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NKS Programme Area:NKS-R
Research Area:Severe accidents
Report Number:NKS-289
Report Title:Simulations of Ex-vessel Fuel Coolant Interactions in a Nordic BWR using MC3D Code
Activity Acronym:DECOSE
Authors:Sachin Thakre, Weimin Ma,
Abstract:Nordic Boiling Water Reactors (BWRs) employ a drywell cavity flooding technique as a nuclear severe accident management strategy. In case of core melt accident where the reactor pressure vessel will fail and the melt will eject from the lower head and fall into a water pool, may be in the form of a continuous jet. It is assumed that the melt jet will fragment, quench and form a coolable debris bed into the water pool. The melt interaction with a water pool may cause an energetic steam explosion which creates a potential risk towards the integrity of containment, leading to fission products release into the atmosphere. The results of the APRI-7 project suggest that the significant damage to containment structures by steam explosion cannot be ruled according to the state-of-the-art knowledge about corresponding accident scenario. In the follow-up project APRI-8 (2012-2016) one of the goals of the KTH research is to resolve the steam explosion energetics (SEE) issue, developing a risk-oriented framework for quantifying conditional threats to containment integrity for a Nordic type BWR. The present study deals with the premixing and explosion phase calculations of a Nordic BWR dry cavity, using MC3D, a multiphase CFD code for fuel coolant interactions. The main goal of the study is the assessment of pressure buildup in the cavity and the impact loading on the side walls. The conditions for the calculations are used from the SERENA-II BWR case exercise. The other objective was to do the sensitivity analysis of the parameters in modeling of fuel coolant interactions, which can help to reduce uncertainty in assessment of steam explosion energetics. The results show that the amount of liquid melt droplets in the water (region of void<0.6) is maximum even before reaching the jet at the bottom. In the explosion phase, maximum pressure is attained at the bottom and the maximum impulse on the wall is at the bottom of the wall. The analysis is carried out using two different triggers, but there is little effect of trigger found on the impulses on wall. The pressure attained and impulses on the wall are higher for bigger jet diameters, bigger melt droplets and higher subcoolings.
Keywords:Steam explosion energetics, Nordic BWR containment, Melt droplets, melt jet.
Publication date:07 Aug 2013
ISBN:ISBN 978-87-7893-364-5
Number of downloads:2344
Download:pdf NKS-289.pdf
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