Ex-vessel debris bed coolability is cornerstone for severe core melt accident mitigation scheme adopted in Swedish type BWRs. Debris agglomeration can significantly affect coolability of the debris bed. The paper discusses an approach for conservative-mechanistic assessment of the debris agglomeration. In order to tackle with considerable aleatory and epistemic uncertainties which present in accident scenario and in the debris agglomeration phenomena we apply mechanistic simulation tool VAPEX code and conservative assumptions about agglomeration related phenomena and accident scenario. Results of systematic analysis are presented in a form of agglomeration mode map. The map shows conditions for transition between different modes of debris agglomeration. Assessments of the influence of epistemic uncertainties and of numerical discretization errors on simulation result are provided. Safety implication aspects of the proposed agglomeration mode map are discussed in the present paper.

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