Study on delayed gamma dose produced by fission products and the secondary gamma dose Produced by neutrons after strong explosion
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摘要: 强爆炸释放的γ辐射剂量评估是核应急防护体系研究的重要方向之一，传统研究多聚焦于瞬发γ（<1 μs）的剂量评估，缓发γ（秒级）因时间延迟常被忽视。本文针对强爆炸后裂变产物在0.2～0.5 s内产生的缓发γ剂量与中子泄露产生的次级γ剂量开展研究，基于蒙特卡罗（MC）方法构建了强爆炸源项-大气输运-地表活化耦合的三维全尺度模型，提出基于MC多步计算的动态剂量评估框架，利用重要性卡降低一定距离内实验模拟的方差，详细对比了其与瞬发γ剂量随时间距离变化的趋势。模拟结果表明，在0.2–0.5秒窗口内，距爆炸源500 m处缓发γ总剂量达0.829 Gy，为瞬发剂量（0.441 Gy）的1.88倍；距爆炸源
1000 m处仅裂变产物产生的缓发剂量（0.0318 Gy）为瞬发剂量（0.0042 Gy）的7.6倍，远距离下危害相较瞬发尤为显著；而中子泄漏产生的γ剂量500 m到1000 m的剂量由0.634 Gy逐步衰减至0.0485 Gy。本文提出动态剂量评估框架，为核应急防护策略优化提供了数据支撑。Abstract:Background The assessment of gamma radiation dose released by strong explosions is an important direction in the research of nuclear emergency protection systems. Traditional research has mostly focused on dose assessment of prompt gamma radiation (duration<1 μs）, while delayed gamma radiation (on the second timescale) is often overlooked due to time delay.Purpose This article focuses on the study of the delayed gamma dose released by fission products after a strong explosion within 0.2-0.5 seconds, as well as the secondary gamma dose generated by neutron leakage, with the aim of systematically evaluating their radiation hazards in the near to medium range.Methods Based on monte carlo (MC) method, a three-dimensional full-scale model coupling strong explosive source term atmospheric transport surface activation was constructed, and a dynamic dose assessment framework based on MC multi-step calculation was proposed. By modifying the importance card method, the variance of the simulation results at medium to close distances was effectively reduced, and a detailed comparison was made between the changing trends of delayed gamma and prompt gamma doses over time and distance.Results The simulation results show that within a time window of 0.2-0.5 seconds: at a distance of 500 meters from the explosion source, the total dose of delayed gamma radiation reaches 0.829 Gy, which is 1.88 times the instantaneous gamma radiation dose (0.441 Gy); At a distance of1000 meters from the explosion source, the delayed gamma dose generated by fission products alone is0.0318 Gy, which is 7.6 times the instantaneous gamma dose (0.0042 Gy), indicating that the hazard of delayed gamma is significantly higher than that of instantaneous gamma at longer distances. The secondary gamma dose generated by neutron leakage decays from 0.634 Gy at 500 meters to0.0485 Gy at1000 meters.Conclusions The dynamic dose assessment framework proposed in this article effectively reveals the significant contribution of delayed gamma radiation in the early radiation field after a strong explosion, especially at a distance where its hazard far exceeds that of instantaneous gamma radiation. This study provides key data support for optimizing nuclear emergency protection strategies.-
Key words:
- fission products /
- delayed gamma dose /
- monte carlo method /
- variance reduction /
- soil modeling
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图 4 不同时间权重取值的散点与拟合曲线
Figure 4. Scatter and fitting curves of weight values at different times
图 5 同条件下的本文MC模拟和文献中裂变产物产生的γ剂量
Figure 5. Gamma dose values generated by fission products in MC simulations under the same conditions
图 6 修改重要性前后不同距离下的方差
Figure 6. Error at different distances before and after modifying importance
图 7 不同距离下裂变产物、瞬发能谱以及中子泄漏能谱产生的γ剂量
Figure 7. Gamma dose generated by fission products, instantaneous energy spectra, and neutron leakage energy spectra at different distances
表 1 235U裂变产物衰变产生的γ辐射能谱
Table 1. Gamma-ray spectra from decay of 235U fission products
Energy interval
midpoint/MeV0.2～0.5 s/
(photons·fission−1·sec−1·MeV−1)1.0～2.0 s 4.0～5.5 s 10～13 s 35-45 s 0.175 1.33 5.37E-01 3.20E-1 1.45E-1 3.85E-2 0.261 6.87E-01 3.37E-01 1.95E-1 9.30E-2 2.48E-2 0.369 4.60E-01 2.35E-01 1.31E-1 6.17E-2 1.86E-2 0.502 4.92E-01 2.98E-01 1.65E-1 7.19E-2 1.67E-2 0.662 3.28E-01 1.94E-01 1.03E-1 4.66E-2 1.19E-2 0.7852 3.00E-01 1.53E-01 6.50E-2 3.10E-2 1.03E-2 1.075 2.18E-01 1.33E-01 5.62E-2 2.55E-2 7.91E-3 1.537 1.45E-01 7.72E-02 3.73E-2 1.95E-2 6.84E-3 1.643 8.68E-02 5.06E-02 2.66E-2 1.20E-2 4.06E-3 1.998 6.17E-02 3.46E-02 1.69E-2 7.56E-3 2.47E-3 2.405 3.86E-02 2.17E-02 1.09E-2 5.12E-3 1.78E-3 2.865 2.87E-02 1.62E-02 7.12E-3 3.51E-3 1.26E-3 3.383 1.85E-02 1.07E-02 5.29E-3 2.19E-3 6.10E-4 3.956 9.71E-03 5.74E-03 2.64E-3 1.24E-3 4.60E-4 4.587 7.96E-03 3.37E-03 1.55E-3 7.12E-4 2.12E-4 5.277 1.67E-03 1.21E-03 5.86E-4 2.94E-4 8.95E-5 6.028 1.94E-03 7.20E-04 4.54E-4 1.65E-4 4.16E-5 
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表 2 典型中子泄漏能谱
Table 2. Neutron leakage spectrum of typical nuclear weapons
Energy interval midpoint/MeV Neutrons/KT Energy interval midpoint/MeV Neutrons/KT 0.05715 2.22E+22 5.21 3E+21 0.6105 3.84E+22 7.27 1.27E+21 1.73 2.52E+22 9.09 7.32E+20 3.205 8.9E+21 _ _
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表 3 权重值代替的缓发γ能谱
Table 3. Delayed gamma spectrum replaced by weight values
Energy interval midpoint/MeV 0.2-0.5 s Energy interval midpoint/MeV 0.2-0.5 s Energy interval midpoint/MeV 0.2-0.5 s 0.175 6.88E+21 1.075 2.68E+21 3.383 6.65E+20 0.261 4.46E+21 1.537 2.10E+21 3.956 4.48E+20 0.369 3.74E+21 1.643 1.67E+21 4.587 3.48E+20 0.502 3.73E+21 1.998 1.58E+21 5.277 1.19E+20 0.662 3.42E+21 2.405 1.31E+21 6.028 1.80E+20 0.7852 3.35E+21 2.865 1.02E+21 _ _
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表 4 修改重要性前后各栅元粒子数
Table 4. Number of particles in each grid element before and after modifying importance
cell Particles (before) Particles (after) 31 3104201 3236183 32 452471 3292005 33 66001 3214688 34 21416 3324995 35 5190 3372750 36 6608 3241357
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