Characterization of oxide breakup by convective currents [fusion reactor]

One safety consideration for fusion reactors is the potential release of activated products during a loss-of-coolant accident (LOCA). In this paper we address the relative role that small, airborne particles, formed by convective current transport and grinding of spalled oxides, may have in predicted releases. We describe the methodology developed to measure the attrition of oxide particles when they are transported in a gas stream. This method has been used to measure the tendency for small "airborne size" particles to form from oxides of a tungsten alloy, a niobium alloy, and a ferritic steel (HT9). We have used these data to compare the magnitudes of elemental releases that may occur by volatilization with those due to the oxide attrition pathway for the tungsten and niobium alloys oxidized in air. Oxide attrition provides significant releases of tungsten and is the dominant mechanism in the release of niobium. These results show that oxide attrition should be considered in safety evaluations and that additional testing is warranted for other materials, such as beryllium, austenitic stainless steel (PCA), and vanadium alloys. Data are also needed for oxides formed in other environments and at other temperatures to establish a database that will allow mapping of dominant release mechanisms for the complex thermal cycles that could exist during a LOCA.