A loss of coolant accident in a fusion reactor could result in the oxidation of activated structural materials and the mobilization of oxide species by volatilization and spalling processes. We have measured the mobilization of various chemical species from austenitic stainless steels (Type 316 and PCA a slightly modified 316 SS) using transpiration test methods. These tests have been performed from 1983 through 1996. We have improved our test methods with time by correcting for contributions from tests components and by using tube specimens with a larger surface area. The latter specimens tested in the Fusion Aerosol Source Test (FAST) facility have allowed us to reduce detection limits by several orders of magnitude and to measure the mobilization of some elements down to temperatures as low as 500°C.

Most of our tests have been performed in our VAPOR (Volatilization of Activation Products Reactor) apparatus. The specimens are contained within a quartz test chamber in this setup. Oxide particles spalled from specimens are more inclined to be attracted and adhered onto the walls of the test chamber and measured as mobilized products in VAPOR. In FAST spalled oxide is more likely to settle-out and be retained in the tube specimen rather than being transported to the collection system located downstream. Conversely, mass flux measurements of elements mobilized by a volatilization process are more likely to be comparable in VAPOR and FAST. We find this to be true at a common test temperature of 800°C for some elements such as molybdenum that volatilizes readily as MoO3 in air. The mass flux obtained for other elements associated with spalled oxide, e.g., iron and manganese, are several times higher in VAPOR than in FAST.

We have developed mobilization flux plots by combining the data from all test series, after eliminating data that was invalid. The lower detection limits resulted in the selection of measurements from FAST below 800°C. VAPOR data used at higher temperature is considered conservatively high due to contributions from spalled oxide. We have used the new cumulative mobilization flux plots, along with codes to calculate radioactivity inventory and dose per Bq of isotope released, to calculate offsite dose values for both air and steam exposures. We compare these temperature-dependent doses per m² exposed surface area, per hour at temperature with the corresponding doses from other fusion-relevant materials.

*This work is supported by the U.S. Department of Energy, Office of Energy Research, Idaho Operations Office, under Contract DE-AC07-94-ID13223.