双重非均匀子群参数制作研究

弥散颗粒燃料元件中燃料颗粒以随机形式弥散在基体中，难以获得确定几何。同时由于共振自屏现象的存在，呈现出一种双重非均匀系统。当前均匀系统产生的共振积分在双重非均匀系统中使用时，会在较低的共振能群产生一定的共振计算误差。为满足现有组件计算程序直接进行双重非均匀性共振计算的需求。基于Sanchez-Pomraning模型下的特征线固定源计算方法，建立一套双重非均匀共振积分表，最后结合子群方法实现随机介质燃料元件的共振计算。数值结果表明，考虑双重非均匀性产生的积分表，在相同的输运条件下和积分表的适用范围内，由子群共振部分对keff计算带来的绝对偏差能保持在200 pcm内。该工作的意义是对于一些不宜改动的传统组件程序，如HELIOS，通过在线修改共振积分表和子群参数，从而使其直接进行弥散颗粒燃料问题的计算成为可能。

Study on Development of Double Heterogeneous Subgroup Parameters

Since the Fukushima nuclear accident, the safety of nuclear fuel under accident conditions has been paid more and more attention. As an accident-tolerant fuel, coated particle fuel has become a research hotspot. In this fuel element, a large number of fuel particles are dispersed in non-fissile matrix materials. The National Oak Ridge Laboratory of the United States proposed a design of fully ceramic micro-encapsulated fuel (FCM) that can be used in pressurized water reactors (PWR). As a promising accident-tolerant fuel, FCM is composed of tri-structural isotope (TRISO) particles and matrix. TRISO fuel particles are divided into five layers from the inside to the outside, followed by fuel core, carbon buffer layer, inner pyrolysis carbon, SiC ceramic layer and outer pyrolysis carbon. TRISO particles and non-fissile matrix are made into cylindrical fuel pellets and filled into the fuel rods. The fuel can withstand higher temperature and maintain the integrity of the fuel element under some extreme accident conditions. The double heterogeneity is introduced in the neutronic model of this fuel. The first heterogeneous effect consists of the fuel rod and its surrounding moderator. The second heterogeneous effect is caused by the random distribution of fuel particles in non-fission matrix. This DH effect brings great challenges to the existing resonance calculation methods. In fact, the Monte Carlo method has the ability to accurately simulate the DH problem. However, Monte Carlo calculation takes a lot of time and is difficult to be competent for time-related burnup calculation. In order to deal with resonance calculation under DH effect from the perspective of deterministic theory, researchers around the world have proposed many feasible methods. However, to implement these methods, existing lattice physics code need to be modified and validated. To solve the above problems, the traditional subgroup method was combined with the DH resonance integral table from the perspective of DH effect. The DH resonance integral tables for 238U and 235U were made respectively for FCM fuel. In the process of developing the DH resonance integral table, the Sanchez-MOC method was used for transport calculation. In order to obtain accurate resonance cross sections, the method of ultrafine group coupled with Sanchez-Pomraning was chosen to calculate the DH problem. In the process of transforming the DH resonance integral table into the subgroup parameters, the fitting method with Pade approximation was used to calculate the subgroup parameters. Finally, the sub-group parameters combined with the traditional MOC solver could recover the real XS under DH condition on the conventional fuel mixing problem. Numerical results show that considering the integral table generated in double heterogeneous system, under the same transport conditions and within the applicable range of the integral table, the absolute deviation caused by the subgroup resonance part to the keff calculation can be kept within 200 pcm. The significance of this work is that for conventional lattice physics code that are not suitable for modification, such as HELIOS, it is possible to directly perform the calculation of the dispersed particulate fuel problem by modifying the resonance integral table and subgroup parameters online.