基于中子平衡研究增殖燃料实现CANDLE模式的最优配置

寿期内中子通量、核素浓度和功率分布的轴向形状均保持恒定(Constant Axial shape of Neutron flux,nuclide densities and power shape During Life of Energy produced,CANDLE)是实现原位增殖-焚烧(Breed-and-Burn,BB)模式的一种燃耗策略。CANDLE堆经易裂变燃料或外中子源进行点火,启动后由增殖燃料的燃烧实现自稳运行。若要CANDLE堆自稳运行于k_(eff)=1,必须对堆芯几何及燃料体积分数进行配置优化。最优配置方案可通过蒙特卡罗方法模拟CANDLE堆芯,根据有效增殖因子筛选得出。但该方法需耗费大量的计算时间,若采用1D模型近似模拟,并结合中子平衡方法进行分析,便可大幅节约计算时间,获得具有指导性意义的结果。本文将论证该方法的可行性,并应用该方法估算钠冷贫铀CANDLE堆半径在100 400 cm、燃料体积分数在35%60%变化时的最优配置。

Using the neutron balance method to access the feed fuel requirements for CANDLE

Background: CANDLE (Constant Axial shape of Neutron flux, nuclide densities and power shape During Life of Energy produced) is a burnup strategy for Breed-and-Burn (B&B) reactors in which the effective neutron multiplication factor at an equilibrium state is equal to unity. The radius and fuel volume fraction of such a core play a very important role in the B&B mode of operation. However, using the Monte Carlo method to model a full CANDLE core for optimization costs too much computational effort and time.Purpose: This study aims to assess the validity of neutron balance analysis with a simplified 1D model for determination of the maximum burnup (BU) attainable in a CANDLE mode, and then apply this 1D methodology to optimize the configurations of feed fuel and core radii for a sodium-cooled core to sustain the CANDLE mode.Methods: When the maximum achievable BU of feed fuel equals the maximum burnup of the CANDLE feed fuel at equilibrium state, thekef value of the core is equal to unity. The neutron balance method with the 1D approach for various core radii and fuel volume fractions of feed fuel can provide a quick way to scope the optimized configuration.Results: Neutron balance analysis with the 1D approach can provide a reasonable estimation of the maximum attainable BU of feed fuel in a CANDLE reactor, despite a small inaccuracy due to the spectrum difference between the 1D and the full core models. A small size of core and a low fuel volume fraction of feed fuel display a small value of the maximum attainable BU, which implies that it is disadvantageous to sustain a CANDLE mode. However, a too big size of core or high fuel volume fraction of feed fuel may lead tokef > 1 at equilibrium state. With the above considerations and optimization, we present a suitable core geometry and its fuel fraction to ensure the stable B&B mode of operation at equilibrium state (kef =1). Conclusion: The neutron balance calculation with 1D approach can save much computational time, and the optimized configurations of feed fuel to sustain a CANDLE mode are proposed based on this methodology.