Development of a HEX-Z Partially Homogenized Benchmark Model for the FFTF Isothermal Physics Measurements

A series of isothermal physics measurements was performed as part of an acceptance testing program for the Fast Flux Test Facility (FFTF). A HEX-Z partially homogenized benchmark model of the FFTF fully loaded core configuration was developed for evaluation of these measurements. Evaluated measurements include the critical eigenvalue of the fully loaded core, two neutron spectra, 32 reactivity effects measurements, an isothermal temperature coefficient, and low-energy gamma and electron spectra. Dominant uncertainties in the critical configuration include the placement of radial shielding around the core, reactor core assembly pitch, composition of the stainless steel components, plutonium content in the fuel pellets, and boron content in the absorber pellets. Calculations of criticality, reactivity effects measurements, and the isothermal temperature coefficient using Monte Carlo N-Particle version 5.1.40 (MCNP5) and ENDF/B-VII.0 cross sections with the benchmark model are in good agreement with the benchmark experiment measurements. There is little agreement between calculated and measured spectral measurements. This benchmark evaluation has been added to the International Handbook of Evaluated Reactor Physics Benchmark Experiments.