PHEBUS FP: a severe accident research programme for current and advanced light water reactors

The ongoing PHEBUS FP programme is the centrepiece of an international co-operation investigating, through a series of integral in-pile experiments, key-phenomena involved in the progression of a postulated severe accident in a light water reactor (LWR). The dedicated PHEBUS facility offers the capability to study the degradation of real core material, from the early phase of cladding oxidation and hydrogen production up to the late phase of melt progression and molten pool formation. The subsequent release of fission products (FPs) and structural materials is also experimentally studied, including their physicochemical interactions, their transport in the cooling system, and their deposition in the containment. The revolatilisation of iodine due to radiochemical effects in the water of the sump and the amount of low-volatility FPs and transuranium elements reaching the containment are receiving a special interest, as large uncertainties related to their modelling subsist. FPT-0 and FPT-1, the first experiments of the programme, performed in December 1993, and July 1996, respectively, have demonstrated that the PHEBUS FP facility is capable of successfully attaining these objectives. They reached very advanced states of degradation, comparable to what was observed in TMI 2, and generated a wealth of results on FP behaviour. The resulting database has been—and will be—applied to develop and validate the computer codes used to assess the safety of the currently operating plants and to check the efficiency of accident management procedures. They will also support the design of future plants having the capacity to confine core melt-down accidents within their containments.