Insights into prolonged solution criticality incidents, using the coupled radiation/multiphase code FETCH

The development of the FETCH transient code has been supported by the UK regulator, the Nuclear Installations Inspectorate, as a generic reference method for modelling fissile solution criticality. The fundamental basis of the method means that it can be applied to situations somewhat beyond the range of available experiment. General geometry finite-element modelling is used for the two linked modules: neutron transport and multiphase computational fluid dynamics. Multiphase modelling includes bubble dynamics, free surface motion (sloshing) and the ability to calculate pressure waves and their interaction with bubble formation. FETCH has been benchmarked against transient power and pressure results from the SILENE experiments and is being tested against published TRACY measurements. Research is being performed in order to use data from the incident at the JCO Company on 30-31 September 1999 to: (a) gain an understanding of the mechanism of criticality to improve the future safety designs of the fuel cycle facilities; and (b) to understand the mechanisms of this accident itself, for example its prolonged nature. The full published geometry of the 'sedimentation tank' has been modelled focusing on, firstly, the role of the assumed initial conditions, such as filling rate. The influence of heat removal rate upon the power during prolonged criticality phase is demonstrated. The paper comments on the role that pressure might have in influencing the consequences of criticality accidents. (author)