A Method of Predicting the Critical Mass and Neutron Flux Distribution of a Reactor by Use of a Physical Model

The detailed design of a nuclear reactor, especially if it is of a new type, must be preceded by experimental work to refine the calculated parameters. A new method is proposed for determining the critical mass and the neutron flux distribution. A model of the reactor is used that is free from fissile material. The fuel channels are filled with a neutron absorber, of cross-section equivalent to that of the intended fuel, and the creation of fast fission neutrons is simulated by a neutron source which is moved along the various channels in turn. Resulting thermal neutron flux distributions are measured with thermal neutron detectors. The required critical mass can be deduced if the absolute source strength and neutron flux are known. The method has been checked on a reactor that operates with uranium hexafluoride. A critical mass was predicted that was in good agreement with the value found on starting up the reactor. The method may be of use in preliminary studies of new reactors in assisting in the choice of lattice dimensions and other parameters. It is simple, and involves neither fissile materials nor high neutron fluxes.