Air ingress into the reactor pressure vessel during a severe accident in a nuclear power plant will lead to a mixed atmosphere mainly consisting of air and steam. Due to the strongly oxidizing character of air this mixed atmosphere leads to an enhanced core oxidation and degradation which affects the fission product release from the core to the primary circuit and subsequently to the containment in case of leaks or through relief valves. Several fission products are of high importance because of their high radiotoxicity and their ability to form highly volatile oxides. It is therefore important to understand the phenomena governing cladding oxidation by air as a prerequisite for source term determination. For investigating the oxidation behaviour under mixed air/steam atmospheres two experiments were conducted in the frame of two EU funded projects, QUENCH-18 (ALISA project) and CODEX-AIT3 (SAFEST project).
For both tests, pre- and post-test simulations were done in the frame of the NUGENIA project QUESA (QUEnch experiment with Steam and Air). This paper deals with the analyses for QUENCH-18 conducted at KIT in 2017. The pre-test benchmark was performed by five participants (EDF, GRS, IBRAE, LEI, and formerly of PSI) using four codes (MAAP, ATHLET-CD of AC2, SOCRAT, twice RELAP/SCDAPSIM) to identify suitable boundary conditions for achieving the targets of the tests, which should consist of the following phases: pre-oxidation, intermediate cooldown, air/steam ingress phase and finally cooldown by water. ... mehrAdditionally, the pre-test analyses were supported by parameter studies from GRS to define the boundary conditions. The results of the pre-test benchmark show mainly comparable behaviour of the simulations except one. Oxygen and steam starvation were predicted, which was one target of the experiment to allow nitride formation as well. For the post-test benchmark nearly the same participants as for the pre-test analyses performed simulations by application of the actual boundary conditions of the experiment. The objective is to evaluate the code capabilities especially related to the air ingress modelling, which is at different development levels for the codes. Primarily the preliminary results of the post-test benchmark and the comparison to the measured data available at the time of this present study will be presented in this paper. The comparison shows in general a good prediction of the measured behaviour by all codes.