Chemical Thermodynamics of the System Cs-U-Zr-H-I-O in the Light Water Reactor Fuel-Cladding Gap

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Abstract
Equilibrium thermodynamic calculations were performed on the Cs-U-Zr-H-I-O system that is assumed to exist in the fuel-cladding gap of light water reactor fuel under in-reactor, steam, and 50% steam—50% air conditions. The in-reactor oxygen potential is assumed to be controlled by either UO2+x + Cs2UO4 or Zr + ZrO2. The important condensed phases in-reactor are UO2+x, CS2UO4, and CsI, and the major gaseous species are Cs, Cs2, CsI, and Cs2I2. The presence of steam does not alter these species, although CsOH also becomes a major gaseous species. In a 50% steam—50% air mixture, the equilibrium condensed phases are U3O8 or UO3 and Cs2U15O46. Under a nonequilibrium situation where zirconium metal can react with iodine, ZrI3 or liquid ZrI2 is present, and the gaseous species ZrI3 and ZrI4 have large partial pressures.