Neutronics and safety analysis of pellet bed reactor for nuclear thermal propulsion

The Pellet Bed Reactor for Nuclear Thermal Propulsion is modeled using the TWODANT discrete ordinance code to determine a reactor point design based on the selection of a fuel fraction in and a diameter of the pellets, dimensions of the reactor core, maximum fuel temperature, and sub-criticality during a water flooding accident. A total excess reactivity of approximately $1.25 (or keff of 1.01), an order of magnitude higher than that estimated at BOM for 15 hours of full power, steady-state operation of the PeBR, is considered. Besides calculating the dimensions of the reactor core to satisfy the excess reactivity at BOM, the results of the neutronics calculations include estimates of the radial and axial fission power density profiles in the PeBR core. These results, in conjunction with a 1-D, steady-state thermal hydraulics analysis are used to select the operation and design characteristics of the PeBR point design, namely: (a) core radius and height of 38.4 cm and 120 cm, respectively, (b) pellet matrix fraction of 0.5, (c) total reactor mass of 3500 kg, excluding those of the radiation shield, the propulsion nozzle, external structure for the propellant flow into the core, and the drive mechanisms of the control drums in the radial reflector, (d) power density of 10 and 15 MW/l for a reactor thermal power of 1000 MW and 1500 MW, submersion calculations show that with all safety rods removed from the core, the 16 control drums are insufficient to maintain the reactor sub-critical. However, when the 8, B4C safety rods are inserted into the reactor, it is possible to maintain the submerged PeBR point design $7.5 sub-critical (keff=0.94)