Abstract The Neutron Standards Laboratory of CIEMAT has conducted the characterization of the independent spent fuel storage installation at the Trillo Nuclear Power Plant. At this facility, the spent fuel assemblies are stored in ENSA-DPT type dual purpose casks. Neutron characterization was performed by dosimetry measurements with a neutron survey meter (LB6411) inside the facility, around an individual cask and between stored casks, and outside the facility. Spectra measurements were also performed with a Bonner sphere system in order to determine the integral quantities and validate the use of the neutron monitor at the different positions. Inside the facility, measured neutron spectra and neutron ambient dose equivalent rate are consistent with the casks spatial distribution and neutron emission rates, and measurements with both instruments are consistent with each other. Outside the facility, measured neutron ambient dose equivalent rates are well below the 0.5 μSv/h limit established by the nuclear regulatory authority. INTRODUCTION The spent fuel casks produced in the Trillo Nuclear Power Plant (NPP) are stored in an independent storage installation (ATI) until the Spanish Centralized Storage Installation (ATC) is completed. Trillo NPP used metal casks, ENS-DPT type, stored in a large concrete warehouse (80 m long, 40 m wide and 20 m tall) with capacity for 80 casks. Several types of spent fuel are stored in the ATI, with different burn-up, enrichment and cooling. For this reason, ATI shows an inhomogeneous H*(10) rate distribution around the casks inside the ATI. From the Radiation Protection (RP) point of view, the measurement of H*(10) inside and especially outside this building is a concern, and routine measurements are made by the RP Service with their own equipment. The employed survey meters are usually calibrated in 252Cf and/or 241Am–Be neutron fields following ISO 8529-1 recommendation, which differ from the realistic neutron fields found around the cask and outside the ATI, and therefore, it is desirable to check the survey meters performance in these fields(1–3). In order to perform an exhaustive and metrological grade characterization of the neutron fields in Trillo NPP storage installation, a Bonner Sphere System (CIEMAT-BSS) was used to measure neutron spectra, determine the integral quantities derived from them and compare with dosimetry measurements with an LB6411 neutron monitor. The comparison of spectrometry and dosimetry measurements allows validating monitor measurements in the neutron fields around the casks. MATERIALS AND METHODS The ENSA-DPT cask is dual purpose metal container, designed and manufactured by the Spanish company Equipos Nucleares S.A. for the dry transport and storage of spent nuclear fuel. It has capacity for 21 Kraftwerk Union 16 × 16–20 light water PWR spent fuel assemblies. The overall dimensions of the cask are 506 cm tall and 236 cm diameter and it consists of a double steel cylinder with gamma shielding lead layer between them and neutron shielding layer outside. The cask has a double lid made of steel with an extra neutron shielding lid, and other neutron shielding layer inside the cask bottom. The neutron shielding material employed in the cask is NS4FR(4), a hydrogenated polymer with boron and alumina. The cask is equipped with four lifting trunnions at the upper part and two rotating trunnion holders at the lower part to allow its manipulation during the transport, and they cause a reduction of the lateral neutron shielding. Measuring instruments The neutron survey meter used is a Berthold LB6411, which consists of a thermal neutron detector of 3He surrounded by a spherical polyethylene moderator. It was calibrated in 252Cf and 241Am–Be neutron sources at CMI (Czech Republic). The 252Cf calibration factor was used to correct LB6411 measurements. The CIEMAT-BSS(5–7) consists of 12 polyethylene spheres with diameters from 3′ to 12′ and a SP9 3He proportional counter. It is calibrated at PTB (Germany) in monoenergetic neutron beams. The codes employed for the unfolding process are GRAVEL and MAXED from UMG 3.3 package. Detailed Monte Carlo simulations of the ENSA-DPT cask and the ATI of Trillo NPP for the SCALE 6.1 code were developed to provide initial spectra for the unfolding. Integral quantities, ϕ and H*(10) with their uncertainties, are determined from measured spectra applying the corresponding ICRP74 fluence-to-dose conversion factors. Measurements Measurements were made in different campaigns taking advantage of the spent fuel load process of the casks. Casks DPT27 and DPT28 were positioned temporarily inside the ATI storage area, but away from the rest of the stored casks to try to minimize their effect on measurements. These measurements around individual casks were carried out at different heights from the floor (100, 250 and 440 cm) and at 1 m from the cask lateral surface. For each height, measurements were made both with BSS and LB6411, in order to compare results of H*(10) rate. Besides, measurements between every group of four casks were performed to establish a grid of neutron H*(10) rate inside ATI. Neutron spectra were also measured with BSS at two positions, one among a group of recently loaded casks and another among a group of casks loaded at the beginning of the operation of the ATI (identified as new and old respectively). Finally, neutron spectra were measured at two points outside the ATI, at 1 m distance from West and South walls, and H*(10) rate was also measured with LB6411 along North and South walls. These measurements took a long time because the measured doses were slightly higher than background neutron dose. It was necessary to protect equipment from weather conditions. RESULTS AND DISCUSSION Individual cask measurements Figure 1 shows BSS measured neutron spectra at different heights for the DPT28 and SCALE calculated initial spectra for the unfolding. They show three components, a degraded fission spectra in fast energy region, a peak in thermal energy region and an intermediate epithermal component. Figure 1. View largeDownload slide Neutron spectra around DPT28 ate three height levels. Figure 1. View largeDownload slide Neutron spectra around DPT28 ate three height levels. BSS measured and SCALE calculated spectra differ in the intensities but the overall shape of the spectrum remains the same. Most of the structures observed in calculated spectra appear also in measured spectra; therefore, they are physical and not an artifact from the numerical unfolding. Some of these structures could be identified from neutron cross section as the Fe window near 0.03 MeV or an O window approximately between 0.5 and 1 MeV. Peaks between them are identified as Fe resonances as consequence of the steel structure of the cask. Differences in intensities could be explained since in the ATI simulations only the individually evaluated cask was included, but not the rest of stored casks. The advantage of using initial spectrum derived from detailed simulations with BSS measurements is that the calculation gets reasonably correct spectrum shape and details and the measurement helps get the absolute value. Spectrum at 250 cm height shows the highest thermal component and spectrum at 440 cm height is significantly harder than at 100 and 250 cm. Measurement at 440 cm height is in front of a lifting trunnion of the cask, and measurement at 100 cm is above a rotating trunnion, located at 60 cm height. Trunnions reduce the neutron shielding of the cask, and therefore resonances are more clearly visible at these heights, especially at 440 cm. For this point the thermal contribution is lower also because the higher distance to floor. Measurement at 250 cm corresponds to the middle height of the cask, with no reductions of neutron shielding, and therefore with higher thermal component and less visible resonances. Thermal peak is shifted from the most probable energy (0.025 eV at room temperature) to higher energies (in this case 0.1 eV). This is a known effect of neutron absorption in moderator media(8). Contributions to thermal peak are backscattered neutrons from concrete walls and floor and thermalized neutrons from neutron shielding of the evaluated casks. From the different intensities between measured and calculated spectra can be deduced that the contribution of the rest of stored casks is significant, and the neutron shielding contains B to absorb thermal neutrons. The comparison between H*(10) rates from BSS and LB6411 are shown in Table 1 for all the positions. Values close to 80 μSv/h are obtained for positions around the cask, except for the 440 cm height where they are lower. No trend to under or overestimate BSS measured H*(10) rate has been observed. The results are statistically compatible and the field factor (FF), obtained as the ratio between LB6411 and BSS measurements of H*(10) rates, shows differences under 10% and only in one case goes up to 14%. From this table is possible to conclude that the use of LB6411 is validated for measurements around these casks. Table 1. Comparison between LB6411 and BSS measurements for H*(10) rate. Magnitude H*(10) (μSv/h) Location LB6411 BSS Field factor DPT27 100 83 ± 3 84 ± 7 0.99 ± 0.09 DPT27 250 82 ± 3 92 ± 6 0.89 ± 0.06 DPT28 100 81 ± 3 78 ± 7 1.05 ± 0.10 DPT28 250 85 ± 3 87 ± 5 0.98 ± 0.06 DPT28 440 61 ± 2 64 ± 5 0.95 ± 0.08 New casks 153 ± 5 147 ± 9 1.04 ± 0.07 Old casks 88 ± 3 78 ± 6 1.14 ± 0.10 Outside West 0.053 ± 0.005 0.056 ± 0.005 0.92 ± 0.11 Outside South 0.047 ± 0.003 Magnitude H*(10) (μSv/h) Location LB6411 BSS Field factor DPT27 100 83 ± 3 84 ± 7 0.99 ± 0.09 DPT27 250 82 ± 3 92 ± 6 0.89 ± 0.06 DPT28 100 81 ± 3 78 ± 7 1.05 ± 0.10 DPT28 250 85 ± 3 87 ± 5 0.98 ± 0.06 DPT28 440 61 ± 2 64 ± 5 0.95 ± 0.08 New casks 153 ± 5 147 ± 9 1.04 ± 0.07 Old casks 88 ± 3 78 ± 6 1.14 ± 0.10 Outside West 0.053 ± 0.005 0.056 ± 0.005 0.92 ± 0.11 Outside South 0.047 ± 0.003 Table 1. Comparison between LB6411 and BSS measurements for H*(10) rate. Magnitude H*(10) (μSv/h) Location LB6411 BSS Field factor DPT27 100 83 ± 3 84 ± 7 0.99 ± 0.09 DPT27 250 82 ± 3 92 ± 6 0.89 ± 0.06 DPT28 100 81 ± 3 78 ± 7 1.05 ± 0.10 DPT28 250 85 ± 3 87 ± 5 0.98 ± 0.06 DPT28 440 61 ± 2 64 ± 5 0.95 ± 0.08 New casks 153 ± 5 147 ± 9 1.04 ± 0.07 Old casks 88 ± 3 78 ± 6 1.14 ± 0.10 Outside West 0.053 ± 0.005 0.056 ± 0.005 0.92 ± 0.11 Outside South 0.047 ± 0.003 Magnitude H*(10) (μSv/h) Location LB6411 BSS Field factor DPT27 100 83 ± 3 84 ± 7 0.99 ± 0.09 DPT27 250 82 ± 3 92 ± 6 0.89 ± 0.06 DPT28 100 81 ± 3 78 ± 7 1.05 ± 0.10 DPT28 250 85 ± 3 87 ± 5 0.98 ± 0.06 DPT28 440 61 ± 2 64 ± 5 0.95 ± 0.08 New casks 153 ± 5 147 ± 9 1.04 ± 0.07 Old casks 88 ± 3 78 ± 6 1.14 ± 0.10 Outside West 0.053 ± 0.005 0.056 ± 0.005 0.92 ± 0.11 Outside South 0.047 ± 0.003 Measurements between stored casks The neutron H*(10) rate grid established by measurements with LB6411 between every group of four loaded casks or empty storage positions is shown in Figure 2. Neutron H*(10) rates values from 90 to 150 μSv/h are found between loaded casks. Neutron H*(10) rate decreases to <10 μSv/h between empty storage positions. In the position where individual casks were measured (storage position 61) the reported value is close to 10 μSv/h. This value is important since it could be considered as the contribution of the rest of stored casks and ATI building to the individual casks measurements at this position. Figure 2. View largeDownload slide H*(10) rate values measured among casks. Figure 2. View largeDownload slide H*(10) rate values measured among casks. Good general correlation can be found between measured H*(10) rate values among every four loaded casks and the neutron emission rate of the casks (Figure 3). In this figure, for every value of H*(10) rate measured among a group of four casks, the sum of the neutron emission rate of each of the four casks is calculated. Similarly, the neutron emission rate of each cask is calculated as the sum of the emission rate of the spent fuel assemblies stored in the cask. The neutron emission rate is calculated for every spent fuel assembly by the NPP using ORIGEN code (Oak Ridge Isotope Generation), which is a general purpose point depletion and decay code to calculate isotopic concentrations, decay heat and radiation source terms, included in the SCALE code package. Since H*(10) rates are measured and neutron emission rate are calculated, this result is a check of the adequacy of both H*(10) rate measurements and neutron source calculations. Figure 3. View largeDownload slide Correlation between neutron H*(10) rate and casks neutron emission rate. Figure 3. View largeDownload slide Correlation between neutron H*(10) rate and casks neutron emission rate. Measurements outside ATI The spectra evaluated outside the ATI (Figure 4) show mainly a thermal component and the H*(10) rate values obtained from them are also compatible with the measurements with the LB6411 (Table 1). After that, neutron H*(10) rates were determined with the survey meter at regular ranges along the South and North faces of the ATI, Figure 5. The reported values are slightly above the background, established as 21 (±17%) nSv/h from measurements far from ATI, with the highest values up to 80 nSv/h always under the limit to public (0.5μSv/h). In South and North faces the effect is very similar although differences are appreciable for values from 30 to 40 m approximately, as consequence of the cask neutron emission rate. Figure 4. View largeDownload slide Neutron spectra measured around outside ATI. Figure 4. View largeDownload slide Neutron spectra measured around outside ATI. Figure 5. View largeDownload slide H*(10) rate measured outside ATI along walls. Figure 5. View largeDownload slide H*(10) rate measured outside ATI along walls. CONCLUSIONS A detailed study of the ENSA-DPT cask at the independent spent fuel storage installation of the Trillo Nuclear Power Plant has been performed combining experimental neutron spectrometry and dosimetry with simulations. H*(10) rate measured with BSS and LB6411 show statistically compatible results, and therefore LB6411 measurements are validated in the neutron workplace field at the installation. H*(10) rate grid between casks inside the installation has been measured and correlated with the neutron emission rate calculated for each cask. Measurements performed outside the installation reported H*(10) rate values slightly over the neutron background and well below the limit of 0.5 μSv/h for public. FUNDING This work was supported by Centrales Nucleares Almaraz-Trillo [Grant number CIEMAT-CNAT 6382]. REFERENCES 1 Buchillier , T. , Aroua , A. and Bochud , F. O. 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Radiation Protection Dosimetry – Oxford University Press
Published: Aug 1, 2018
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