Abstract
In the event of a severe accident, it is necessary to prevent the release of radioactive materials into the environment due to damage to a containment vessel. It is also important to mitigate the consequences of such a severe accident. If debris (core melt and molten structure) falls down from a reactor vessel onto a pedestal floor, concrete will be eroded by the debris as a result of thermal decomposition and fusion. In addition, concrete decomposition will cause the production of non-condensable gases such as hydrogen and carbon monoxide, resulting in pressurization of the containment vessel. Therefore, we have been developing a safety system to stabilize the core melt and avoid a molten core concrete interaction. For these reasons refractory materials with high melting points should be installed. The refractory layer can protect concrete by the system like core-catcher. In order to estimate the effect of the refractory layer, it is necessary to consider about the reaction between debris and refractory layer. In this study, we investigate a eutectic reaction using a phase diagram. To apply the phase diagram to the evaluation method of refractory layer erosion, phase diagrams of debris and candidate refractory materials, such as ZrO_2, Al_2O_3, and MgO are required. In previous works, UO_2-ZrO_2 were studied and reported by many seniors as a debris composition, but there are no data for UO_2-ZrO_2-Al_2O_3 and UO_2-ZrO_2-MgO, such containing debris and refractory material composition. However there are some methods to predict phase diagrams from a database (TDnucl) base on existing binary phase diagrams. Therefore, in this work, phase diagram analyses were carried out and experimental verification data of UO_2-ZrO_2-Al_2O_3 and UO_2-ZrO_2-MgO were collected. At the representative debris composition, quasibinary phase diagram calculations and experiments were carried out. Experiments for the phase diagram were carried out by elevated temperature test. During these test, plateaus were detected from the temperature curve. From the results, for Al_2O_3 or ZrO_2 as refractory layer, it would be possible to use TDnucl. However, for MgO as refractory layer, it would be better to consider about oxidation states of phase diagrams. Using these phase diagrams, it would be possible to include the eutectic reaction to the evaluation method and it would be possible to predict more realistic situations.
