Norihiro Yamano

Phenomenological studies on melt-coolant interactions in the ALPHA program

Abstract Two series of experiments to investigate melt-coolant interactions have been performed as part of the ALPHA program at JAERI. In the melt drop steam explosion experiments, melt simulating a molten core was dropped into a pool of water. Volume fractions of the melt, water and steam in the mixing region prior to the occurrence of spontaneous steam explosions were quantified. Other characteristics of melt-coolant interactions were evaluated for settling velocity of the melt in water, propagation and expansion velocities, energy conversion ratio and debris size distribution. It was found that the probability of the occurrence of spontaneous steam explosions could be reduced by using a melt dispersion device. Measurement of void fraction in the mixing region clearly showed that the melt dispersion device enhanced steam generation. However, one experiment indicated that the use of the dispersion device could possibly result in a more energetic steam explosion. It was found that the mixing of non-condensable gas in the steam phase of the mixing region during melt dispersion played an important role for the suppression of the spontaneous steam explosion. Knowledge of the parametric effects of melt mass, ambient pressure and water temperature was extended. In the melt coolability experiments, water was poured onto the melt to investigate melt-coolant interactions in a stratified geometry where water overlies on a melt layer. Melt eruptions which could induce an explosive interaction were observed when the subcooled water was poured through a pipe nozzle. The eruption was not observed when the water was near the saturation temperature or supplied through a spray nozzle. The explosive interaction in the stratified geometry was found to be much smaller in magnitude than the steam explosion in the melt drop configuration.

Experimental study on in-vessel debris coolability in ALPHA program

Abstract In-vessel debris coolability experiments were performed in ALPHA program at JAERI. Aluminum oxide (Al 2 O 3 ) produced by a thermite reaction was used as a debris simulant. Approximately 30 and 50 kg of Al 2 O 3 were poured into a pool of nearly saturated water at the ambient pressure of approximately 1.3 MPa formed in a lower head experimental vessel. The post-test visual observation and measurement using an ultrasonic technique indicated the formation of a thin porous layer at the vicinity of the surface of the solidified Al 2 O 3 and the interfacial gap between the solidified Al 2 O 3 and the lower head experimental vessel wall. Thermal transient characteristics on the lower head experimental vessel wall observed in the experiments implied that the interfacial gap and the thin porous layer in the solidified Al 2 O 3 layer acted as a thermal resistance during the initial heat-up stage, and water subsequently penetrated into the interfacial gap to effectively cool the lower head experimental vessel wall. The maximum heat flux removed from the experimental vessel was ranged from approximately 190 to 360 kW m −2 while the temperature of the vessel wall decreased rapidly.

Nuclear aerosol codes

Codes used to simulate aerosol behavior inside containments of nuclear power plants after assumed severe accidents are described. The basic aerosol physical equations of all codes are the same worldwide. Only minor differences can be detected regarding some special aerosol physical processes. These differences are not inherent but caused by boundary conditions, which are of special interest for the code users. The comparison of the single codes also shows that the general agreement achieved by the numerical treatment of the aerosol equation requires an appropriate discretization of the distribution function to yield stable solutions under all arbitrary conditions. The application of solutions based on special distribution functions should, therefore, be restricted to certain scenarios.

Small-scale component experiments of the penetration leak characterization test in the ALPHA program

A small-scale penetration leak characterization test has been performed as a part of the ALPHA program at Japan Atomic Energy Research Institute (JAERI). Two series of experiments were performed using test sections which simulate relevant parts of an EPA (Electrical Penetration Assembly) used in Japanese PWR containments. One of the test sections simulates an alumina module and the other includes the silicone resin portion of the EPA. The test section was heated in a leak test vessel which simulated thermal-hydraulic conditions inside and outside of the containment in a severe accident. From the experimental results, it was concluded that although the silicone resin may melt at high temperature, the alumina module will remain intact under severe accident conditions. The EPA as a whole is estimated to maintain leak-tightness during a severe accident. It was found in the experiments that heat conduction along the metal portion of the test section had a strong influence on the melt progression of the resin. It was also found that the measured strain of the alumina module was predominantly caused by the elevated temperature. Therefore, the thermal load will be more of a threat to the EPAs integrity rather than the pressure load.

Study on premixing phase of steam explosion at JAERI

Abstract Melt jet breakup and fragmentation has been studied in ALPHA program at JAERI. In the first two experiments of the MJB series, a jet of molten lead–bismuth eutectic alloy was released into a deep pool of saturated water. The steam generation rate was measured and correlated with the jet behavior observed by a high-speed camera. The jet breakup length and debris size distribution were also evaluated. In parallel with the experimental study, JASMINE code has been developed for the simulation of a steam explosion process. The models of melt jet breakup and the particle breakup in the code were assessed by analyzing FARO-L14 and ALPHA MJB experiments.