Steam oxidation of fuel in defective LWR rods

Oxidation of UO2 by pure steam at pressures of 7 and 70 atm and 500 degrees C and 600 degrees C was measured in a thermogravimetric apparatus. The kin etics are linear, vary as the square root of the steam pressure, and are co nsistent with initial rates extrapolated from higher-temperature experiment s in l-atm steam. At temperatures characteristic of normal operation of def ective fuel rods, the rate of hydrogen production by thermal oxidation of t he fuel in steam is small compared with that due to cladding corrosion. The presence of H-2 in the steam has a much greater retarding influence on fue l oxidation than on cladding oxidation. Other potential sources of fuel che mical reactivity in steam, including reaction in cracks in the hot pellet i nterior and radiolysis of steam by recoiling fission fragments, do not resu lt in significant fuel oxidation. During the incubation stage of fuel-rod d egradation, the bulk of the evidence indicates that fuel oxidation is not a major source of the hydrogen in the fuel-cladding gap that eventually may cause secondary-hydriding failure of the rod. (C) 1999 Elsevier Science B.V . All rights reserved.