On the use of importance factors from Monte Carlo calculations for efficient fast neutron fluence prediction

The implementation of a simplified approach for efficient fast neutron fluence estimations is discussed in the paper. The analysis is based on the utilisation of neutron importance factors obtained by means of standard forward neutron transport calculations with a Monte Carlo code. The approach is supposed to be useful for a number of practical problems when precise calculations are either not necessary or not feasible. The calculations have been performed with the MCNPX-2.4.0 code using realistic models of a Swiss PWR. Verification studies were performed based on comparison with results from detailed routine calculations that were validated against experimental data. The results obtained indicate that the simplified approach yields rather satisfactory efficiency in terms of accuracy and computational expenditures, despite the fact that during the analysed reactor history the transition from an 'out-in-in' to a 'low-leakage' fuel management scheme caused a significant disturbance of the neutron source distribution. Thus, provided that the reactor power and burnup distributions are known or reasonably well predicted, the proposed approach may be applied for the effective prediction of the fast neutron flux response and examples of some practical cases requiring such approximate techniques are also outlined. (authors)