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

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 UO_2+x ⁺ Cs₂UO₄ or Zr + ZrO₂. The important condensed phases in-reactor are UO_2+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.