Analysis of Fuel-Coolant Interaction Potential in Sodium Cooled Fast Reactor Safety Experiments

Improved utilization of nuclear fuels, burning of actinides and advancements in safety have brought about renewed interest in sodium cooled fast reactor technology. In regards to this and in light of recent events which have focused attention on new concerns involving safety, analyses have recently been performed addressing beyond design basis accident conditions for which the existence of large quantities of vaporized fuel and coolant have been postulated. Specifically, for very low probability, but highly energetic core disruptive accidents in sodium cooled fast reactors, molten and vaporized fuel in contact with coolant can bring about a fuel coolant interaction leading to transformation of a significant inventory of coolant from liquid to vaporforme, with a potential for subsequent release of fuel as aerosol from the reactor vessel due to transport of fuel fragments and fuel aerosol by the vaporized fuel and coolant. Because general statements concerning the nature of these events were largely speculative, out-of-reactor experiments were conducted in the mid 1980’s in the FAST-CRI-III facility at Oak Ridge National Laboratory to study the transport in sodium of aerosol-bearing UO2 vapor bubbles. Although codes featuring detailed multi-physics models [1] were in various stages of development at the time of program cessation, a simplified thermo-mechanical model, free of requirements for detailed thermophysical property data, has recently been developed for purposes of evaluating FAST outcomes. The model consists of a hydrodynamic module which is used to predict the movement of a pulsatile, aerosol-bearing vapor bubble through the surrounding coolant and a heat transfer module which accounts for thermal interactions as the bubble thermally radiates to the surrounding coolant. The model predictions are consistent with key experimental trends, namely: (1) significantly reduced aerosol release as the coolant level increased, (2) greatly reduced aerosol release in sodium tests compared to release levels measured in a series of baseline water tests. The consistency of these trends is discussed in terms of thermo-mechanical characteristics of the respective coolants. Specifically, the inertia of the surrounding coolant impedes bubble transport to the free surface which addresses the first point above, and, relative to the second point, bubble lifetimes are sufficiently short relative to time estimates for transit to the free-surface, due principally to the effectiveness of quenching by radiation which is particularly pronounced in the case of sodium, owing to high reflectivity values. Additionally, pool subcooling was found to have a cross-cutting influence on aerosol release. Only in tests in which pool subcooling was reduced to ∼10 Kelvin was significant aerosol release detected. For those tests, which occurred in water, measurements suggest that coolant vaporization occurred at intensities above 3000 kg/m2-s, well beyond what has generally been reported from fuel coolant interaction studies in which the coolant interactions are with molten fuel forms. This set of findings will permit a more general assessment of the implications of fuel coolant interactions on the progression of core disruptive accidents, particularly with regards to assessing probable modes of in-vessel aerosol transport within sodium cooled reactors.© 2012 ASME