JPS60230349A - Nuclear-fission-type ionization chamber for neutron detection - Google Patents

Abstract

PURPOSE:To prevent any deterioration of measurement accuracy by attaching a coaxial cable, one end of which opens into an airtight case and the other end is sealed with ceramic, to an airtight case in which an outer electrode having a nuclear fission substance and an inner electrode are installed. CONSTITUTION:When constituting a nuclear-fission-type ionization chamber used as the detector of a local output area neutron detection assembly for measuring neutrons in an atomic furnace for power supply, firstly, a columnar inner electrode 4 is installed inside an outer electrode 2, formed by packing a nuclear fission substance 3 in an air-tight case 1, with a space 5 allowed between the electrodes 2 and 4. Next, the space 5 is packed with argon. At the same time, a coaxial cable 6 is attached to the case 1 to achieve electric conduction; one end 6a of the cable 6 opens into the case 1, the outer end of the cable 6 is sealed with a ceramic member 9 and the internal area of the cable 6 is packed with an insulating material 12. Because of the above constitution, since ceramic does not exist in the end 6a, measurement accuracy can be improved without influencing the pressure of argon gas in the case 1.

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. Concerning 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 fission type ionization chamber has conventionally been constructed as shown in FIG. 1, and is housed in a cylindrical airtight case 1 made of, for example, austenitic stainless steel.

A cylindrical outer electrode (cathode) 2 containing fissile material 3 is arranged along the inner wall. 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. Moreover, the coaxial cable 6 has both ends 6a and 6b.
The ceramics 8 and 9 are hermetically sealed with ceramics 8 and 9, and the ceramics 8 and 9 are made of alumina porcelain (A40g), for example, so that the cable jacket 10 and the core wire 7 can be electrically insulated.
is used. Furthermore, inside the coaxial cable 6, there is a semi-sintered insulator 12 filled with alumina powder.

An inert gas 13 is filled in order to maintain high insulation resistance between the cable sheath 10 and the core wire 7.

This inert gas 13 is usually the same type of gas as in the airtight case 1, such as argon. Note that section 1, section 1, and section 22 are exhaust pipes.

The operating principle of such a nuclear fission type ionization chamber is that a fissile material 3, which has an ionized gas filled between both electrodes on one of the electrodes,
For example, fission products produced when thermal neutrons collide with uranium-235 ionize the filling gas, creating electron-ion pairs, which are collected by applying a voltage between the two electrodes, and a signal is generated in the form of an electric current. is extracted.

[Problems with background technology]

When a conventional fission-type ionization chamber as described above is installed in a nuclear reactor and used for a long period of time, stress accumulates inside the ceramic 8 located within the airtight seal 1 due to irradiation with fast neutrons, which eventually causes cracks. Occurred, and in some cases led to penetration. This is a phenomenon in which the ceramic 8 undergoes curing and expansion due to the irradiation itself with fast neutrons. Figure 3 shows an example of the occurrence of the above-mentioned cracks. 14 indicates a penetrating crack, and 5 indicates a crack that occurred in the middle.

When such a crack 14 leading to penetration occurs, the f-gas filled in the airtight case 1 and the gas 13 filled in the coaxial cable L move according to the pressure difference.

For example, if the gas pressure in the airtight case 1 is higher than the gas pressure in the coaxial cable 6, the gas in the airtight case 1 moves to the opposite side of the coaxial cable. As a result, the number of ionized argon atoms per unit length of the fission products increases, so
The signal current becomes smaller, generating a signal that looks like the thermal neutron flux has decreased. Conversely, when the pressure on the coaxial cable 6 side is high, the number of ionized argon atoms increases, so the signal current increases by 7JI. Usually, the latter combination is used.

The reason for this is that, as shown in Fig. 4(a), in a 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 will decrease.

Therefore, in the former case, a crack 14 leading to penetration occurs in the airtight seal 1 as shown in FIG. 1(b). lshi giant spear enemy color Ike a small sumata darken 與6b To reach the apparent lifespan quickly (2). In some cases, as shown in the same figure (C) This is because some detectors may cross the line, forcing the detector to be replaced.In the latter case (=, in the same figure (
As shown in d), by normalizing the signal current after the increase to the signal current before the increase and raising the service life line by that amount, the service life remains unchanged and continuous use is possible without any operational problems. . However, cracks leading to penetration 14
If this occurs, the coaxial cable 6 side cannot be used until the gas pressure on the side of the coaxial cable 6 and the gas pressure on the airtight case 1 side reach equilibrium, so it is necessary to prevent cracks from occurring.

[Purpose 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, in which one end of a coaxial cable located in an airtight case is open, and a position away from the opening is hermetically sealed with ceramic.

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 l made of, for example, austenitic stainless steel, a cylindrical outer electrode 2 provided with a fissile material 3 is disposed along the inner wall. This outer electrode 2
An inner electrode 4 having a cylindrical shape 17 with a small diameter portion at both ends is coaxially disposed with a predetermined gap 5 therebetween. It is filled with air pressure.

Furthermore, a coaxial cable L is fixed to one side of the airtight case 1 to ensure electrical continuity. in this case,
One end of the coaxial cable (located inside the airtight case 1) 6
A is open unlike the conventional case, and a position away from this opening, for example, a bent end 6b of the coaxial cable, is hermetically sealed with a ceramic 9.

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 ends 6a of the coaxial cables are open and no ceramic is present, the conventional problems caused by the occurrence of cracks are eliminated, and deterioration of measurement accuracy can be prevented.

[Embodiment of the second invention] The nuclear fission type ionization chamber according to the invention of s2 is constructed as shown in FIG. 6, and the same parts as those of the conventional example and the first invention are given the same reference numerals.

That is, the inside of the airtight case 1 has the same structure as the conventional example and the first invention, so explanation and illustration are omitted, but a coaxial cable L with the same name is provided in one side of the airtight case 1 to ensure electrical continuity. There is.

In this case, similarly to the first invention, the coaxial cable 6 (D-
One end (located inside the airtight case 1) 6a is open, and the coaxial cable L
The curved end 6b of is hermetically sealed with ceramic 9.

However, the length of the coaxial cable L in the second invention is set to be shorter than the coaxial cable L in the first invention. In order to compensate for the total length, in the second invention, a connecting portion 11J is added to the coaxial cable L.
Another coaxial cable Z7 is connected via. This coaxial cable 17 has the same interior as the coaxial cables described above, but both ends 1711 and 17b are hermetically sealed with ceramics 22 and 23 such as alumina porcelain, respectively. Further, the connecting portion 1B- includes an airtight case 20 for establishing conduction between each cable sheath of both coaxial cables 6 and 17, a metal piece 19 for connecting each core wire, and an infill filled in the airtight case 20. It consists of two active gases.

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 connecting portion L! By placing the screw at an appropriate distance from the twisting material 16 and at a position where there are no or very few fast neutrons, cracking of the ceramic can be prevented and a highly reliable nuclear fission type ionization chamber can be provided. In FIG. 7, 24 is a reactor pressure vessel, and 25 is a neutron detector.

[Brief explanation of drawings]

The first cause 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. Figure 3 is also the occurrence of cracks. 4 (8) to (dl are characteristic curve diagrams showing the relationship between I-signal current and neutron irradiation amount in the conventional branched ionization chamber, and FIG. 5 is one of the first inventions. FIG. 6 is a cross-sectional view showing a fission type ionization chamber according to an embodiment of the second invention; FIG.
The figure is a sectional view showing the state of use of the second invention. 1...Airtight case, 2...Outside 1 electrode, 3...Fissile material, 4...Inside (111j electrode, 5...Gap, 6
... Coaxial cable, 6a... One end of the coaxial cable,
6b...-Iketsuge of coaxial cable, 7... Core wire, 9...
-Ceramic, 10...Cable jacket, 12...Insulator, Ll...Coaxial cable, 17a...One end of coaxial cable, 17b...C end of coaxial cable, 1B...
・Connection part, J9...Metal piece, 20...Airtight case,
22, 23... Ceramic. Representative Patent Attorney Takehiko Suzue Figure 1 Figure 2 Figure 1 Figure 3

Claims (2)

[Claims] (1) An outer electrode containing fissile material and an inner electrode are arranged with a predetermined gap in an airtight case. In a nuclear fission type ionization chamber for neutron detection, the gap is filled with ionized gas and a coaxial cable is fixed to one side of the airtight case. A nuclear fission type ionization chamber for neutron detection, characterized in that one end of the coaxial cable located in the airtight case is open and hermetically sealed with ceramic at a position spaced from the opening. (2) An outer electrode containing fissile material and an inner electrode are arranged with a predetermined gap in an airtight case. In a nuclear fission type ionization chamber for neutron detection, the gap is filled with ionized gas and a coaxial cable is fixed to one side of the airtight case. One end of the coaxial cable located within the hermetic case is open, and hermetically sealed with ceramic at a position spaced from the opening. A fission type ionization chamber for neutron detection, further comprising another coaxial cable connected to the coaxial cable.