[en] Purpose: To reduce the corrosion rate and suppress the increase of radioactive corrosion products in reactor water of nuclear fuel assemblies for use in BWR type reactors having spacer springs made of nickel based deposition reinforced type alloys. Constitution: Spacer rings made of nickel based deposition reinforced type alloy are incorporated and used as fuel assemblies after applying treatment of dipping and maintaining at high temperature water followed by heating in steams. Since this can remove the nickel leaching into reactor water at the initial stage, Co-58 as the radioactive corrosion products in the reactor water can be reduced, and the operation at in-service inspection or repairement can be facilitated to improve the working efficiency of the nuclear power plant. The dipping time is desirably more than 10 hours and more desirably more than 30 hours. (Horiuchi, T. )

[en] Purpose: To suppress the increase of radioactive corrosion products in reactor water dissolved from springs used in nuclear power plants. Constitution: Springs made of nickel-based deposition enforced type alloy are heat-treated at an oxidative atmosphere from 400 to 730 deg C after molding and aging. In a case where springs are used in the reactor water of a nuclear power plant directly after the molding and aging, a great amount of nickel is leached out from the springs. However, by applying the heat treatment within the abovementioned temperature range in an oxidative atmosphere, the leaching of nickel in the reactor water is reduced and thus the concentration of Co-58 is decreased. Accordingly, the operation upon periodical inspection or maintenance can be facilitated to improve the working efficiency of the nuclear power plant. (Kawakami, Y.)

[en] Purpose: To use a wear proof nickel base alloy in check pins of a guide roller and a limiter roller, thereby to completely remove cobalt produced by wear and reduce the radiation dose. Constitution: Check pins of a guide roller and a limiter roller are formed by an alloy consisting of 30 to 40% by weight of chromium, 2.0 to 4.0% by weight of aluminum, and the residual part of nickel. Chromium is necessary for improving wear resistance. With less than 30% of thereof, the alloy lacks the wear resistance, whereas with more than 40% thereof, the forge processing of the alloy becomes difficult. Aluminum is also necessary for improving the wear resistance and with less than 2.0% thereof, the alloy lacks the wear resistance, whereas with more than 4.0% the forge processing of the alloy becomes difficult. (Sekiya, K.)

[en] Purpose: To prevent cobalt-60 production and to improve the impact shock strength of control rods from the viewpoint of material composition. Constitution: A guide roller is supported by a retaining pin in the aperture of an upper handle and a limiter roller is rotatably provided by a retaining pin in the recess of a lower handle of a control rod blade. Both of the rollers are made with a nickel base alloy containing 0.08% or less of carbon, 14 - 17% of chromium, 5 - 9% of iron, 0.7 - 1% of niobium, 2.25 - 2.75% of titanium, 0.4 - 1% of aluminium (by weight ratio). Both of the retaining pins are made with a cold worked iron base alloy containing 0.1% or less of carbon, 16 - 26% of chromium, 5.5 - 15% of nickel, 0.4 - 5% of silicon, 5.5 - 15% of manganese, 0.05 - 0.6% of nitrogen (by weight ratio). (Horiuchi, T.)

[en] Purpose: To enable substantially permanent wastes storage by calcinating radioactive wastes, storing them in the recesses of metal plates and laminating and pressurizing the metal plates in a container to solidify them. Method: Radioactive wastes are calcinated and stored in the recesses formed in metal plates. The metal plates are made of a metal, i.g., lead metal, having a good fittability to the calcinated products and the recesses are formed corresponding to the configuration of the calcinated products. Then, the metal plates are moved laterally and vertically so that the metal plates containing the calcinated products in their recesses may be laminated in the most compacted structure, and the entire contents are compressed to solidify. Accordingly, the calcinated products are disposed in the metal matrix, can be stored with safe for a long time and the volume reduction ratio can be decreased significantly. (Yoshino, Y.)

[en] Purpose: To compression mold, sinter and solidify the calcined bodies of radioactive wastes together with a metal, thereby to form solidified bodies having good heat conductivity and excellent mechanical strength, thermal shock resistance, and mechanical shock resistance, and durable in permanent storage. Method: Powders of calcined bodies of radioactive wastes and metal powders (Pb, Zn, Sn, Cu, Ag, Al, Fe and Ni or alloys of these metals) of more than 10% in apparent volume are mixed with each other and thereafter the mixtures are compression molded into a compression density of more than 40%. Subsequently, the compression molded bodies are sintered at a temperature of below 600⁰C. Alternatively, the powders of the calcined bodies are granulated and sintered beforehand, and metal coated layers are formed on the surfaces of particles by the non-electrolytic plating method, evaporating method or the like. If necessary, metal powders may be mixed therein, and thereafter the mixtures may be compression molded and sintered in the reducing atmosphere. (Sekiya, K.)

[en] In order to reduce the radiation exposure in nuclear power plants, it is necessary to reduce the corrosion products of cobalt and iron crud. The cobalt-base alloy used for pins and rollers of control rods is considered to constitute the major source of the radiation. Toshiba developed a new cobalt-free alloy for this use. It is a nickel-base alloy (CFA alloy) for control rod pins and rollers, comparable to cobalt-base alloy in material characteristics. (author)

[en] Inconel X-750, which is used in the neutron field, is currently considered to be the major source of activated corrosion products, such as ⁵⁸Co and ⁶⁰Co. In order to reduce these corrosion products, Inconel X-750 was pre-oxidized in air, and the effect of pre-oxidation on metal release into high temperature water was examined by autoclave tests. The oxide film formed on the metal surface was characterized and the relationship between the metal release and the characteristics of the oxide film was discussed. The release rates of nickel, chromium and iron from Inconel X-750 into high temperature water were decreased by pre-oxidation. The oxidation effect on inhibition of nickel release is greater for the higher oxidation temperature and the longer time. The nickel release rate for Inconel X-750 pre-oxidized at 973K for 72ks in air decreased to about 1/5 of that for Inconel X-750 autoclaved at 663K. The oxide film effective for metal inhibition was composed of double oxide layers. The outer layer mainly consists of NiFe₂O₄ and NiO, and the inner layer consists mainly of Cr₂O₃. Such double oxide layers were stable in the high temperature water. It is believed that the aging treatment could be effective for inhibiting the metal release, provided that an appropriate atmospheric condition during the treatment is employed. (author)

[en] This study is aimed at developing a cobalt-free hardfacing alloy alternative to Co based alloy, whose use in a nuclear environment is not desirable. The new alloy, presently under development, consists of Cr, Mo, Nb and Ni. Effects of these alloy contents on wear resistance were investigated at room temperature. The alloys studied contain 15 ∼ 35 % Cr, 7 ∼ 12.5 % Mo and 5 ∼ 14 % Nb. The wear loss of Ni-Cr-Mo-Nb alloys decreased with increasing Cr content, but showed a minimum at 30 % Cr content. It also decreased with increasing Mo content, but little improvement was observed above 10 % Mo. The wear loss decreased with increasing Nb content. Above 10 % Nb content, it seemed to increase slightly. An increase of (Cr, Ni)₂(Nb, Mo) intermetallic compound and matrix strengthening with increasing Cr, Mo and Nb contents seemed to improve the wear resistance. Enlarged Cr₃Ni₂ intermetallic compounds, however, decreased the wear resistance. Ni-(25 ∼ 30 %)Cr-(10 ∼ 12 %)Mo-(8 ∼ 12 %)Nb alloys are desirable from a viewpoint of wear resistance. The Ni-25 % Cr-11 % Mo-10 % Nb alloy powder was manufactured for trial and welded on carbon steel using the plasma-transfer-arc process. Sound deposits were obtained and the weld deposit showed wear and corrosion resistance equivalent to that for Co based alloy Stellite 6. (author)