JPS60159670A - Leakage radiation detector - Google Patents

Abstract

PURPOSE:To cover the detection of leakage radiation over a wide area by making light having specified intensity incident on one end of a quartz optical fiber coil, detecting the intensity of the light emitted from the other end and displaying the output thereof or the amplified output. CONSTITUTION:When the radiation leaked from a nuclear reactor vessel 1, the radiation pass through the inside of an optical fiber coil 2 and therefore the transmission loss of the core of the optical fiber is increased by the gamma rays among the radiation. The quantity of the light passing through the coil 2 decreases consequently and the photocurrent of a photodetector 6 decreases. The value of an output level meter 8 decreases as well and an alarm is sounded when the value decreases down to the critical value or below.

Description

[Detailed Description of the Invention] C7') Technical Field This invention relates to a device for detecting leakage radiation in a nuclear reactor or the like.

There is a risk of a large amount of radiation leaking from a nuclear reactor.
This must be constantly monitored.

Medical and research facilities also use radiation for treatment and experiments. Radiation has various adverse effects on the human body and is extremely harmful.

However, since the presence of radiation cannot be easily detected, the danger is doubled.

Radiation includes alpha rays, beta rays, gamma rays, and neutron rays.

Alpha and beta rays have short ranges, so they do not pose much of a problem in terms of leakage radiation.

Leakage is a problem for gamma rays and neutron rays. This is because these are not streams of charged particles and do not decay easily.

The relative danger to the human body of neutron rays compared to gamma rays is called RBE (biological effectiveness ratio).

This value is not fixed, and is often calculated assuming a value between 1 and 25.

It is said that exposure to gamma rays can increase the risk of cancer and leukemia. However, the probability of inducing cancer is not proportional to the amount of radiation exposure, and even low-dose irradiation can increase the frequency of cancer.

Therefore, it is not okay to have a small amount of gamma ray leakage; it is better to have no leakage at all.

(b) Conventional technology Geiger and Muller counters are well known as devices that can quantitatively measure the intensity of radiation.

Electrodes are placed inside a cylindrical container, filled with gas, and a high voltage is applied between the electrodes. When radiation enters, the gas is ionized and a current flows between the electrodes, which is amplified to determine the intensity of the radiation.

Instead of such a detector, a detector using Ge or Si semiconductor is often used to detect gamma m, XM, etc. Such solid-state detectors are compact, convenient to handle, and can also measure gamma-ray spectra, making them extremely useful.

Detection sensitivity is also high.

Its small size and high detection sensitivity make it ideal as a sensor. However, on the other hand, it is expensive and 1.
Conventionally, radiation detectors were installed discretely around equipment that generates radiation, such as nuclear reactors, to monitor whether there was any radiation leakage. .

However, since detectors are not lined up all around the reactor, there is no guarantee that there will be a radiation detector at the location where a radiation leak occurs.

On the contrary, in the case of parabolas with strong straightness such as gamma rays, they can leak even through gaps in outer walls in a narrow range, and it is thought that narrow gamma ray beams like this often do not reach the detector. It will be done.

Since it is not possible to know in advance where gamma ray leakage will occur, the location of the radiation detector cannot always be optimal. It would be better to increase the number of detectors, but there is a limit to this as it is expensive.

One possibility is to increase the size of each detector. However, it is not easy to pull a large-diameter Ge or Si single crystal, and a detector that requires a large-area Ge or Si wafer would be extremely expensive.

(1) Purpose of the Invention The object of the present invention is to provide an inexpensive and simple radiation detector capable of detecting leakage radiation from a nuclear reactor or the like over a wide range.

) Increased loss due to gamma ray irradiation of optical fibers Optical fibers can be divided into quartz-based, multi-component glass-based, and plastic-based fibers.

Silica-based optical fibers have the lowest transmission loss, making them suitable for long-distance transmission.

There are two types of silica-based optical fibers.

An optical fiber is a wire consisting of a central core with a high refractive index and a ground surrounding it with a slightly low refractive index, which is coated with a primary coat and a secondary coat.

Two types of optical fibers are manufactured based on combinations of core and cladding materials.

One is that the core is quartz, TiO□, GeO2, P2O5
It is a fiber whose cladding is made of quartz. This is an optical fiber manufactured by Corning and others. Quartz is a transparent medium with an extremely low refractive index, and since we could not find a cladding with a lower refractive index than quartz as a core, we used quartz as the cladding. to quartz,
When a small amount of Ti5Ge and P oxide is mixed, the refractive index increases, so this is used as the core material.

Another type of optical fiber uses a core made of quartz and a cladding made of quartz doped with fluorine or boron. The invention was based on the discovery of fluorine and boron as additives that can lower the refractive index of quartz.

Even now, the mainstream of quartz optical fibers is to use a material in which quartz and oxides of Ti, p, and Ge are added for the core. The refractive index difference between the cladding and the core can be arbitrarily determined by the amount of oxide added, but since the refractive index difference only needs to be small,
The amount added is also small. Often a few percent by weight, up to 15
% by weight or less.

In this way, optical fibers containing TiSP, Ge, etc. in the core are ordinary fibers.

As already mentioned, a Ge detector is used to detect radiation (gamma rays, X-rays). Ge constituting the single crystal,
Ge as a dopant slightly added to quartz is of course physically different.

However, isn't the Ge in glass fiber also sensitive to radiation? it is conceivable that.

FIG. 2 is a graph showing the increase in transmission loss due to gamma ray irradiation of a Ge- and P-doped gray≠light-index (GI) optical fiber.

The horizontal axis is the gamma ray irradiation dose (rad). rad
is a physical unit that expresses the amount of radiation. When 100 ergs of energy is absorbed into 1 g of an irradiated object, this dose is called l rad.

The vertical axis represents the amount of increase in transmission loss due to gamma ray irradiation, and the unit is dB/km.

The black circles in the data that make up this graph indicate the dose rate of 2.2.
X 10 rad/H is shown. That is, for 1 gram of core per hour, 0.022
This is the case when Juyur's energy is absorbed.

The data marked with an X indicates a dose rate of 7.5 X 10' rad.
/ H, and the white circle indicates the dose rate is 9.8 x 105ra
The case of d/H is shown.

Any data can be plotted on a single irradiation dose vs. transmission loss increase graph.

This means that the increase in transmission loss of the optical fiber is determined by the total irradiation dose, regardless of the energy per time of gamma ray irradiation.

Although this data is for graded-index optical fibers, the same can be said for step-index fibers.

As already mentioned, the core is often doped with quartz and oxides such as GeO3 and P2O5 to increase its refractive index. An optical fiber containing Ge1F is one of the most common silica-based optical fibers.

Since it is already mass-produced, it is easy to obtain and inexpensive.

g) Structure of the invention FIG. 1 is a diagram showing the overall structure of the leakage radiation detector of the invention.

1 refers to nuclear reactor vessels, medical equipment that generates radiation, and experimental equipment. Although there are various shapes such as a cylinder, a cube, and a rectangular parallelepiped, the shape is simply cylindrical. Hereafter, for simplicity, it will be referred to as reactor vessel 1.

Optical fibers are wrapped around the reactor vessel 1 many times. This may be wound without any gaps, or may be wound with certain intervals.

This is the already mentioned quartz-based optical fiber whose core is doped with Ge.

Optical connectors 3.4 are attached to both ends of the optical fiber coil 2.

A light emitting element 5 is provided near one optical connector 3, and a light receiving element 6 is provided near the other optical connector 4.

Light of a constant intensity is emitted from the light emitting element 5, and enters the optical fiber coil 2 from the optical connector 3 through a lens or the like.

This light propagates through the optical fiber coil 2, exits from the optical fiber connector 4 at the other end, and enters the light receiving element 6. The light receiving element 6 generates a current 7 according to the light intensity. This is amplified by amplifier 7.

The amplified voltage signal is displayed on the output level meter 8. In addition to the level meter 8, a recorder may be used to record over time.

When the output drops below a certain value, an alarm 9 is activated. It issues an alarm either by sound or by means of a flashing signal.

Although the light emitting element emits light of a constant intensity, this does not necessarily have to be continuous light, but may be pulsed light. It suffices if the pulse interval and pulse height are constant, and the same pulse waveform appears on the light receiving element, but this pulse height is displayed on the output level meter 8, and it is determined whether or not to issue an embezzlement signal based on the value of the pulse height. do.

(Power) Operation When no radiation leaks from the nuclear reactor 1, the light emitted from the light emitting element 5 is attenuated by the original transmission loss of the optical fiber coil 2 and appears at the output end, and is attenuated by the light receiving element 6. Detected. Alarm if the output level is below the critical value
A9 doesn't ring. The output level meter 8 also shows a high value.

However, if radiation leaks from the reactor vessel 1, it will pass through the optical fiber coil 2. Gamma rays in the radiation increase the transmission loss in the core of the optical fiber.

Therefore, the amount of light passing through the optical fiber coil 2 decreases, and the photocurrent of the light receiving element 6 decreases. Output level meter 8
The value also becomes low, and when it falls below a critical value, a first alarm will sound.

This makes it immediately clear that radiation is leaking from the reactor.

This detector can only detect gamma rays. However, even if alpha and beta rays leak, they attenuate over a short distance, and gamma rays are usually emitted along with them.

Even if a neutron beam leaks, it will be accompanied by gamma rays because it will emit gamma rays when it collides with gas. Monitoring gamma rays means monitoring almost all radiation.

(1) Effect The entire optical fiber coil is a sensor, and the optical fiber coil 2 widely covers equipment that may leak radiation, so even if there is only one leakage point, the leakage can be detected immediately.

Since the sensor covers a very large area, radiation leakage can be monitored over a wide area.

It is much cheaper than semiconductor sensors made of metal, germanium, silicon, GaAs, etc., and is easy to install.

In addition, since the radiation generating device and the monitoring device are connected by optical fiber instead of electric cable, they are not affected by noise and do not malfunction.

Compared to the conventional method of installing small semiconductor radiation detectors here and there to monitor radiation leakage, the presence of leakage can be detected more completely. Once a leak is identified, immediate action can be taken.

This invention is extremely useful in increasing the safety of operating nuclear reactors, medical equipment, research equipment, etc.

[Brief explanation of the drawing]

FIG. 1 is an overall configuration diagram of the leakage radiation detector of the present invention. FIG. 2 is a graph showing the increase in transmission loss due to gamma ray irradiation of a Ge and P doped graded index optical fiber. The horizontal axis represents the gamma ray irradiation dose (rad), and the vertical axis represents the amount of increase in transmission loss in units of dB/km. 1 ...... Reactor vessel 2 ...... Optical fiber coil 3.4...
...... Optical connector 5 ...... Light emitting element 6 ...
...... Photo-receiving element 7 ......
Amplifier 8 ・・・・・・ Output level meter 9 ・・・
・・・・・・ Shigeru Inventor: Yoshiko Kobayashi, Nobunura, Hiroshi Nashi, Patent Applicant: Sumitomo Electric Industries, Ltd. Application Agent, Patent Attorney: Shigeru Kawase, Koki゛□゛1, Tomi゛11, II −;：

Claims (2)

[Claims] (1) G wrapped around a device that generates radiation
a silica-based optical fiber coil 2 whose core is doped with e;
A light emitting element 5 that makes light of a constant intensity enter one end of the optical fiber coil 2, a light receiving element 6 that detects the intensity of light emitted from the other end of the optical fiber 2, and an output or an amplified output of the light receiving element 6. A leakage radiation detector comprising: a display device; and a display device. (2) The leakage radiation detector according to claim (1), further comprising an alarm device that issues an alarm when the detection signal of the light-receiving element 6 becomes lower than a critical value.