Fuel motions in the multi-pin PFR/TREAT test series

Fuel motions during the early stages of failure were determined for the seven multi-pin PFR/TREAT tests from an integrated analysis of data from the Fast Neutron Hodoscope and the loop instrumentation. It was observed that motion in TUCOP simulations is in two stages; an initial slow movement at failure followed by abrupt dispersal. It was also found that axial movement of fresh fuel begins promptly at initial failure but that pre-irradiated fuel motion is delayed by 15 ms in both TOP and TUCOP simulations.