Royal Institute of Technology, Nuclear Power Safety AlbaNova University Center, SE Stockholm, SWEDEN 1 The SGTR Event in EFIT Some relevant experimental information B.R. Sehgal Emeritus Professor Royal Institute of Technology Nuclear Power Safety AlbaNova University Center SE Stockholm

Royal Institute of Technology, Nuclear Power Safety AlbaNova University Center, SE Stockholm, SWEDEN 2 INTRODUCTION In my presentation at the last progress meeting for WP1.5, I had indicated that a SGTR event in a vessel full of Pb may create quite difficult consequences: - generation of pressure in the constant-volume vessel beyond the setting of safety valves, - possibility of an energetic event loading the core and the vessel, - possibility of a core-compaction event, - possibility of reactivity addition due to Pb voiding and steam entry into the core region. The thermal-hydraulics of the event: blow-down of water and steam at sonic velocity in a pool of liquid metal at low pressure is extremely complicated. A knowledge of the physics of the event can only be obtained if some experimental information becomes available. Experiments on a water jet injection into a pool of high temperature LBE were reported in the last NURETH meeting, held in Oct The experiments were performed in Japan. These were careful experiments with visualization of the interaction through neutron radiography.

Royal Institute of Technology, Nuclear Power Safety AlbaNova University Center, SE Stockholm, SWEDEN 3 EXPERIMENTAL INFORMATION-1 A 10 mm. Dia water jet with water velocities of 5.8 and 7.8 m/sec was injected in a LBE pool at temperatures from 230°C to 547°C. The LBE pool was maintained in a slice hemispherical vessel of 170 mm dia. and 150 mm height. The test configuration and the test matrix are shown in next two slides. This is not a big system, but it is a representative experiment with prototypic temperature for the LBE melt. The interaction was visualized by using neutron radiography in a research reactor test cell. Thus, it could be seen how far the water penetrated into the LBE pool. Void fraction, pressure and temperature measurements were made with fast-response instrumentation. Pictures of cavity formation and the extent of penetration were obtained and are shown in the next slide. The water jet at 7.8 m/sec velocity penetrated to the bottom of the vessel. There were several different modes of interaction. In some the steam formed was continously released. In others, there was an accumulation of sub-cooled water in the cavity undergoing film boiling, which later became unstable. There was an energetic reaction in one experiment, which deformed the walls of the vessel. In another experiment LBE discharged out of the vessel due to pressure increase.

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Royal Institute of Technology, Nuclear Power Safety AlbaNova University Center, SE Stockholm, SWEDEN 9 EXPERIMENTAL INFORMATION-2 The LBE temperature and the water subcooling both affect the mode of interaction and the stability of film boiling. It appears that as the subcooled water heats up in the cavity, the film boiling can become unstable.Film collapse and direct contact of melt and water becomes energetic. This is particularly true when the instantaneous contact temperature exceeds the homogenous nucleation temperature. Such processes were first observed and anlysed for the sodium-cooled fast breeder reactor core-disruptive accident, wherein sodium vapor explosions were possible, when interaction occurred with molten fuel. The LWR steam explosions also are a product of a similar type of interaction, except, in this case hot molten core material jet enters into a pool of water, which is the vaporizing fluid. The experiment reported here is valuable, but, it does not fully represent the interaction that we are worried about. The water-steam discharge at high pressure to a tank at low pressure will lead to water flashing. There would be penetration of two phase jet into LBE, but, perhaps, it would be a steam jet and there may be less instability. The mode of interaction may be quite different from that observed in the Japanese experiment.

Royal Institute of Technology, Nuclear Power Safety AlbaNova University Center, SE Stockholm, SWEDEN 10 THE EUROTRAN EXPERIMENTS A set of experiments on the water-LBE interaction at practically prototypic conditions are scheduled to be performed in ENEA. Very high pressure water blow-down into a pool of LBE is supposed to be performed. We are waiting for good observations from this projected experiment to learn about the physics and mode of the interaction. It is not clear that the ENEA experiments would illuminate us sufficiently to develop a model for these processes. There will not be any visualization, but it is not clear that the instrumentation will be sufficiently responsive, detailed and sophisticated to provide the data needed.

Royal Institute of Technology, Nuclear Power Safety AlbaNova University Center, SE Stockholm, SWEDEN 11 PREDICTION OF THE CONSEQUENCES OF THE SGTR EVENT IN EFIT The prediction of the consequences of the SGTR event, I believe, presently is not credible, since we do not know the physics of the interaction. Calculations can be performed, perhaps, with some codes, e.g. SIMMER, RELAP-5, but how accurate their predictions are is unknown. These codes, should at least be applied to the analysis of the experiment described above to check, if there are any models in these codes to describe such an interaction. The steam explosion codes: MC3D, MATTINA, etc. could also be used for analysis of these experiments, and, later, for prediction of the SGTR event in EFIT. However, knowing these codes, I do not have much hope that we would succeed, since they describe the interaction of a corium jet with a water pool, which is the reverse of what happens in the SGTR event. We have to wait for the data from the DM4 experiment in ENEA. Meanwhile we should investigate the application of various codes. I believe we will hear about the application of the SIMMER code to this event. We would also perform some extrapolatory modelling work.