Experimental Results on Pouring and Underwater Liquid Melt Spreading and Energetic Melt-coolant Interaction

In a hypothetical light water reactor (LWR) core-melt accident with corium release from the reactor vessel, the ultimate containment integrity is contingent on coolability of the decay-heated core debris. Pouring of melt into a pool of water located in the reactor cavity is considered in several designs of existing and new LWRs as a part of severe accident (SA) management strategies. At certain conditions of melt release into the pool (e.g. large ratio of the vessel breach size to the pool depth), liquid melt can spread under water and reach a coolable configuration. Coolability of the melt depends on decay heat generated per unit area of the spread melt which is directly proportional to the terminal spread thickness of the melt layer. Thus a success of the debris bed coolability depends on the efficacy of the molten core materials spreading which is limited by rapid solidification of the melt due to melt-coolant heat transfer. Among the factors which can reduce spreading effectiveness are heat and mass losses of the liquid melt due to fragmentation, cooling and solidification in the process of melt jet pouring into the pool. Previous extensive experimental and analytical works on liquid melt spreading and solidification were focused mostly on analysis of melt spreading in case of melt release through an inclined channel. Melt spreading under water as a result of a jet pouring into a pool, has not been addressed systematically. This paper summarizes first experimental results obtained in the frame of Pouring and Underwater Liquid Melt Spreading (PULiMS) research program. The work is an extension of previously reported by Kudinov et al. [1-4] studies on debris bed formation and agglomeration (DEFOR-A) phenomena. In contrast to DEFOR-A experiments, PULiMS exploratory tests (PULiMS-E) discussed in this work have been performed with a shallow (20 cm) water pool. Up to 78 kg of high melting temperature core melt simulant materials (eutectic mixtures of the binary oxides such as Bi 2 O 3 -WO 3 and ZrO 2 -WO 3 ) is used in each test. Initial melt superheat varied from 70 up to 300ºC. In the paper we discuss: (i) experimental observations of the jet pouring into a shallow pool and underwater liquid melt spreading on a flat surface; (ii) characterization of solidified melt debris; (iii) key physical processes as well as melt material properties and experimental conditions most influencing the melt spreading and solidification phenomena. Produced experimental data can be used for validation of the models for prediction of the underwater liquid melt spreading in case of melt jet pouring in a shallow water pool.QC 20131212