Hideo Maki

Heat-transfer characteristics of Zircaloy-2 oxide film

Oxide film was made to form on the outside surface of zircaloy-2 tube (inside diameter=12.672mm, wall thickness=0.826mm) under the following three different conditions, (a) 300°C saturated LiOH solution, (b) 450°C and 15 ata steam and (c) 450°C air. The resulting oxide presented colors ranging from white to black, depending on the oxidation temperature and duration of exposure.Each of these oxidized specimens, together with a control specimen (unoxidized) was heated directly by alternating current in high temperature coolant water sealed in an autoclave.The apparent thermal conductivity of the oxide film was obtained by comparison of the temperatures inside tube measured on the oxidized and clean specimens under conditions as similar as possible. The experiment was conducted under 110°, 150°, 200°, 250° and 300°C of coolant temperature.At near 110°C of coolant temperature, the apparent thermal conductivity was not affected significantly by difference in the method adopted for oxidization, and the values obtained were distinctly lower than that of sintered ZrO2 stabilized with CaO. At higher temperatures, the oxide films formed under different conditions each showed individual heat transfer characteristics. The apparent thermal conductivity of white oxide film was smaller than that of black oxide film at near 300°C of coolant temperature.

Effect of Stress on Hydride Precipitation Behavior in Zircaloy-2

Hydride directionality in zircaloy is reportedly affected by thermal cycling and stress. Experiments were performed to study these effects quantitatively. The first experiment was thermal cycling applied repeatedly on rolled zircaloy-2 plate under tensile load. It was observed that at the first cycle the hydride directionality changed by an amount depending on the stress level, and thereafter, upon repetition of the thermal cycling, the rate of change was slow and steady, and independent of the level of applied stress. In arranging the data, a new concept, which was termed the “average precipitation angle”, is proposed to express hydride directionality, in which account is taken of the length and the angle of precipitation of the hydride platelets. This concept should at times be more suitable than the currently adopted directionality factor Fn for methodically expressing hydride directionality. Based on the results of this experiment, the average precipitation angle in zircaloy-2 cladding in reactors can...

Out-of-Reactor Study on External-Pressure Creep of Zircaloy-2 Fuel Cladding Tubes

An out-of-reactor experiment was conducted for the purpose of studying the external- pressure creep behavior of zircaloy-2 fuel cladding tubes. The hour-glass shape of UO2 pellets acquired by thermal distortion was simulated by flanges machined out at both ends of stainless steel pellets 21 mm long. The mock-up specimens thus formed were pressurized externally in a furnace, and the changes brought upon the tube diameter were measured at intervals. The external-pressure creep deformation was observed to proceed in three steps- diametral decrease, elliptical deformation and final collapse. In the case of hollow tubes tested devoid of the pellets, elliptical deformation was observed, which accelerated with time until abrupt final collapse. Elliptical deformation was not observed on the pellet-filled tubes. Empirical equations were derived from the experimental results as functions of time, of hoop stress and of temperature, to express the external-pressure creep strain behavior of the stress-relieved tube, p...

Effect of chemical composition on corrosion resistance of zircaloy fuel cladding tube for BWR.

Effects of Fe and Ni contents on nodular corrosion susceptibility and hydrogen pick-up of Zircaloy were investigated. Total number of 31 Zr alloys having different chemical compositions; five Zr-Sn-Fe-Cr alloys, eight Zr-Sn-Fe-Ni alloys and eighteen Zr-Sn-Fe-Ni-Cr alloys, were melted and processed to thin plates for the corrosion tests in the environments of a high temperature (510°C) steam and a high temperature (288°C) water.In addition, four 450kg ingots of Zr-Sn-Fe-Ni-Cr alloys were industrially melted and BWR fuel cladding tubes were manufactured through a current material processing sequence to study their producibility, tensile properties and corrosion resistance.Nodular corrosion susceptibility decreased with increasing Fe and Ni contents of Zircaloys. It was seen that the improved Zircaloys having Fe and Ni contents in the range of 0.30[Ni]+0.15[Fe]≥0.045 (w/0) showed no susceptibility to nodular corrosion.An increase of Fe content resulted in a decrease of hydrogen pick-up fraction in both steam and water environments. An increase of Fe and Ni content of Zircaloys in the range of Fe≤0.25w/0 and Ni≤0.1w/0 did not cause the changes in tensile properties and fabricabilities of fuel cladding tube. The fuel cladding tube of improved Zircaloy, containing more amount of Fe and Ni than the upper limit of Zircaloy-2 specification showed no susceptibility to nodular corrosion even in the 530°C steam test.