Safety analysis models for the irradiation of 237NP targets at the high flux isotope reactor

A campaign is underway to provide a new domestic supply of plutonium-238 using existing nuclear research reactors at the Oak Ridge National Laboratory (ORNL) and Idaho National Laboratory (INL) and existing chemical recovery facilities at ORNL. Validation and testing activities for new irradiation target designs have been conducted in three phases over a 2 year period to provide data to support an increased throughput toward a continuous production phase. Design, qualification, and fabrication of "fully loaded" targets of NpO₂/Al pellets have been completed, and target irradiation is ongoing at the High Flux Isotope Reactor (HFIR) at ORNL. In order to qualify experiments for irradiation at the HFIR, bounding accident conditions established in the HFIR safety analysis report (SAR) must be analyzed. Target design drawings, pellet fabrication data, and post-irradiation examination (PIE) measurements are input to computational safety analyses that calculate conservative parameters of interest in the target including maximum internal temperatures, coolant surface temperatures, and structural stress/strain maxima. Heat generation and decay rates in the target are analyzed using the neutronics codes MCNP, SCALE, and VESTA. Steady-state thermal-structural analysis of the target is performed using COMSOL Multiphysics, and transient thermal hydraulic analysis is performed using RELAP5. The primary physics phenomena explored include heat conduction, structural mechanics, thermal hydraulics, neutron transport, and isotopic transmutation with specific challenges in gas-gap/contact conductance, coupled thermal-structural responses, and pellet irradiation behavior.