Ryukichi Nagasaki

Metallographic investigation on the cladding failure in the pressure vessel of a BWR

Abstract Samples were removed for metallographic analyses from one of the cracked portions of the multi-layer stainless-steel weld overlay in the JPDR pressure vessel. Metaliographic structure and microchemical analysis were consistent with each other in suggesting that the failure was caused by the significant depletion in the chromium content in the weld metal. The lack in the compositional balance was found to have originated at the overlaying of the first layer on the dissimilar base metal. Morphological features of the cracks indicated the operation of stress corrosion cracking during the process of crack propagation. The chemical composition of the weld metal, thermal history, tensile stress and environment were judged to have favoured the occurrence of cracking. Some of the cracks were found to have reached the base metal across the weld boundaries and further penetrated into the vessel wall in the form of localized corrosion. The attack was of pitting-type corrosion and it was considered to have been accelerated by the concentration-cell action due to the formation of a concentration gradient of dissolved oxygen in water within the narrow cracks. In consideration of the future security of the vessel, containing the defects in the corrosion-resistant liner, some possible problems and further necessary studies were discussed. The probable influence of the high-temperature aqueous environment on the fatigue character of the reactor vessel was pointed out.

The effect of cold working on creep rupture properties for helium-injected austenitic stainless steel

Abstract In order to study the effect of helium on the high temperature embrittlement of stainless steel, helium (7.5 × 10 −6 atomic fraction) was injected into cold-worked stainless steels by using a cyclotron. At 650°C, it appeared that the reduction in creep-rupture strength due to helium was larger as cold-working was increased, but a loss of rupture elongation was less for a particular degree of cold-working. The 10% cold-worked material showed particularly good creep-rupture properties in the presence of helium. The loss of ductility was more pronounced in the creep test than in the tensile test.

Behavior of Tritium in Aluminum-Lithium Alloys

The hehavior of radiation induced gas bubbles in Al-Li alloys was observed by means of 3H autoradiography. The autoradiographs were compared with photomicrographs of the corresponding areas. Autoradiography proved to reveal the gas bubbles in the alloys more distinctly than photomicrography. It was found that the bubbles first appeared at the grain boundaries and subgrain boundaries, and there also appeared depleted zones along the grain boundaries. This was followed by crystallographic alignment of the bubbles when heated to higher temperatures. In the case of large gas concentrations, bubbles as large as 5 μ in diameter were observed in specimens heated for 1 hr at 500°C. Bubbles precipitated along the boundaries of recrystallized grains in a specimen of irradiated Al-2.7 w/0 Li when it was tension stretched and then heated for 1 hr at 300°C.

Behavior of Helium Gas Bubbles in Neutron-Irradiated Beryllium

Transmission electron microscopic observations were made on hot-rolled Be sheet prepared from cast ingots irradiated to about 5 × 1019 nvt(fast) in JRR-2. The aim of the study was to elucidate the effect of irradiation on changes in microstructure and the effect of pre-irradiation heat treatment on the formation of He gas bubbles during post-irradiation annealing. The formation and growth of He bubbles during post-irradiation annealing was distinctly less inhibited in the Be irradiated as hot-rolled than in the metal irradiated after recrystallization treatment. In the former case, He bubbles could be observed upon post-irradiation treatment of only 3hr at 750°C, while in the latter case, a similar formation of bubbles could, only be seen after a final treatment at 900°C for 50 hr. The difference thus observed in the tendency of bubble formation depends upon the grain boundary mobility during post-irradiation heat treatment. The nucleation and growth of bubbles occurred preferentially at the grain boundar...

Improvement of Creep Properties in a Helium Injected Austenitic Stainless Steel

The effect of helium on creep properties was studied in AISI Type 316 austenitic steels after two kinds of treatment, namely, cold rolling and grain-boundary strengthening. Helium was injected into the specimens using a cyclotron, and the creep-rupture tests were performed at 650°C in air. In the cold-rolled specimens, the creep-rupture strengths increased with cold working. The loss of ductility due to the helium injection was minimum at about 10 percent of cold rolling. In the grain-boundary strengthened treatment, serrated grain boundaries were produced by the two step solution treatment. The specimens with serrated grain boundaries had good creep-rupture strength in both the presence or absence of helium. However, a specimen with extreme grain growth during the two step solution treatment had low creep ductility and severe helium embrittlement. A specimen which had serrated grain boundaries without extreme grain growth had excellent creep properties in both helium uninjected and injected cases.

DEVELOPMENT OF A DEVICE FOR MEASURING CIRCUMFERENTIAL MECHANICAL PROPERTIES OF CLADDING TUBES.

A method of measuring the mechanical strength properties in the circumferential direction of cladding tubes for a fast breeder was developed. Circumferential yield stress, ultimate tensile strength and elongation were obtained by analyzing the stress-strain curve. The proposed device is sipmle in operation, and lends itself to application in remoting controlled systems. Another advantage is that the specimens used in the device are very small, so that the method could prove useful for studying the radiation effects on cladding materials.

Metallographic Observations on Creep Deformation of Magnox AL 80

A study, with emphasis on metallographic observations, has been made on the creep deformation of Magnox AL 80 at 250°, 300°, 350° and 400°C. The creep deformation at 250°C is quite different from those at 300°C and above. With deformation at 250°C, grains break clown into cells, that are marked and are smaller in size nearer the grain boundaries. This phenomenon however cannot be observed in specimens subjected to creep at higher temperatures. Wavy grain boundaries, caused by uneven migration, are seen at 250°C, while projection of grain boundaries, caused by localized migration, often occurs at higher temperatures. It is thought that creep cavities are nucleated at the tips of the ripples and projections of the grain boundary, which act like jogs and triple points.

Fatigue Behavior of Magnox AL80 and Irradiation Effect on Fatigue Properties

Bending fatigue tests were carried out at room temperature on Magnox AL80 of different grain size specimens as well as on welded specimens. Also, the structure of the surface of the fatigue fractured specimens was examined. The fatigue strength of the fine-grained specimens was much larger than that of the coarse-grained. The fatigue behavior of the welded specimens was not appreciably different from that of the fine-grained specimens, although the S-N curves for these specimens were slightly different. In the fine-grained specimens, the fatigue cracks nucleated in the slip striations within the grains and propagated intergranularly for the most part. In the coarse-grained specimens the cracks propagated along slip bands within the grains or along the grain boundaries. The irradiation effect on the fatigue properties of Magnox AL80 was also investigated. The fatigue resistance of the fine-grained specimens, including the welded specimens, was lowered by irradiation to an exposure of 1×1017nvt when tested ...

Irradiation Effect on the Tensile Properties of Magnox AL80

A study has been made on the effect of neutron irradiation on the tensile properties at room temperature of Magnox AL80, Mg-Al alloy and unalloyed magnesium as a function of grain size. Heat treated wire specimens were encapsulated and then irradiated in the JRR-2 reactor up to exposures of 1.2x1019nvt (epi-Cd energy). As a result the parameters σ0 and k in the Petch relationship increased in magnitude after irradiations exceeding 1x1017nvt. Among the materials tested Magnox AL80 was found to be the most sensitive to neutron irradiation and unalloyed magnesium the least effected. The effect of neutron irradiation on the strain rate (e) dependence of the tensile stress (σ) was also investigated. The effect of neutron irradiation was to increase A and to decrease B in the equation σ=A+B log e, when σ represented the yield stresses.

Some Studies on Metallurgical Behavior of Magnox Alloy, (II)

Creep tests have been carried out on Magnox, Mg-1% Al and Mg-1% Zr alloys at several temperatures in the range from 300° to 500°C under several stresses in the range from 0.15 to 0.38kg/mm2. After the creep test specimens were cut out to investigate the phenomenon of cavitation in these alloys during creep test.Average creep rates of Magnox alloys under the condition described above are the order of 10-310-1%/hr and Magnox A12 possesses higher creep strength than other Magnox alloys. Creep specimens show ductile fracture at the temperature of 400°C or above and brittle fracture at the temperature of 300°C or below. The surface of the specimen becomes rough by creep deformation.Cavities are formed preferentially in grain boundaries normal to the stress axis at early stage of secondary creep mainly caused by grain boundary sliding. In the case of higher stress cavities and cracks are mainly created at triple points of grain boundaries. At constant temperature, for a given amount of strain the number of cavities increases with decreasing strain rate and for a given strain rate it increases with total amount of strain. Cavities are formed easily at the temperature range from 300° to 400°C and they are not formed at the temperature of 500°C and above.