Low pressure corium dispersion experiments in the DISCO test facility with cold simulant fluids

Abstract

In a severe accident special pressure relief valves in the primary circuit of German Pressurized Water Reactors (PWR) will transfer a high pressure accident into a low pressure scenario. However, there may be a time window during late in-vessel reflooding scenarios where the pressure is in the order of 1 or 2 MPa at the moment of the reactor vessel rupture. A failure in the bottom head of the reactor pressure vessel, followed by melt expulsion and blowdown of the reactor cooling system, might disperse molten core debris out of the reactor pit, even at such low pressures. The mechanisms of efficient debris-to-gas heat transfer, exothermic metal/oxygen reactions, and hydrogen combustion may cause a rapid increase in pressure and temperature in the reactor containment. Integral experiments are necessary to furnish data for modeling these processes in computer codes, that will be used to apply these result to the reactor case. The acquired knowledge can lead to realize additional safety margins for existing or future plants. The test facility DISCO-C (Dlspersion of Simulant COrium - Cold) models the annular reactor cavity and the subcompartments of a large European reactor in a scale 1:18. The fluid dynamics of the dispersion process was studied using model fluids, water or bismuth alloy instead of corium, and nitrogen or helium instead of steam. The effects of different breach sizes and locations, and different failure pressures on the dispersion were studied, specifically by testing central holes, lateral holes, horizontal rips, and complete ripping of the bottom head. 22 experiments were performed in a basic cavity geometry with holes at the bottom of the lower head to study the similarity relations. Variables were the hole diameter, the initial pressure in the RPV and the fluids used. The only flow path out of the reactor pit was the annular gap between the inner wall of the reactor pit and the RPV, and then along the main coolant lines into the subcompartments. Dispersal rates between 0% with small holes (0.3 m scaled) and low pressure (0.3 MPa) and 78% with large holes (0.9 m) and higher pressures (> 1 MPa) were observed. A certain amount of liquid (approximately 25% of 0.0034 m 3 ) is trapped in the space near the RPV support structure, independent of the initial conditions. The dispersed fractions are lower with metal than with water, but still high. The results concerning the dispersed fractions could be correlated by the Kutateladze number Ku = ρ G u G2 /(ρ L g σ) 1/2 , with u G , the maximum gas velocity in the annular space around the RPV, for all hole sizes, both driving gases, nitrogen and helium, and both liquids, water and Bi-alloy, with f d = 0.4 log10(Ku) < 0.76. The Kutateladze number represents the conditions to levitate droplets against gravity. No sharp threshold velocity or pressure could be found, below which no dispersion occurred. From the similarity correlations we can deduce that the results from the liquid metal tests represent the lower bound for the dispersed melt fractions in the reactor case. In a second test series the effect of the position and the shape of the breach in the lower head was investigated. Four different opening were studied: two different hole sizes at an angle of 45 degrees, one horizontal slot, and the unzipping and tilting of the lower head. With lateral breaches the liquid height in the lower head relative to the upper and lower edge of the breach is an additional parameter for the dispersion process. In most cases not all the liquid is discharged out of the RPV. If the initial liquid level is above the upper edge, the blowdown starts with the single-phase liquid discharge, driven by the pressure difference between vessel and cavity, as for central holes. However, the gas blowthrough occurs earlier than with central holes. In the subsequent stage the liquid is carried out of the lower head by entrainment. The gas velocity, the density ratio of gas and liquid, the surface area of the liquid pool, and the duration of the blowdown govern this entrainment process.