Source Term And Timing Uncertainty in Severe Accidents
Sergey Galushin, Anders Riber Marklund, Dmitry Grishchenko, Pavel Kudinov, Tuomo Sevón, Sara Ojalehto, Ilona Lindholm, Patrick Isaksson, Elisabeth Tengborn, Naeem Ul-Syed,
One set of representative accident scenarios and one set of relevant deterministic modelling parameters that can affect Nordic boiling water reactor (BWR) severe accident progression and the magnitude of the source term released to the environment were identified. To achieve this, a set of activities was performed, including review of the safety design of the Swedish and Finnish BWRs; review of the PSA L2 for a typical Nordic BWR and identification of risk significant accident sequences; review of severe accident phenomena and respective modelling in the MELCOR code as well as identification of epistemic modelling parameters that can affect severe accident progression and the source term.
The scenario set was based on review of PSA L2 for a typical Nordic BWR design, as well as insights from the emergency preparedness and response and national regulators, including accident scenarios that lead to acceptable release (diffuse leakage from the intact containment, filtered containment venting in case of transient or LOCA), as well as scenarios that lead to unacceptable release (either due to containment rupture due to ex-vessel phenomena or unfiltered containment venting in case of failed containment isolation, or containment bypass sequences).
In total, 50 MELCOR code parameters were selected for further analysis based on the review of the MELCOR modelling of severe accident phenomena and uncertain epistemic (phenomenological) modelling parameters that can affect severe accident progression and the source term released to the environment. These parameters involved in the modelling of core degradation and relocation, fission products release from fuel, debris behaviour in the core region and vessel lower head, vessel lower head failure, fission products behaviour in the RCS and the containment, as well as modelling of the filter trapping, containment sprays and pool scrubbing.
Best-estimate and bounding assessments of the magnitude of fission products released to the environment were performed for the set of selected scenarios and parameters using MELCOR simulations performed at KTH, VTT and Vysus Group. A preliminary screening of the parameters and scenarios was performed using the obtained results and proposals for further study in phase two of the project were made.
Severe accident, Source term, PSA L2, Boiling Water Reactor, MELCOR, Uncertainty analysis