Since the very beginning, the Phébus FP programme of
integral experiments was considered as a necessary complement to the
qualification “one by one” of physical models through separate effects
tests. Small-scale analytical experiments are obliged to introduce
hypotheses on the additivity of phenomena and do not allow to be sure
that no important phenomenon has been omitted. Also the physico-chemical
nature of a number of species can best be determined in integral type
of experiments. For all those purposes, a series of five in-pile
integral experiments has been performed. The facility provided
prototypic reactor conditions which allowed the study of basic phenomena
governing core degradation through to the late phase (melt pool
formation), hydrogen production, fission product (FP) release and
transport, circuit and containment phenomena, and iodine chemistry.
For each of these topics, key lessons have been learnt and are described. Amongst the most important, one can cite:
- The need to revisit cladding oxidation modelling, that impacts the hydrogen production kinetics.
- The fuel collapse (transition from rod-like geometry towards a molten pool) at temperatures far below what was expected.
The fission product release from degrading fuel.
chemical form of fission products when transported in the Reactor
Coolant System, especially for iodine and caesium, the most important
in-containment behaviour of iodine especially the reactions between
iodine and paints and the trapping of iodine by silver under certain
From these findings, the
physical models implemented in simulation tools used for safety studies
have been improved. Simulation tools have recently been extensively used
for the understanding of the Fukushima accident events.