Modelling the criticality consequences of free surface motion in fissile liquids

To date the criticality assessment, code FETCH has been applied to how and high powered transients in fissile liquids, where free surface motion was not of major significance. Additional code features now allow us to model a ramp reactivity insertion, obtained by pumping uranyl nitrate solution, at a constant rate, into a shallow tank. The result is a transient with complex free surface motion and interacting radiolytic gas bubble generation. The case simulated is from the CRAC series. Free surface modelling has also been tested by simulating a collapsing water column. (author)