Patent Number: 042343852
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is based on the discovery, as a result of numerous experiments, that if silicon, titanium and carbon as essential components are added to the austenitic stainless steel, the silicon content is made under 0.7% by weight, and further, the contents of titanium and carbon are controlled so as to satisfy a certain relationship, the swelling of steel would be suppressed to the extremely low order (for example, 1.5% at maximum at 1.times.10.sup.23 nvt irradiation), without damaging the physical and mechanical properties of steel. It is found according to the invention that there exist complementary relations in the contents of silicon, titanium and carbon in the suppression of the swelling, though the mechanism of suppressing the swelling is not known in detail. The term "austenitic stainless steel" as used in this specification and the appended claims means an austenitic stainless steel containing chromium in a normally prescribed amount, nickel in a normally prescribed amount, manganese in an amount of up to 2% by weight, molybdenum in an amount of up to 3% by weight, silicon, carbon and titanium in amounts as hereinafter described, and the balance of iron. In addition the steel may contain incidental components which may be incorporated from the ordinary process of manufacturing steel. Such incidental components include nitrogen of up to 3%, oxygen of up to 0.02%, aluminium of up to 0.05%, arsenic of up to 0.03%, boron of up to 0.002%, cobalt of up to 0.1%, niobium of up to 0.05%, copper of up to 0.1%, sulphur of up to 0.03%, and vanadium of up to 0.2%. In general, the chromium content of the steel is in the range of 9 to 26% by weight. If the content is more than 26 wt.%, neither the stable austenitic system can be obtained, nor the void suppression may be attained. On the contrary, if the content is below 9 wt.%, a sufficient oxidation resistance cannot be obtained. Preferably, the content of chromium is 16 to 20% by weight. Usually, the nickel content of the steel is 6 to 30% by weight. Although the nickel content of more than 30% by weight is effective in the suppression of the swelling, a steel containing such an amount of nickel decreases in the corrosion resistance to a liquid metal, and is readily attacked by the nuclear fission products. Moreover, it not only aggravates neutron economy, but also causes a large amount of cobalt as an impurity to be contained in nickel. As a result, cobalt 60 originated from the cobalt may be turned into the most cumbersome radioactive corrosion product and interfere with the operation of the nuclear reactor. Whereas, with the nickel content of less than 6 wt.%, there can be obtained no stable austenite system, and the effect of suppressing the void would be lost. A preferable content of nickel is in the range of 10 to 16% by weight. While the silicon added to the austenitic stainless steel according to this invention is a component essential for suppressing the void swelling, addition of a large amount of silicon may destroy the physical and mechanical properties of the steel. Accordingly, the silicon content is preferable to be kept at a low level, if the contents of carbon and titanium to be added together with the silicon satisfy the relationship as described hereinafter. Generally, the silicon content is kept below 0.7% by weight (i.e., more than 0% to less than 0.7% by weight), preferably 0.5% to less than 0.7% by weight. It has been found, as a result of the various experiments, that there exists a correlation between the titanium content and the carbon content in suppressing the void swelling, and that if carbon and titanium in amounts satisfying the following relationship are added to the austenitic stainless steel which contains the silicon kept within the abovementioned quantitative limits, the void swelling can be suppressed to a very small extent (i.e., 1.5% at maximum at 1.times.10.sup.23 nvt irradiation). The relationship between the contents of carbon and titanium will be given by the following formula. EQU 17[C]+27[Ti].gtoreq.2.1 (A) where [C] represents the carbon content indicated in % by weight and exceeding 0, and [Ti] represents the titanium content indicated in % by weight and exceeding 0. The upper limits of the carbon content and the titanium content can not be determined unconditionally, and increasing both the carbon and titanium contents causes the mechanical properties of steel to be aggravated, for example, embrittled. The upper limits with respect to the carbon content and the titanium content capable of sufficiently suppressing the void swelling without substantially aggravating the mechaical properties of the steel are 0.1% by weight and 0.5% by weight, respectively. Preferably, the minimum carbon content is 0.01% by weight, and a preferred range based on this minimum is 0.01 to 0.1% by weight. Further, the minimum titanium content is preferably 0.03% by weight, and a preferred range based on this minimum is 0.03% to 0.3% by weight. The variables [C] and [Ti] in the formula (A) are interdependent, and if either of the variables is fixed, then the other one varies between the above-mentioned maximum value and the minimum value calculated from the formula (A) with the formula (A) taken as an equation. That is, for example, where [C] is 0.02, [Ti] varies between the maximum value of 0.5 and the minimum value of about 0.065 calculated with the formula (A) taken as an equation. This applies also to the preferable cases of the carbon content and the titanium content as stated above. Namely, if [C] is 0.02, then [Ti] is preferably in the range of 0.065 to 0.3 (% by weight). FIG. 1 shows the relationship between the carbon content and the titanium content by a graph. In the figure, line AJ stands for the formula: 17[C]+27[Ti]=2.1. A general range of the carbon and titanium contents fall within the region encircled by lines AB, BC, CD and DA, and the region encircled by lines EF, FG, GH, HI and IE shows a preferable range of carbon and titanium contents. Further, it has been found according to the invention that the void swelling can be further suppressed and the mechanical properties of steel is improved by subjecting the austenitic stainless steel to a final heat treatment at a temperature enough to permit the carbon and titanium contained in the steel to be formed substantially completely into a solid-solution. The final heat treatment is such a treatment as to be effected at the end of the process to provide the product and is generally carried out at a temperature of more than 1120.degree. C. up to 1200.degree. C. A heat treatment at a temperature of 1120.degree. C. or less can not form the solid-solution satisfactorily, while a heat treatment at a temperature of more than 1200.degree. C. can not attain the necessary mechanical strength of the steel and is uneconomical. The final heat treatment is preferably carried out at a temperature of 1150.degree. C. to 1200.degree. C. and for one to ten minutes. FIG. 2 shows a nuclear fuel element 10. A solid fuel body 3 composed of a plurality of pellets 2 is housed in a cylindrical clad 1 which is formed of the austenitic stainless steel according to this invention. The pellets 2 may be obtained by compression-moulding a nuclear fuel material such as the oxides, nitrides and carbides of uranium, plutonium and thorium, or a mixture thereof into a cylindrical shape and sintering it at a high temperature. The clad 1 is sealed tight by means of plugs 4 and 5 provided at the both ends thereof and a spring 6 for preventing the pellets 2 from the movement is installed in a plenum chamber 7 located between the fuel body 3 and the plug 4. A gap 8 is present between the clad 1 and the fuel body 3. In the plenum chamber 7 and the gap 8 is filled herium gas. FIG. 3 shows a nuclear fuel subassembly A. A plurality of the nuclear fuel elements 10 as shown in FIG. 2 are supported by grids (not shown) and received in a duct 11, with any adjoining two of the elements separated from each other by a spacer (not shown). It is preferred that the duct 11 be formed of the austenitic stainless steel according to this invention. This invention will be more fully understood from the following examples. Examples. Sample steels, each 750 g, were prepared by melting a steel component of high purity in a vacuum. The steel was cast into a rod of approximately 3.5 cm in diameter and then made up to a plate of approximately 3 mm thick by hot-rolling at 1150.degree. C. Then, the steel plate was rapidly cooled after annealed at 1150.degree. C. for 10 minutes, and cold-rolled into a thin plate of approximately 0.5 mm thick. Thereafter, it was heat-treated at 1200.degree. C. for 5 minutes. A small test piece (8 mm.times.14 mm.times.0.2 mm thick) was cut out of the thin steel plate and then finished up with an electrolytic polishing after smoothing the surface thereof with an abrasive paper. Voids formed in the test piece by irradiation of carbon ions at high temperature was observed under an electron microscope, and the whole volume of the voids was determined, thereby estimating the amount of the swelling of steel. The evaluation of the quantity of the swelling was made by using the experimental correction (reported in Journal of Nuclear Science and Technology, Vol. 13, pp. 743-751) by SHIMADA et al. In the simulative experiment by irradiation of the carbon ions, the temperature at which the swelling reaches the peak shifts higher by approximately 100.degree. C., because the damaging speed of a sample is fast as much as 1000 times that by irradiation of neutrons. Accordingly, irradiation of the carbon ions at about 525.degree. C. corresponds to irradiation of the neutrons at about 625.degree. C. Meanwhile, pre-injection of herium into the test piece had been carried out so that herium ions injected in the test piece distirbuted uniformly over the surface to the depth of 4000 A by changing the intensity of the herium ion energy. Table 1 shows the components of the 316 steel which is an austenitic steel on market and various steels used for the above-mentioned experiments. TABLE 1 __________________________________________________________________________ Sample steel No. Ni Cr Mo Mn Si C Ti S P Co N O __________________________________________________________________________ 316 10.5 17.5 2.63 0.78 0.44 0.050 &lt;0.01 0.005 0.03 0.24 -- -- M79 14.1 17.5 2.48 0.91 1.05 0.068 &lt;0.01 0.005 0.025 0.041 0.0048 0.0136 M82 13.5 17.4 2.52 1.67 0.59 0.041 0.02 0.006 0.002 0.042 0.0058 0.0088 M83 13.3 16.8 2.46 1.69 0.58 0.018 0.17 0.006 -- -- 0.0049 0.0070 M84 13.8 16.9 2.44 1.77 0.52 0.101 0.22 0.007 -- -- 0.0048 0.0079 M85 13.2 16.7 2.44 1.65 0.48 0.072 0.025 0.006 0.001 0.043 0.0049 0.0066 M90 13.3 16.6 2.47 1.59 0.55 0.087 0.039 0.007 0.002 0.041 0.0062 0.0068 M91 13.1 16.7 2.47 1.65 0.51 0.055 0.084 0.007 -- -- 0.0038 0.0050 M92 13.0 16.7 2.46 1.67 0.59 0.022 0.019 0.007 -- -- 0.0044 0.0058 M93 13.3 16.7 2.47 1.63 0.63 0.016 0.034 0.007 0.002 0.042 0.0043 0.0072 __________________________________________________________________________ Note: Unit % by weight; Fe balance The results are shown in FIG. 4 which shows the amount of the swelling of each steel obtained by the irradiation of ions corresponding to that of high-speed neutrons at 1.times.10.sup.23 nvt under an expected condition of LMFBR, dotted against the titanium content. As seen from the figure, the sample steels can be classified into two groups with the titanium content at 0.03% as a border. That is, a first group is the high-swelling group composed of the steels 316, M79, M82, M85 and M92 with Ti content of less than 0.03%, and a second group is the low- and the extremely low-swelling group composed of the steels M83, M84, M90, M91 and M93 with Ti content of 0.03% or more. The high-swelling group shows a swelling of the order of 8% as represented by the 316 steel on the market, while the low- and the extremely low-swelling group shows an excellent resistivity to the swelling (i.e., the swelling of about 1% to less than 0.1%). The second group can completely satisfy the desired value of the swelling such as several percentages, for example, 6%, which is the swelling limit of the steel covering the nuclear fuel in the fast breeder reactor under design or construction at present. The reason is that since the amount of the high-speed neutrons at the end of the operation of the reactor is about 2.times.10.sup.23 nvt under the expected operation condition of the LMFBR and at such an amount of the neutrons the amount of swelling is proportional to the square of the amount of the neutrons with respect to 300 series stainless steel, the amount of swelling of 1% at the amount of the neutrons of 1.times.10.sup.23 nvt corresponds to the amount of swelling of 4% at the amount of the neutrons of 2.times.10.sup.23 nvt. Further, FIG. 4 clearly shows that the amount of swelling varies to a large extent between titanium contents of 0.02% and 0.03%.