Yuanzineng kexue jishu (Jan 2022)
Heterogeneous Leakage Model for PWR Pin-by-pin Homogenization Calculation
Abstract
The assembly-homogenized two-step scheme was developed to provide efficient solutions with an acceptable accuracy for pressurized water reactor (PWR) core calculation. However, because of the effective boundary condition performed in the assembly calculations, neither the axial nor radial leakage rate is considered within the heterogeneous assembly calculation. The heterogeneous flux spectrum is far from the actual flux spectrum in the core named global spectrum. The previous studies show that the homogeneous leakage model works effectively for the assembly homogenization. The leakage model is used to modify the flux to get the more consistent assembly-homogenized few-group constants, it is also employed to obtain the diffusion coefficients for core calculation. Within the homogeneous leakage model, the heterogeneous assembly is treated as an equivalent homogeneous medium and the axial or radial leakage rate is exactly same under the specific buckling. However, it becomes different and unsettled for the Pin-by-pin homogenization, the assumption that the neutron flux is homogeneous and the leakage rate is uniform over the lattice pitch is no longer valid for Pin-by-pin calculation, especially when poison pins or empty channels are present in the fuel assembly. Thus, homogeneous and heterogeneous leakage models used in Pin-by-pin calculation scheme were evaluated in this paper. The derivation of the formula of the leakage model was introduced, and the implements of homogeneous and heterogeneous leakage models used in Pin-by-pin homogenization of the lattice calculation were studied. The homogeneous leakage model applied in Pin-by-pin homogenization will lead to the same corrective factor to correct the neutron flux and the current, while the diffusion coefficient based on Fick’s law would be utilized in the core calculation. When the heterogeneous leakage model is applied, a consistent method of cooperation between the heterogeneous leakage model and the pin-cell homogenization theory is proposed. Before buckling search of the heterogeneous leakage model, pin-cell discontinuity factor must be generated to preserve the pin-cell-homogenized problem unchanged compared with the original assembly problem. After buckling search and spectrum leakage modification, the homogenization method should be super-homogenization method rather than GET due to the different critical spectra of adjacent pin cells. The heterogeneous leakage model can provide space-dependent leakage coefficient as the pin-cell diffusion coefficient for core Pin-by-pin calculation. For practical reactor-core applications, different leakage models together with the diffusion coefficients determined by the transport cross-section or by the heterogeneous leakage model are compared with each other to determine which one is more accurate for the PWR Pin-by-pin calculation. Numerical results demonstrate that the homogeneous leakage model works effectively for Pin-by-pin homogenization, while the heterogeneous leakage model together with the space-dependent leakage diffusion would provide better accuracy of PWR Pin-by-pin calculation.
