JPS5913146B2 - Neutron detection device for radiation emitters - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to radiation emitters that emit very strong gamma rays and small amounts of neutrons, such as spent fuel that has been irradiated with neutrons in a nuclear reactor, or temporarily used during use. The present invention relates to a neutron measurement device that efficiently and accurately measures neutrons from nuclear heat materials extracted from a nuclear reactor.

Conventionally, this type of measurement device covers the radiation emitter with a lead shield to attenuate gamma rays, provides a neutron moderator on the outside of the lead shield, and inserts a thermal neutron detector into the moderator. was used.

Japanese Patent Publication No. 51-17480 discloses a neutron detection device that improves neutron detection efficiency while maintaining gamma ray shielding ability by arranging lead shields in appropriate locations.

However, since this device focuses on shielding gamma rays, there is a lack of consideration for the behavior of neutrons around the neutron detector, and therefore, if the position of the neutron detector deviates slightly from the planned position, measurement errors may occur due to this. This error is comparable to that of conventional devices.

FIG. 1 is a diagram showing the configuration of this device.

The radiation emitter 1, the neutron detector 2, and the lead shield 3S that shields gamma rays are surrounded by light water 4, which is a neutron moderator.

Thermal neutron flux distribution 6 plotted in the direction of arrow 5 is as shown.

The thermal neutron flux distribution is almost flat inside the radiation emitter 1, but the gap 7 between the radiation emitter 1 and the lead shield 3S
At a, the neutron absorption effect disappears, so the thermal neutron flux increases slightly.

In lead 3S, the thermal neutron flux shows a gentle slope and decreases as shown by curve 1b.

Near the position Ic of the neutron detector 2, the fast neutrons that have passed through the lead 3S are decelerated and become thermal neutrons, so the thermal neutron flux distribution tends to increase somewhat.

However, if the relationship between the thickness tl of the lead 3S and the distance t2 between the lead and the neutron detector 2 cannot be determined optimally, for example, if the gamma ray intensity is large and the thickness tl needs to be increased, or if the neutron detector If the shape is large, a rise in thermal neutron flux is unlikely to occur at the position 7c, and therefore the neutron detector 2 cannot be installed at the position of the rise.

In such a case, the thermal neutron flux distribution near the location where the neutron detector is installed will have a large gradient, so
If the position of the neutron detector deviates even slightly from the planned position, the detection efficiency of the neutron detector changes considerably, resulting in measurement errors.

SUMMARY OF THE INVENTION An object of the present invention is to provide a neutron measuring device for a radiation emitter that eliminates the above-mentioned drawbacks.

Another object of the present invention is to provide a neutron measurement device that actively increases the amount of thermal neutrons counted.

Another object of the present invention is to provide a neutron measuring device that effectively guides increased thermal neutrons to a neutron detector.

Still another object of the present invention is to provide a neutron measurement device with fewer measurement errors caused by positional deviation of a neutron detector by flattening the thermal neutron flux distribution as much as possible.

Still another object of the present invention is to provide a neutron measurement device that effectively blocks background neutrons.

Next, embodiments of the present invention will be described in detail with reference to FIG. 2 and subsequent figures.

FIG. 2 is a diagram showing a first embodiment of the present invention.

In the figure, a radiation emitter 1 faces a neutron derivative 3 across a fast neutron moderator material 4 of suitable thickness, for example, water 4 with a thickness of 2 to 3 crrl, and a neutron detector 2 surrounded by this dielectric material 3 is provided. It is.

The neutron derivative 3 uses a medium with a very large thermal neutron diffusion coefficient, such as carbon, lead, air, heavy water beryllium, etc., but the fast neutrons emitted from the emitter 1 are absorbed by water 4, which is a moderator.
The thermal neutrons are actively converted into thermal neutrons, and these thermal neutrons reach the neutron detector 2 through the neutron derivative 3.

The neutron flux distribution plotted in the direction of arrow 5 becomes curve 8 and increases significantly.

Since the neutron flux distribution is proportional to the neutron detection efficiency, the neutron detection efficiency is greatly improved.Since the neutron detector 2 is installed in the neutron derivative 3 with a very large thermal neutral diffusion coefficient, the neutron detector Detector efficiency due to misalignment is reduced.

Thermal neutron flux distribution within the neutron derivative exhibits a gradual shape due to the extremely large diffusion coefficient.

FIG. 3 is a diagram showing a second embodiment of the present invention.

In the figure, a neutron diffusion preventer 9 is provided around the neutron derivative 3 shown in the first embodiment, and is further surrounded by a neutron absorber 10.

If there is no neutron diffusion preventer 9 and the neutron derivative 3 is directly surrounded by a neutron absorber 10, thermal neutrons move very easily within the neutron derivative 3 and are absorbed by the neutron absorber 10, resulting in a steep decline in the neutron flux distribution. .

The neutron diffusion preventer 9 provided to prevent this is preferably made of a material with a small thermal neutron diffusion coefficient and a small neutron absorption cross section.

Specifically, light water polyethylene and zirconium hydrogen containing a large amount of hydrogen are preferable.

By disposing the neutron diffusion preventer 9, the neutron flux distribution is improved from a steep drop to a curve 11, resulting in a shape almost similar to the neutron flux distribution curve 8 shown in the first embodiment.

FIG. 4 is a diagram showing a third embodiment of the present invention.

In the figure, a radiation emitter 1 is surrounded by fast neutron moderators 4, 12, 13, and 14, and a neutron derivative 3 is further placed outside to surround these moderators.

The thermal neutron diffusion distance of fast neutron moderator is neutron derivative 3
Since it is much smaller, the fast neutron moderator section 4 is somewhat thicker than the other moderator sections 12, 13, 14.

The portion of the neutron derivative 3 facing the moderator portions 12, 13, and 14 also functions as a moderator more effective than the fast neutron moderator 4.

This moderator and derivative effectively reflects the neutrons that once leaked from the radiation emitter back to the emitter, increasing the neutron multiplication factor of the neutron multiplication system centered on the reflector.

Increased neutron emission rate.

Due to this neutron multiplication effect, the thermal neutron flux distribution 15 is further raised than the distribution 11 shown in the second embodiment.

FIG. 5 is a diagram showing a fourth embodiment of the present invention.

In the figure, the neutron detector 2 is inserted into a gamma ray shield 16 provided within the neutron guide 3.

The shielding body 16 provided across the detector is thicker on the side facing the emitter 1, and its anti-glare law is weaker.

When lead is used for the shielding material, lead can have properties similar to those of the neutron derivative 3 because lead has a good neutron diffusion coefficient.

The region indicated by 17 is a cavity provided inside the neutron derivative 3.

The neutron induction characteristics of this cavity can also be improved depending on how it is arranged.

The illustrated cavity 1γ serves to effectively guide neutrons from the neutron moderator/inductor facing the moderators 12 and 13 toward the neutron detector 2.

These configurations further increase the neutron flux distribution 18 as shown.

As described in detail above, according to the present invention, the amount of thermal neutrons counted is actively increased, the increased thermal neutrons are effectively guided, and the error caused by the positional shift of the neutron detector is reduced. A neutron detection device that improves neutron detection efficiency and effectively blocks background neutrons by reducing the number of neutrons can be obtained.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the arrangement of the constituent parts and neutron flux distribution of the detection device which is the basis of the present invention, and Figs. 2 to 5 show the constituent parts and neutron flux of the present invention, which is an improved version of the detection device shown in Fig. 1. It is a figure showing distribution. 1... Radiation emitter, 2... Neutron detector, 3... Neutron derivative, 4, 12, 13,
14... Fast neutron moderator, 5... Arrow, 6, 8, 11. 15.17,18...neutron flux distribution curve, 9.
...neutron diffusion preventer, 10...neutron absorber, 16...gamma ray shield, 17...
cavity.

Claims (1)

[Claims] 1. A device for detecting neutrons emitted from a radiation emitter, comprising: a moderator that decelerates fast neutrons emitted from the radiation emitter; and a moderator that guides neutrons thermalized by the moderator. A neutron detection device for a radiation emitter, comprising a neutron derivative and a neutron detector disposed inside the neutron derivative. 2. In the neutron detection device according to claim 1, the radiator is configured such that neutrons emitted from the radiation emitter and heated are not prevented from reaching the neutron detector through the neutron derivative. A neutron sample device for a radiation emitting body, characterized in that a neutron diffusion preventive body is provided around the neutron derivative, and a neutron absorber is further provided around the neutron diffusion preventive body. 3. Neutron detection of a radiation emitter according to claim 2, characterized in that a substance that increases the neutron multiplication factor of the radiation emitter made of nuclear fuel material is disposed outside the radiation emitter. Device.