Reaction Kinetics of Iodine and Cesium in Steam/Hydrogen Mixtures

The chemical reaction kinetics of fission product iodine and cesium released from fuel to a steam/hydrogen atmosphere are investigated at conditions associated with severe core damage accidents. The results are used to assess the time to establish equilibrium and the ultimate chemical form of iodine and cesium as a function of gas mixture concentration and temperature conditions. Illustrative calculations are presented for interpretation of the chemical form of iodine and cesium during the Three Mile Island Unit 2 accident, as well as for recent severe fuel damage experiments. At low fission product concentrations (fission product/steam mole ratio < 10⁻⁸), the time to establish equilibrium may be on the order of tens of seconds, with the principal species being CsOH and HI. However, at fission product/steam mole ratios exceeding 10⁻⁵, the principal species are CsOH and Csl, with an equilibrium time of ∼10⁻⁴ s. Concentration conditions thus influence the ultimate chemical form of fission products in a steam/hydrogen gas mixture and the time to establish thermochemical equilibrium. Fission product concentration conditions should therefore be considered in the specification of the chemical form of iodine and cesium gas-phase transport for nuclear plant accident consequence analysis.