OECD/NEA Burnup Credit Criticality Benchmarks Phase IIIB: Burnup Calculations of BWR Fuel Assemblies for Storage and Transport

The report describes the final results of the Phase IIIB Benchmark conducted by the
Expert Group on Burnup Credit Criticality Safety under the auspices of the Nuclear Energy
Agency (NEA) of the Organization for Economic Cooperation and Development (OECD).
The Benchmark was intended to compare the predictability of current computer code and
data library combinations for the atomic number densities of an irradiated BWR fuel
assembly model. The fuel assembly was irradiated under specific power of 25.6 MW/tHM
up to 40 GWd/tHM and cooled for five years. The void fraction was assumed to be uniform
throughout the channel box and constant, at 0, 40 and 70%, during burnup. In total, 16
results were submitted from 13 institutes of 7 countries. The calculated atomic number
densities of 12 actinides and 20 fission product nuclides were found to be for the most part
within a range of ± 10% relative to the average, although some results, esp. 155Eu and
gadolinium isotopes, exceeded the band, which will require further investigation. Pin-wise
burnup results agreed well among the participants. The results in the infinite neutron
multiplication factor k∞ also accorded well with each other for void fractions of 0 and 40%;
however some results deviated from the averaged value noticeably for the void fraction of
70%.