TREAT multi-serttaneutronics design and analysis support

Transient Reactor Test (TREAT) experiment vehicle design is necessary in preparation for TREAT facility restart and the resumption of transient testing to support current Accident Tolerant Fuel (ATF) characterization needs and future fuels testing requirements. Currently the most mature vehicle design is the Multi-SERTTA (Static Environments Rodlet Transient Test Apparatuses), which can accommodate up to four concurrent rodlet-sized specimens under separate environmental conditions. Development of this robust test vehicle design requires neutronics analyses to support optimization of the Power Coupling Factor (PCF) to efficiently maximize energy generation in the fuel rodlets and to perform essential safety analyses. Current Multi-SERTTA calculations support final design of this vehicle, the design process for other TREAT vehicles, and the development of analyses practices for upcoming transient testing experiments. Models of the preliminary design Multi-SERTTA vehicle with rodlets were prepared and neutronics calculations were performed using MCNP6.1 with ENDF/B-VII. 1 nuclear data libraries. Calculations of PCF for both dry and wet operational conditions were performed, with PCF values approximately 28 % greater in a wet environment due to thermalization of neutrons with the experiment medium. Use of neutron absorbing components in the Multi-SERTTA experiment vehicle can significantly reduce PCF values in the experiment. It is estimated that with high-power transients, testing of fuel pins depleted up to 120 GWD/MTU is quite possible in TREAT. It is expected that transient correction factors (TCF) for Multi-SERTTA would be similar to those measured in M8CAL. Actual calibration testing as part of reactor restart and experimental campaign support will be utilized to validate expected PCF and TCF values for Multi-SERTTA experiments.