JPH0324737B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a fission-type neutron detection device which is one detection element of a local power range neutron detector assembly used as a means of neutron instrumentation in a power reactor. Regarding improvements to ionization chambers.

[Technical Background of the Invention] Generally, a fission type ionization chamber is used as one detection element of a local power range neutron detector assembly used as a means of neutron instrumentation in a power reactor.

This type of nuclear fission type ionization chamber has conventionally been constructed as shown in FIG. An electrode (cathode) 2 is provided. Inside this outer electrode 2, an inner electrode (anode) 4 having a cylindrical shape with a small diameter portion at both ends is disposed coaxially with a predetermined gap 5, and is fixed to one side of the airtight case 1. It is connected to the coaxial cable 6 for electrical continuity. The gap 5 is a gas space, and is filled with an ionized gas such as argon.
Further, the coaxial cable 6 has both ends 6a and 6b hermetically sealed with ceramics 8 and 9, and the ceramics 8 and 9 are made of, for example, alumina porcelain ( Al ₂ O ₃ ) is used. Furthermore, the inside of the coaxial cable 6 is filled with an inert gas 13 in order to maintain high insulation resistance between the cable sheath 10 and the core wire 7, in addition to a semi-sintered insulator 12 filled with alumina powder. There is. This inert gas 13 is usually the same type of gas as in the airtight case 1, such as argon. In addition, 22 in the figure is an exhaust pipe.

The operating principle of such a fission-type ionization chamber is that an ionized gas filled between both electrodes is filled with fission products produced when thermal neutrons collide with a fissile material 3, such as uranium-235, provided on one of the electrodes. Gas is ionized to generate electron-ion pairs, ions are collected by applying a voltage between both electrodes, and a signal is extracted in the form of a current.

[Problems in the Background Art] When a conventional nuclear fission type ionization chamber as described above is installed in a nuclear reactor and used for a long period of time, the inside of the ceramic 8 located in the airtight case 1 is exposed to radiation due to fast neutron irradiation. Stress accumulated, eventually causing cracks and, in some cases, penetration. This is a phenomenon in which holes are created in the ceramic 8 and expanded by the irradiation with fast neutrons itself. FIG. 3 shows an example of the occurrence of the above-mentioned cracks, where 14 indicates a penetrating crack and 15 indicates a crack that occurs in the intermediate portion.

When a crack 14 leading to such penetration occurs, the gas filled in the airtight case 1 and the gas 13 filled in the coaxial cable 6 will move according to the pressure difference. for example,
When the gas pressure inside the airtight case 1 is higher than the gas pressure inside the coaxial cable 6, the gas inside the airtight case 1 moves toward the coaxial cable 6 side. As a result, the number of ionized argon atoms per unit length of the range of the fission product decreases, so the signal current becomes smaller, generating a signal that looks like the thermal neutron flux has decreased. Conversely, when the gas pressure on the coaxial cable 6 side is high, the number of ionized argon atoms increases, so the signal current increases. Usually, the latter combination is used.

The reason for this is that, as shown in Figure 4a, in a nuclear fission type ionization chamber, the absolute amount of fissile material 3 provided on the inner wall of the outer electrode 2 gradually decreases due to neutron irradiation, and the signal current also decreases accordingly. It's coming. Therefore, in the former case, the airtight seal 1 is
When a crack 14 that leads to penetration occurs within the cap, the signal current also decreases rapidly, approaching the life line and apparently reaching the end of the life earlier. This is because, in some cases, as shown in Figure c, the life span may be exceeded at once, and the detector must be replaced. In the latter case, as shown in Figure d, by normalizing the signal current after the increase to the signal current before the increase and raising the life line by that amount, the life will not change and there will be no operational problems. , continuous use is possible. However, if a crack 14 that leads to penetration occurs, it becomes unusable until the gas pressure on the coaxial cable 6 side and the gas pressure on the airtight case 1 side reach equilibrium, so the occurrence of cracks is prevented. There is a need.

[Object of the Invention] An object of the present invention is to provide a fission type ionization chamber for neutron detection that prevents deterioration of measurement accuracy due to ceramic cracks.

[Summary of the Invention] The first invention is a nuclear fission type ionization chamber for neutron detection, which has an open end at one end of a coaxial cable located in an airtight case, and is hermetically sealed with ceramic at a position away from the opening. be.

In a second invention, one end of a coaxial cable located in an airtight case is open, and a position away from this opening is hermetically sealed with ceramic, and further, another coaxial cable is connected to this coaxial cable. This is a fission type ionization chamber for neutron detection.

[Embodiment of the first invention] The nuclear fission type ionization chamber according to the first invention is constructed as shown in FIG. 5, and the same parts as in the conventional example are given the same reference numerals.

That is, in a cylindrical airtight case 1 made of, for example, austenitic stainless steel, a cylindrical outer electrode 2 provided with a fissile material 3 is disposed along an inner wall. A predetermined gap 5 is provided inside this outer electrode 2.
An inner electrode 4 having a cylindrical shape with small diameter portions at both ends is disposed coaxially, and the gap 5 is filled with an ionized gas such as argon at 1 atmosphere.

Furthermore, a coaxial cable 6 is fixed to one side of the airtight case 1 to ensure electrical continuity. In this case, one end 6a of the coaxial cable 6 (located inside the airtight case 1) is open, unlike the conventional case, and a position away from this opening, for example, the other end 6b of the coaxial cable 6, is sealed by the ceramic 9. Sealed.

Note that even if one end 6a of the coaxial cable 6 is open, the insulator 12 inside the coaxial cable 6 will not easily scatter, and since the one end 6a is used facing upward, some of the outer and inner electrodes 2 and 4 It does not scatter into space.

[Effects of the First Invention] Since the one end 6a of the coaxial cable 6 is open and there is no ceramic, the conventional problems caused by the occurrence of cracks are eliminated, and deterioration in measurement accuracy can be prevented.

[Embodiment of the second invention] The nuclear fission type ionization chamber according to the second invention is constructed as shown in FIG. 6, and the same parts as in the conventional example and the first invention are given the same reference numerals.

That is, since the inside of the airtight case 1 has the same structure as the conventional example and the first invention, explanations and illustrations will be omitted, but a coaxial cable 6 is fixed to one side of the airtight case 1 for electrical continuity. .

In this case, like the first invention, one end 6a of the coaxial cable 6 (located inside the airtight case 1) is open, and a position away from this opening, for example, the other end 6b of the coaxial cable 6, is made of ceramic. 9, hermetically sealed. However, the coaxial cable 6 in the second invention is set shorter in length than the coaxial cable 6 in the first invention. In order to compensate for the total length, in the second invention, another coaxial cable 1 is connected to the coaxial cable 6 via the connection part 18.
7 is connected. This coaxial cable 17 is
The inside is the same as the coaxial cable 6, but both ends 17a and 17b are hermetically sealed with ceramics 22 and 23, such as alumina porcelain, respectively. Further, the connection portion 18 is connected to both coaxial cables 6,
It consists of an airtight case 20 for establishing conduction between the outer sheaths of each of the 17 cables, a metal piece 19 for connecting each core wire, and an inert gas 21 filled in the airtight case 20. As this inert gas 21, argon or the like similar to the gas inside the airtight case 1 is used.

[Effects of the second invention] Also in the case of the second invention, the same effects as in the case of the first invention can be obtained.

In addition, in the second invention, as shown in FIG. 7, the occurrence of cracks in the ceramic can be prevented by providing the connecting portion 18 at an appropriate distance from the fuel 16 and in a position where there are no or very few fast neutrons. , it is possible to provide a highly reliable nuclear fission type ionization chamber. In FIG. 7, 24 is a reactor pressure vessel, and 25 is a neutron detector.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view of a conventional fission-type ionization chamber for neutron detection, Figure 2 is an enlarged cross-sectional view of the vicinity of the airtight seal in the fission-type ionization chamber of Figure 1, and Figure 3 is also a cross-sectional view showing the occurrence of cracks. A cross-sectional view showing the condition,
Figures 4a to 4d are characteristic curve diagrams showing the relationship between signal current and neutron irradiation amount in a conventional fission type ionization chamber;
FIG. 5 is a sectional view showing a fission type ionization chamber according to an embodiment of the first invention, FIG. 6 is a sectional view showing a fission type ionization chamber according to an embodiment of the second invention, and FIG. FIG. 7 is a cross-sectional view showing a state in which the second invention is used. DESCRIPTION OF SYMBOLS 1...Airtight case, 2...Outer electrode, 3...Fissile material, 4...Inner electrode, 5...Gap, 6...Coaxial cable, 6a...One end of coaxial cable, 6b...Other end of coaxial cable, 7... Core wire, 9... Ceramic, 10...
Cable jacket, 12... Insulator, 17... Coaxial cable, 17a... One end of coaxial cable, 17b... Other end of coaxial cable, 18... Connection part, 19... Metal piece,
20... Airtight case, 22, 23... Ceramic.

Claims (1)

[Claims] 1. An outer electrode and an inner electrode provided with a fissile material are disposed in an airtight case with a predetermined distance between them.
In a nuclear fission type ionization chamber for neutron detection, which is filled with ionized gas and has a coaxial cable fixed to one side of the airtight case, an end of the coaxial cable on the side of the airtight case is open and spaced apart from the opening. A nuclear fission type ionization chamber for neutron detection, characterized in that the chamber is hermetically sealed with ceramic at the position where the chamber is positioned. 2. An outer electrode containing fissile material and an inner electrode are placed in an airtight case with a predetermined distance between them.
In a fission type ionization chamber for neutron detection, which is formed by filling an ionized gas with a coaxial cable and fixing a coaxial cable to one side of the airtight case, an end of the coaxial cable on the airtight case side is open, and from the opening A nuclear fission type ionization chamber for neutron detection, characterized in that the chamber is hermetically sealed with ceramic at a spaced apart position, and further has another coaxial cable connected to the coaxial cable.