Neutronic modeling of debris beds for a criticality evaluation

Abstract

After the Fukushima accident, the interest of the scientific community in severe accident research has been renewed. One of the severe accident research issues that needs to be further investigated is the recriticality potential of the debris bed that is formed after the core meltdown. The uncertainty regarding fuel debris conditions is very high. Consequently, one of the challenges of the criticality evaluation is the neutronic modeling of the debris itself. Conservative assumptions and simplifications have to be performed to overcome the uncertainties and to achieve a computationally feasible debris bed model. This paper presents a suitability analysis of several debris bed models for a criticality evaluation. The objective is to identify the most suitable model, i.e. the model with the best compromise between accuracy and simplicity, from a set of candidates. For that, a detailed near-to-reality model was developed whose higher fidelity results have been used as a reference to evaluate the adequacy of various simplifications. This work focuses on the modeling of the porous internal structure of the debris, concretely on the fuel particles, which are characterized by their shape, size and spatial distribution. The final proposed neutronic model represents the debris bed as a regular 3D arrangement of monosized spherical particles. This is a very suitable model delivering accurate results as long as an appropriate equivalent diameter deq is chosen. However, it was proved that not always the same deq value is adequate for representing the debris in the whole porosity range. The Monte Carlo codes MCNP6.1 and Serpent 2.1 were used to construct the debris bed models and to compute the infinite multiplication factor(k∞).