Alloy suitable for use in a radioactive radiation environment and a reactor core component formed therefrom

Abstract: An alloy suitable for use in an environment exposed to neutron radiation consists principally of Cr-Ni austenite stainless steel. To increase its resistance to radiation, the steel contains nitrogen in an amount exceeding the impurity level and has principally an austenite structure. The alloy is useful for reactor core components such as a core shroud, core supporters, control rod etc., which are exposed to the neutron radiation. The high N content prevents embrittle­ ment by the radiation.

Description: This invention relates to the use of a Ni-Cn-austenite alloy in an environment exposed to, neutron radiation, and more specifically to the use of a Ni-Cr-austenite steel in a nuclear reactor and to reactor core components formed at least partly from the steel. Reactor core members, such as core supportors, the core shroud, control rods etc. disposed inside a nuclear reactor are exposed to neutron radiation during use. This causes damage to the materials, which can markedly change their characteristics. Deterioration of the material characteristics is critical to the safety and reliability of the reactor. Therefore, the reactor core member material must be selected with this difficulty in mind. In light-water reactors, it is feared that the material of internal instruments and appliances may suffer radiation-embrittlement during operation due to the neutron radiation. Besides the embrittlement due to the neutron radiation, the SCC phenomenon in water at high temperature and high pressure must also be taken into account in selecting the material for the core. In a fast breeder reactor, damage to a fuel covering tube, a core tube or the like has specifically been a critical problem. In such a reactor, the temperature of the coolant (liquid sodium) is relatively high, e.g. 350 to 500°C, and the amount of high speed neutron radiation is far greater than in a light-water reactor. Consequently, voids can occur in the material exposed to the neutron radiation, causing a serious problem of swelling (of volume). In fusion reactors, the neutron radiation of such high energy as to be incomparable with that in fission reactors would take place. Hence, the first wall material encompassing the plasma is exposed to severe radiation damage. Damage due to gas atoms (hydrogen and helium atoms) generated by the nuclear conversion process is an extremely critical problem, in addition to the above-mentioned swelling phenomenon. There are various proposals to prevent swelling of the core material exposed to neutron radiation. For example, in Japanese Laid-open Patent Application 54-36498, an austenite stainless steel including titanium, niobium and carbon is disclosed, and in Japanese Laid-open Patent Application 54-84197, there is disclosed a method of treatment of austenite stainless steel in which the steel is subjected to solid solution treatment at a temperature from 950 to 1200°C after being finally formed, and thereafter undergoes an aging treatment at a temperature of about 600 to 800°C for about 50 hours. US-A-3,856,517 discloses a stainless steel alloy said to be particularly suited for use in fast neutron reactors and containing inter alia 0.04 to 0.8% C and 0.04 to 0.06% N. It is stated that larger amounts of nitrogen are undesirable. An article in "Werkstoffe und Korrosion" Vol. 23, No. 11, November 1972, pages 973-983 discusses a steel alloy for use in condensors of radioactive waste water. The amount of carbon is less than 0.04% and that of nitrogen 0.15%. This alloy is not proposed for use in an environment exposed to intense neutron-containing radiation. DE-B-1,533,158 discloses reactor core components of a steel which may contain inter alia 0.02 to 0.04% C and 0.02 to 0.08% N. An object of the invention is to make possible the use of a Ni-Cr-austenite alloy in an environment exposed to radioactive radiation. Essentially, the present invention proposes that the Ni-Cr-austenite alloy, which is used in an environment exposed to neutron-containing radiation of at least 10²⁰ nvt, contains nitrogen in an amount of up to 0.15 % by weight and 0.003 to 0.01 % by weight of carbon, the total amount of carbon and nitrogen being at least 0.09% by weight. The method of achieving the desired nitrogen content is preferably to use a base alloy which contains large quantities of nitrogen or to add an alloy which contains a large amount of nitrogen to the base alloy. The amount of nitrogen is up to 0.15 wt% and preferably is such an amount that the formation of a nitride in the alloy is substantially not permitted. Preferably nitrogen exists in the alloy substantially in solid solution. The unit "nvt" used herein has the same meaning as "n/cm²", being the product of n=number of neutrons in a unit volume (n/cm³) v=neutron velocity (cm/s) t=time (s). Preferably this steel contains not more than 1 wt% Si, not more than 2 wt% Mn, 15 to 25 wt% Cr and 8 to 35 wt% Ni and has primarily an austenite structure. Especially preferred is an austenite steel having a full austenite structure. The conventional thinking hitherto has been that nitrogen present in austenite steel would result in helium damage at a high temperature due to helium atoms generated by the nuclear reaction resulting from neutron radiation. Hence, steps have been taken to reduce the nitrogen content. However, the inventors of the present invention have examined in detail the effects of nitrogen on the radiation damage, using an ultrahigh voltage electron microscope, and have found that, on the contrary, the nitrogen atoms tend to reduce the damage due to the atoms introduced into the lattice by the radiation and to the interaction between crystal defects such as the void points and the nitrogen atoms. In other words, the inventors have discovered that when nitrogen is added, austenite steel exhibits higher radiation resistance. For example, when irradiated with neutrons in doses of at least 10²³ n/m² (0.1 MeV), stainless steel (SUS 304) stretches less than when it is not irradiated with neutrons. Through research in developing materials that have resistance against neutron radiation and that may be substituted for SUS 304, the inventors have discovered that stainless steels are made brittle bv neutron radiation chiefly due to dislocation loops formed in the steel by the radiation, and they have thus attempted to control the dislocation loops that are formed by the neutron radiation by using an austenite stainless steel containing 0.003 to 0.01 % carbon and up to 0.15 wt% nitrogen. The chemical components of the austenite steel of the present invention will next be described. For good radiation resistance, precipitation of C as carbide is not preferred. Hence, the carbon content is preferably low so as to prevent precipitation of carbide. For increased SCC resistance (in the environment of pure water at high temperature and high pressure in a light-water reactor), the carbon content is preferably also such that it does not permit precipitation of carbide. The carbon content is therefore from 0.003 to 0.01%. To reduce radiation damage, the N content is as specified. If the N content is increased, the beneficial effect is also increased but a large N content tends to permit formation of a nitride. Precipitation of the nitride reduces the solid solution N content in the matrix and forms a Cr nitride, thus having an adverse effect upon SCC resistance. For these reasons, the N content is up to 0.15%. To make up for the decrease in strength due to the decrease in the C content by the addition of N, the total amount of C and N is at least 0.09%. In addition to C and N, impurity elements such as P, S and the like may also be present. Austenite stainless steel containing 1 to 3% Mo is especially suitable. Besides C and N contents as described above, the preferred ranges for this steel are Cr: 15-20%, Ni: 10-15%, Mo: 2-3%. The material of the present invention may be used in the form having a full austenite structure after solid solution treatment, but it may also be used after cold working subsequent to the solid solution treatment. The alloy of the invention preferably comprises an alloy containing nitrogen in the specified amount and Cr in such an amount as not to permit the formation of a substantial α phase. Preferably, the Cr content from 15 to 25%. The alloy may contain considerable amounts of elements such as Mo, W, Al, Ti, Nb, Zr and the like. In an aspect of the present invention, the austenite stainless steel serves as a material for forming reactor core components including machine parts, that receive neutron irradiation in reactor cores. All of the core components subject to neutron radiation need not be made of the austenite stainless steel. Only those core members disposed in regions which receive particularly intense neutron irradiation should be made of the austenite stainless steel. For example, as already mentioned SUS 304 stretches less when it is irradiated with neutrons in doses of at least 10²³ n/m² (0.1 MeV), compared with when it is not irradiated with neutrons. Therefore, core members disposed in the places irradiated with neutrons in doses of at least 10²³ n/m² (0.1 MeV), such as control rods, neutron counter tubes, core supporters, core shrouds, neutron source pipes etc. should be made of the austenite stainless steel of the invention. An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which:- Fig. 1 is a graph of the relation between amount of swelling and radiation temperature; Fig. 2 is a graph of the relation between void density and radiation temperature; Figs. 3(A) and 3(B) are electron microphotographs of sectioned specimens illustrating the formation of dislocation loops by neutron radiation; Figs. 4 and 5 are graphs of the relations between growth of dislocation loops and neutron radiation dose when specimens are irradiated at temperatures of 550°C and 470°C respectively; and Fig. 6 is a sectional view schematically showing the construction of a reactor core having components embodying the present invention. Example The chemical compositions of the samples used are given in the following table. Sample 1 is a comparative material and sample 2 is a material of the present invention. The carbon content is substantially the same in the two samples, but their nitrogen contents are remarkably different. The two steels have an austenite structure. Each sample was subjected to solid solution treatment by heating at 1050-1100°C for 30 minutes, and then electrolytically polished. Electron radiation was effected with a ultra-high voltage electron microscope. Neutron radiation damage corresponding to approximately 5×10²³ n/cm² was applied at a work voltage of 1,000 keV to permit observation of the structure rearrangement in the sample and the formation of voids. The results are shown in Figures 1 and 2, where the reference numbers 1 and 2 indicate the curves for the two samples. As Figure 1 shows, sample 2 having a higher N content exhibits less swelling than sample 1. The same improvement appears clearly in the difference of void density shown in Figure 2. As will be appreciated, the presence of nitrogen serves to restrict swelling due to the void formation, and the addition of nitrogen is therefore extremely effective for improving radiation resistance. Specimens having the same contents as above were subjected to solution treatment at 1050°C for 15 minutes, and then irradiated with electrons in an ultrahigh-voltage electron microscope (acceleration voltage 1MV). Figs. 3(A) and 3(B) show the formation of dislocation loops when these specimens 2 and 1 respectively, are irradiated at a rate of 4.8×10²³ e/sec (2.2×10⁻³ dpa/sec) which corresponds to a neutron radiation of (1×10²⁷ n/m² at a temperature of 500°C. (dpa is a unit of damage and stands for displacement per atom. 1 dpa corresponds to a single displacement of each atom in a lattice.) Specimen 2 (Fig. 3(A)) which contains a large amount of nitrogen only permits the dislocation loops to grow very little compared with specimen 1 (Fig. 3(B)). This indicates that specimen 2 is embrittled very little. Figs. 4 and 5 (irradiation at 550°C and 470°C respectively) show that in specimen 2, the growth of dislocation loops is restrained even when it is irradiated at these temperatures. By adding nitrogen to the austenite stainless steel, therefore, the core members made of the austenite-type stainless steel can be prevented from being embrittled by neutron irradiation. Though the characteristics of material damage due to electron radiation are different from those of damage due to neutron radiation, the material of the present invention can be expected to show excellent radiation resistance to neutron radiation from comparison with the degree of damage of conventional materials. Fig. 6 shows the core of a BWR-type reactor, having neutron source pipes 1, a core support member 2, neutron counter tubes 3, control rods 4 and a core shroud 5. These core members are subjected to intense neutron radiation, and hence are according to the invention, made of austenite stainless steel which contains not more than 0.03% by weight of carbon and at least 0.06%, preferably less than 0.15%, by weight of nitrogen. It is, of course, allowable to make other fine parts using this austenite stainless steel, in addition to the core members 1 to 5. Furthermore, materials of the invention can be used for, for example, the core shroud, core supporters, control rods etc. of a PWR-type reactor core, and the fuel pins, wrapper tubes etc. of a FBR-type reactor core. The prevention or reduction of embrittlement by neutron radiation can increase the reliability of the reactor core, and can lengthen the life of the core components and internal instruments and appliances.

Claims: The use, in an environment exposed to neutron-containing radiation, of a Ni-Cr austenite stainless steel wherein said neutron-containing radiation has an intensity of at least 1020 nvt and the steel contains 0.003 to 0.01 wt.% C and up to 0.15 wt.% N, the total amount of carbon and nitrogen being at least 0.09 wt.%. The use, according to claim 1 of a steel which comprises Fe and contains not more than 1 wt.% Si, not more than 2 wt.% Mn, 15 to 25 wt.% Cr and 8 to 35 wt.% Ni. The use, according to claim 1 or claim 2, of a steel having full austenite structure. The use, according to any one of the preceding claims, of a steel in which the nitrogen content is such that nitrogen does not precipitate as a nitride within the carbide precipitation temperature of the steel. Use according to any one of claims 1 to 4 of a steel comprising Fe and 15 to 20% Cr, 10 to 15% Ni and 2.0 to 3.0% Mo. Use of an Ni-Cr austenite steel according to any one of claims 1 to 5 as a reactor core component.

Inventors: ITOW HISAWO, MASAOKA ISAO, NAKATA KIYOTOMO, YOSHIDA TOSHIMI, ITOW, HISAWO, MASAOKA, ISAO, NAKATA, KIYOTOMO, YOSHIDA, TOSHIMI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1982
Main classification: C22C38
IPC codes: C22C38