Preliminary validation and application of shutdown dose rate calculation method based on cell-in-mesh
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摘要: 停堆剂量率计算分析是核反应堆辐射安全的重要内容。为了分析车载微型移动核电源的停堆剂量率，研究了基于栅元嵌套网格的停堆剂量率计算方法。该方法在严格两步法的计算框架下，对活化光子源的抽样方法进行了改进。通过构建几何简单的包围盒，抽样得到源栅元内的粒子分布，从而提高了抽样效率和精度。在国际热核聚变实验堆(ITER)停堆剂量率基准题中进行了验证，该方法与参考解符合较好。基于该方法开展了兆瓦级车载微型移动核电源Megapower的停堆剂量率分析，计算结果表明热管贯穿端剂量率水平相对较高。该方法可用于反应堆停堆剂量率的计算分析，能够准确评估结构材料活化源及其产生的剂量率，对于反应堆屏蔽设计、维修计划的制定及退役具有重要的参考意义。Abstract:
Background Shutdown dose rate (SDR) analysis plays a critical role in ensuring radiation safety during reactor maintenance, transportation, and decommissioning. Traditional methods such as the direct one-step (D1S) method and the rigorous two-step (R2S) method face limitations in accuracy and implementation, especially for compact and complex geometries like vehicle-mounted micro-nuclear power systems.Purpose This study aims to develop and validate a cell-in-mesh-based R2S method for SDR calculations, with enhanced sampling efficiency and spatial resolution. The goal is to enable accurate prediction of post-shutdown radiation fields for both benchmarking and practical reactor applications.Methods An improved R2S methodology was implemented by integrating nested cell-in-mesh geometry with a Monte Carlo (MC) transport framework. Photon source sampling was optimized using bounding box division and local mesh-based distribution sampling. The method was validated using the ITER shutdown dose rate benchmark and applied to the Megapower microreactor model, which employs HALEU fuel, heat pipe cooling, and composite shielding.Results The developed method produced SDR distributions with statistical deviations below 2% and matched international benchmark results within 4% deviation. In the Megapower case, the highest dose rate (16.3 mSv/h) at a radial location 30 cm occurred near the heat pipe outlet, primarily due to activated structural materials and neutron streaming along the heat pipe path.Conclusions The cell-in-mesh-based R2S method improves the accuracy and resolution of SDR calculations without significantly increasing computational costs. It is suitable for advanced shielding analysis of compact nuclear systems and provides a reliable tool for guiding safety design, maintenance planning, and decommissioning strategies.-
Key words:
- shutdown dose rate /
- rigorous two-step method /
- source particle sample /
- ITER benchmark /
- Megapower
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图 1 基于栅元嵌套网格的停堆剂量率计算方法流程图
Figure 1. Flowchart of shutdown dose rate calculation method based on cell-in-mesh
表 1 ITER停堆剂量率基准题材料信息
Table 1. ITER shutdown dose rate benchmark material compositions
nuclide nuclide fraction nuclide nuclide fraction steel and water mixture steel steel and water mixture steel H 1.46E-01 − Mn 1.42E-02 1.82E-02 B 4.02E-05 5.14E-05 Fe 5.03E-01 6.44E-01 C 8.14E-04 1.04E-03 Co 3.68E-04 4.71E-04 N 2.17E-03 2.78E-03 Ni 9.06E-02 1.16E-01 O 7.29E-02 6.95E-05 Cu 2.05E-03 2.62E-03 Al 8.04E-04 1.03E-03 Zr 9.52E-06 1.22E-05 Si 7.73E-03 9.89E-03 Nb 4.67E-05 5.98E-05 P 3.50E-04 4.48E-04 Mo 1.13E-02 1.45E-02 S 1.02E-04 1.30E-04 Sn 7.31E-06 9.36E-06 K 5.55E-06 7.10E-06 Ta 2.40E-05 3.07E-05 Ti 1.36E-03 1.74E-03 W 2.36E-06 3.02E-06 V 3.41E-05 4.36E-05 Pb 1.68E-06 2.14E-06 Cr 1.46E-01 1.87E-01 Bi 1.66E-06 2.13E-06 
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表 2 ITER停堆剂量率基准题辐照历史
Table 2. ITER shutdown dose rate benchmark irradiation history
neutron source/(1017 s−1) time repetition 1.0714 2 y 1 8.25 10 y 1 0 0.667 y 16.607 1.33 y 1 0 3920 s200 400 s 17 0 3920 s280 400 s 3 0 3920 s280 400 s 1 0 106 s
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