中子相互作用对多体系统核临界安全的影响与评估方法研究

Research on Influence and Assessment Method of Neutron Interaction for Nuclear Criticality Safety in Multiple Units System

中子相互作用,即中子在两个或两个以上裂变系统之间的交换,使多体系统的核临界安全问题变得非常复杂。针对核燃料循环领域不断涌现的多体具体场景,提出了一种保守、便捷、适用性较好的评估多体系统核临界安全(反应性变化)的方法。基于立体角法建立了距离因素与反应性变化之间的定量关系;通过反照率来评估单体之间、单体与屏蔽体之间的中子散射贡献;通过容器壁效应函数考虑容器壁材料与厚度的影响。利用中国原子能科学研究院的临界装置,通过临界实验对本方法进行了验证。本方法在评估距离效应实验时,与实验结果最大偏差不超过0.12Δk/k,评估屏蔽效应实验时,与实验结果最大偏差不超过0.035Δk/k,对比结果能够证明本方法的可行性与安全性。

Neutron interactions,the neutron exchanges among two or more fissile systems,make the nuclear criticality safety problems very complicated in multiple units system.Unplanned neutron interaction between two fissionable bodies is involved in two criticality accidents.In recent years,the pressure of production capacity for nuclear fuels is growing rapidly with the growth of nuclear energy.A lot of multiple units systems are emerging in nuclear fuel manufacturing plant.The criticality safety of multiple unit systems is more and more important in nuclear fuel cycle.Towards the increasing specific applications of multiple units in fields of nuclear fuel cycle,a conservative,convenient and applicable assessment method was proposed to assess the nuclear criticality safety(reactivity changes)in multiple units system.The assessment method based on solid angle theory,the primary relations from distance factor to reactivity changes were established for double units with simple geometries,such as slabs,cylinders,and spheres.The reactivity change in one fissile unit from another one was treated as the neutron leakage change,which was quantitatively linked with the solid angle changes from other fissile units.The solid angle could be calculated by surface integral method.Then application scenarios for the assessment method were investigated and summarized to build up the numerical validation plans,such as nuclear fuel types,enrichments,container shapes,distance and shielding between containers.Double slabs,double cylinders and double spheres with 3 different 235 U enrichments in uranium metal,uranium dioxide and uranyl nitrate systems were analyzed with Monte Carlo criticality calculation code.The contributions of neutron scattering between single units in slab system are stronger than other cases.This situation is also notable in shielding.Assessment method will not satisfy conservative condition.It’s corrected by considering secondly scattering of leakage neutrons,through setting appropriate albedo conditions to adjust the solid angle values.The effects of materials and thickness of container wall were considered by container wall attenuation functions.Criticality experiments were performed to validate the assessment method on criticality assembly in China Institute of Atomic Energy.The criticality assembly is consisted of two parallel slabs with uranyl nitrate solutions with 235 U enrichment 19.75%.The distance effect experiments include six different distances.The shielding effect experiments include three different types of shielding materials.The maximum bias between assessments and measurements is less than 0.12Δk/k in distance effect experiments.The maximum bias between assessments and measurements is less than 0.035Δk/k in shielding effect experiments.The feasibility and conservation of assessment method could be validated by the comparison results.