Benchmark Evaluation of HTR-PROTEUS Pebble Bed Experimental Program

Benchmark models were developed to evaluate 11 critical core configurations of the HTR-PROTEUS pebble bed experimental program. Various additional reactor physics measurements were carried out as part of this program; currently, only a total of 37 absorber rod worth measurements have been evaluated as acceptable benchmark experiments for cores 4, 9, and 10. Dominant uncertainties in the experimental K_eff for all core configurations come from uncertainties in the ²³⁵U enrichment of the fuel, impurities in the moderator pebbles, and the density and impurity content of the radial reflector. Calculations of K_eff with MCNP5 and ENDF/B-VII.0 neutron nuclear data are greater than the benchmark values but are within 1% and also within the 3σ uncertainty, except for core 4, which is the only randomly packed pebble configuration. Repeated calculations of k_eff with MCNP6.1 and ENDF/B-VII.1 are lower than the benchmark values but are within 1% (∼3σ), except for cores 5 and 9, which calculate lower than the benchmark eigenvalues by <4σ. The primary difference between the two nuclear data libraries is the adjustment of the absorption cross section of graphite. Simulations of the absorber rod worth measurements are within 3σ of the benchmark experiment values. The complete benchmark evaluation details are available in the 2014 edition of the International Handbook of Evaluated Reactor Physics Benchmark Experiments.