Development of the Fission Product Release Analysis Code COPA-FPREL

The COPA-FPREL computer code has been developed to estimate the releases of gaseous and metallic fission products (FPs) from high-temperature gas-cooled reactor (HTGR) fuel into coolant. The COPA-FPREL code treats FP release from a coated fuel particle (CFP), diffusion in a fuel element, and leakage into the coolant considering the temperature distribution within a CFP and a fuel element. The code uses a finite difference method to calculate FP migration and heat transfer. In the finite difference method, the kernel, buffer, and coating layers of a CFP and the fuel element are divided into small finite difference intervals. A steady-state heat transfer equation and the Fickian diffusion equation are applied to these intervals. A relatively high diffusion coefficient is assigned to the buffer and the broken coating layers to describe fast diffusion in those regions. Sorption equilibrium is set up between the concentration at the fuel element surface facing the coolant and the vapor pressure at the graphite side of the boundary layer that forms on the fuel element surface. Mass transfer occurs through the boundary layer into the bulk coolant. In a prismatic HTGR, sorption equilibrium is assumed to form between the concentrations at the compact and structural graphite surfaces and the vapor pressure in the gap between the compact and the structural graphite. For ¹³⁷Cs, ⁹⁰Sr, ^110mAg, and ⁸⁵Kr isotopes, the fractional releases from a CFP, a pebble, and a fuel block during simulated heating processes and reactor operations were calculated using COPA-FPREL.