Patent Number: 062332996
Section: summary

BACKGROUND OF THE INVENTION 1. The Field of the Invention The present invention relates in general to an assembly which is loaded into a fast reactor for the purposes of transmutation treatment of a long-lived radioactive material. More particularly, the present invention relates to an assembly for transmutation of a long-lived radioactive material, in which the transmutation assembly is composed solely of FP (fission product)--containing pins, in which each of the FP pins has a cladding tube containing therein a moderator and a radioactive material including long-lived fission product (LLFP) nuclides in such a manner that the radioactive material is surrounded by, or in other words covered with, the moderator material in the cladding tube. 2. Description of Prior Art Utilization of nuclear energy is inevitably followed by the generation, more or less, of long-lived fission products (LLFP). High level radioactive waste which has been disposed of by glassification generally contains long-lived radioactive FP nuclides, such as technetium-99 and iodine-129, which have a half-lives of about 210 thousand years and about 1.6 million years, respectively. Technetium-99 is water-soluble and there is fear that it is, after a long period of time, eluted and released out of a barrier in the form of ions of TcO.sub.4.sup.-, etc. following geologic disposal. With respect to iodine-99, considering its safety from the viewpoint of migration into the ground water when the iodine-99 is solidified and then subject to geologic disposal, there are the problems that this material has a high solubility and has a low absorption by the barrier. Thus, from a viewpoint of reduction of the environmental load, these long-lived FP nuclides should preferably be transformed into the other stable nuclides by a suitable method before a final disposal thereof. For example, the technetium-99 and iodine-129 are transformable, by a neutron absorption reaction, into non-radioactive and stable ruthenium (Ru) and xenon (Xe). Technetium-99 and iodine-129, which are long-lived FP nuclides, have larger neutron absorption cross sections for thermal neutron energy than for fast neutron energy, and have larger resonance absorption regions in a lower energy region (approximately 5 eV, etc.). Therefore, in order to transmutate these long-lived FP nuclides by neutron absorption reactions, it is advisable to slow down to some extent fast neutrons which have been generated by nuclear fission, and then use the same as resonance energy neutrons or thermal neutrons. Both thermal neutron reactors and fast neutron reactors are considered as reactors in which to transmute long-lived FP nuclides. A thermal neutron reactor utilizes moderated or slowed-down neutrons and, therefore, the nuclides can be transmuted, to some extent, by loading pins which contain technetium-99 or iodine-129 into the reactor core. A fast neutron reactor, on the other hand, requires the fast neutrons to be moderated and, therefore, has utilized an assembly having a structure in which long-lived FP nuclides-containing pins and moderator-containing pins are housed together in a wrapper tube, for the purpose of transmutation of these long-lived FP nuclides. A conventional assembly for the purpose of transmutation is shown in FIG. 6, in which moderator-containing pins 12 and FP-containing pins 14 are disposed in a dispersed arrangement in a wrapper tube 10 as illustrated. The moderator pin 12 is composed of a cladding tube which contains moderator material and nothing else, and the FP pin 14 is composed of a cladding tube which contains the material including long-lived FP nuclides, and nothing else. In the case where the assemblies for the transmutation purposes as described above are loaded into a blanket region of the fast reactor, the transmutation rate of the long-lived FP nuclides is approximately 2.0-2.5% which is not as much as the requirement. The inventor of the present invention considers that the reason for such a low transmutation rate resides in a remarkably high self-shielding effect of the neutrons in the FP pins such that the nuclear reactions between the FP and the neutrons are carried out only near the surface of the FP pins, with the result that the neutrons are prevented from entering deep into the FP pins. This problem would be solved to a limited extent by an attempt at forming thinner FP pins but this measure would raise other serious problems in that the necessary number of pins is increased, resulting in deficiencies in production, working effect and cost performance. Further, in the conventional structure of the transmutation-purpose assemblies as described above, the arrangement of the FP pins and the moderator pins in the space of the assembly is complex, resulting in less working efficiency and difficulties in inspection of the pins. SUMMARY OF THE INVENTION Accordingly, the present invention is proposed to solve the above problem, and it is therefore an object of the present invention to provide an improved assembly for transmutation which permits an efficient transmutation (i.e., transformation into stable nuclides by a nuclear transformation), in a nuclear reactor, of a long-lived radioactive material (especially, long-lived FP nuclides such as technetium-99 or iodine-129, etc.) which was produced in a nuclear reactor. Another object of the invention is to provide an assembly for transmutation of a long-lived radioactive material which permits a high workability of production and a high operability of inspection and also meets with a requirement for reduction of costs. In a first aspect of the present invention, there is provided an assembly for transmutation of a long-lived radioactive material comprising: wire-type members of a long-lived radioactive material comprised of metals, alloys or compounds including long-lived fission product nuclides, PA1 a moderator material surrounding the wire-type members, PA1 cladding tubes each containing therein the wire-type members surrounded by the modulator material to thereby form FP (fission products) pins, and PA1 a wrapper tube housing therein the FP pins and nothing else. PA1 thin ring-type (or tubular) members of a long-lived radioactive material comprised of metals, alloys or compounds including long-lived fission product nuclides, PA1 a moderator material surrounding an inner surface and an outer surface of the thin ring-type members, PA1 cladding tubes each containing therein the thin ring-type members surrounded by the moderator material to thereby form FP pins, and PA1 a wrapper tube housing therein the FP pins and nothing else. PA1 wire-type members of a long-lived radioactive material comprised of metals, alloys or compounds including long-lived fission product nuclides, PA1 a moderator material surrounding the wire-type members, PA1 cladding tubes each containing therein the wire-type members surrounded by the moderator material to thereby form FP pins, and PA1 a wrapper tube housing therein the FP pins solely. PA1 thin ring-type members of a long-lived radioactive material comprised of metals, alloys or compounds including long-lived fission product nuclides, PA1 a moderator material surrounding an inner surface and an outer surface of the thin ring-type members, PA1 cladding tubes each containing therein the thin ring-type members surrounded by the moderator material to thereby form FP pins, and PA1 a wrapper tube housing therein the FP pins solely. In the transmutation assembly described above, the wire-type members of the long-lived radioactive material may be located in a dispersed state and each of the dispersed wire-type members is surrounded by the moderator material. In a second aspect of the invention, there is provided an assembly for transmutation for a long-lived radioactive material comprising: In a third aspect of the invention, there is provided a reactor core for a fast reactor comprising a core region, a blanket region and a shield region, wherein transmutation assemblies are selectively and at least partly loaded into the core region, the blank region or the shield region, each of said transmutation assemblies comprising: In a fourth aspect of the invention, there is provided a reactor core for a fast reactor comprising a core region, a blanket region and a shield region, wherein transmutation assemblies are selectively and at least partly loaded into the core region, the blanket region or the shield region, each of said transmutation assemblies comprising: In the present invention, wire-type members of a long-lived radioactive material are surrounded by a moderator material and then the wire-type members surrounded by the moderator material are installed in cladding tubes to form FP pins and the FP pins, and nothing else, are housed in a wrapper tube. The long-lived radioactive material is comprised of metals, alloys or compounds including long-lived fission product nuclides. The wire-type members described above can be placed in a dispersion arrangement with the enclosure by the moderator material. In another embodiment of the invention, the wire-type members described above can be replaced by a single or a plurality of ring-type or thin-walled tubular member(s), and the thin ring-type member(s) is or are surrounded by the moderator material on the outer and inner surfaces thereof and then placed into the cladding tubes to thereby form the FP pins. Only the FP pins are housed in the wrapper tube. In order to carry out a transmutation treatment, the transmutation assemblies of the long-lived radioactive material as described above can be loaded selectively and at least partly into a core region, a blanket region or a shield region of a reactor core of a fast reactor. From a viewpoint of reducing possible influence on the reactor core characteristics, it is optimal to load the transmutation assemblies into the blanket region in the reactor core.