Precalculations for the bundle for the first Phebus-FP test FPT0

Phebus-FB is a 1:5000 scale facility where a series of six experiments will be carried out that aim to model the release, retention and chemical behaviour of fission products during a severe accident in a light water thermal reactor. The core, coolant circuit and containment are all modelled and the heating power to the bundle is supplied by nuclear fission in the fuel rods. In this paper we consider calculations that have been performed by different organisations, and with different computer codes, in preparation for the first test FPT-0. In particular we study the phenomena in the bundle. Here we are primarily concerned with thermalhydraulics, core degradation and fission product release. All of these are considered. We note that predictions for the temperature field in the bundle depend less on the code used than on the conductivity assumed for the surrounding shroud. Unlike a reactor the Phebus bundle will be hottest in the mid-plane. There will be three phases in the experiment. First a heat-up with considerable heat-generation from zircalloy oxidation and then, once the zircalloy is mostly consumed, a slower heat-up till the melting point of fuel is reached. The third phase is the cool-down. Because of the high degree of clad oxidation, little interaction between zircalloy and uranium dioxide is expected. Modelling of fission product release and transport is crude. Release is expected to be slow because the fuel in this first test is fresh but correlations used to model it can be expected to over-predict because they are based on irradiated fuel. We have so far made no calculations of transport, nucleation and deposition of fission products and aerosols in the core. (author). 15 refs, 28 figs, 1 tab