A neutronics optimization approach for preliminary design and safety of nuclear reactors for nuclear thermal propulsion

Nuclear thermal propulsion is a key technology for long-range spaceflight, as demonstrated by the rising interest from space agencies, especially NASA. A preliminary design involves the exploration of many reactor configurations, until a configuration that meets all the design requirements is found. Trade-offs among different design fields, such as neutronics, thermal-hydraulics, safety and rocket performances are unavoidable. Design engineers have to run a large amount of simulations because of the multitude of possible reactor configurations and the different analyses that must be carried out. The present work investigates an optimization approach for the design of a LEU reactor with CERMET fuel, to find a configuration that fulfils neutronics, safety and NASA requirements for a nuclear thermal rocket. A neutronics analysis constitutes the basis of the work, but accidental scenarios and thermal-hydraulics analysis are performed as well, to assess reactor safety and rocket performances. The optimization procedure simulates different configurations and gradually reduces the design space to be explored, until an optimal region is found. In this way, unnecessary simulations are avoided. A Python script is developed to handle the whole analysis, from pre-processing to post-processing, including the integration between the neutronic code Serpent and MATLAB®. Once the optimal region is found, the most promising configurations are identified by comparing different performance metrics retrieved from the neutronics analysis. A safety analysis that simulates the reactor behaviour in four accidental scenarios is carried out on this smaller group of reactor configurations. Finally, reactors that fulfil safety requirements undergo a thermal-hydraulic analysis, which verifies that thermal limits are not exceeded and evaluates rocket performances. The approach successfully reduces the number of configurations to explore, and limits additional analyses to those configurations that are truly competitive. Furthermore, configurations in the optimal region achieve better performance than the initial configuration. It is also found that the most demanding constraints relate to safety: many promising configurations were discarded because they cannot meet safety criteria. This highlights the necessity of including safety analyses in the initial phase of reactor designs. It is concluded that a LEU, CERMET-fuelled reactor design can be considered challenging but feasible, and the approach presented in the work may be applied to find an optimal configuration for a preliminary reactor design.