Advanced thermionic reactor systems design code

An overall systems design code is under development to model an advanced in-core thermionic-energy-conversion-based nuclear reactor system for space applications at power levels of 10 to 50 kWe. The design code will consist of a series of design modules, each responsible for the determination of specific system parameters. The code modules include one for neutronics and core criticality, one for thermionic fuel element performance, a radiation shielding module, a module for waste heat transfer and rejection, and modules for power conditioning and control. The neutronics and core criticality module determines critical core size, core lifetime, and shutdown margins and is achieved by utilizing the criticality calculation capability of the Monte Carlo neutron and photon transport code system (MCNP). The effects of enriched tungsten on the neutron multiplication factor for relatively fast and thermal reactor configurations is considered using a series of core calculations. It is shown that, based on neutronic considerations alone, reactors based on natural tungsten emitters and collectors will be required to utilize driver fuel rods to achieve criticality. Otherwise, enriching the tungsten in isotope 184 will be necessary. Driven reactor cores are also considered, including configurations with distributed driver rods as well as segregated driver and thermionic fuel element regions.