Chapter 13 - Fuel Burnup and Transmutation

Abstract

During nuclear reactor operation, the fission products created build up over time and gradually poison the reactor. Eventually, these fission products along with the depletion of the fissionable material can reduce the reactor keff to the point where criticality can no longer be maintained. For nuclear thermal rockets, fuel burnup generally does not pose much of a problem since the time of operation is so short that there is not sufficient time for fission products to accumulate to any great extent. For power-producing reactors which operate for long periods of time at relatively high neutron fluxes, however, fuel burnup can be significant and has to be accounted for in the design of the reactor system. Such long-term operation would be required if, for instance, a nuclear reactor were being used to power some type of electric propulsion system. While there are many fission product nuclides created during the fission process, there are two nuclides in particular which, because of their extremely high neutron absorption cross sections and high fission yield probabilities, should be accounted for in the design of even nuclear thermal rocket engines. These nuclides are X135e and S149m. Of the two, X135e is considerably more important than S149m due to the fact that X135e has a significantly higher neutron absorption cross section.