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| NKS Programme Area: | NKS-R | | Research Area: | Severe accidents | | Report Number: | NKS-503 | | Report Title: | Source Term and Timing Uncertainty in Severe accidents - NKS-STATUS Phase 3 report | | Activity Acronym: | STATUS | | Authors: | Sergey Galushin, Govatsa Acharya, Dmitry Grishchenko, Pavel Kudinov, Sara Ojalehto, Tuomo Sevón, Ilona Lindholm, Patrick Isaksson, Naeem Ul-Syed, | | Abstract: | The overall goal of the NKS-STATUS project is to advance knowledge on the uncertainty in the magnitude of fission product release during potential severe accidents in Nordic Boiling Water Reactors (BWRs). The work aims to provide insights into the effect of various types of uncertainty on the source term predictions.
This 4th phase of the NKS-STATUS project builds on previous work by focusing on fission product behavior inside the containment, with particular attention to remobilization, mitigative actions, and probabilistic evaluation of uncertainty.
Results obtained for the SBO scenario leading to filtered containment venting showed a limited contribution of cesium remobilization to total release, with dominant sensitivity to aerosol shape factors (GAMMA, CHI). The role of independent containment spray systems was also assessed in this scenario. Sprays slightly reduced cesium release to the environment, primarily affecting the initial release phases, but had minimal influence on remobilization.
In LOCA scenarios, significant remobilization of Cs and I2 deposited in the reactor pressure vessel and containment was observed, with the mode of debris ejection (IDEJ) identified as the main driver of uncertainty. The project further examined vessel lower head failure and debris ejection, identifying conditions under which steam explosion loads increase containment failure probabilities. Debris ejection conditions were shown to significantly influence the probability of containment failure due to ex-vessel steam explosions. A statistical approach using TEXAS-V demonstrated that non-reinforced hatch doors are highly vulnerable, with failure probabilities exceeding 80% in some scenarios. Updated empirical CDFs for Cs and I2 releases show variability depending on containment failure timing; later failures generally result in lower releases due to increased deposition and scrubbing.
Probabilistic evaluation of uncertainty analysis results for fission product release in filtered containment venting scenarios initiated by LOCA (RC7B) indicates that despite significant phenomenological uncertainties, the overall impact on unacceptable release frequency remains limited, with containment bypass sequences and Interfacing System LOCAs (IS-LOCAs) scenarios continuing to dominate the risk profile. | | Keywords: | Severe accident, uncertainty quantification, MELCOR, Nordic BWR, fission products, source term | | Publication date: | 02 Sept 2025 | | ISBN: | 978-87-7893-601-1 | | Number of downloads: | 6 | | Download: |  NKS-503.pdf |
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