Analysis of TREAT transient overpower experiments using the PLUTO codes

Pre- and post-test computer analyses of TREAT transient overpower test H6 are presented and compared with an earlier analysis of E8. The computer models used were the PLUTO code and its successor, the PLUTO2 code. A good agreement with several events could be achieved in the post test simulations, which supports the basic modeling in the codes used. The pre-test analyses suffered from the uncertainty in the pin failure sequence. Fission-gas pressures in the pins at failure time were on the order of 15 to 30 MPa in both tests. Plenum fission-gas release may have played an important role in both tests. Although a stronger FCI tended to be promoted by the larger hydraulic forces in the H6 test with its higher pump pressure and flow velocity, the voiding was less pronounced for the same reasons. Fuel sweepout from short EBR-II irradiated pins failing into non-voided channels tended to be on the order of 5 to 20 g of fuel per pin