Monte Carlo simulation of the γ-radiation dose field from fission products
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摘要: 针对随时间变化的裂变产物γ体源在冲击波扰动形成的非均匀大气中的输运问题，采用冲击波流场演化的LAMBR理论计算方法构建冲击波扰动下大气密度分布计算模型，基于辐射输运的时间离散理论，发展了用于计算裂变产物γ辐射场参数的瞬态变步长蒙特卡罗模拟方法，并开展了空旷地面条件下235U裂变产物γ辐射环境场流体力学增强效应的模拟计算。计算结果表明，建立的裂变产物γ辐射变步长多时间步MC模拟方法能够反映出冲击波扰动大气密度场对裂变产物γ辐射环境的流体力学增强效应，与静态大气模型相比，流体力学增强效应可使部分位置的总剂量增强2～3倍。Abstract:
Background Extreme nuclear events typically generate intense explosions and release radioactive fission products. Fission product γ, emitted during radioactive decay of fission products, can affect radiation dose fields for 10−5 to 15 seconds. During this period, the source intensity, spectrum, and spatial distribution exhibit significant temporal variations. Concurrently, shock-waves induce complex atmospheric density changes, creating hydrodynamic enhancement effects.Purpose This study aims to develop a computational model for simulating time-varying fission product γ transport in non-uniform atmospheres perturbed by shock-waves, specifically quantifying the hydrodynamic enhancement effect on γ radiation dose fields.Methods A computational model for atmospheric density distribution was constructed using the LAMBR theory for shock-wave flow-field evolution. Based on radiation transport time-discrete theory, a transient variable-time-step Monte Carlo (MC) method was developed using the PHEN particle transport code.Results A validation via 20 kt TNT-equivalent detonation simulations at 400 m altitude was conducted to evaluate the hydrodynamic enhancement effect of fission product γ of 235U. The results demonstrate that, compared to a uniform atmospheric model, the hydrodynamic enhancement effect can amplify the γ dose by 2-3 times at some locations.Conclusions The proposed transient variable-time-step Monte Carlo simulation method can effectively capture the hydrodynamic enhancement effect of the shock wave-perturbed atmospheric density field on the fission product γ radiation fields. -
图 3 源400 m、20 kt TNT当量爆炸不同时刻的大气密度场分布随时间变化关系
Figure 3. Variation of the distribution of atmospheric density field vs. time with a source height 400 m, 20 kt TNT equivalent explosion
图 5 裂变产物γ的瞬态变步长蒙特卡罗模拟步骤
Figure 5. Transient variable step-size multi-time step Monte Carlo simulation method for fission product γ
图 6 地面水平距离500m处的归一化剂量响应函数曲线
Figure 6. Normalized dose response function vs. time at GR=500 m
图 7 高度400 m威力20 kt TNT当量爆炸典型时刻轴线(x=0,y=0)上空气密度分布随高度的变化关系
Figure 7. Distribution of air density on the axis z at different times after explosion with a height of 400m and a power of 20 kt TNT
图 8 不同测点处的剂量率与累积剂量
Figure 8. Dose rate and cumulative dose at different measuring points
表 1 半径500 m的等效空腔对裂变产物γ辐射传播的影响
Table 1. Effect of equivalent cavity with a radius of 500 m on γ radiation propagation of fission products
ground range/m Dc/(10−24 Gy/particle) RSD/% Du/(10−24 Gy/particle) RSD/% Dc/Du cavity model uniform atmospheric model 100 28.7 1.4 4.69 3.8 6.11 300 24.7 1.5 3.94 4.7 6.26 500 18.8 1.5 3.11 3.3 6.03 700 12.8 1.5 2.17 3.7 5.93 900 8.23 2.0 1.36 3.5 6.05 1000 5.88 1.6 1.07 4.6 5.50 1200 3.35 2.1 0.618 6.6 5.43 1500 1.41 3.5 0.247 6.1 5.72 
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表 2 高度400m威力20ktTNT当量爆炸裂变产物源的极半径、赤道半径及中心高度
Table 2. The polar radius, equatorial radius and height of the fission product with explosion geight of 400m and power of 20ktTNT Equivalent
time/s polar radius/m equatorial raidus/m height/m time/s polar radius/m equatorial raidus/m height/m 0.001 26.589 26.589 400.005 0.400 134.157 153.864 408.868 0.002 34.182 34.182 400.010 0.500 138.050 162.659 412.442 0.003 41.438 41.438 400.015 0.600 141.853 171.237 416.497 0.004 48.352 48.354 400.020 0.700 145.568 179.597 421.010 0.005 54.921 54.923 400.025 0.800 149.194 187.738 425.956 0.006 61.141 61.143 400.031 0.900 152.730 195.659 431.311 0.007 67.006 67.009 400.036 1.000 156.177 203.359 437.053 0.008 72.514 72.517 400.041 2.000 185.671 268.007 510.433 0.009 77.659 77.663 400.046 3.000 205.976 309.561 596.145 0.010 82.438 82.442 400.051 4.000 217.808 329.289 674.347 0.020 117.042 116.990 400.099 5.000 230.135 347.923 764.869 0.030 118.987 119.448 400.222 6.000 242.753 367.006 844.379 0.040 119.413 120.417 400.354 7.000 254.535 384.827 912.429 0.050 119.838 121.383 400.492 8.000 265.382 401.215 974.151 0.060 120.262 122.348 400.636 9.000 275.463 416.464 1032.285 0.070 120.685 123.310 400.786 10.000 284.968 430.822 1088.199 0.080 121.107 124.270 400.942 11.000 294.002 444.480 1142.567 0.090 121.529 125.228 401.104 12.000 302.657 457.573 1195.753 0.100 121.949 126.184 401.271 13.000 310.917 470.058 1248.006 0.200 126.107 135.626 403.259 14.000 319.045 482.171 1297.545 0.300 130.176 144.853 405.799 14.900 327.117 492.861 1324.902
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