The aim of this paper is to propose a new approach for optimizing the posit
ion of fuel assemblies in a nuclear reactor core. This is a control problem
for the neutronic diffusion equation where the control acts on the coeffic
ients of the equation. The goal is to minimize the power peak (i.e. the neu
tron flux must be as spatially uniform as possible) and maximize the reacti
vity (i.e. the efficiency of the reactor measured by the inverse of the fir
st eigenvalue). Although this is truly a discrete optimization problem, our
strategy is to embed it in a continuous one which is solved by the homogen
ization method. Then, the homogenized continuous solution is numerically pr
ojected on a discrete admissible distribution of assemblies.