Thermal behaviour and tritium management for in-pile testing of the pebble bed assemblies in the HFR in Petten

Four pebble-bed assemblies are to be irradiated in the HFR in Petten with the objective to study the thermo-mechanical behaviour of the breeder ceramic pebble beds during irradiation. The thermo-mechanical behaviour of the pebble bed assemblies was calculated in a 2D axi-symmetric model in MARC. In this approach there could not be accounted for the influence of thermocouple tubes on the temperature distribution in the assembly, because these are distributed in the assembly in a non axi-symmetric manner. The solution for this problem was to expand the model to a 3D model used for thermal computations only. For safety reasons the tritium production in the breeder and permeation through the first and second containment must be estimated before the in-pile experimentation begins. In order to do so, the calculated thermal distribution is used as input for the enhanced two-dimensional finite element model in MARC. Adaptations are made in the 2D model by adding the capability of performing mass flux calculations. This paper describes the finite element models used for computation of the temperature distribution and the tritium flux through the pebble bed assembly. The results of these calculations are critical for a safety assessment of the in-pile operation of the experiment and will give a better understanding of the in-pile behaviour on temperature and tritium management in advance.