 (0.1%) 7.25E-02 (0.4%) 90° 9.43E-02 (8.7%) 8.94E-02 (0.1%) 7.59E-02 (0.4%) 180° 8.53E-02 (8.2%) 1.05E-01 (0.1%) 7.75E-02 (0.4%) 270 5.39E-02 (7.1%) 9.51E-02 (0.1%) 6.53E-02 (0.4%) Angle Eye lens (Sv/h) Skin (Sv/h) Effective dose (Sv/h) 0° 7.70E-02 (10.1%) 9.40E-02 (0.1%) 7.25E-02 (0.4%) 90° 9.43E-02 (8.7%) 8.94E-02 (0.1%) 7.59E-02 (0.4%) 180° 8.53E-02 (8.2%) 1.05E-01 (0.1%) 7.75E-02 (0.4%) 270 5.39E-02 (7.1%) 9.51E-02 (0.1%) 6.53E-02 (0.4%) (): Relative error. Table 5. Comparison of the lens, skin dose equivalent and effective dose at different angles. Angle Eye lens (Sv/h) Skin (Sv/h) Effective dose (Sv/h) 0° 7.70E-02 (10.1%) 9.40E-02 (0.1%) 7.25E-02 (0.4%) 90° 9.43E-02 (8.7%) 8.94E-02 (0.1%) 7.59E-02 (0.4%) 180° 8.53E-02 (8.2%) 1.05E-01 (0.1%) 7.75E-02 (0.4%) 270 5.39E-02 (7.1%) 9.51E-02 (0.1%) 6.53E-02 (0.4%) Angle Eye lens (Sv/h) Skin (Sv/h) Effective dose (Sv/h) 0° 7.70E-02 (10.1%) 9.40E-02 (0.1%) 7.25E-02 (0.4%) 90° 9.43E-02 (8.7%) 8.94E-02 (0.1%) 7.59E-02 (0.4%) 180° 8.53E-02 (8.2%) 1.05E-01 (0.1%) 7.75E-02 (0.4%) 270 5.39E-02 (7.1%) 9.51E-02 (0.1%) 6.53E-02 (0.4%) (): Relative error. Lens dose using the voxel phantom vs the sensitive lens dose using a mesh phantom As noted above, the ICRP 110 voxel phantom is reportedly less accurate when used to describe the sensitivity of the eye lens to radiation as compared to a mesh phantom. Thus, the sensitive eye lens dose using the head part of the mesh phantom was simulated at an angle of 0° in order to compare the simulated results using the ICRP voxel phantom to investigate any differences between them. As shown in Table 6, the sensitive lens dose using the mesh phantom was found to be 3.27% less than the eye lens dose using the voxel phantom. In consideration of the average gamma energy of 409 keV radiating from the SG, this result is in good agreement with the findings of Nguyen et al.(9), who found a maximum difference within 5% between the dose conversion coefficients for the sensitive lens with the mesh phantom and the eye lens dose conversion coefficients with the ICRP 110 voxel phantom. This result is also supported by the findings of Behrens and Dietze, who showed that the sensitive lens dose was within an error range of −3 to 2.5%(19). As the lens dose by beta rays in the SG is negligible, the dose effect of the former type has not been studied in detail. Table 6. Simulated eye lens dose vs the sensitive eye lens with the head parts of phantoms at an angle of 0°. Organ Head mesh Head voxel Voxel Sensitive lens (lens) 6.08E-02 6.37E-02 6.35E-02 Entire lens (bulbs) 6.52E-02 5.62E-02 5.40E-02 Organ Head mesh Head voxel Voxel Sensitive lens (lens) 6.08E-02 6.37E-02 6.35E-02 Entire lens (bulbs) 6.52E-02 5.62E-02 5.40E-02 Table 6. Simulated eye lens dose vs the sensitive eye lens with the head parts of phantoms at an angle of 0°. Organ Head mesh Head voxel Voxel Sensitive lens (lens) 6.08E-02 6.37E-02 6.35E-02 Entire lens (bulbs) 6.52E-02 5.62E-02 5.40E-02 Organ Head mesh Head voxel Voxel Sensitive lens (lens) 6.08E-02 6.37E-02 6.35E-02 Entire lens (bulbs) 6.52E-02 5.62E-02 5.40E-02 Comparison with Measurements and the Previous Study The results of this study were compared with those of Behrens and Dietze’ study(18) and the measured TLD readings for the SG maintenance work conducted on the OPR1000 design from 2015 to 2017(20), as presented in Table 7. The TLD system (Harshaw 6600), which OPR1000s use for dose evaluation, has four elements and is capable of measuring lens doses with a 300 mg/cm2 filter and effective doses with a 1000 mg/cm2 filter simultaneously. The SG maintenance workers wear two TLDs, one on the chest and the other on the back. According the plant procedure, the effective dose is calculated with the equation 0.55 × chest dose + 0.5 × back dose. Because there is no specific procedure, lens dose is calculated as the chest or the back dose, whichever is greater for a conservative approach. Records of the TLD readings of the lens doses to workers who performed the SG maintenance were investigated. If the skin dose is used for control of the dose limits to the lens dose for SG maintenance work, it could overestimate the lens dose by up to 76%. Therefore, the skin dose can be used as a substitute for the lens dose simply for conservative radiation protection purposes, as there is no risk of exceeding the lens dose limit. If the lens dose is considered to be 123% of the effective dose, any overestimation of the lens dose will be less severe as compared to when using the skin dose for the lens dose. Therefore, it is more reasonable to use the effective dose as a substitute for the lens dose incurred in the SG. Table 7. Comparison with earlier findings and the measurements. Ratio This study Behrensa Measured OPR1000 E [Hp(10)] Hlens [Hp(3)] 0.81–1.21 0.9–1.2 1.07 ± 0.18 Hskin [Hp(0.07)] Hlens [Hp(3)] 0.95–1.76 1.1 (1–500)b 1.12 ± 0.1 Ratio This study Behrensa Measured OPR1000 E [Hp(10)] Hlens [Hp(3)] 0.81–1.21 0.9–1.2 1.07 ± 0.18 Hskin [Hp(0.07)] Hlens [Hp(3)] 0.95–1.76 1.1 (1–500)b 1.12 ± 0.1 aPhoton > 30 keV; belectron (<1.5 MeV) + photon. Table 7. Comparison with earlier findings and the measurements. Ratio This study Behrensa Measured OPR1000 E [Hp(10)] Hlens [Hp(3)] 0.81–1.21 0.9–1.2 1.07 ± 0.18 Hskin [Hp(0.07)] Hlens [Hp(3)] 0.95–1.76 1.1 (1–500)b 1.12 ± 0.1 Ratio This study Behrensa Measured OPR1000 E [Hp(10)] Hlens [Hp(3)] 0.81–1.21 0.9–1.2 1.07 ± 0.18 Hskin [Hp(0.07)] Hlens [Hp(3)] 0.95–1.76 1.1 (1–500)b 1.12 ± 0.1 aPhoton > 30 keV; belectron (<1.5 MeV) + photon. CONCLUSION Since ICRP (2011) reduced its dose limit to 20 mSv/y from 150 mSv/y according to the reevaluated risk of the radiation dose to the eye lens, a variety of studies to evaluate the lens dose have been carried out mostly for medical research and in relation to ideal cases with simple geometries and mono-energetic beta and gamma particles. Although maintenance work in the SG is one of the tasks with the highest CRE levels at nuclear power plants, no research has attempted to evaluate the eye lens doses to workers in the SG, where a non-uniform radiation field and a complex geometry environment exist. In order to evaluate the lens dose in the OPR1000 SG, the corresponding radioactive source term was initially characterized based on the CRUD activity and the average energy levels of gamma and beta particles were determined to be 427 and 2.6 keV, respectively. Secondly, the lens dose in the SG of the OPR1000 design was simulated and found to be 5.35E-02–9.39E-02 Sv/h using MCNP6 and the ICRP voxel phantom. The lens dose was mostly caused by the gamma component of the source, with <1% of the dose by beta particles. Based on the lens dose rate, if a SG worker stays in the SG for 12.9 min or longer, he could reach the new annual lens dose limit. However, the risk of exceeding the dose limit is not anticipated in consideration of plant practices for workforce allocations for SG maintenance Thirdly, the skin dose and effective dose calculated according to ICRP 103 methodology were found to be respectively 0.95–1.76 and 0.81–1.21 times the simulated lens doses depending on the phantom angle. These values are in good agreement with the findings of Behrens regarding the gamma dominant radiation field. In addition, the ratios of the skin dose and effective dose to the lens dose measured in the OPR1000 SG were found to be between the ranges simulated in this study. Thus, 123% of the effective dose was proposed as a substitute for the lens dose for the implementation of ALARA, though the skin equivalent dose can be used to estimate the lens dose for conservative radiation protection purposes. This study is the first in-depth investigation of the lens dose in the non-uniform and highly radioactive conditions present at nuclear power plants, and the findings here will be very useful if the lens dose limit is revised based on the new ICRP recommendation in Korea. For more accurate evaluations of the lens dose, it will be necessary to measure the energy spectrum of the SG, improve the MCNP modeling to include factors such as blinking of the eyelids, and the use of masks, and to simulate actual working postures. In addition, evaluations of the lens dose for tasks involving high radiation levels other than those related to the SG at nuclear power plants should be conducted. ACKNOWLEDGEMENTS The authors would like to thank Professor C.H. Kim and coworkers at Hanyang University for allowing us to use the digital phantom and for the precious advice during this study. REFERENCES 1 International Commission on Radiation Protection . Statement on tissue reactions. International Commission on Radiological Protection ( 2011 ). 2 International Commission on Radiation Protection . Recommendations of the international commission on radiological protection. ICRP Publication 103 ( 2007 ). 3 Nuclear Safety and Security . Radiation exposure status in 2015. Nuclear Safety Yearbook 2015. pp. 283–284 ( 2016 ). 4 Korea Hydro and Nuclear Power Co., Ltd. Annual radiation management report ( 2015 ). 5 Deshon , J. Modeling PWR fuel corrosion product deposition and growth process. Technical report 101173 ( Palo Alto, CA : EPRI ) ( 2005 ). 6 OECD/NEA . Radiation protection aspects of primary water chemistry and source-term management. (17–30) April 2014 7 International Commission on Radiation Protection . Adult reference computational phantoms. ICRP 110. 2009. Ann. ICRP 39(2) ( 2009 ). 8 Kim , C. H. et al. . The reference phantoms: voxel vs polygon . Ann. ICRP 45 , 188 – 201 ( 2016 ). Google Scholar CrossRef Search ADS PubMed 9 Nguyen , T. T. et al. . Incorporation of detailed eye model into polygon mesh version of ICRP 110 reference phantoms . Radiat. Prot. Dosim. 168 ( 1 ), 8695 – 8707 ( 2016 ). 10 Yeom , Y. S. , Jeong , J. H. , Han , M. C. and Kim , C. H. Tetrahedral-mesh-based computational human phantom for fast Monte Carlo dose calculations . Phys. Med. Biol. 59 , 3173 – 3186 ( 2014 ). Google Scholar CrossRef Search ADS PubMed 11 Electric Power Research Institute . Impact of plant design and Chemistry on PWR releases and dose rates. TR 1013507 ( 2006 ). 12 KHNP . Shinkori 1,2 final safety analysis report, Ch. 11, 12. 13 Kim , C. et al. . Methodology for posture changes of mesh-type computational phantoms . Proc. Korean Assoc. Radiat. Prot. 196 – 197 ( 2017 ). 14 Radiation Dose Assessment Resource . Beta Spectrum Files for Nuclides.Available on http://www.doseinfo-radar.com/RADARDecay.html 15 International Commission on Radiation Protection . Conversion coefficients for radiological protection quantities for use for external radiation exposures. ICRP Publication 116. Ann. ICRP 40(2–5) ( 2010 ). 16 Chosun University . Source term prediction study for PWR and PHWR. pp. 89–96 ( 2016 ). 17 Frame and Abelquist . Use of smears for assessing removable contamination . Health Phys. 76 ( Supplement 2 ), S57 – S66 ( 1999 ). 18 Behrens , R. and Dietze , G. Monitoring the eye lens: which dose quantity is adequate? Phys. Med. Biol. 55 , 4047 – 4062 ( 2010 ). Google Scholar CrossRef Search ADS PubMed 19 Behrens , R. and Dietze , G. Dose conversion coefficients for photon exposure of the human eye lens . Phys. Med. Biol. 56 , 415 – 437 ( 2011 ). Google Scholar CrossRef Search ADS PubMed 20 KHNP . Outage radiation management reports of Hanbit nuclear power plants 5,6 ( 2015 –2017). © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

EVALUATION OF EYE LENS DOSE TO WORKERS IN THE STEAM GENERATOR AT THE KOREAN OPTIMIZED POWER REACTOR 1000

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Abstract

Abstract ICRP (2011) revised the dose limit to the eye lens to 20 mSv/y based on a recent epidemiological study of radiation-induced cataracts. Maintenance of steam generators at nuclear power plants is one of the highest radiation-associated tasks within a non-uniform radiation field. This study aims to evaluate eye lens doses in the steam generators of the Korean OPR1000 design. The source term was characterized based on the CRUD-specific activity, and both the eye lens dose and organ dose were simulated using MCNP6 combined with an ICRP voxel phantom and a mesh phantom, respectively. The eye lens dose was determined to be 5.39E-02–9.43E-02 Sv/h, with a negligible effect by beta particles. As the effective dose was found to be 0.81–1.21 times the lens equivalent dose depending on the phantom angles, the former can be used to estimate the lens dose in the SG of the OPR1000 for radiation monitoring purposes. INTRODUCTION A recent epidemiological study of the radiation effect on the eye lens suggested that the associated cataract risk is far higher than that proposed in a previous study, which had been the basis for the ICRP 60 dose limit. As a result of the review, ICRP revised the dose-equivalent limit to the eye lens from 150 to 20 mSv/y in 2011(1). Extensive research thereafter has attempted to evaluate lens doses more accurately under various radiation conditions and geometric configurations of the source to the receptor so as to meet the new reduced dose limit. However, the current Korean regulatory framework for radiation protection is based on ICRP 60. The Korean nuclear regulatory body is performing a feasibility study to apply the new ICRP recommendations, including ICRP 103(2), which is expected to be codified in the National Nuclear Safety Act in the future. The current lens dose-equivalent limit for radiation workers (150 mSv/y) in Korea is 7.5 times higher than the effective dose limit of 20 mSv/y for an average of 5 years. Due to the fact that it is highly unlikely to expect a non-uniform radiation field where the dose rate in space is marginally different at the general access area in nuclear power plants, common understanding in Korea has prevailed between the regulatory body and utilities that the lens dose equivalent would be below the limit (150 mSv/y) if the effective dose is within its limit (20 mSv/y). Thus, it is a general practice at nuclear power plants regularly to measure workers’ effective doses and skin doses but not to evaluate the dose equivalent of the eye lens, except in special cases. The occupational radiation dose incurred at nuclear power plants amounts to ~34% of the total national radiation exposure in Korea as of 2015(3). The Korean optimized power reactor (OPR1000) is a 1000 MWe pressurized water reactor with two coolant loops developed based on System 80 by the Korea Hydro and Nuclear Power Co., Ltd. (KHNP). Currently, 12 OPR1000 systems are in operation in Korea. During normal operations and outages at nuclear power plants, their employees and contractors are generally exposed to various radiation sources, such as the reactor coolant and contaminated equipment and pipes. Figure 1 shows the average cumulative radiation exposure (CRE) at OPR1000s in 2016(4). It is necessary to investigate radiation tasks including the assembly and disassembly of the reactor where monitoring of the lens dose is required. As a first step in this study, an evaluation of the lens dose to the steam generator (SG) maintenance workers has been selected due to the high radiation level, the harsh working environment and the limited working time (a few minutes) allowed. Among the radiation tasks performed at nuclear power plants, SG-related maintenance is among those during which workers incurred high CRE levels at OPR1000 in 2016. In addition, the CRE associated with the maintenance of the SG primary side amounts to nearly 15% of the total CRE, as shown in Figure 1, as SGs become highly contaminated by activated corrosion deposits, i.e. Chalk River Unidentified Deposits (CRUD), on their surfaces during operation(5). If the regulatory limit for the eye lens were to be identical to the effective dose limit, it would be necessary to investigate whether the abovementioned practice of radiation protection at nuclear power plants remains valid or needs to be revised. In addition, guidance on evaluations of the lens dose equivalent must be established. However, very limited work has been carried out to evaluate the eye lens dose to people working in highly active SGs. In this regard, this study aims to characterize the radiation sources of the SGs of OPR1000s and to evaluate the eye lens dose to plant workers, particularly those involved in the maintenance of the SGs in preparation for the possible revision of the dose limit for the eye lens in Korea. In addition, the present study proposes a technically credible relationship between the effective dose or skin and lens equivalent doses. Figure 1. View largeDownload slide OPR CRE distribution by task. Figure 1. View largeDownload slide OPR CRE distribution by task. METHODOLOGY For an investigation and evaluation of the eye lens dose, the radioactive source term of the SG is one of the most important factors. However, it is generally not accessible for direct measurements of radioactive materials owing to the high dose rate (~tens of mSv/h) and the complexity of the geometry of the SG. In addition, the source term of the SG is known to vary depending on the plant operational history and the primary shutdown chemistry, among other factors(6). Thus, a conservative analysis based on the plant design report was conducted for radiation protection purposes to estimate the concentration of the CRUD deposited on the surface of the SG. The Monte Carlo N-Particle transport code (MCNP6) with a continuous energy spectrum and the latest nuclear cross-section library (END/B-VII.1) were used in this study. The ICRP 110 male voxel phantom was adopted in order to evaluate the eye lens dose equivalent and the effective dose(7). As a voxel of the male phantom is composed of a 2.137 × 2.137 × 8 mm3 cube, it is larger than very small organs such as the eye lens such that the associated dose calculation may be less accurate for weak beta particles(8). Therefore, the dose to the sensitive lens in the SG chamber was calculated for comparison purposes using a tetrahedral-mesh phantom developed based on ICRP 110 by Hanyang University under ICRP Committee 2(9). Figure 2 shows the eye model of the mesh phantom. For the dose calculation, the mesh phantom was directly implemented, without voxelization, in the MCNP6 code using the unstructured mesh geometry(10). Figure 2. View largeDownload slide Enlarged eye shape of the mesh phantom. Figure 2. View largeDownload slide Enlarged eye shape of the mesh phantom. SOURCE TERM The OPR1000 design has two SGs with cold and hot water chambers for each, in which maintenance tasks such as the opening and closing of manways are implemented. The maintenance of the SG primary side takes place mostly in water chambers with diameters of 2 m, where the whole body of the worker is exposed directly to the radiation source, as shown in Figures 3 and 4. The radiation source of the SG typically has CRUD deposited on its inner surface and is known to be highly dependent on the materials of the reactor coolant system, the chemical treatment practices of the reactor coolant, and the operational history(6). In addition, most of the radioactive sources present at nuclear power plants which cause external doses are contained in tanks, pipes and equipment generally made of steel, which beta particles cannot penetrate. However, beta particles from CRUD, such as 58Co, which is the main source used with SGs, can relatively easily reach the bodies of SG maintenance workers and present a dose to the eyes and skin(11). CRUD is deposited on the surfaces of the water chambers, dividers and tubes during the circulation of the reactor coolant, as sketched in Figure 4. On the other hand, it is very difficult directly to measure nuclides and the activity and distribution of the source inside the SG due to the high dose rates and the complex geometry. Therefore, it is a generally accepted practice for this source term to be estimated based on the equilibrium thickness of the CRUD and its specific activity. With regard to the SG tubes, their surface contamination levels are converted to the volume contamination for an efficient simulation assuming that the tubes are homogeneously distributed with water outside and air inside. The chamber and the divider are the surface source. Their activities are calculated as follows: Ai(Bq/cm2)=D(g−CRUD/cm2)×Si(Bq/g−CRUD), where Ai is activity of nuclide i per unit area; D (equilibrium thickness of CRUD): CRUD weight per unit area of the SG compartments (g-CRUD/cm2); Si (Specific activity of CRUD): activity of nuclide i per unit CRUD weight (Bq/g-CRUD)—Table 2; and D for the chamber and the divider is 1.0E-03g-CRUD/cm2, whereas D for the tubes is 1.0E-04g-CRUD/cm2. Figure 3. View largeDownload slide Schematic diagram of the SG. Figure 3. View largeDownload slide Schematic diagram of the SG. Figure 4. View largeDownload slide Geometric configuration of the SG sources and the phantom Figure 4. View largeDownload slide Geometric configuration of the SG sources and the phantom For the tubes, the surface activity was calculated according to the equation above. The total activity of the tubes was then calculated by multiplying its total surface area. The total activity was divided by the weight of the tubes, the air inside the tubes, and the water surrounding the tubes outside. For this study, the final safety analysis report of OPR1000 was used for the calculation of the SG source term, with the associated main parameters described in Tables 1 and 2(12). In comparison, a smear sample was taken from the cold leg manway of the SG of one of the OPRs and analyzed for nuclides using an HPGe detector system. Table 1. Main parameters for the estimation of the SG source term. Parts Dimension (mm) Material CRUD thickness (g/cm2) Chamber 2083(ID) Carbon steel 1.00E-03 Divider 2020 SA240 1.00E-03 U tubes (No. 8,214) 19.1(OD) 16.9(ID) Alloy 690TT 1.00E-04 Parts Dimension (mm) Material CRUD thickness (g/cm2) Chamber 2083(ID) Carbon steel 1.00E-03 Divider 2020 SA240 1.00E-03 U tubes (No. 8,214) 19.1(OD) 16.9(ID) Alloy 690TT 1.00E-04 Note: OD, outer diameter; ID, inner diameter. Table 1. Main parameters for the estimation of the SG source term. Parts Dimension (mm) Material CRUD thickness (g/cm2) Chamber 2083(ID) Carbon steel 1.00E-03 Divider 2020 SA240 1.00E-03 U tubes (No. 8,214) 19.1(OD) 16.9(ID) Alloy 690TT 1.00E-04 Parts Dimension (mm) Material CRUD thickness (g/cm2) Chamber 2083(ID) Carbon steel 1.00E-03 Divider 2020 SA240 1.00E-03 U tubes (No. 8,214) 19.1(OD) 16.9(ID) Alloy 690TT 1.00E-04 Note: OD, outer diameter; ID, inner diameter. Table 2. CRUD-specific activity of the SG. Nuclides CRUD activity (Bq/g, CRUD) 51Cr 7.92E09 54Mn 2.79E07 59Fe 4.49E07 8Co 2.53E09 60Co 6.57E07 95Zr 5.96E07 Nuclides CRUD activity (Bq/g, CRUD) 51Cr 7.92E09 54Mn 2.79E07 59Fe 4.49E07 8Co 2.53E09 60Co 6.57E07 95Zr 5.96E07 Table 2. CRUD-specific activity of the SG. Nuclides CRUD activity (Bq/g, CRUD) 51Cr 7.92E09 54Mn 2.79E07 59Fe 4.49E07 8Co 2.53E09 60Co 6.57E07 95Zr 5.96E07 Nuclides CRUD activity (Bq/g, CRUD) 51Cr 7.92E09 54Mn 2.79E07 59Fe 4.49E07 8Co 2.53E09 60Co 6.57E07 95Zr 5.96E07 DOSE SIMULATION MCNP6 was used to simulate the eye lens dose incurred by workers in the SG together with effective dose and the skin dose. The ICRP 110 male voxel phantom was incorporated in this study, as noted above. Although various postures of workers in the SG can be expected in actual situations, here, the phantom was placed in a standing position in the SG, as the posture of the ICRP 110 phantom cannot be changed and because phantoms for which the posture can be thus adjusted have not been fully developed(13). In addition to the lens dose, the skin dose and organ dose were evaluated at angles of 0, 90°, 180° and 270° in order to investigate the effect of the angular position of the phantom on the dose. The position of the phantom is shown in Figures 4 and 5. Once the source is estimated, the full energy spectra of each source location are input into MCNP6 for the dose simulation. In addition, the beta energy (bin) and yield of each source are separately input to investigate the contribution of the doses to the lens by beta and gamma energy. RADAR was used to convert the continuous energy spectrum of the beta sources into discrete energy bins for the MCNP input(14). The track length estimate of the energy deposition (F6:E tally) was used to determine the dose absorbed by the organs, and the maximum number of source particles for the dose simulation was set to 5E09. Furthermore, a variance-reduction technique, in this case source biasing, was applied such that the relative error associated with the voxel simulation would be less than the MCNP recommended value of 10%. Primary photons and secondary electrons were tracked down to levels of 1 and 14 keV, respectively, for the simulation. The lens dose was calculated by averaging the doses to the right lens and the left lens(2, 15). The organ equivalent dose for the male phantom was calculated based on ICRP 103 organ weighting factors and the ICRP 116 methodology for the calculation of the doses to red bone marrow and the bone surface(15). Most previous studies of eye lens doses focused on a few idealized geometries and mono-energetic gamma or beta sources in the medical area but not in nuclear power plants. There have been no in-depth studies of eye lens doses in non-uniform radiation fields incurred by equipment such as a SG at a nuclear power plant. Moreover, high levels of concern over the contribution of the dose to the lens by beta particles have been raised due to the short distance from the radioactive sources of the SG to the receptor. Thus, the doses to the eye lens by beta and gamma rays were separately calculated for an evaluation of their dose contributions and the special need to monitor beta radiation. In addition, the presence of any correlation of the skin dose equivalent or organ dose to the eye lens was investigated because the eye lens lies geometrically between the skin and the organs. Therefore, the skin and organ dose equivalents were evaluated along with the lens dose equivalent in this study. A voxel phantom is reported to be less accurate when used to simulate the eye lens equivalent due to the size of the voxel, which is found to be relatively large compared to the corresponding human anatomical size(8). Therefore, the dose to the sensitive lens was additionally evaluated using the head part of the mesh phantom in comparison with the voxel dose result. Figure 5. View largeDownload slide Angular position of the phantom in the SG. Figure 5. View largeDownload slide Angular position of the phantom in the SG. RESULTS AND DISCUSSION Source inventory of the SG The estimated radionuclide inventory is summarized in Table 3 for the main parts of the SG, including the water chambers, the divider and the tubes. It was used as the input of the radionuclide sources into the Monte Carlo dose simulation code. The amount of surface contamination on the chambers and dividers was calculated and found to be to be 1.06E07 Bq/cm2, and contamination of the tubes was 3.56E05 Bq/cm3, stemming from the assumption that the surface activity of the tubes was distributed throughout the volume of the tubes, air and water both inside and outside of them. The gamma and beta energy spectra for each source location are presented in Figures 6 and 7. The main nuclides of the SG were found to be 58Co and 51Cr. The average gamma and beta energy levels were analyzed and found to be 427 and 2.6 keV, respectively. Although there are a few models which can be used to estimate radionuclides on the surfaces of the primary side of reactors(16), they estimate only CRUD activities while excluding fission product activities, which can be neglected unless fuel defects arise. Furthermore, there is very limited research on measurements of actual inventories of SGs. Thus, as a comparison indicator, one of the water chambers at the OPR was smear-sampled and analyzed for radionuclides using an HPGe detector system because there was no way to access the other parts of the SG without waiting for dismantling during the plant’s decommissioning stage. The composition of the radionuclides from the actual smear test results was compared with the estimated inventory, as shown in Table 2. However, these values could only be qualitatively compared because smear tests do not give credible quantitative results pertaining to contamination, as the results strongly depend on the fraction of fixed and non-fixed contaminants on the surface and their collection efficiency(17). The two main nuclides for both the smear samples and the estimated source term were found to be 58Co and 51Cr. Table 3. Estimated radionuclide inventory of the SG. Nuclides Chamber (Bq/cm2) Divider (Bq/cm2) Tubes (Bq/g) Measurements (Bq/300 cm2) 51Cr 7.92E06 7.92E06 2.65E05 6.64E02 54Mn 2.79E04 2.79E04 9.27E02 7.04E02 59Fe 4.49E04 4.49E04 1.49E03 7.44E01 58Co 2.53E06 2.53E06 8.40E04 5.60E03 60Co 6.57E04 6.57E04 2.18E03 2.15E03 95Zr 5.96E04 5.96E04 1.98E03 4.16E03 Nuclides Chamber (Bq/cm2) Divider (Bq/cm2) Tubes (Bq/g) Measurements (Bq/300 cm2) 51Cr 7.92E06 7.92E06 2.65E05 6.64E02 54Mn 2.79E04 2.79E04 9.27E02 7.04E02 59Fe 4.49E04 4.49E04 1.49E03 7.44E01 58Co 2.53E06 2.53E06 8.40E04 5.60E03 60Co 6.57E04 6.57E04 2.18E03 2.15E03 95Zr 5.96E04 5.96E04 1.98E03 4.16E03 Table 3. Estimated radionuclide inventory of the SG. Nuclides Chamber (Bq/cm2) Divider (Bq/cm2) Tubes (Bq/g) Measurements (Bq/300 cm2) 51Cr 7.92E06 7.92E06 2.65E05 6.64E02 54Mn 2.79E04 2.79E04 9.27E02 7.04E02 59Fe 4.49E04 4.49E04 1.49E03 7.44E01 58Co 2.53E06 2.53E06 8.40E04 5.60E03 60Co 6.57E04 6.57E04 2.18E03 2.15E03 95Zr 5.96E04 5.96E04 1.98E03 4.16E03 Nuclides Chamber (Bq/cm2) Divider (Bq/cm2) Tubes (Bq/g) Measurements (Bq/300 cm2) 51Cr 7.92E06 7.92E06 2.65E05 6.64E02 54Mn 2.79E04 2.79E04 9.27E02 7.04E02 59Fe 4.49E04 4.49E04 1.49E03 7.44E01 58Co 2.53E06 2.53E06 8.40E04 5.60E03 60Co 6.57E04 6.57E04 2.18E03 2.15E03 95Zr 5.96E04 5.96E04 1.98E03 4.16E03 Figure 6. View largeDownload slide Energy spectrum of the SG chamber/divider. Figure 6. View largeDownload slide Energy spectrum of the SG chamber/divider. Figure 7. View largeDownload slide Energy spectrum of the SG tubes. Figure 7. View largeDownload slide Energy spectrum of the SG tubes. Simulated dose with the voxel phantom The dose to the eye lens of the ICRP male voxel phantom in the SG at OPR1000 was simulated. Both the organ dose and the skin dose equivalent were calculated for comparison. In addition, the highest dose rate in the SGs of OPR1000s measured using an Automess Teletector 6112 MH was plotted in order to investigate the validity of the dose simulation results as well as the SG source. The eye lens dose equivalent with the ICRP 110 voxel phantom The eye lens dose rate at an angle of 0° was determined to be 7.65E-02 Sv/h. As shown in Table 4, gamma rays from the SG were found to play a dominant role in the dose to the eye lens, whereas the dose effect by beta rays was minimal. This phenomenon could be explained by the low average energy of beta particles, i.e. 2.6 keV. According to Behrens and Dietze, the equivalent dose to a sensitive lens per photon fluence (pSv cm2) at 400 keV is 2.281, whereas the beta conversion factor at the average energy level of 2.6 keV was not presented but is <1.9E-03 (pSv cm2)(18). In consideration of this ratio, the dose contribution by beta rays is likely negligible. In addition, 83% of the lens dose was identified to have been caused by the water chamber and the divider. On the other hand, radiation originating from the tubes is greatly reduced owing to the self-shielding material of the tubes, alloy 690, as well as the water outside the tubes. Table 4. Simulated organ dose using the voxel phantom at an angle of 0°. Radiation Sources Lens (Sv/h) Skin (Sv/h) Organs (Sv/h) Gamma Chamber 2.37E-02 3.67E-02 2.35E-02 Divider 4.03E-02 4.61E-02 3.73E-02 Tubes 1.30E-02 1.05E-02 1.27E-02 Beta Chamber 8.60E-07 1.29E-04 1.43E-06 Divider 1.47E-06 5.58E-04 6.16E-06 Tubes 9.90E-10 4.68E-08 7.69E-10 Total 7.70E-02 9.36E-02 6.88E-02 Radiation Sources Lens (Sv/h) Skin (Sv/h) Organs (Sv/h) Gamma Chamber 2.37E-02 3.67E-02 2.35E-02 Divider 4.03E-02 4.61E-02 3.73E-02 Tubes 1.30E-02 1.05E-02 1.27E-02 Beta Chamber 8.60E-07 1.29E-04 1.43E-06 Divider 1.47E-06 5.58E-04 6.16E-06 Tubes 9.90E-10 4.68E-08 7.69E-10 Total 7.70E-02 9.36E-02 6.88E-02 Table 4. Simulated organ dose using the voxel phantom at an angle of 0°. Radiation Sources Lens (Sv/h) Skin (Sv/h) Organs (Sv/h) Gamma Chamber 2.37E-02 3.67E-02 2.35E-02 Divider 4.03E-02 4.61E-02 3.73E-02 Tubes 1.30E-02 1.05E-02 1.27E-02 Beta Chamber 8.60E-07 1.29E-04 1.43E-06 Divider 1.47E-06 5.58E-04 6.16E-06 Tubes 9.90E-10 4.68E-08 7.69E-10 Total 7.70E-02 9.36E-02 6.88E-02 Radiation Sources Lens (Sv/h) Skin (Sv/h) Organs (Sv/h) Gamma Chamber 2.37E-02 3.67E-02 2.35E-02 Divider 4.03E-02 4.61E-02 3.73E-02 Tubes 1.30E-02 1.05E-02 1.27E-02 Beta Chamber 8.60E-07 1.29E-04 1.43E-06 Divider 1.47E-06 5.58E-04 6.16E-06 Tubes 9.90E-10 4.68E-08 7.69E-10 Total 7.70E-02 9.36E-02 6.88E-02 Skin dose equivalent with the ICRP 110 voxel phantom The skin dose rate at an angle of 0° in the SG as shown in Table 4 was determined to be 9.36E-02 Sv/h. The contaminated chambers and the divider were found to contribute to the skin dose rate most prevalently, as in case of the eye dose rate mentioned above. The skin dose in this study was found to be greater than the lens dose by 22% at 0° and by maximum of 76% at 270°. This stems from the fact that the skin equivalent depth (7 mg/cm2) is less than the lens equivalent depth (300 mg/cm2). The ratio of the simulated skin to the lens dose was found to be 0.95–1.76 in comparison with 1.1 as determined by Behrens, seemingly due to difference in the source configuration and energy spectra(18). Organ dose with the ICRP 110 voxel phantom The organ dose rate at an angle of 0° was simulated and found to be 6.88E-02 Sv/h with an error of 0.4%. This was in relatively good agreement with the measured exposure rate, as shown in Figure 8, in consideration of the detector error of ± 10% and variation in the dose rate (at a factor of 3 at most) at measuring points. As indicated in Table 4, gamma rays also played a dominant role in the effective dose, whereas the beta ray contribution to the dose was found to be <1% due to the shielding effect of the skin covering the organs of the phantom and considering that the maximum range of beta particles with an average energy level of ~3 keV in water is ~2E-03 mm. Figure 8. View largeDownload slide Simulated angular dose rate inside the SG. Figure 8. View largeDownload slide Simulated angular dose rate inside the SG. Dose at the angular position of the phantom Considering how workers move in the SG and use different postures when performing their designated work tasks, the phantom was placed for the dose simulation at four different angles ranging from 0° to 270°. Table 5 and Figure 8 show the simulated dose rates for each angle. Lens doses at 0°, 90° and 180° were found to be identical within the simulated range of error, except for that at 270°, where the lens of the phantom is opposite to the divider. The skin doses at four angles were found to be greater than the lens dose and the effective dose, except at 90°. The lens dose rate was determined to be between 5.35E-02 and 9.39E-02 Sv/h depending on the phantom angle. The average dose rate in the SG chambers of OPR1000s was measured and found to be 46 mSv/h, with maximum and minimum values of 70 and 28 mSv/h, respectively, during the first to the fourth outages(4). As shown in Figure 8, the simulated doses at the four different angles were relatively well in the range of the exposure rate in consideration of the uncertainties involved in the simulation and measurements. This can also support the contention that the source inventory of the SG used in the study was technically reasonable. Table 5. Comparison of the lens, skin dose equivalent and effective dose at different angles. Angle Eye lens (Sv/h) Skin (Sv/h) Effective dose (Sv/h) 0° 7.70E-02 (10.1%) 9.40E-02 (0.1%) 7.25E-02 (0.4%) 90° 9.43E-02 (8.7%) 8.94E-02 (0.1%) 7.59E-02 (0.4%) 180° 8.53E-02 (8.2%) 1.05E-01 (0.1%) 7.75E-02 (0.4%) 270 5.39E-02 (7.1%) 9.51E-02 (0.1%) 6.53E-02 (0.4%) Angle Eye lens (Sv/h) Skin (Sv/h) Effective dose (Sv/h) 0° 7.70E-02 (10.1%) 9.40E-02 (0.1%) 7.25E-02 (0.4%) 90° 9.43E-02 (8.7%) 8.94E-02 (0.1%) 7.59E-02 (0.4%) 180° 8.53E-02 (8.2%) 1.05E-01 (0.1%) 7.75E-02 (0.4%) 270 5.39E-02 (7.1%) 9.51E-02 (0.1%) 6.53E-02 (0.4%) (): Relative error. Table 5. Comparison of the lens, skin dose equivalent and effective dose at different angles. Angle Eye lens (Sv/h) Skin (Sv/h) Effective dose (Sv/h) 0° 7.70E-02 (10.1%) 9.40E-02 (0.1%) 7.25E-02 (0.4%) 90° 9.43E-02 (8.7%) 8.94E-02 (0.1%) 7.59E-02 (0.4%) 180° 8.53E-02 (8.2%) 1.05E-01 (0.1%) 7.75E-02 (0.4%) 270 5.39E-02 (7.1%) 9.51E-02 (0.1%) 6.53E-02 (0.4%) Angle Eye lens (Sv/h) Skin (Sv/h) Effective dose (Sv/h) 0° 7.70E-02 (10.1%) 9.40E-02 (0.1%) 7.25E-02 (0.4%) 90° 9.43E-02 (8.7%) 8.94E-02 (0.1%) 7.59E-02 (0.4%) 180° 8.53E-02 (8.2%) 1.05E-01 (0.1%) 7.75E-02 (0.4%) 270 5.39E-02 (7.1%) 9.51E-02 (0.1%) 6.53E-02 (0.4%) (): Relative error. Lens dose using the voxel phantom vs the sensitive lens dose using a mesh phantom As noted above, the ICRP 110 voxel phantom is reportedly less accurate when used to describe the sensitivity of the eye lens to radiation as compared to a mesh phantom. Thus, the sensitive eye lens dose using the head part of the mesh phantom was simulated at an angle of 0° in order to compare the simulated results using the ICRP voxel phantom to investigate any differences between them. As shown in Table 6, the sensitive lens dose using the mesh phantom was found to be 3.27% less than the eye lens dose using the voxel phantom. In consideration of the average gamma energy of 409 keV radiating from the SG, this result is in good agreement with the findings of Nguyen et al.(9), who found a maximum difference within 5% between the dose conversion coefficients for the sensitive lens with the mesh phantom and the eye lens dose conversion coefficients with the ICRP 110 voxel phantom. This result is also supported by the findings of Behrens and Dietze, who showed that the sensitive lens dose was within an error range of −3 to 2.5%(19). As the lens dose by beta rays in the SG is negligible, the dose effect of the former type has not been studied in detail. Table 6. Simulated eye lens dose vs the sensitive eye lens with the head parts of phantoms at an angle of 0°. Organ Head mesh Head voxel Voxel Sensitive lens (lens) 6.08E-02 6.37E-02 6.35E-02 Entire lens (bulbs) 6.52E-02 5.62E-02 5.40E-02 Organ Head mesh Head voxel Voxel Sensitive lens (lens) 6.08E-02 6.37E-02 6.35E-02 Entire lens (bulbs) 6.52E-02 5.62E-02 5.40E-02 Table 6. Simulated eye lens dose vs the sensitive eye lens with the head parts of phantoms at an angle of 0°. Organ Head mesh Head voxel Voxel Sensitive lens (lens) 6.08E-02 6.37E-02 6.35E-02 Entire lens (bulbs) 6.52E-02 5.62E-02 5.40E-02 Organ Head mesh Head voxel Voxel Sensitive lens (lens) 6.08E-02 6.37E-02 6.35E-02 Entire lens (bulbs) 6.52E-02 5.62E-02 5.40E-02 Comparison with Measurements and the Previous Study The results of this study were compared with those of Behrens and Dietze’ study(18) and the measured TLD readings for the SG maintenance work conducted on the OPR1000 design from 2015 to 2017(20), as presented in Table 7. The TLD system (Harshaw 6600), which OPR1000s use for dose evaluation, has four elements and is capable of measuring lens doses with a 300 mg/cm2 filter and effective doses with a 1000 mg/cm2 filter simultaneously. The SG maintenance workers wear two TLDs, one on the chest and the other on the back. According the plant procedure, the effective dose is calculated with the equation 0.55 × chest dose + 0.5 × back dose. Because there is no specific procedure, lens dose is calculated as the chest or the back dose, whichever is greater for a conservative approach. Records of the TLD readings of the lens doses to workers who performed the SG maintenance were investigated. If the skin dose is used for control of the dose limits to the lens dose for SG maintenance work, it could overestimate the lens dose by up to 76%. Therefore, the skin dose can be used as a substitute for the lens dose simply for conservative radiation protection purposes, as there is no risk of exceeding the lens dose limit. If the lens dose is considered to be 123% of the effective dose, any overestimation of the lens dose will be less severe as compared to when using the skin dose for the lens dose. Therefore, it is more reasonable to use the effective dose as a substitute for the lens dose incurred in the SG. Table 7. Comparison with earlier findings and the measurements. Ratio This study Behrensa Measured OPR1000 E [Hp(10)] Hlens [Hp(3)] 0.81–1.21 0.9–1.2 1.07 ± 0.18 Hskin [Hp(0.07)] Hlens [Hp(3)] 0.95–1.76 1.1 (1–500)b 1.12 ± 0.1 Ratio This study Behrensa Measured OPR1000 E [Hp(10)] Hlens [Hp(3)] 0.81–1.21 0.9–1.2 1.07 ± 0.18 Hskin [Hp(0.07)] Hlens [Hp(3)] 0.95–1.76 1.1 (1–500)b 1.12 ± 0.1 aPhoton > 30 keV; belectron (<1.5 MeV) + photon. Table 7. Comparison with earlier findings and the measurements. Ratio This study Behrensa Measured OPR1000 E [Hp(10)] Hlens [Hp(3)] 0.81–1.21 0.9–1.2 1.07 ± 0.18 Hskin [Hp(0.07)] Hlens [Hp(3)] 0.95–1.76 1.1 (1–500)b 1.12 ± 0.1 Ratio This study Behrensa Measured OPR1000 E [Hp(10)] Hlens [Hp(3)] 0.81–1.21 0.9–1.2 1.07 ± 0.18 Hskin [Hp(0.07)] Hlens [Hp(3)] 0.95–1.76 1.1 (1–500)b 1.12 ± 0.1 aPhoton > 30 keV; belectron (<1.5 MeV) + photon. CONCLUSION Since ICRP (2011) reduced its dose limit to 20 mSv/y from 150 mSv/y according to the reevaluated risk of the radiation dose to the eye lens, a variety of studies to evaluate the lens dose have been carried out mostly for medical research and in relation to ideal cases with simple geometries and mono-energetic beta and gamma particles. Although maintenance work in the SG is one of the tasks with the highest CRE levels at nuclear power plants, no research has attempted to evaluate the eye lens doses to workers in the SG, where a non-uniform radiation field and a complex geometry environment exist. In order to evaluate the lens dose in the OPR1000 SG, the corresponding radioactive source term was initially characterized based on the CRUD activity and the average energy levels of gamma and beta particles were determined to be 427 and 2.6 keV, respectively. Secondly, the lens dose in the SG of the OPR1000 design was simulated and found to be 5.35E-02–9.39E-02 Sv/h using MCNP6 and the ICRP voxel phantom. The lens dose was mostly caused by the gamma component of the source, with <1% of the dose by beta particles. Based on the lens dose rate, if a SG worker stays in the SG for 12.9 min or longer, he could reach the new annual lens dose limit. However, the risk of exceeding the dose limit is not anticipated in consideration of plant practices for workforce allocations for SG maintenance Thirdly, the skin dose and effective dose calculated according to ICRP 103 methodology were found to be respectively 0.95–1.76 and 0.81–1.21 times the simulated lens doses depending on the phantom angle. These values are in good agreement with the findings of Behrens regarding the gamma dominant radiation field. In addition, the ratios of the skin dose and effective dose to the lens dose measured in the OPR1000 SG were found to be between the ranges simulated in this study. Thus, 123% of the effective dose was proposed as a substitute for the lens dose for the implementation of ALARA, though the skin equivalent dose can be used to estimate the lens dose for conservative radiation protection purposes. This study is the first in-depth investigation of the lens dose in the non-uniform and highly radioactive conditions present at nuclear power plants, and the findings here will be very useful if the lens dose limit is revised based on the new ICRP recommendation in Korea. For more accurate evaluations of the lens dose, it will be necessary to measure the energy spectrum of the SG, improve the MCNP modeling to include factors such as blinking of the eyelids, and the use of masks, and to simulate actual working postures. In addition, evaluations of the lens dose for tasks involving high radiation levels other than those related to the SG at nuclear power plants should be conducted. ACKNOWLEDGEMENTS The authors would like to thank Professor C.H. Kim and coworkers at Hanyang University for allowing us to use the digital phantom and for the precious advice during this study. REFERENCES 1 International Commission on Radiation Protection . Statement on tissue reactions. International Commission on Radiological Protection ( 2011 ). 2 International Commission on Radiation Protection . Recommendations of the international commission on radiological protection. ICRP Publication 103 ( 2007 ). 3 Nuclear Safety and Security . Radiation exposure status in 2015. Nuclear Safety Yearbook 2015. pp. 283–284 ( 2016 ). 4 Korea Hydro and Nuclear Power Co., Ltd. Annual radiation management report ( 2015 ). 5 Deshon , J. Modeling PWR fuel corrosion product deposition and growth process. Technical report 101173 ( Palo Alto, CA : EPRI ) ( 2005 ). 6 OECD/NEA . Radiation protection aspects of primary water chemistry and source-term management. (17–30) April 2014 7 International Commission on Radiation Protection . Adult reference computational phantoms. ICRP 110. 2009. Ann. ICRP 39(2) ( 2009 ). 8 Kim , C. H. et al. . The reference phantoms: voxel vs polygon . Ann. ICRP 45 , 188 – 201 ( 2016 ). Google Scholar CrossRef Search ADS PubMed 9 Nguyen , T. T. et al. . Incorporation of detailed eye model into polygon mesh version of ICRP 110 reference phantoms . Radiat. Prot. Dosim. 168 ( 1 ), 8695 – 8707 ( 2016 ). 10 Yeom , Y. S. , Jeong , J. H. , Han , M. C. and Kim , C. H. Tetrahedral-mesh-based computational human phantom for fast Monte Carlo dose calculations . Phys. Med. Biol. 59 , 3173 – 3186 ( 2014 ). Google Scholar CrossRef Search ADS PubMed 11 Electric Power Research Institute . Impact of plant design and Chemistry on PWR releases and dose rates. TR 1013507 ( 2006 ). 12 KHNP . Shinkori 1,2 final safety analysis report, Ch. 11, 12. 13 Kim , C. et al. . Methodology for posture changes of mesh-type computational phantoms . Proc. Korean Assoc. Radiat. Prot. 196 – 197 ( 2017 ). 14 Radiation Dose Assessment Resource . Beta Spectrum Files for Nuclides.Available on http://www.doseinfo-radar.com/RADARDecay.html 15 International Commission on Radiation Protection . Conversion coefficients for radiological protection quantities for use for external radiation exposures. ICRP Publication 116. Ann. ICRP 40(2–5) ( 2010 ). 16 Chosun University . Source term prediction study for PWR and PHWR. pp. 89–96 ( 2016 ). 17 Frame and Abelquist . Use of smears for assessing removable contamination . Health Phys. 76 ( Supplement 2 ), S57 – S66 ( 1999 ). 18 Behrens , R. and Dietze , G. Monitoring the eye lens: which dose quantity is adequate? Phys. Med. Biol. 55 , 4047 – 4062 ( 2010 ). Google Scholar CrossRef Search ADS PubMed 19 Behrens , R. and Dietze , G. Dose conversion coefficients for photon exposure of the human eye lens . Phys. Med. Biol. 56 , 415 – 437 ( 2011 ). Google Scholar CrossRef Search ADS PubMed 20 KHNP . Outage radiation management reports of Hanbit nuclear power plants 5,6 ( 2015 –2017). © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

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Radiation Protection DosimetryOxford University Press

Published: Mar 15, 2018

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