Melting Temperatures and Thermal Conductivities of Corium Prepared from UO 2 and Zircaloy-2
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Melting Temperatures and Thermal Conductivities of Corium Prepared from UO2 and Zircaloy-2 Masato Kato, Teppei Uchida, Shun Hirooka, Masatoshi Akashi, Akira Komeno and Kyoichi Morimoto Advanced Nuclear System Research and Development Directorate, Japan Atomic Energy Agency, 4-33, Muramastu, Tokai-mura, Ibaraki, 311-1194, Japan ABSTRACT Corium which simulates the molten core of a Boiling Water Reactor was prepared as a parameter of Zr content, and melting temperatures and thermal conductivities were measured. The melting temperatures were measured by the thermal arrest method and were 2622 oC, 2509 o C and 2540 oC, respectively, in the specimens of 24.3 at%, 49.0 at% and 73.5 at% Zr content. Thermal conductivities had low values of 2.0-3.5 W/m oC at temperatures of 400 to 1600 oC. INTRODUCTION To understand a severe reactor accident scenario, the temperature profile inside the molten core must be known. And to get this profile, it is essential to understand physical properties such as thermal conductivity and melting temperature and to evaluate melting behavior of the reactor core. In a severe accident in an Light Water Reactor (LWR), steam oxidation of Zircaloy (Zry) cladding occurs and corium forms by melting of the core [1, 2]. Many studies on the corium have been carried out so far, but the data related to physical properties are limited [3-6]. Nagase and Uetsuka [3] reported physical properties of sintered (U0.6Zr 0.4)O2 which simulated the core debris of a Pressurized Water Reactor (PWR). A Boiling Water Reactor (BWR), however, has a larger Zry inventory than a PWR has. The content of Zr is estimated to be about 60-65 at% Zr in the molten BWR core. Corium with various compositions is formed inside the molten core depending on the temperature condition; analysis results of the once molten fuel after the accident at Three Mile Island Unit 2 (TMI-2) in 1979 showed that the core debris varied in composition. It is important to investigate the physical properties depending on composition for assessment of an accident scenario. In this work, corium was prepared from UO2 and Zry-2, and its melting temperature and thermal conductivity were investigated as a function of composition. MEASUREMENTS Powders of UO2 and Zry-2 were used as starting materials. The powders were mixed and pressed into three kinds of pellets which were 24.3 at%, 49.0 at% and 73.5 at% Zr in metal. The pellets were sintered at 1650 oC3 h in Ar/0.05% H2 with added moisture. Large swelling was observed due to steam oxidation of Zry-2 during sintering. Subsequently the pellets were held at 1650 oC1 h in dry flowing gas of Ar/0.05% H2. These specimens were crushed and sealed in a tungsten capsule in a vacuum atmosphere by electronic beam welding. The capsules were heated, and melting temperatures were measured. The measurement apparatus and technique were described previously in detail [7]. After the measurement, the specimens were taken from the
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capsules, and densities of the melted specimens were measured by Archimedes' method. A melted specim
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