Fast reactor designs are currently being revisited aiming at having a consolidated safety dossier. In that frame, studying any perturbation of nominal operating condition is mandatory.

Among different initiators, particular attention is being paid to reactivity insertion due to core assembly bowing and deformation and induced lattice readjustments as a consequence of events such as earthquakes.

In this study, a deterministic calculation scheme based on the mesh projection method has been used in order to evaluate the reactivity changes occurring in a deformed sodium fast reactor core.

With the microscopic cross sections calculated by ECCO, full three-dimensional core calculations are being conducted with ERANOS (DIF3D), VARIANT, and SNATCH to solve neutron transport equations in either diffusion, nodal variational, or Sn transport approximations.

A simple analytical model based on perturbation theory has been developed to identify the main phenomena leading to changes in the core reactivity. Reactivity changes induced by small deformations can be estimated as a summation of reactivity perturbations of individual subassemblies.

The results obtained with this method have been checked by comparing them to those obtained with Monte Carlo simulations. A good agreement is being found allowing the use of this method in realistic problems with significant computer resource reduction.

The different contributions to the reactivity changes confirm the results of the analytical model.