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The release of the occluded gases in uranium-plutonium mixed carbide and uranium carbide pellets for irradiation tests were measured successfully using a multipurpose high temperature apparatus. A new type of gas line filter was set up in the gas exhausting line, where the filter could be exchanged easily without plutonium contamination and degassed by heating. The release of the occluded gases in the carbide fuels was measured at 1,473, 1,673 and 1,873 K. At 1,473 K, most of the gases evolved was H2 gas and above this temperature, the oxygen contained in the fuel was released continuously in the form of CO gas. It was shown that the H2 gas was generated by the reaction of the fuel with moisture adsorbed at the fuel surface during storage and the CO gas was released in proportion to its equilibrium pressure of the oxycarbide solid solution. The release of the gases from the fuels scarcely affects the increase of the inner gas pressure of fuel pins since the amount of H2 gas is small, about 100 μl·atm/g-fu...

In-pile release of fission gas from sintered UC pellets in the presence of 8–230 ppm of water vapor in the He sweep gas was measured over the temperature range of 160°–1,000°C. A very complex release behavior was observed and the mechanisms of release were deduced from the manner in which the release depended on the decay constant. It was established that the release of short-lived fission gases during irradiation was controlled mainly by pseudo-recoil, while chemical reaction between UC and water vapor, as well as knock-out, appeared to contribute much more significantly in the case of the longer-lived fission gases. The release of fission gas after reactor shutdown was shown to be governed by the UC-H2O reaction. The ratio of the release due to this reaction in reference to the total release was found to be dependent not only on the concentration of the water vapor but also on the amount present of the accumulated reaction products. Also, a discussion is given on the inordinately high release of 135mXe ...