Process according to claim 3, wherein for each mesh j 1???j???J, all the second neutron flux components ??1 (j,g)(m) 1???g???G is determined iteratively, the values ??1 (j,g)(n,m) obtained at the nth iteration being calculated as the solution of the system with G linear equations: ##EQU25## in which m is a time index, the neutron fluxes being calculated at instants t=t0 +m.??t, n is the iteration