The analysis of transient criticality in wetted uranium dioxide systems and in fissile solutions

The use of uranium oxide powders and granules is common place in the nuclear industry and is manufactured and held in containers subject to stringent criticality revues. However inadvertent events such as the ingress of water might hypothetically cause criticality, and it is for the analysis of such events that a computer code POWDER has been developed. Similarly fissile solutions are used in factory processing and in experimental reactors, and interest in transient criticality in these solutions has continued over many years, with experiments both in the USA and in France. In order that understanding of the various physical mechanisms be obtained, and thus to be able to analyse hypothetical accidents, a computer code CRITEX has been written. This paper describes the physical mechanisms believed to be operating in the two situations, and outlines the methods used in the two codes. Some examples of the use of the codes POWDER and CRITEX are given, with comparison with experimental results. The work was partly funded by BNFL. (author)