Patent Number: 062332996
Section: description

PREFERRED EMBODIMENTS OF THE INVENTION FIG. 1 is a transverse sectional view of an assembly for transmutation (ie., transmutation assembly) of a long-lived radioactive material according to an embodiment of the present invention and an example of a FP pin loaded into the transmutation assembly. The FP pin 20 has a structure in which a single wire-type member 22 of the long-lived radioactive material composed of metals, alloys or compounds including long-lived fission product (LLFP) nuclides is disposed at a center of the FP pin and the wire-type member 22 is surrounded by a moderator material 24 to form a pellet-like or rod-like structure and loaded in a cladding tube 26. A plurality of such FP pins 20 are prepared and arranged in a bundle-like structure and located in a hexagonal wrapper tube 28 to thereby form the transmutation assembly 30 of the long-lived radioactive material of the present invention. The long-lived fission product (LLFP) nuclides include, for example, technetium-99 and iodine-129 and are used in the form of a metal, an alloy or a compound. In the case of technetium, for example, Tc (metal), TcO.sub.2 and so forth are used. In the case of iodine, AgI, NaI, PdI.sub.2, CeI.sub.3 and so forth can be used. The wire-type member 22 of the long-lived radioactive material is preferably formed such that it has a diameter of about 1 to 2 mm. This will permit moderator to slow down neutrons in a suitable manner and restrict as much as possible the self-shielding effect of the neutrons (that is, the effect of preventing the neutrons entering deep into the FP) so that a high transmutation rate can be achieved. The wire-type member 22 having a diameter of 1 mm or more can be produced relatively easily. As the moderator material 24, zirconium hydride or beryllium oxide, for example, can be used. The pellet or rod, which is formed by surrounding the wire-type member 22 of the long-lived radioactive material by means of the moderator material 24 as described above, is inserted into the cladding tube 26 along a substantially entire length thereof and sealed at its upper and lower ends by end plugs (not shown) in a manner similar to the case of general fuel pins. The wrapper tube 28 in which a plurality of FP pins 20, and nothing else, are located in a regular arrangement has a similar structure to that of a general fuel assembly and has an entrance nozzle (not shown) at its lower portion and a handling head (not shown) at an upper portion for facilitating the loading work by the use of a fuel loading/unloading machine so that a coolant can flow inside the wrapper tube 28. FIG. 2 shows another embodiment of the FP pin. In this embodiment, a plurality of wire-type members (seven wire-type members in the illustrated embodiment) 32 are disposed in a dispersed arrangement and each of the wire-type members 32 is surrounded by a moderator material 34 to form a pellet or rod structure, and loaded into a cladding tube 36. Installation of a plurality of wire-type members 32 of the long-lived radioactive material can increase the amount of transmutation. Similar to the case of the previous embodiment, it is desirable that the diameter of the wire-type member 32 of the long-lived radioactive material is selected from a range from about 1 mm to 2 mm. It is appreciated that the number of the wire-type members 32 and the location thereof are optional and can be selected as desired in accordance with requirements. FIG. 3A, FIG. 3B and FIG. 3C show further embodiments of the FP pin, in which a thin ring-type or thin-wall tubular member 42 of a long-lived radioactive material is used. In the embodiment of FIG. 3A, a thin ring-type member 42 of the long-lived radioactive material has a relatively small diameter and is positioned at the center of the pin so that the thin ring-type member 42 is surrounded at both its inner and outer surfaces by a moderator material to form a pellet-like or rod-like structure and loaded into the cladding tube 46. In the embodiment of FIG. 3B, a thin ring-type member 48 of the long-lived radioactive material having a relatively large diameter is provided so that the thin ring-type member 48 is surrounded at it both inner and outer surfaces by the moderator material 44 to form a pellet-like or rod-like structure and then loaded into the cladding tube 46. In the embodiment of FIG. 3C, the above-described thin ring-type member 42 of a small diameter and the thin ring-type member 48 of a large diameter are concentrically located and surrounded at their inner and outer surfaces by the moderator material 44 to form a pellet-like or rod-like structure, and loaded into the cladding tube 46. This structure having a plurality of thin ring-type members 42 and 48 of the long-lived radioactive material can increase its transmutation performance. In the embodiments of FIGS. 3A, 3B and 3C, it is desirable that each of the thin ring-type members 42, 48 has a thickness of about 1 to 2 mm. The FP pins shown in FIGS. 3A, 3B and 3C are suitable for decreasing or lowering the self-shielding effect of neutrons and increasing the transmutation rate. The thin ring-type members each having a thickness of 1 mm or more can be produced relatively easily. The transmutation assembly of a long-lived radioactive material can be formed by using one of the types of those FP pins described above and, therefore, production and inspection of the transmutation assembly can be performed quite simply, with the result of a reduction in costs. The transmutation assemblies of the present invention can be loaded selectively and partly into a core region, a blanket region or a shield region of a reactor core in a fast reactor. When the transmutation assemblies are loaded into the blanket region, all of the blanket assemblies may be replaced by the transmutation assemblies. In order to effectively use the excess of neutrons and restrict an influence upon the reactor core characteristics, it is optimal that the transmutation assemblies are loaded in the blanket region. EXAMPLE An experiment was made with reference to a fast reactor which has transmutation assemblies of a long-lived radioactive material loaded in the blanket region and detailed analysis was made by using a Monte Carlo Code. Table 1 below shows the results. The transmutation assembly which is the scope of the present invention (Invention 1 and Invention 2, below) had a structure as shown by FIG. 1, in which long-lived technetium-99 was formed into a thin metal wire and surrounded by a moderator material of zirconium hydride to form pellets, and then the pellets were loaded into a cladding tube to form a FP pin. A plurality of FP pins surrounded by a wrapper tube form a transmutation assembly. The transmutation assemblies were loaded into a blanket region of a fast reactor of 1 million kWe as shown in FIGS. 4 and 5 and a one-year term transmutation rate was obtained. FIG. 4 shows the core structure of the fast reactor and the loading position of the transmutation assemblies and FIG. 5 shows the dimensions of the reactor core. In this structure, it is seen that the transmutation assemblies are loaded in the position of a radial blanket. For comparison purposes, analysis was made for the conventional transmutation assembly of a long-lived radioactive material shown in FIG. 6, as shown by "Conventional Example (Conv. Ex.)1" and "Conventional Example (Conv. Ex. 2)" in Table 1 below. TABLE 1 No. of No. Diameter Trans- Trans- Loading pins in of of Loaded muted mutation method of assem- FP FP pins amount amount rate FP pins bly pins (mm) (kg) (kg/year) (%/year) Conv. Ex. 127 37 10 3750 67.5 1.8 1 Conv. Ex. 127 22 10 1883 45.8 2.5 2 Invention 127 127 1.3 183 17.9 9.8 1 Invention 217 217 1.3 313 28.5 9.1 2 In Table 1, the "No. of FP pins" of the conventional examples (Conv. Ex. 1 and Conv. Ex. 2) represent the FP pins into which only a long-lived radioactive material is loaded and the "No. of FP pins" of the present invention ("Invention 1" and "Invention 2") represent the FP pins loaded with pellets which are composed of wire-type long-lived radioactive material surrounded by the moderator material. Accordingly, the radius of the conventional FP material is coincident with the pin radius, and the radius of FP material of the invention is equal to the radius of the wire type material and the radius of the actual pins (FP pins) is 5 mm, which is the same as that of the conventional ones. As shown by Table 1, the transmutation rate is low (that is, 1.8 to 2.5 %/year) in the case where the conventional transmutation assembly is used and is therefore not so effective. By contrast, use of the transmutation assembly for long-lived radioactive material of the present invention successfully achieved a high transmutation rate of about 9 to 10%/year, which is 4-5 times as high as the conventional structure, regardless of the number of pins used in the assembly. From the analysis, the number of pins in the transmutation assembly in the present invention can be extended to about 127-271 as shown in Table 1. In the example, the wire-type long-lived radioactive material had a diameter of 1.3 mm but any wire-type long-lived material can be used if it has a diameter in a range between about 1 mm and about 2 mm. According to the present invention, the transmutation assembly is composed solely of FP-containing pins each of which is formed of a cladding tube and wire-type or thin ring-type members of a long-lived radioactive material surrounded by a moderator material; this allows the transmutation rate of the long-lived FP nuclides to achieve an extraordinarily high level. Further, the transmutation assembly can be formed with FP pins of the same type, with a consequent simplification of production and inspection and reduction of costs.