Reverse engineering of GETTER : a fission product release code for PBMR

Abstract

Fission product release from spherical fuel spheres under different irradiation and heat-up conditions is one of the key criteria used in High Temperature Reactor (HTR) design. Accurate analyses of fuel performance and fission product behaviour is therefore essential in justifying the safe behaviour of the Pebble Bed Modular Reactor (PBMR). GETTER proved to be a very versatile tool for evaluating fission product transport problems; ranging from heating experiments (up to 1 800°C) to full core analyses under operating and accident conditions. GETTER includes subroutines, which calculate the burn-up and power history of a fuel sphere in the core, and the temperature distribution in the sphere based on given neutron cross-sections and gas temperatures. These values are then used to determine fission product inventories in the fuel materials. Transport through the coating layers of TRISO-particles, graphitic matrix material, and into the coolant gas are based on Ficks' laws of diffusion and evaporative mass transfer. A description is given of the physical laws and correlations used by GETTER to simulate fission product, activation product, or heavy metal migration in spherical HTR fuel elements. Reverse engineering of GETTER was carried out to obtain the mathematical derivations from these laws to the equations that were implemented in the source code of GETTER. In addition, some important anomalies regarding the derivations and implementation in the code are presented in this document; together with suggested resolution mechanisms in certain cases. The flow of theory implementation in the code is also discussed briefly.