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NKS Programme Area:NKS-R
Research Area:Severe accidents
Report Number:NKS-487
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 project is to generate a body of knowledge regarding the uncertainty in the magnitude of fission products release in the case of a potential severe accident in Nordic nuclear power plants. The work aims to provide insights into the effect of various types of uncertainty on the source term predictions. The results of the work will be useful both for probabilistic and deterministic safety assessments as well as for emergency response applications. The third phase of the project focused on the assessment of the source term uncertainty in Nordic BWR-type reactors, in cases of containment bypass, containment failure, and filtered release scenarios. In the case of containment bypass scenarios, the results showed that structures, systems, and components outside containment can reduce conservatism in source term analysis. Further refinement of the level of detail in modeling release paths and system effects could enhance the accuracy of the source term analysis results. Analysis of release pathways for fission products in the case of containment failure, especially through failed containment hatch doors can lead to significant changes in the source term. Critical concerns include basemat melt-through and non-coolable debris beds. The analysis of the effect of the independent spray system on the source term released to the environment in filtered containment venting scenarios showed significant reduction in the Cs release compared to that in the unmitigated scenarios. The analysis includes quantification of uncertainties related to the MELCOR code's epistemic modeling parameters and options, and their effects on the timing and magnitude of the source term. Furthermore, for the Finnish BWR the Morris method and SNAP/DAKOTA analyses highlighted the relative influence of parameters, with some unexpected influential parameters likely due to numerical noise. Containment spraying effectively reduced Cs and I releases, although it did not affect FCV opening times. The analysis of the effectiveness of pool scrubbing in the Multi-Venturi Scrubbing System and the refined modelling of the MVSS showed how the efficiency of pool scrubbing changes during different stages of severe accident progression. It also showed the impact of MVSS structures on the behavior of fission products in the scrubber and the source term released to the environment.
Keywords:Severe accident, uncertainty quantification, MELCOR, Nordic BWR, fission products, source term
Publication date:27 Jun 2024
Number of downloads:92
Download:pdf NKS-487.pdf
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