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| NKS Programme Area: | NKS-R | | Research Area: | Severe accidents and Reactor Physics | | Report Number: | NKS-97-RAK2-TR-A3 | | Report Title: | On recriticality during reflooding of a degrading boiling water reactor core | | Activity Acronym: | RAK-2 | | Authors: | J. Miettinen, L. Nilsson, H. Sjövall, F. Højerup, E.K. Puska, M. Anttila, I. Lindholm | | Abstract: | In-vessel core melt progression in Nordic BWRs has been studied as a part of the RAK-2 project
within the Nordic Nuclear Safety Programme 1994-1997. A part of this study was the evaluation of
possibility and consequences of recriticality in a reflooded, degraded BWR core.
The objective of the study was to examine, if a BWR core in a Nordic nuclear power plant can reach
critical state in a severe accident, when the core is reflooded with unborated water from the emergency
core cooling system and what is the possible power augmentation related to recriticality. The
containment response to elevated power level and consequent enhanced steam production was
evaluated.
The first subtask was to upgrade the existing neutronics/thermal hydraulic models to a level needed
for a study of recriticality. Three different codes were applied for the task: RECRIT, SIMULATE-3K
and APROS. Preliminary calculations were performed with the three codes. The results of present
studies showed that reflooding of a partly control rod free core gives a recriticality power peak of a
substantial amplitude, but with a short duration due to the Doppler feedback. The energy addition is
small and contributes very little to heat-up of the fuel. However, with continued reflooding the fission
power increases again and tend to stabilise on a level that can be ten per cent or more of the nominal
power, the level being higher with higher reflooding flow rate.
A scoping study on TVO BWR containment response to a presumed recriticality accident with
a long-term power level being 20% of the nominal power was performed. The results indicated that
containment venting system would not be sufficient to prevent containment overpressurization and
containment failure would occur about 3-4 h after start of core reflooding. In the case of station
blackout with operating ADS the present boron system would be sufficient to terminate the criticality
event prior to containment failure, but in case of feedwater LOCA and boron dilution to the whole
containment water pool, the present boron concentration would not be sufficient to ensure subcriticality
in the core.
The project succesfully initiated development of adequate analytical tools for recriticality studies and
thus laid foundation for the continued work in the field in the framework of the EU SARA project
(Severe Accident Recriticality Analyses) 1997- 1998. | | Publication date: | 01 Febr 1997 | | ISBN: | ISBN: 87-7893-010-3 | | Number of downloads: | 5337 | | Download: |  NKS-97-RAK2-TR-A3.pdf |
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