Abstract Material activation assessment of a proposed accelerator-based boron neutron capture therapy facility was performed using the FLUKA Monte Carlo code to quantify the magnitude of the problem in terms of the isotope inventories, induced activities, and residual dose rates. Two simplified operation scenarios were considered: a 30-min proton bombardment to simulate a typical session of patient treatment and a long-term 1 year continuous operation to estimate the accumulation of long-lived radionuclides. Following the generation and transport of decay radiation, the space- and time-dependent inventories of induced radionuclides in materials and residual dose rates after shutdown were obtained. The predicted results were compared with the corresponding regulatory limits. Moreover, the effectiveness of various measures to reduce the impact of material activation was demonstrated. INTRODUCTION The Tsing Hua open-pool reactor was successfully renovated for boron neutron capture therapy (BNCT) in 2004. Clinical trials for patients with locally recurrent head and neck cancer began on 1 August 2010. To date, more than 20 patients have been treated, and the overall response is encouraging(1). The BNCT group at Nat’l Tsing Hua University has proposed an accelerator-based BNCT (AB-BNCT) facility to replace the ageing reactor and expand related research in Taiwan. The proposed facility adopted a similar design to the cyclotron-based neutron source at KURRI(2), where neutrons are produced from the bombardment of a beryllium target using 1 mA 30 MeV protons. To deliver a high-quality epithermal neutron beam, the emitted fast neutrons from the target will be moderated using a specially designed beam shaping assembly (BSA)(3). Operating such a high-power accelerator inevitably produces intense secondary neutrons that may result in significant activation in the surrounding components, mostly those materials used to build the BSA and shielding structure. MATERIALS AND METHODS FLUKA and activation analysis The FLUKA Monte Carlo code is not only a multi-particle transport code but also an integrated code capable of making direct prediction of the production and decay of radionuclides following hadronic and electromagnetic showers initiated by high-energy particles. Furthermore, the generation and transport of decay radiations are also included in the same simulation(4, 5). The one-step simulation capability provides users with a valuable tool for assessing prominent challenges related to induced radioactivity in various scenarios. Two simplified irradiation scenarios were considered here to evaluate the characteristics of isotope inventories, induced activities and residual dose rates associated with this facility’s operation. The first scenario was a 30-min proton bombardment to simulate a typical session of patient treatment and the second was a 1 year continuous operation to estimate the accumulation of long-lived radionuclides. BSA and facility layout As shown in Figure 1, the basic layout of the proposed facility includes an accelerator room and a patient treatment room; the BSA is embedded in the partition wall between the two rooms to reduce radiation leakage and simplify the shielding design. For simplicity in simulation and conservative scoping analysis, the cyclotron and beamlines are not included in the model of the accelerator room, and no phantom is located in the treatment room to intercept the epithermal neutron beam from the BSA exit. The beryllium target and the BSA design were modeled in detail to provide an accurate radiation field around the facility. Figure 1. View largeDownload slide Cross-sectional view of the proposed AB-BNCT facility, showing the BSA and bulk shielding structure. Figure 1. View largeDownload slide Cross-sectional view of the proposed AB-BNCT facility, showing the BSA and bulk shielding structure. During normal operation, a monoenergetic 30 MeV proton beam of 1 mA is launched to impinge on a beryllium target located inside the BSA, which comprises iron, lead, polyethylene, bismuth and FLUENTAL. The cylindrical BSA is 160 cm in diameter and 125 cm in length, and the inner dimensions of the treatment room and accelerator room are 4 × 4 × 3 m3 and 5 × 4 × 3 m3, respectively. The bulk shielding walls are 2 m thick, made of ordinary concrete (type TSF-5.5) with a density of 2.34 g/cm3 and a composition by atom fraction of 10.6% hydrogen, 25.3% carbon, 44.5% oxygen, 2.3% magnesium, 0.8% aluminum, 2.1% silicon, 14.2% calcium and 0.2% iron. Two colored solid rectangles in Figure 1 indicate the locations of interest for scoring residual dose rates in the accelerator and treatment rooms after shutdown. Figure 2 shows the FLUKA-predicted neutron spectra that emerged from the front, lateral and back surfaces of the BSA under the irradiation of a 1 mA 30 MeV proton beam directed at the beryllium target. The neutron spectrum scored on the front surface demonstrates the design purpose of the BSA, providing high-quality epithermal neutrons for BNCT purposes. It is worth noting that the secondary neutrons that emerged from the back and lateral surfaces, mainly fast and thermal neutrons, respectively, are substantially more intense than the epithermal neutrons from the BSA exit. This characteristic could pose major challenges not only in shielding of prompt radiation during operation but also in residual activity after shutdown, especially in the accelerator room where a non-negligible amount of fast neutrons produced in the target could immediately leak from the BSA’s back surface. Figure 2. View largeDownload slide Neutron spectra that emerged from the front, lateral and back surfaces of the BSA under the irradiation of a 1 mA 30 MeV proton beam. Figure 2. View largeDownload slide Neutron spectra that emerged from the front, lateral and back surfaces of the BSA under the irradiation of a 1 mA 30 MeV proton beam. RESULTS AND DISCUSSION Induced radioactivity The radioactivity in material gradually accumulates during irradiation. After shutdown, short-lived radionuclides rapidly decay, leaving only medium- and long-lived radionuclides. Identifying the reaction mechanisms and radionuclides that contribute most to residual activities and dose rates is crucial in material activation assessment because it facilitates developing strategies to mitigate the impact. Except for the beryllium target itself, the induced radioactivity in this facility originates primarily from secondary neutrons generated through proton bombardment. The BSA is deemed to be highly activated by neutrons because of its designed purpose as a spectrum-shaping device. The surrounding concrete is also expected to be activated to some extent by stray neutrons. Figure 3 shows two examples of the FLUKA activation analysis, depicting the time evolution of the total induced radioactivity of the facility after 30 min and after 1 y of operation, respectively. The contributions of dominant radionuclides in different periods of cooling time are identified and indicated in the figure. Figure 3. View largeDownload slide Time evolution of the total induced radioactivity of the AB-BNCT facility after 30 min and after 1 year of operation, emphasizing the contributions of dominant radionuclides after various periods of cooling. Figure 3. View largeDownload slide Time evolution of the total induced radioactivity of the AB-BNCT facility after 30 min and after 1 year of operation, emphasizing the contributions of dominant radionuclides after various periods of cooling. After 30 min of operation, the total activities of the facility at immediate shutdown mostly came from 28Al (T1/2 = 2.24 min), which is the product of the neutron capture by 27Al in concrete and the FLUENTAL in the BSA. 56Mn (T1/2 = 2.58 h), the result of the 56Fe(n,p) reaction in the BSA, was the dominant radionuclide after several minutes to ~1 d of cooling. Subsequently, 7Be (T1/2 = 53.2 d) in the target, the product of the 9Be(p,t) reaction, was the dominant radionuclide. Finally, 55Fe produced in the iron of the BSA with a 2.6-year half-life lasts several years after irradiation but engenders practically negligible external exposure, emitting only low-energy 5.9 keV X rays. For the 1 year of operation, 7Be accumulates in the beryllium target and plays the most essential role in radioactivity at all times until ~120 d of cooling. Subsequently, 55Fe in the iron becomes the dominant contributor. In addition to 7Be, in this case, the pure beta-emitter 45Ca (T1/2 = 162.6 d), the product of the neutron capture by 44Ca in concrete, is generally the second-most important radionuclide in residual activity, as depicted in the right-hand side of Figure 3. Residual dose rates Electrons, positrons and photons emitted from the decay of radionuclides result in residual dose rates around an activated component. The generation and transport of these secondary radiations could be included in the same FLUKA simulation, providing a useful dose map around the activated component. Figure 4 shows the time evolution of residual dose rates in the accelerator and treatment rooms of the facility after 30 min and 1 year of operation. Overall, the residual dose rates in the accelerator room are at least two orders of magnitude higher than those in the treatment room. This can be anticipated because of the characteristics of the prompt radiation field around the BSA. As shown in Figure 2, intense fast neutrons streaming back from the BSA tend to cause more activation. Additionally, decay photons in the proximity of the target could leak from the BSA entrance back into the accelerator room. Figure 4. View largeDownload slide Time evolution of residual dose rates in the accelerator and treatment rooms of the AB-BNCT facility after 30 min and after 1 year of operation. Figure 4. View largeDownload slide Time evolution of residual dose rates in the accelerator and treatment rooms of the AB-BNCT facility after 30 min and after 1 year of operation. Examining the details of residual dose rates reveals that decay gammas from radionuclides 28Al, 56Mn and 7Be are primary sources in the accelerator room. After the operations cease, 1.779 MeV decay gammas from 28Al play the dominant role in residual radiation. Up to ~10 min of cooling, three decay gammas with energies of 0.847, 1.811 and 2.113 MeV from 56Mn are the main contributors. After several days of cooling, 0.477 MeV decay gammas from 7Be dominate the residual radiation. The radiation sources in the treatment room are different, mainly short-lived 28Al and 49Ca (T1/2 = 8.72 min) in concrete. For both operation scenarios, the effect of residual dose rates in the treatment room is not severe, ~450 μSv/h at the immediate shutdown, which quickly decays to ~4 μSv/h after 30 min of cooling. However, the residual dose rates in the accelerator room are high and can pose a radiation hazard to nearby workers during a maintenance period after machine shutdown. Even for only 30 min of operation, at least a 1 d wait is therefore required for the dose rate to reduce to ~10 μSv/h. The situation worsen as the operation time lengthens. Measures to mitigate the impact of material activation in the accelerator room are necessary for practical operation. According to the inventory and residual activity analysis, we propose the following approaches to reduce unwanted residual radiation: Firstly, inner-room surfaces should be covered using a layer of 4-cm-thick Li-doped polyethylene to reduce neutron activation in concrete walls. Secondly, we suggest installing shutters to block decay gammas leaking directly from the BSA entrance and exit. In addition, we suggest installing an extension lead collar covering the wall surface of the BSA exit to isolate decay gammas from the highly activated partition wall near the BSA. Combining these measures can effectively reduce residual dose rates by more than a factor of ~20 in the accelerator room. Figure 5 shows the time evolution of residual dose rates in the accelerator and treatment rooms of the facility after 1 year of operation, demonstrating the effectiveness of these measures to reduce the impact of material activation. Figure 5. View largeDownload slide Time evolution of residual dose rates in the accelerator and treatment rooms of the AB-BNCT facility after 1 year of operation, showing the effectiveness of various measures to reduce the impact of material activation. Figure 5. View largeDownload slide Time evolution of residual dose rates in the accelerator and treatment rooms of the AB-BNCT facility after 1 year of operation, showing the effectiveness of various measures to reduce the impact of material activation. CONCLUSIONS Realizing an AB-BNCT facility requires a high-power proton accelerator, and the induced radioactivity in various components of the facility require appropriate assessment. Targeting the proposed facility in Taiwan, we examined possible inventories of induced radioactivity and their contributions to residual activities and dose rates under worst-case operation scenarios. Based on extensive FLUKA calculations, the scoping analysis provides us with useful guidance for continued optimization of detailed design and future radioactive waste management. REFERENCES 1 Wang , L. W. et al. . Fractionated boron neutron capture therapy in locally recurrent head and neck cancer: a prospective Phase I/II Trial . Int. J. Radiat. Oncol. Biol. Phys. 95 , 396 – 403 ( 2016 ). Google Scholar CrossRef Search ADS PubMed 2 Tanaka , H. et al. . Characteristics comparison between a cyclotron-based neutron source and KUR-HWNIF for boron neutron capture therapy . Nucl. Instrum. Methods B 267 , 1970 – 1977 ( 2009 ). Google Scholar CrossRef Search ADS 3 Liu , H. Y. W. and You , J. F. Filter and neutron beam source including the same. ROC Patent No. TW201515011A ( 2015 ). 4 Fasso’ , A. et al. . FLUKA: a multi-particle transport code. CERN-2005-10, INFN/TC_05/11, SLAC-R-773 ( 2005 ). 5 Hedberg , V. et al. . Predicting induced radioactivity in a large high-energy physics apparatus: the example of the ATLAS experiment . Nucl. Instrum. Methods A 592 , 230 – 246 ( 2008 ). Google Scholar CrossRef Search ADS © The Author(s) 2018. Published by Oxford University Press. All rights reserved. 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Radiation Protection Dosimetry – Oxford University Press
Published: Aug 1, 2018
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