Status of the FETCH transient criticality code and application to TRACY data and dosimetry applications

The FETCH code has been developed to model criticality transients in multi-phase media of fissile solution and is applied to three TRACY transients with step reactivity insertions of 2$, 2.5$ and 2.8$. The solutions used in TRACY are of a lower uranium enrichment (10% U235) and higher concentrations (nominal 400 gU/litre). These transients are characterized by radiolytic gas evolution. It has been investigated if FETCH can be used to predict TRACY data using the fundamental parameters (determining rate and quantity of radiolytic gas production). The sensitivity of the TRACY simulations to these parameters is also examined. To complement modelling the behaviour of TRACY over short times, investigation is made of the behaviour over 10's of seconds after the initial pulse - still in a time range where heat transfer out of the system is of minor significance. It is shown that there can be a second fission peak some 10 to 30 seconds after the initial peak and that the time evolution of this peak is determined by the assumed radiolytic gas bubble size used in the gas-liquid drag laws. The use of FETCH in modelling dosimetric quantities is discussed on the basis of results from a transient analysis. Prediction of the dose to various organs for a worker is studied, especially the neutron dosimetry for the worker who received the largest dose during the JCO Company criticality accident. (H. Yokoo)