On core debris behaviour in the pressure vessel lower head of Nordic boiling water reactors

In-vessel melt progression in Nordic BWRs has been studied as part of the RAK-2 project within the Nordic Nuclear Safety Programme 1994-1997. A part of the study was the evaluation of the late phase melt progression phenomena and the thermal behaviour of core debris, the pressure vessel wall and the lower head penetrations during a severe accident.

The investigations presented here focus on BWR cases. The MELCOR/Bottom Head Package was applied to investigate the core debris bed behaviour and thermal response of structures in the case of the Olkiluoto 1 and 2 reactor vessel lower head. Both low and high pressure scenarious were analysed with sensitivity studies addressing the effects of debris bed porosity, debris particle size and reflooding of the dry debris bed.

Lower head failure mechanisms and timing were examined by allowing instrument tube failure (normal case) or by deactivating the penetration failure model with an input option. Due to modelling assumptions in MELCOR, all presented calculations examine thermal behaviour of a rubble bed in the lower head.

Calculated results are evaluated against experimental data. Studies using Forsmark 3 input data were carried out with the MAAP4 code. Studied cases covered also low and high pressure sequences, and a number of sensitivity calculations varying a few key parameters were performed. Only creep rupture of the reactor pressure vessel (RPV) was considered in the MAAP4 analyses.

The reason for discarding penetration failures was that the current MAAP4 model for ejection of penetration tubes is not deemed to be applicable to ABB reactor specific penetrations. The current MAAP4 model with entrainment and fragmentation of the debris jet from the core to the lower plenum results in creep rupture close to the bottom of the RPV.