Criticality Benchmarks for the Epithermal Test Assembly Experiments

The Epithermal Test Assembly (ETA) experiments were performed to test the adequacy of ²³³U, ²³⁵U, and ²³²Th cross sections in epithermal spectra in support of the Light Water Breeder Reactor (LWBR) Program. The ETA design contained a central heavy water–moderated test region surrounded by a light water–moderated annular driver region. Two series of experiments were performed: ETA-I with ²³⁵UO₂-ThO₂ fuel rods in the test region and ETA-II with ²³³UO₂-ThO₂ fuel rods in the test region. The dominant uncertainties in the critical configurations include the test-rod pitch pitch for ETA-I; the test-region fuel-rod fuel density and ²³³U to (²³³U + Th) weight ratio for ETA-II; and the driver-region fuel-rod outer diameter, uranium enrichment, and pitch for both ETA experiments. Benchmark model results using MCNP5 are provided for ENDF/B-V, ENDF/B-VI, ENDF/B-VII.0, and ENDF/B-VII.1 cross sections with only the ENDF/B-VII.0 results falling within three standard deviations of the benchmark model k_eff. The ETA-I and ETA-II benchmark evaluations have been included in the International Handbook of Evaluated Criticality Safety Benchmark Experiments and are replicated in the International Handbook of Evaluated Reactor Physics Benchmark Experiments.