Stabilization of Tungsten-Uranium Dioxide Composites Under Thermal Cycling Conditions

Uranium losses during thermal cycling of tungsten - uranium dioxide composites to 2500 C in flowing purified hydrogen for specimens initially containing 35 volume percent uranium dioxide were reduced by the use of thorium dioxide, calcium oxide, or yttrium oxide as additives in solid solution with the uranium dioxide. The effectiveness of the additives decreased in the order yttrium oxide, calcium oxide, and thorium dioxide. Stabilization of uranium dioxide by calcium oxide or yttrium oxide can be explained in part on the basis of the introduction of oxygen vacancies into the fluorite lattice and the associated lowered partial molar free energy of oxygen. The difference in the effectiveness of calcium oxide and yttrium oxide is discussed, and a possible explanation of the observed behavior is proposed. Of the concentrations of yttrium oxide tested (i.e., 2. 5, 5, and 10 mole percent), 10 mole percent of yttrium oxide was the most effective in pre venting loss of uranium from composities

Coating life prediction

The investigation combines both experimental studies and numerical modeling to predict coating life in an oxidizing environment. The experimental work provides both input to and verification of two numerical models. The coatings being examined are an aluminide coating on Udimet 700 (U-700), a low-pressure plasma spray (LPPS) Ni-18Co-17Cr-24Al-0.2Y overlay coating also on U- 700, and bulk deposits of the LPPS NiCoCrAlY coating

Fuel-Retention Properties of Tungsten-Uranium Dioxide Composites

Although tungsten-uranium dioxide composites appear very promising, as fuel element materials for high-temperature nuclear rocket reactors, one of the major problems with these materials is the loss of fuel at reactor operating temperatures (near or above 4500 F). Substantial fuel losses occur by vaporization whenever the uranium dioxide is directly exposed to elevated temperature environments and by fuel migration and vaporization when reactor operating conditions that involve thermal cycling are simulated. Several methods of minimizing fuel loss have been evaluated and appear quite promising