Verification of HELIOS and HELIOS/AFEN against PWR critical experiments loaded with high plutonium content MOX fuels

We have verified a neutron and gamma transport code, HELIOS1.4, and HELIOS/ AFEN core nuclear analysis procedure against two mock-ups (ALL-MOX and GD-M OX) of PWR critical experiments loaded with high plutonium content MOX fuel s. HELIOS slightly overestimates the criticality but within an acceptable e rror bound, and calculates pin power distribution with an RMS error less th an 1.1% for both ALL-MOX and GD-MOX mock-up cores. The HELIOS/AFEN core nuc lear analysis procedure, based on the single assembly calculation for nodal group constants generation, has been also verified against the same two mo ck-ups. It was found that the energy group dependent power form function sh ould be applied in pin power reconstruction for MOX fuel assembly. The AFEN reconstructed pin powers in MOX and UO2 fuel assemblies are in good agreem ent with the experiments; the RMS error is about 1.7% for the MOX assembly of both mock-ups, which is comparable to 1.9% for UO2 fuel assembly. This r esult assures the quality of HELIOS/AFEN procedure in accurately calculatin g the pin power distribution in PWR core intermixed with high plutonium con tent MOX and UO2 fuel assemblies. (C) 1999 Elsevier Science Ltd. Ail rights reserved.