Modelling of criticality accidents in aqueous solutions of a nuclear reprocessing plant

The following results were obtained by applying a model to criticality excursions in nuclear reprocessing plants: The vessel geometry influences the nuclear power and the duration of criticality. Low liquid levels in large vessels produce higher nuclear power levels and criticalities of the order of a few minutes (nucleate boiling). High liquid levels in small vessels, however, produce low power and a duration of the accident of several hours (surface boiling). When liquid addition is interrupted in due time criticality terminates by itself by increasing the concentration up to the subcritical geometry. When the boiling temperature is reached criticality is generally not terminated by adding a solution free of fissile material but the number of fissions still increases. For the typical concentrations and quantities of fissile material in a nuclear reprocessing plant, it can be practically excluded that a criticality accident of more than 1020 fissions is produced. (orig.)