Criticality safety of fuel assemblies with missing fuel rods

Abstract

The present work is concerned with the problem of determining the reactivity of an assembly from which an unknown number of fuel pins are removed at random locations. Because it is usually designed to be under-moderated, the reactivity of an incomplete assembly will pass by an optimum when the number and position of the missing rods vary. Estimating this maximal reactivity is paramount for criticality safety but is difficult to achieve in practice, because of a prohibitive combinatorial factor, that forbids any direct approach of the problem.It is shown that the maximum reactivity of an isolated PWR assembly with missing fuel rods can be estimated with very few calls on time expensive computer simulations. Drawing a parallel with the field of neutron transport in random geometries, it is indeed shown that the problem can be reduced to an optimum search in an appropriate bounded, bi-dimensional space.