Research On Lattice Homogenization Via Continuous-Energy Monte Carlo Method

The validity of the standard two-step reactor physics calculation relies onthe accuracy of multi-group cross sections and constants generated in theassembly-level homogenization. The conventional deterministic lattice codes areincompetent in the more and more complex geometries and complicated physics.In contrast, generating the homogenization multi-group constants via MonteCarlo method overcomes the difficulties in geometry and treats energy incontinuum, thus provides more accuracy parameters. Besides, the same code anddata library can be used for a wide range of applications due to the versatility.This study focuses on the Monte Carlo method based homogenization.The track length scheme has been used as the foundation of cross sectiongeneration, which is straightforward. The scattering matrix and Legendrecomponents, however, require special techniques. The scattering event methodhas been proposed to solve this problem. There are no continuous-energycounterparts in the Monte Carlo calculation for diffusion coefficients. Threemethods have been developed to calculate the diffusion coefficients for diffusionreactor core codes.BNtheory, which is widely used in deterministic homogenization to take theleakage effect into account, has been applied in this study when the infinitelattice of identical symmetric motives is assumed.The multi-group constants generated via continuous-energy Monte Carlomethod do not satisfy the equivalence between reference calculation and macrocalculation applied in reactor core analysis. General equivalence theory and thesuperhomogenization method have been applied to the Monte Carlo methodbased multi-group constants to meet the requirement of equivalence. The superequivalence method has been developed in this paper to overcome the limitationsof the two conventional equivalence techniques. The reaction rates and surfacecurrents are preserved, and no additional homogenization parameters are neededin the super equivalence method.The MCMC code has been developed and used to generate homogenizedmulti-group constants for diffusion reactor core code CITATION and multi-group Monte Carlo code MCMG. The accuracy of the developed methods has beendemonstrated both at assembly and core level. The numerical results showedimproved agreements with the references. The MCMC scheme can be seen as apotential alternative to the widely used deterministic lattice codes.This thesis has realized the homogenized multi-group constants generationusing Monte Carlo method. After the leakage effect correction and equivalencetreatment were carried out, Monte Carlo method based homogenization has beenprimarily achieved. This study will be a remarkable foundation for the design ofadvanced reactor systems.