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| NKS Programme Area: | NKS-R | | Research Area: | Severe accidents and Reactor Physics | | Report Number: | NKS-97-RAK2-TR-A4 | | Report Title: | On core debris behaviour in the pressure vessel lower head of Nordic boiling water reactors | | Activity Acronym: | RAK-2 | | Authors: | I. Lindholm, K. Hedberg, K. Thomsen, K. Ikonen | | Abstract: | 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 scenarios 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.
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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. For the reflooding cases
both at high and low system pressure, the postulated critical heat flux gap boiling model proves to
be very efficient in saving the RPV from creep rupture even if reflooding is started late in the
sequence. This is because of the assumption that heat can be removed effectively from both crust
and the RPV wall more or less immediately after the start of reflooding. The results indicate that
the MAAP4 lower plenum model with several layers (particulate debris, metal layer and oxidic
debris) requires a finer nodalization at the bottom of the vessel, where MAAP4 predicts the creep
rupture is most likely to take place. | | Publication date: | 01 Oct 1997 | | ISBN: | ISBN: 87-7893-020-0 | | Number of downloads: | 5560 | | Download: |  NKS-97-RAK2-TR-A4.pdf |
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