TRIGA mixed HEU - LEU thermal hydraulic analysis

It is generally known that now TRIGA SSR has a mixed HEU-LEU core. In order to increase the reactor fuel utilization by slightly rising the excess reactivity, one idea was to extract the central fuel pin from the fuel cluster. The paper is dedicated to the core safety thermal hydraulic analysis of the modified cluster configuration. Thermal hydraulic analysis was done by neutronic computation with 3DDT (three-dimensional diffusion) computer code. The results of these computations, regarding pin-power factors, were input for the thermal hydraulic analysis. We use for our analysis COBRA IV computer code. The configuration analyzed was of a TRIGA fuel cluster of 25 fuel elements in 5 x 5 square configuration. It was considered multi-channel geometry specific for the COBRA computations. For the reason of analysis two cooling modes were chosen: - two pumps with 660 l/sec flow rate; - one cooling pump with 330 l/s. The computations were done for 14 MW rating power. In both cooling modes the heat transfer is produced by fluid forced convection. The gap heat transfer coefficient used is 1.36 +4 W/m2K, which corresponds for a gap width of 1.27-3 cm according to the Safety Report. It is to emphasize that both HEU and LEU fuel types share the same thermal characteristics. The important safety characteristics for both fuels is maximum central temperature, which is limited to 750 deg. C. The reason for this limitation is the fuel composition. Above this temperature the U-ZrH alloy changes the phase and hydrogen is generated what increases the stresses into the fuel. Working with COBRA computer code and with power peaking factors resulting from neutronic analysis, the effect of central pin removal is given by comparing the temperatures of the fuel elements in normal and modified cluster. A small increase of about 1% is observed in latter case as compared with the normal one. At the same time the temperatures at 4 pins in the vicinity of the central water-filed space is decreased due to additional cooling. Also, it was found that the 750 deg. C central fuel limit temperature is not reached in the modified fuel cluster