Gas Core Reactor Rocket - Neutronic Considerations

Neutronic Considerations

Since steep temperature gradients will be present in any such gas core reactor, several implications for neutronics must be considered. The open-cycle gas-core reactor (OCGCR) is typically a thermal/epithermal reactor. Most types of OCGCR require external moderation due to the steep temperature gradients inside the gaseous core. Neutrons born in the fuel region travel relatively unimpeded to the external moderator where some are thermalized and sent back into the gas core. Due to the high core temperatures, however, on the return trip the neutrons are up scattered in the fuel region, which leads to a significant negative reactor worth. To achieve criticality, this reactor is operated at very high pressure and the exterior radial wall is made up of a moderator of some sort, generally barium oxide. Moderation can also come from introducing moderating particles into either the fuel or propellant streams, but by doing so, the benefits in neutronics is canceled by loss of rocket performance.