The fission product (FP) behaviour in PHEBUS FPT0 (fresh fuel rods) and FPT1 (irradiated fuel) in-pile experiments is interpreted. The main experimental results in FPT0 and FPT1 are first summarized, and the FP release rates measured in both tests are reported and related to degradation events. The interpretation is performed in two steps. First, using general knowledge from the open literature and from previous interpretations of VERCORS analytical experiments, a qualitative description of the possible behaviour of fission gases, Cs, Mo and Ba is given by considering only intact fuel geometry. This interpretation is illustrated by calculations performed with the mechanistic MFPR code. Then, in a second step, possible effects of fuel dissolution, foaming and interactions with structural materials are considered.
For FPT1, most of the FP release might be explained by mechanisms acting in intact geometry. On the contrary, for FPT0, it is necessary to consider the fuel dissolution by molten non-oxidised Zr, mainly at grain boundaries to explain the early and large FPs release observed in this test.
In addition, it is shown that in both tests, the release from the corium molten pool is not significant and that interactions with Zr and Fe from structural materials can strongly reduce the Ba release.