NEW SIMPLIFIED FORMULAE OF ESTIMATION THE TOTAL NUMBER OF FISSIONS BASED ON THE NORDHEIM-FUCHS MODEL

摘要

This article presents simplified formulae for the estimation of the total number of fissions for criticality accidents. One of these formulae is based on the Nordheim-Fuchs model. An additional formula takes into account the boiling for solution systems. These formulae are adapted to homogeneous media and can be used for solution and metal accidents. The principle of these formulae, associated with their assumptions, is described. Then, these formulae are compared with practical applications for SILENE and CRAC reactors (solution systems) and past criticality accidents with metal assemblies as well as a previous simplified formula. The proposed new formulae give bounding estimates of the total number of fissions, provided that their area of applicability are respected and that the simulations performed to calculate the necessary input data are representative of the criticality accident configuration. These new formulae require the determination of more parameters than previous simplified formulae, which make them more complicated to use, but may point out and quantify the impact of some physical phenomenon (for instance for the solution boiling). In particular, the comparison with past criticality accidents with metal assemblies shows that more detailed simulations (in terms of geometry and reflection conditions) might be performed in future work to improve the estimate of the number of fissions based on the Nordheim-Fuchs model.