On the effect of MOX fuel conductivity in predicting melting in FR fresh fuel by means of TRANSURANUS code

Paper in proceeding, 2015

The capability of the fuel to operate at high power without melting is important to Fast Reactors. Reactor design limits normally require that there be a low probability of fuel melting during steady-state operation, including overpower conditions. This requirement has a direct effect on the steady-state power limit of the fuel pin and hence on the reactor power. The development of computational tools that are able to capture the occurrence of high temperature phenomena and mechanisms is thus an important step in reducing the margin of conservatisms and increasing the reactor efficiency. Among the experiments that were conducted for this purpose, HEDL P-19 experiment has been selected and simulated using TRANSURANUS code to exploit its capability to capture the inception of MOX fuel melting. The experiment included 8 fresh pins with cladding Outside Diameters (OD) 5.84 mm, and 8 fresh 6.35 mm OD pins. It was performed during 1971 to investigate the effect of the initial fuel-to-cladding diametric gap size (from 0.086 to 0.25 mm) on the linear heat rate needed to initiate incipient melting at beginning-of-life. All 5.84 mm OD pins with fuel-to-cladding gaps equal to or less than 0.14 mm had no fuel melting. The remaining 5.84 mm OD pins and all the 6.35 mm OD pins experienced partial fuel melting. This work reported hereafter consists of two parts. The main objective of the first one is to assess the capability of TRANSURANUS to predict the measured melting heights of the tested rods and its implications on the thermal conductivity correlations implemented in the code. The second part included modifications that targeted the high temperature thermal conductivity term in two TRANSURANUS correlations. The modifications were incorporated into the code that was recompiled.