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[en] Thermal shock behavior of UO2 pellets has been investigated by means of out-of-pile experiments and a theoretical analysis which particularly emphasized the porosity effect on the thermal shock damage. In the experiments, specimens of porosity range 0.05-0.15 were thermal-shocked by heating and then quenching in a water bath at various quenching temperature differences (ΔT). Results showed that with increasing porosity, ΔT values cause a first damage (ΔTsub(c)) and bring about destructive failure of the specimens increased, while the strength loss at ΔTsub(c) was reduced. These findings suggested that the higher the porosity, the higher the pellet integrity during rise to power. Theoretical equations expressing the thermal shock damage were introduced. Good agreements were obtained between observed and predicted values. (orig.)
[en] Problems of ablation mechanisms of cometary nuclei are discussed in the light of some specific observations of comets and meteors. Estimates of the mass in the Geminid meteor stream are given. The outbursts of Comet P/Tuttle-Giacobini-Kresak are compared with those of the more distant comet P/Schwassmann-Wachmann 1. A formal solution of heat shock effects in comets near perihelion is given as an upper limit of the efficacy of this process for cometary disruption
[en] The thermoelastic interaction in an unbounded medium with a spherical cavity is studied using two-temperature generalized thermoelasticity theory. The medium is assumed to be initially quiescent. The inner surface of the cavity is taken traction free and subjected to a thermal shock. By the Laplace transformation, the basic equations are expressed in the form of a vector-matrix differential equation, which is solved by an eigenvalue approach. Some comparison have been shown in figures to estimate the effect of the two-temperature parameter.
[en] Carbon-carbon (C/C) composites have been widely used for high-temperature structural applications because they possess excellent mechanical properties such as high specific strength and thermal shock resistance. In the nuclear industry, the composites have been also considered for plasma facing materials in fusion reactors and high-temperature structural parts in gas cooled reactors. In the high-temperature gas cooled reactor, the composites are being considered for the application of various high-temperature structural parts such as control rod components, core restraint belts, tie rods, upper plenum shroud, hot duct insulation cover sheets, and floor blocks. However, the carbon based materials are susceptible to oxidation at temperatures above 400 .deg. C. Therefore, the oxidation behavior and property degradation at high temperature under impure He or in air in case of accidental air ingress should be evaluated before the deployment of composites. In this study, the oxidation behavior and microstructure change of nuclear-grade C/C composites, Toyo Tanso CX-270G and SGL Sigrabond 1501YR, were evaluated after oxidation at various temperatures. The oxidation experiments were performed in range of 500 to 1200 .deg. C under air and He atmosphere
[en] In this work, a ZrB2-SiC-Ta4HfC5/Ta4HfC5 oxidation-resistant dual-layer coating was fabricated to improve the oxidation resistance of carbon–carbon (C/C) composites. The coating was prepared by pack cementation combined with the slurry paste method. Spark plasma sintering was used in the procedure. The as-prepared dual-layer coating was approximately 110 μm, and no distinct interface was observed between the coating and the matrix. The isothermal oxidation test results showed that the dual coating effectively prevented C/C composites from oxidizing. The weight loss of the coated samples was only 3.3 and 9.5% after oxidation at 1773 K for 20 h and ten thermal shock cycles between 1773 K and room temperature in air, respectively. The pores and microcracks were the main reason for the failure of the coating.
[en] A summary is given of a report prepared for the NRC which surveys licensee event reports of operating American PWRs as part of a study of the capability of PWR pressure vessels to withstand thermal shock. Seven generalized event sequences leading to pressure vessel thermal shock (PVTS) and five severity categories of PVTS are defined. A listing of 34 significant events is presented. In total 99 events were identified from the data bank. The characteristics of these events are analysed according to mode of reactor operation before the event, initiating sequences of the event and causes of initiation. (U.K.)