Induced nuclear reactions: processes – systems – and elements – Control component for a fission reactor
Reexamination Certificate
1998-10-14
2001-06-12
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Control component for a fission reactor
C376S333000, C428S034500, C428S698000
Reexamination Certificate
active
06246740
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cylindrical sleeve comprising an SiC fiber-reinforced SiC composite material, which is capable of exhibiting excellent durability even under a severe condition such that it is irradiated with radioactive rays or radiation, etc. The composite material sleeve according to the present invention can suitably be used, particularly, as a component (or element) to be disposed in a nuclear reactor, a component constituting a control rod for controlling a nuclear reactor, a high-heat flux component for constituting a nuclear fusion reactor, a component for constituting a high-temperature heat exchanger, etc.
2. Related Background Art
A ceramic material has been well known as a material which is excellent in heat resistance. However, it is a relatively recent event that such a ceramic material has further overcome its disadvantage of “brittleness (or fragility)” so that it may retain a high strength even under a high-temperature condition. In addition, because of the “hard and brittle” characteristics of the ceramic material, it has required a special measure or device to use a ceramic material having a desired heat resistance and high strength as a structural material, i.e., to precisely process or form the ceramic material into a desired shape.
For example, with respect to a “sleeve” which is a hollow cylindrical member, when a monolithic ceramic material is used, the ceramic material is liable to cause brittle fracture during the processing, forming or machining thereof. Accordingly, it is difficult to prepare a long sleeve (e.g., one having a length of about 1 m) with a wall thickness of 0.2 to 0.4 mm.
When a carbon fiber-reinforced carbon composite material is used as the material for constituting a sleeve, it is possible to prepare the above-mentioned long sleeve. However, the carbon fiber-reinforced carbon composite material has a problem such that it is liable to cause a carburizing reaction with a cladding (or covering tube) comprising stainless steel (i.e., diffusion and permeation of carbon into the surface layer of the stainless steel).
When a heat-resistant steel is used as the material constituting a sleeve, it is also possible to prepare a long sleeve. However, the heat-resistant steel has a problem such that it is liable to cause a carburizing reaction with a boron carbide (B
4
C) pellet as a neutron absorber (or neutron absorbing material).
When an alumina fiber-reinforced CMC (ceramic matrix composite material) is used as a sleeve material, such a CMC material has a problem such that it is liable to cause volume expansion or swelling when irradiated with neutrons.
Heretofore, as a control rod for controlling a nuclear reactor such as fast breeder reactor, one comprising a stainless steel cladding and a B
4
C pellet which has been inserted into the cladding.
When the control rod disposed in a nuclear reactor is irradiated with neutrons, the temperature of the control rod becomes higher, and the stainless steel constituting the cladding can be carburized by B
4
C so that it becomes brittle, whereby the cladding can be broken. In addition, as the B
4
C absorbs neutrons, it generates helium and is expanded, and the B
4
C pellet is broken due to a thermal stress caused by the heat generation and the resultant fragments are moved so that they fill the gap between the B
4
C and the stainless steel cladding. When the B
4
C pellet is continuously irradiated with neutrons in such a state, the resultant stress may act between the thus expanded B
4
C and the stainless steel tube, whereby the stainless steel cladding can be damaged in some cases.
In consideration of these circumstances, for the purpose of preventing the fragments of boron carbide B
4
C from moving in the cladding, there has been proposed a structure wherein a thin-wall pipe (usually, referred to as a “shroud”) comprising austenite stainless steel, ferrite steel-stainless steel, etc., is disposed in the above-mentioned stainless steel cladding so that the shroud covers the entire length of the neutron absorber pellet (see, Japanese Patent Publication (KOKOKU) No. Hei 6-31769 (i.e., 31769/1994)).
However, when the above-mentioned control rod equipped with the shroud is used for a long period of time, the shroud comprising the stainless steel has a problem such that it is reacted with the boron carbide pellet and loses its ductility due to the carburization thereof, etc., thereby to lower its function or performance as a shroud tube.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a sleeve which is excellent in heat resistance, corrosion resistance, high strength, and precision in formability (or processability), even under a severe condition such that the sleeve is exposed to radiation.
Another object of the present invention is to provide a sleeve which can exhibit an excellent performance or function as a shroud tube to be used for a nuclear control rod for controlling a nuclear reactor.
A further object of the present invention is to provide a sleeve having an excellent compatibility with boron carbide as a neutron-absorbing material.
As a result of earnest study, the present inventors have found that a thin-wall sleeve exhibiting an excellent durability even under a severe condition can be produced by using an SiC fiber-reinforced SiC composite material, which has heretofore been considered as a material which is difficult to be formed into a thin-wall sleeve.
The SiC fiber-reinforced SiC composite material sleeve according to the present invention is based on the above discovery, and comprises an SiC fiber-reinforced SiC composite material (SiC/SiC), which has a porosity of 40% or less and a wall thickness of 5 mm or less.
The present invention also provides a process for producing a cylindrical SiC composite material sleeve, wherein an SiC sleeve having a porosity of 40% or less and a wall thickness of 5 mm or less is formed by repeating a step of impregnating three or less laminated layers of a circularly knitted SiC continuous fiber with an organic silicon compound and then calcining the thus impregnated SiC continuous fiber so as to densify the circularly knitted SiC continuous fiber.
As described above, when the SiC fiber-reinforced SiC composite material is formed into a thin-wall sleeve having a specific porosity as mentioned above, there is provided a sleeve having a particularly excellent characteristic as a structural component (or element) or structural member, which is usable in an environment under which heat resistance, corrosion resistance, high strength, and/or precision in formability are required.
Particularly, when the sleeve comprising an SiC fiber-reinforced SiC-base composite material according to the present invention is inserted, as a shroud, between a stainless steel cladding and a B
4
C pellet (neutron absorber), there is provided a control rod which has an excellent characteristic such that the shroud is very little damaged by the irradiation thereof with neutrons, it has a tolerance for mechanical damage (i.e., durability in a case where a part of the shroud is mechanically damaged), the movement or migration of the B
4
C in the stainless steel cladding can be prevented, and the stainless steel cladding is less liable to be damaged. When the sleeve according to the present invention is used as a shroud, unlike the case where a shroud comprising stainless steel is used, the carburization of the SiC fiber-reinforced SiC-base composite material constituting the sleeve according to the present invention is suppressed to a very low level.
In addition, since the sleeve according to the present invention can be formed into a thin-wall sleeve having a wall thickness of 0.5 mm or less, unlike a case where a thick-wall sleeve is used as a shroud, it is avoided that the amount of the B
4
C pellet to be inserted into the shroud is decreased so as to invite a reduction in the neutron-absorbing capacity of the control rod.
The following Tables 1 to 3 show comparisons between various k
Maruyama Tadashi
Mitsuno Shiro
Onose Shoji
Carone Michael J.
Japan Nuclear Cycle Development Institute
Mun K. Kevin
Pillsbury & Winthrop LLP
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