Fission product speciation in Phebus tests FPT0 and FPT1 and the possible influence of boron

The thermochemical calculations performed with a large number of elements to describe the chemical system in a Phebus FP test allow the following conclusions: The metallic absorber materials play a significant role in fission product speciation. At around 1000 C and above, silver iodide is the dominant iodine carrier in a full steam atmosphere in both FPT0 and FPT1. At lower temperatures, cadmium iodide takes over. In a reduced environment, indium iodide becomes important. Caesium iodide does certainly appear; at certain test conditions it becomes the major iodine transporting vehicle. But its role is not greater than that of the other iodides. It has more chances in FPT1 than in FPT0. Chlorine, which was present in FPT0 as a contaminant in relatively large amounts, does not prevent the iodine from forming Csl. Both share the caesium like partners. The evolution of caesium chloride and iodide proceeds in parallel. When CsCl is abundant, Csl is strong, too. Should boron be added to the steam injection, the effect on fission product chemistry would be minor. It would certainly not lead to more formation of the highly volatile iodine species Hl and elemental iodine. All that boron can do is to shift some of the iodine from Csl onto the absorber materials. The effect of boron on caesium redistribution is significant only at temperatures above 1000 C - which are virtually not determining for the final fission product speciation in tests with spent fuel pins - or in a strongly reduced atmosphere, which is not available for more than some five to 10 minutes during a test. An important role in the formation of caesium species is played by molybdenum. For the results of code calculations it is therefore important what release rate is attributed to this fission product, especially if low values are involved. (orig./HP)