Modeling of fission product release during severe accidents with the fuel performance code ALCYONE

Abstract

This paper presents simulations of four tests performed on medium to high burnup fuel during the VERCORS and VERDON experimental programs. The tests are representative of a Severe Accident (SA) sequence with a temperature increase up to fuel-clad melting and oxidizing/reducing conditions within the furnace. The simulations are performed with the fuel performance code ALCYONE where irradiated fuel thermochemistry and fission gas release are coupled. In this paper, the impact of the radial burnup and Fission Product (FP) profiles within the fuel pellet on the FP release from the sample during the SA sequence is studied. Simulations of the fuel sample behavior during nominal irradiation in commercial reactors are first performed to assess the initial state of the fuel. The simulations of the SA sequences include a burnup dependent fission gas release model. The simulated release curves of various volatile and semi-volatile FPs (Xe, I, Te, Cs, Mo and Ba) are compared successfully to on-line measurements. The impact of the burnup and FP radial profiles on both the thermochemical equilibria within the pellet and the FP release kinetics is discussed. It is shown that the FP release from the fuel pellets is not significantly increased by the consideration of the burnup and FP radial profiles. This conclusion is due to the limited radial extension of the peaked burnup radial profile in the fuel pellet and to the uniform temperature of the fuel samples.