Cotels Project (2): Fuel Coolant Interaction Tests Under Ex-Vessel Conditions

Extensive efforts have been performed to resolve in-vessel fuel coolant interaction (FCI) concerning ct-mode failure issue. However, there are several basic differences between in-vessel and ex-vessel FCI conditions. FCI phenomena when a molten cerium falls into water pool on containment floor during a sever accident (SA) were experimentally investigated in COTELS Test A. Typical ex-vessel FCI conditions of SA in actual plant were pertinently simulated in the tests based on SA scenario analysis of LWR. In the tests, about 60kg of UO2 mixture was poured into water pool. In most tests conducted, measured pressure in the test vessel reached the first peak value as long as 0.5 s compared with the time observed in steam explosion (SE), then it gradually approached the final quasi-steady state pressure within 10 s. No violent SE was observed in any tests. Most of poured cerium was broken up and only small amount of ingot cerium was found on the pool bottom. The first peak pressure was well correlated by the cerium particle size which was much larger than that usually observed in SE. Measured final quasi-state pressure was larger than that evaluated by the initial stored energy in cerium due to non-condensible gas generation as a result of oxidation of metallic components in the cerium. The non-dimensional first peak pressure was well correlated by one non-dimensional parameter not only for COTELS data but also for FARO data.