TOPAZ-Ⅱ反应堆慢化剂正温度效应研究

Study on Positive Temperature Effect of Moderator in TOPAZ-ⅡReactor

TOPAZ-Ⅱ空间核反应堆电源采用热离子静态热电转换方式,系统输出电功率为5 kW,寿命可达3 a,是当前可迅速工程化的最先进的空间核电源系统之一。然而TOPAZ-Ⅱ反应堆的慢化剂温度系数是一个较大的正值,并导致了全堆的温度系数也是正值,这对反应性控制系统的可靠性提出了很高要求,因而有必要对其产生的机理进行详细的研究。采用MCNP程序构建了TOPAZ-Ⅱ反应堆堆芯三维精确几何模型,从中子能谱的变化、中子平衡关系的变化以及中子循环因子的变化3个方面详细分析了TOPAZ-Ⅱ反应堆慢化剂正温度效应产生的原因。通过设计验证算例验证了分析的正确性,并找到了可减小慢化剂正温度效应的方法,为未来热离子反应堆的优化设计提供了一个指导方向。

TOPAZ-Ⅱspace nuclear reactor power supply adopts thermionic static thermoelectric conversion mode,the output power of the system is 5 kW,and its service life can reach 3 years.It is one of the most advanced space nuclear power supply systems that can be rapidly engineered at present.However,the temperature coefficient of moderator in TOPAZ-Ⅱreactor is a large positive value,which leads to the positive temperature coefficient of the whole reactor,which puts forward high requirements on the reliability of the reactivity control system,so it is necessary to study its mechanism in detail.A three-dimensional accurate MCNP model of TOPAZ-Ⅱreactor core was established,and the causes of the positive temperature effect of moderator in TOPAZ-Ⅱreactor were analyzed in detail from three aspects:The change of neutron spectrum,the change of neutron equilibrium relationship and the change of neutron cycle factors.It is found that the positive temperature effect of moderator is the result of the joint action of three factors.First of all,TOPAZ-Ⅱadopts zirconium hydride solid moderator.Secondly,unlike TRIGA,TOPAZ-Ⅱadopts the heterogeneous arrangement of rod-shaped fuel inserted in moderator matrix.Finally,TOPAZ-Ⅱuses highly enriched uranium with235U enriched to 96%,and the fuel pellets are thick.Under the synergistic effect of these three factors,the thermal neutron utilization factor of fuel increases obviously after the temperature of moderator increases,which leads to the positive temperature effect.From the point of reducing the heterogeneity in the reactor core,a verification example was established,and it verifies the correctness of the analysis.On this basis,a method to reduce the positive temperature effect of moderator was found,which provides a guiding direction for the optimization design of thermionic reactors in the future.