THORIA AND THORIA-URANIA REINFORCED BY METAL FIBERS

Abstract

Thoria compacts containing refractory metal fibers in quantities as low as 5 wt% showed significantly better resistance to thermal shock spalling than thoria alone. Of the metals and alloys evaluated, molybdenum and niobium gave the best results. Values are presented for room- and elevated-temperature properties of fiber-reinforced thoria. Measured properties included compressive strength, modulus of rupture, impact strength, thermal shock resistance, thermal conductivity, thermal expansion, and oxidation resistance. The widespread presence of microcracks in these compacts resulted in significantly lower strengths and elastic moduli than those of thoria. Fibers improve resistance to thermal spalling by suppressing and limiting crack propagation and by structurally reinforcing the cracked body. Room-temperature thermal conductivity of reinforced thoria is slightly higher than that of thoria, but at 1600 deg C it is 3 times greater. Oxidation resistance of molybdenum-reinforced thoria is best of all combinations investigated and improves with increased specimen density. However, because of the presence of microcracks, even the densest specimens are severely attacked after 24 hr in air at 1000 deg C. Thoria-urania compacts reinforced with molybdenum fibers were bonded together by means of conventional brazing techniques. Irradiation of metal-reinforced thoria-urania specimens indicated that molybdenum fibers aided the heat transfer from the fuel, whereas niobium fibers reacted with the fuel and lost their effectiveness. (auth)