核事故缓解任务的最优工作路径算法

Optimal working path algorithm for mitigation tasks under nuclear accidents

[目的]核电厂在发生严重事故后,放射性物质可能从安全壳泄漏到核电厂厂房中,给执行事故缓解任务而在厂内移动的工作人员造成一定的辐射剂量损害.为了尽可能降低工作人员执行任务期间所受的辐射剂量,对其最优工作路径进行算法研究.[方法]使用自主研发的厂内放射性评估软件REMADA,模拟计算CPR1000核电厂在一回路小破口事故、主蒸汽管道破裂事故和全厂断电事故期间厂房内的放射性物质浓度分布,并基于REMADA的辐射剂量评估模块建立辐射剂量最优路径算法,搜索不同缓解任务在限定通行时间内所有的可行工作路径,给出工作人员在厂内通行受辐射剂量最低的路径.[结果]该算法寻找到的最优路径能有效地降低厂内工作人员受到的辐射剂量损害,并且该算法具有较高的执行效率,可满足实际应用中严重事故应急决策的需求.[结论]该研究可为降低工作人员受到的放射性风险、制定事故应急策略提供有力的数据支撑与技术支持.

[Objective] In an event of a severe nuclear accident,the reactor core may be at risk of meltdown.In such case,radioactive materials may leak into the reactor building from the containment,exerting a radiological impact on individuals who move in the reactor building to perform mitigative missions.It is thus important to estimate the potential radiological consequences of the mission and identify optimized working paths before taking actions to minimize radiation dose of people working in the reactor building and to avoid putting them in harm's way unintentionally.[Methods] A self-developed simulation software,REMADA,is used to simulate the distribution of radioactive materials in CPR1000 reactor building during small break loss of coolant accident(SBLOCA),main steam line break(MSLB),and station blackout(SBO).Based on the dose estimation module of REMADA,an optimal path algorithm based on depth-first search is established to search for all valid paths and determine the optimal path for individuals involved in different mitigative missions within the reactor building.[Results] The Modular Accident Analysis Program MAAP5 is used to simulate the leakage rates of three different radioactive materials(including inert gas,cesium iodide,and metallic oxide) from the containment under SBLOCA,MSLB,and SBO in the study.By establishing a mathematical model of the CPR1000 reactor building and selecting typical atmospheric boundary conditions,REMADA simulates the airflow inside the reactor building.Based on aerosol models,the air and ground distributions of radioactive materials in different compartments are calculated,and the total radioactivity in the fields of all compartments after decay is estimated.According to severe accident management guidance,this study assumes three mitigative tasks for SBLOCA,MSLB,and SBO,respectively,assuming departure time,starting locations,destinations,and time limit for all tasks.Depth-first search algorithm is applied and optimized by pruning unqualified cases based on restrictive conditions,which results in obtaining all possible working paths for timely completing mitigation tasks.As a result,the algorithm has found 4 463 possible paths for SBLOCA,9 763 possible paths for MSLB,and 4 283 possible paths for SBO.Radiation doses received by people working on each path for mitigation tasks are estimated using the dose evaluation module of REMADA.The optimal working paths for all tasks are obtained through dual ordering of time consuming and radiation doses.The results show that the optimal paths found by the algorithm can effectively reduce the radiation doses for people.Compared with conventional paths which take less time for traveling,the optimal paths can almost eliminate radiation doses in SBLOCA and reduce radiation doses by about 50% and 55% for MSLB and SBO,respectively.The running time of the algorithm in each case is less than 3 s,proving high efficiency to meet the requirements of emergency decision making in nuclear accidents.[Conclusion] This study demonstrates the feasibility of dynamic dose evaluation of people under nuclear accidents and presents a method for searching minimum doses paths for individuals carrying out mitigative tasks based on a depth-first search.Based on the highly extensible modules of REMADA,the algorithm established in this study can be applied to various practical scenarios,including different nuclear power plants and nuclear accidents.The calculation results of the algorithm indicate that the identified optimal paths can effectively reduce the radiation dose damage to people working in the reactor building.The algorithm is efficient enough to meet the requirements of practical application for emergency decision making during severe accidents.In conclusion,the algorithm can effectively aid in minimizing radioactive risks for individuals and facilitate the formulation of rational emergency decisions.