Fuel Cycle Depletion to Spent Fuel Gamma Facility Dose Rate Validation Studies

The primary neutron science-based missions of Oak Ridge National Laboratory's versatile 85 MW High Flux Isotope Reactor (HFIR) are neutron scattering, isotope production, and materials irradiation. However, HFIR also has the unique ability to irradiate materials with gamma radiation from spent fuel elements in its Gamma Irradiation Facility (GIF). Gamma dose rate measurements were performed in the mid-1980s with a nitrogen-filled gamma ionization chamber and fuel assemblies that operated at 100 MW, which was HFIR's original design power. The experimentally obtained results have been used for more than three decades to aid researchers with their experiments. Utilizing experimental measurements is most appropriate; however, modeling and simulation is the best way to account for experimental geometries and materials within the GIF experiment chamber (e.g., self-shielding/attenuation effects). This study develops and validates a calculation methodology with the dose rate measurement data. The systematic methodology includes fuel cycle depletion calculations with the HFIRCON and VESTA codes, gamma source characterization calculations with the SCALE/ORIGEN code, and gamma transport analyses with the Monte Carlo N-Particle code. The calculated dose rate results were typically 0-10% greater than the experimentally obtained measurements, which is considered good agreement and provides confidence that the methodology can be used to optimize experiment designs and accurately estimate as-irradiated experimental dose rates to enhance the fidelity of gamma irradiation research at HFIR.