Method of restraining diffusion of tritium and apparatus for same

Abstract: A method of restraining a diffusion of tritium and an apparatus for same are disclosed. The method includes the step of disposing a hydrogen-absorbing metal (16) in a tritium passage in a direction of diffusion of tritium so that the hydrogen-absorbing metal absorbs tritium to be diffused. The apparatus includes a hydrogen-absorbing metal (16) surrounding a device of a fast breeder reactor. The tritium to be diffused can be readily captured. An arrangement of capturing the tritium permeating the device is simplified in the present invention.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method of and an apparatus for restraining the diffusion of tritium and an apparatus for same. In general, a fast breeder reactor comprises an arrangement of devices as shown in FIG. 3. In FIG. 3, a reactor vessel made of stainless steel is indicated at 1. The reactor vessel 1 contains a reactor core 2 in which a nuclear fuel assembly comprising a fissionable material is placed. The reactor core 2 receives a control rod 3 including boron so that the control rod 3 can be pulled up and inserted into the reactor core 2 from above. The fast breeder reactor has a primary cooling system piping 4 made of stainless steel and causing liquid sodium as a coolant to pass through the reactor core 2 in the reactor vessel 1 and to circulate by means of a pump 5 between the reactor vessel 1 and an intermediate heat exchanger 6. The fast breeder reactor also has a secondary cooling system piping 7 made of stainless steel and causes liquid sodium as a coolant of a secondary cooling system to circulate by means of a pump 8 between the intermediate heat exchanger 6 and a steam generator 9. A steam system piping feeding water to the steam generator 9 to receive steam therefrom is indicated at 10. Both the primary and secondary cooling system pipings 4 and 7 have heat reserving means. The arrangement of each of the heat reserving means is as follows: FIG. 4 illustrates the arrangement of a piping 11 which is assumed to represent each of the primary and secondary cooling system pipings 4 and 7. As shown in FIG. 4, a stainless steel strip 13 surrounds both the piping 11 made of stainless steel in view of both resistance against corrosion caused by liquid sodium and a high-temperature strength and a preheater 12 arranged in parallel to the piping 11, a heat reserving material 14 surrounds the stainless steel strip 13, and a thin steel strip 15 surrounds the heat reserving material 14. The thickness of each of the stainless steel strip 13 and the steel strip 15 is normally about 0.1 mm - 0.2 mm. The reserving material 14 normally is a mixture including calcium oxide or silicon oxide as a main component. Turning back to FIG. 3, a fission reaction in the reactor or a nuclear transformation caused by neutron irradiation of the boron used as a moderator of the control rod 3 produces tritium. For example, in a nuclear reactor of one million KW tritium of about 1 g/year occurs. The tritium is mixed with liquid sodium passing through the reactor core 2 and transferred through the primary cooling system piping 4 to the intermediate heat exchanger 6. Then, the tritium permeates the wall of a heat-transfer pipe of the intermediate heat exchanger 6 and enters liquid sodium contained in the secondary cooling system piping 7 to reach the steam generator 9. Thus, the tritium permeates the pipings and devices, so that the tritium tends to readily permeate and diffuse in atmospheres outside of the devices. In particular, it is well known that tritium permeates stainless steel used to constitute the devices of a nuclear power plant. Thus, tritium permeating e.g. the piping 11 inside to outside permeates the heat reserving means to diffuse to the outside thereof. Since as described above, the tritium which has once diffused continues to diffuse and in the worst case, might diffuse in the atmosphere, the diffusion of the tritium must be restrained. US-A-4 528 003 discloses an apparatus and a method for removing hydrogen isotopes (P, T, D) from a fluid (gas stream of liquid sodium), in which method structures comprising hydrogen-absorbing metals are disposed in the fluid passage. The same feature is disclosed in JP-A-54/02389. It is the object of the present invention to provide a method and an apparatus of restraining the diffusion of tritium. In order to achieve this object, a method of the invention for restraining a diffusion of tritium comprises the step of disposing a hydrogen-absorbing metal on the external side of the enclosure wall of a tritium passage so that the hydrogen-absorbing metal absorbs tritium to be diffused. An apparatus of the invention for restraining a diffusion of tritium, comprises a hydrogen-absorbing metal surrounding a device of an fast breeder reactor. According to the invention, since the hydrogen-absorbing metal well absorbs tritium constituting an isotope of hydrogen, the hydrogen-absorbing metal disposed on the external side of the enclosure of the tritium passage in the direction of diffusion of tritium absorbs and captures the tritium, the tritium is restrained in diffusing anywhere. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-section of a piping showing one embodiment of the present invention; FIG. 2 is a cross-section of a heat reserving arrangement showing another embodiment of the present invention; FIG. 3 is a block diagram illustrating a primary cooling system of a fast breeder reactor; FIG. 4 is a cross-section of a heat reserving arrangement of a prior-art piping; FIG. 5 is section of the wall of a piping showing a still another embodiment of the present invention; and FIG. 6 is a cross-section of a heat reserving arrangement of a piping showing another embodiment of the present invention. Hereinafter, the preferred embodiments of the present invention will be described with reference to the drawings. FIG. 1 illustrates one embodiment of the present invention which has been applied to a piping of a fast reactor. Hereinafter, the same portion of FIGS. 2, 5 and 6 as that of FIGS. 1 and 3 carries the same reference number. A tubular thin strip 16 made of a hydrogen-absorbing metal surrounds a piping 11 and a preheater 12 mounted in parallel to the outer surface of the piping 11. The thin strip 16 was 0.1 mm to several mm in thickness. However, the thickness of the thin strip 16 is not, of course, restricted to this value. The hydrogen-absorbing metal for the thin strip 16 is, e.g., Mg₂Ni and a Ti-Mn based alloy wuch as TiMn 1.5 . The outer cylindrical surface of the thin strip 16 has a heat reserving material or heat insulator 14 disposed thereon. Along the outer circumference of the heat reserving material 14 is disposed a thin steel strip 15 to confine the material 14. A sheet of a hydrogen-absorbing metal e.g. Ti-Mn based alloy may be used instead of the thin steel strip 15. Thus, there are three cases, that is, a first case in which along the inner periphery of the heat reserving material 14 is disposed the hydrogen-absorbing metal, a second case in which along the outer periphery of the heat reserving material 14 is disposed the hydrogen-absorbing metal, and a third case in which along both the outer and inner peripheries of the heat reserving material 14 are disposed the hydrogen-absorbing metals. FIG. 2 illustrates a cross-section of a heat reserving arrangement showing another embodiment of the invention. A heat reserving material 14 includes a hydrogen-absorbing metal 17 scattered therein at a rate of 0.1 to 0.3 g/cm² in the form of fiber (alternatively, chip or powder) so that the heat reserving material 14 and the hydrogen-absorbing metal 17 are uniformly mixed with each other. The heat reserving material 14 is normally divided into blocks for easy handling. FIG. 5 illustrates the wall arrangement of a piping showing still another embodiment of the present invention. The wall arrangement of the piping 11 comprises a wall 18 and a layer made of a hydrogen-absorbing metal 19 e.g. Ti-Mn based alloy which layer is jointed by spot-welding or by use of fittings such as bands, bolts and nuts, or is sprayed or coated integrally to the outer surface of the wall 18. FIG. 6 illustrates a piping showing another embodiment of the invention. A piping has an arrangement in which the inner cylindrical wall 20 or outer cylindrical wall 21 for combining the heat reserving material 14 has a layer made of the hydrogen-absorbing metal 19 e.g. Ti-Mn based alloy which layer is jointed or sprayed or coated integrally to the inner or outer cylindrical wall 20 or 21. While Ti-Mn alloy composition was used as the hydrogen-absorbing metal, the following alloy compositions may be alternatively employed: Mg-Ni, Mg₂Ni, Mg₂Ni 0.9 Cr 0.1 , LaNi₅, MmNi₅, MmCo₅, MmNi 4.5 Mn 0.5 , MmNi 4.5 Al 0.5 , MmNi 4.5 Cr 0.5 , TiFe, TiCr, TiCr₂, TiFe 0.9 Nb 0.1 , Ti-Zr-Mn-Mo, Ti-Mn-Fe-V, Ti-Zr-Mn-Fe and CaNi₅. The character "Mm" represents a misch metal consisting of a mixture of La, Ce, Pr, Nd, Sm and the like. In accordance with the above embodiments of the invention, tritium which occurs in the reactor core and may escape through the wall of the piping of the reactor to an outside atmosphere can be captured near the surface of the piping, so that a contamination of the outside atmosphere i.e. radiation exposure can be reduced and a gas cleaner in a containment vessel containing the reactor vessel can be simplified.

Claims: A method of restraining the diffusion of tritium, comprising the step of: disposing a hydrogen-absorbing metal (16, 19) on the external side of the enclosure wall of a tritium passage so that the hydrogen-absorbing metal absorbs tritium to be diffused. An apparatus for restraining the diffusion of tritium out of a fast breeder reactor, characterized by a hydrogen-absorbing metal surrounding a device of the fast breeder reactor. An apparatus according to claim 2, characterized by a heat reserving means (14) which is mounted to a device of a fast breeder reactor and which includes a hydrogen-absorbing metal. An apparatus of restraining a diffusion of tritium as recited in claim 3, wherein said heat reserving means (14) comprises an inner wall surrounding the device, a heat reserving material surrounding the exterior of the inner wall, and an outer wall surrounding the exterior of the heat reserving material, at least one of the inner and outer walls being made of the hydrogen-absorbing metal (16). An apparatus of restraining a diffusion of tritium as recited in claim 3, wherein said heat reserving means (14) comprises an inner wall surrounding the device, a heat reserving material surrounding the exterior of the inner wall, and an outer wall surrounding the exterior of the heat reserving material, at least one of the inner and outer walls being provided with the hydrogen-absorbing metal (19) mounted thereto. An apparatus of restraining a diffusion of tritium as recited in claim 3, wherein said heat reserving means (14) comprises an inner wall surrounding the device, a heat reserving material surrounding the exterior of the inner wall, and an outer wall surrounding the exterior of the heat reserving material, said heat reserving material (14) being made of the hydrogen-absorbing metal. An apparatus of restraining a diffusion of tritium as recited in claim 3, wherein said heat reserving means comprises an inner wall surrounding the device, a heat reserving material (14) surrounding the exterior of the inner wall, and an outer wall surrounding the exterior of the heat reserving material, said heat reserving material being provided with the hydrogen-absorbing metal mixed therewith. An apparatus of restraining a diffusion of tritium as recited in claim 3, wherein said heat reserving means (14) comprises an inner wall surrouding the device, a heat reserving material surrounding the exterior of the inner wall, and an outer wall surrounding the exterior of the heat reserving material, said heat reserving material being provided with the hydrogen-absorbing metal mixed therewith in the form of fiber, chip or powder.

Inventors: KAWABE RYUHEI, SHIMOYASHIKI SHIGEHIRO, KAWABE, RYUHEI, SHIMOYASHIKI, SHIGEHIRO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1989
Main classification: C01B3
IPC codes: G21D1, C01B3, C01B4