Recession of ZrO2 Scale on Zircaloy Cladding in Molten Metals Consisting of Stainless Steel and B4C

摘要

In sever accidents in boiling water type reactors with water cooling system shutdown, the Zircaloy fuel cladding is oxidized in high-temperature steam atmosphere. Subsequently, temperature of the fuel cladding is increased by radiation heat from the nuclear fuel and oxidation reaction of the Zircaloy itself. At that time, temperature of the control rods consisting of stainless steel and B4C powder is also increased. Eutectic reaction in the control rods occurs around 1200C. The eutectic melt is reacted with the Zircaloy cladding with ZrO_2 scale. In this situation, oxygen in ZrO_2 scale diffuses into the Zircaloy, resulting in recession of the ZrO_2 scale. Breakage and spallation of the ZrO_2 scale also occurs. The recession rate of the ZrO_2 scale thickness is quite fast. For example, ZrO_2 scale with 90 microns disappears at 1200C for 30 min. The kinetic analysis of the recession of ZrO_2 scale on the Zircaloy is discussed in the present report. This study is performed partially in Advanced Multi-scale Modeling and Experimental Tests for Fuel Degradation in Severe Accident Conditions, supported by Ministry of Economy, Trade and Industry.