裂变燃耗计算中裂变产额数据的适用性评估

Evaluation of the Applicability of Fission Yield Datain Fission Burn-up Calculation

针对裂变燃耗计算中低中子注量率条件下实验测量和评价的裂变产额能否应用于高中子注量率条件下裂变燃耗的计算问题,通过裂变核消失量和裂变产物生成量,用解析方法分别计算了低中子注量率(1014cm-2·s-1)和高中子注量率(1030cm-2·s-1)下的裂变燃耗。结果表明,裂变燃耗低时无关中子注量率高低,两种方法的计算结果一致,燃耗增大时差异增大。分析燃耗差异的主要原因为:低中子注量率下是裂变产物生成量的饱和,高中子注量率下是裂变产物在中子作用下的“消失”。用燃耗计算程序CINDER90及其自带的核数据库模拟计算了热中子在高低两种注量率下235U和87Kr,88Kr,144Ce,147Nd等裂变产物的核子数密度随时间的变化关系,据此用两种燃耗计算方法的计算结果与解析分析的结果一致。因此,裂变燃耗深度是影响裂变产额数据适用性的重要因素。当裂变燃耗较低时,无论中子注量率高低,实验室条件下测定的裂变产额等核数据适用于裂变燃耗的计算,但当裂变燃耗较高时,在高中子注量率下需准确评估裂变产物的表观消失截面,在低中子注量率下则需注意短寿命裂变产物的饱和问题。

The fission burn-up of a nuclear material can be calculated according both to the depletion of the fissile nuclide and to the production of fission products,and nuclear data such as fission and annihilation cross sections of the fissile nuclides as well as yields of fission products are needed.As the fission yields of some products are insensitive to the energy spectrum of neutrons inducing fission,the calculated burn-ups according to the production of the fission products are more accurate when the neutron spectrum is complicate.Then an issue is widely focused on that whether the fission yields measured in laboratories are applicable for the burn-up calculation under the condition of super-high neutron flux.The burn-ups according to the equation of the depletion of the fissile nuclide and the equation of the production of fission products are calculated under the conditions of low fluence rate(1014 cm-2·s-1)and high fluence rate(1030 cm-2·s-1)of neutrons.It is concluded by comparing the burn-up results that no matter the neutron flux is high or low,the burn-ups calculated via the production of fission products and via the depletion of the fissile nuclide are consistent when the value of burn-up is small whereas there are significant deviations when the value of burn-up is high.The burn-up calculation code CINDER90 and its built-in nuclear database are used to simulate the variation of nucleon number density with time for 235 U,and fission products,such as 87 Kr,88 Kr,144 Ce,and 147 Nd at high and low flux rates of thermal neutrons.Therefore,the nuclear data accurately measured in laboratories are applicable for burn-up calculations in high neutron flux.However,when the value of burn-up is high enough,say great than 20%,the annihilation cross sections of fission products in high flux and the saturation of production of the short-life fission products in low flux should be considered carefully.