Influence of Delayed Neutron Spectra on Fast Reactor Criticality

For high-accuracy criticality calculations, one should take into account the difference in the energy distributions between prompt and delayed fission neutrons. In steady-state reactor calculations, it is usually assumed that delayed and prompt neutrons are emitted with the same energy distribution. This approximation may lead to systematic deviations in k_effof between −0.2 and +0.05%. While for typical cores of liquid-metal-cooled fast reactors and corresponding critical assemblies the effect is usually fairly small, it may become more important for low-enriched k_∞ experiments and for highly enriched, high-leakage cores. For group cross-section adjustment procedures usually covering a wide range of critical assemblies with fairly different nuclear characteristics, a proper treatment of the energy distributions of delayed neutrons could be particularly important for excluding systematic differences as far as possible.