微型反应堆高速撞击下核临界安全的初步分析

Preliminary analysis of nuclear criticality safety of micro-reactor under high-speed impact

微型反应堆可作为月球表面动力、航天器动力等用于太空探索,反应堆发射前需要对发射事故的影响进行安全分析。反应堆高空坠落并撞击到混凝土是主要的临界安全分析场景,高速撞击后反应堆可能重返临界。本文以两种简化的反应堆模型高速撞击场景为例,利用连续介质力学有限元程序ABAQUS与粒子输运蒙特卡罗程序耦合,完成了纯燃料堆芯垂直撞击地面和带径向反射层和屏蔽层圆柱堆30°倾角撞击地面的模拟,预测了两种场景反应堆高速撞击下keff随时间变化的物理特性。结果表明:纯燃料反应堆垂直撞击地面的keff增加最高可达1000×10^(-5),而具有反射层和屏蔽层反应堆30°倾角撞击地面的keff增加最高为200×10^(-5)。均匀密度变化条件下,采用表面非结构网格与内部非结构网格的蒙卡程序的计算结果符合较好,内部非结构网格能够更真实地捕捉材料的非均匀密度变化效应。本文的研究为微型反应堆高速撞击下的临界安全研究奠定了重要基础。

[Background]Micro-reactors can be used as a lunar surface power or spacecraft power source for space exploration.Before launching the reactor,a safety analysis should be conducted to prevent a launch accident.Currently,the safety analysis of the radioactive isotope power system does not fully include the safety analysis of the reactor.The main critical safety analysis scenario is that the reactor falls and hits the concrete ground from a high altitude.The reactor may return to criticality after high-speed impact.[Purpose]This study aims to investigate the nuclear safety characteristics of a space reactor subjected to dynamic shock under high-speed impact conditions.[Methods]First of all,based on internal and surface unstructured grids,two simplified reactor models corresponding to two high-speed impact scenarios,i.e.,pure fuel reactor vertical impact with ground,and cylinder reactor with a reflector layer and shielding layer impact the ground at a 30°angle were established.Then,the ABAQUS finite element method and unstructured mesh Monte Carlo method of particle transport were combined to predict the criticality properties of the pure fuel and cylindrical reactor during high-speed impact.Based on the surface and internal unstructured mesh Monte Carlo transport technology,the criticality safety analysis platform of micro-reactor under high speed impact was established.[Results]The results show that the keff induced by the deformation may increase with time for the above mentioned two simplified reactors.The maximum increase in the keff of the pure fuel reactor can reach 1000×10^(-5),whereas for the cylinder reactor,the keff is improved to a maximum of 200×10^(-5).Considering the non-uniform density effect,reactivities of-666×10^(-5) and-132×10^(-5) are introduced into the two reactors.[Conclusions]The critical safety characteristics of the reactor under different impact conditions should be evaluated to ensure sufficient safety margins under such accident conditions.