12.116. The alkali metals, such as cesium and rubidium, are both re­active and volatile in their elemental form. In the fuel, gaseous cesium tends to exist in thermodynamic equilibrium with several cesium com­pounds. Upon failure of the fuel, a fraction of the cesium present will combine with most of the iodine, but the remainder is likely to react with steam present to form cesium hydroxide. Other reactions could occur with the various substances present but it is likely that the compounds formed will have low volatility.

Tellurium

12.117. Tellurium is the most significant fission product of the chalcogen class, which also includes selenium. This importance is a result of both the concentration of its several isotopes (about 1 percent of the fission prod­ucts) and their radiotoxicity, primarily because they are iodine precursors. Under accident conditions, it is expected to be released from the fuel in elemental form and then react with various structural materials. Tellurium behavior appears to be dependent on the particular accident scenario and is the subject of ongoing studies.