JP2012047758A - Soundness evaluation method and device for used fuel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To evaluate fuel soundness without arranging a neutron source or a neutron detector in a storage container during storage and transportation of used fuel.SOLUTION: A used fuel 7 is stored in a storage container 4 shielded by a neutron shield and a γ-ray shield, and a gas collection space is provided in the storage container 4 above the used fuel 7. A target 1 made of a material which generates neutrons through light nuclear reaction is arranged in a gas collection space 11, and an X-ray generation device 2 and a neutron detector 3 are arranged outside the storage container 4. The neutron detector 3 counts neutrons generated when X rays emitted by the X-ray generation device 2 and transmitted through the neutron shield and γ-ray shield collide against the target 1. Based upon the counted value thereof, abnormality of the used fuel 7 is detected to evaluate soundness of the used fuel.

Description

  The present invention relates to a soundness evaluation method and apparatus for evaluating the soundness of spent nuclear fuel.

  Spent fuel used and burned in the reactor is temporarily stored in a fuel storage pool, and stored in a spent fuel storage container (hereinafter also simply referred to as “storage container”) to be transported to a reprocessing facility. Is done.

  On the other hand, the neutron multiplication method is the easiest method for measuring the subcriticality of spent fuel. In this method, a neutron generated from a neutron source having a known intensity is incident on a spent fuel, and the neutron generated from the spent fuel is measured. However, it is generally difficult to use this method for a spent fuel storage device configured to prevent neutrons from leaking outside.

  For example, as a system for measuring the subcriticality of spent fuel stored in a spent fuel storage container, a neutron detector is previously installed in a spent fuel transport container (storage container) as shown in Patent Document 1. Subcriticality measurement systems are known.

Japanese Patent Laid-Open No. 11-118981

Kyoritsu Publishing Experimental Physics Course 29 "Reactor" Chapter 8 edited by Koji Fushimi

  However, in the case of the system shown in Patent Document 1, the storage container is sealed for a long period of time, and it is not realistic to arrange the neutron detector inside the storage container.

  It is also possible to place a neutron detector outside the storage vessel for measurement, but since the storage vessel is designed to sufficiently shield neutrons, use neutrons generated from the spent fuel itself. It is difficult. In addition, in order to detect neutrons, a neutron source is built in the storage container in advance, and neutron multiplication can be performed so that neutrons can be detected from the outside of the storage container. Therefore, it is difficult to adopt.

  In addition, if a neutron source is incident from the outside of the storage container and neutrons are similarly measured by a neutron detector arranged outside the storage container, a powerful neutron generator that can penetrate the neutron shield of the container is required, which is very In addition to being expensive, the storage vessel is activated by neutrons, increasing the environmental dose.

  Therefore, an object of the present invention is to provide a method and apparatus for evaluating the soundness of spent fuel during storage or transportation.

  The present invention achieves the above object, and one aspect of the present invention provides a gas pool space above the spent fuel in a storage container storing spent fuel, and the gas pool space is provided inside the gas pool space. A target made of a substance that generates neutrons by photonuclear reaction is disposed, an X-ray generator and at least one neutron detector are disposed outside the storage container, and the X-rays emitted by the X-ray generator are A method for evaluating spent fuel integrity, comprising: counting neutrons generated when colliding with a neutron detector, and detecting an abnormality of the spent fuel based on a count value obtained by the neutron detector. is there.

  Yet another embodiment of the present invention provides a gas collection container with gamma ray shielding above the spent fuel in the storage container storing the spent fuel so that the height is less than the height of the spent fuel. A target made of a substance that generates neutrons by photonuclear reaction is disposed, an X-ray generator and at least one gamma-ray detector are disposed outside the storage container, and the X-rays emitted by the X-ray generator are the target. By irradiating the spent fuel with neutrons generated when it collides with the spent fuel, the spent fuel undergoes fission, and fission products generated by fission are collected in the gas collection container. The used gamma ray is counted by the gamma ray detector, and an abnormality of the spent fuel is detected based on the counted gamma ray. It is a value method.

  Still another embodiment of the present invention is to generate a neutron by a gas pool space provided above the spent fuel in a storage container storing the spent fuel and a photonuclear reaction provided in the gas pool space. A target made of a substance to be used, an X-ray generator provided outside the storage container, at least one neutron detector provided outside the storage container, and X-rays emitted by the X-ray generator are And a means for detecting an abnormality of the spent fuel by counting neutrons generated at the time of collision with the neutron detector.

  Still another embodiment of the present invention is a gas collection container provided with a gamma ray shield for collecting fission products generated by fission provided above a fuel assembly in a storage container storing spent fuel. A target made of a substance that generates a neutron by photonuclear reaction provided at a height equal to or lower than that of the spent fuel, an X-ray generator provided outside the storage container, and provided outside the storage container By irradiating the spent fuel with neutrons generated when the X-rays emitted by the at least one gamma ray detector and the X-ray generator collide with the target, the spent fuel is caused to undergo fission. Means for counting generated gamma rays with the gamma ray detector, and means for detecting an abnormality of the spent fuel based on a count value obtained by the gamma ray detector. It is a health evaluation apparatus of the spent fuel to be.

  According to the present invention, it is possible to evaluate the soundness of spent fuel during storage or transportation without arranging a neutron source in a storage container or the like.

FIG. 3 is a schematic plan sectional view showing the configuration of the first or second embodiment of the subcriticality measuring apparatus for a fuel storage container according to the present invention. It is a figure which shows the structure of 2nd Embodiment which concerns on the subcriticality measuring apparatus of the fuel storage container by this invention, Comprising: The II-II typical arrow sectional view of FIG. The typical plane sectional view showing the composition of the 3rd or 4th embodiment concerning the subcriticality measuring device of the spent fuel storage pool by the present invention. FIG. 4 is a diagram showing a configuration of a fourth embodiment according to a subcriticality measuring apparatus for a spent fuel storage pool according to the present invention, and is a schematic sectional elevation view taken along line IV-IV in FIG. 3. FIG. 9 is a schematic sectional elevation view showing the configuration of a fifth embodiment of the spent fuel integrity evaluation apparatus according to the present invention. FIG. 10 is a schematic sectional elevation view showing a configuration of a sixth embodiment of the spent fuel integrity evaluation apparatus according to the present invention. The FP gas collection container expansion sectional view of FIG.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Here, the same or similar parts are denoted by common reference numerals, and redundant description is omitted.

[First Embodiment]
FIG. 1 is a schematic view showing a basic configuration of a first embodiment of a spent fuel subcriticality measuring apparatus according to the present invention.

  In FIG. 1, spent fuel 7 is built in a storage container 4 shielded by a neutron shield 6 and a γ-ray shield 5. The spent fuel 7 is composed of a plurality of spent fuel assemblies, and a target 1 made of a neutron generating material that generates neutrons by a photonuclear reaction is disposed at the center thereof. Further, neutron detectors 3 a and 3 b for detecting neutrons are arranged on the outer periphery of the storage container 4. Although it is possible to measure even one of these neutron detectors, it is desirable to arrange two neutron detectors at symmetrical positions in order to prevent erroneous signals and malfunctions.

  Here, as the neutron generating material constituting the target 1, beryllium which has a low threshold for the photonuclear reaction and is not contained in the structural material or heavy water which is present in a small amount in water is suitable. It is known that these beryllium and deuterium react with high-energy X-rays of about several MeV and cause a photonuclear reaction that emits neutrons.

  Further, an X-ray generator 2 that emits X-rays that collide with a target 1 that is a neutron generating substance by a photonuclear reaction to generate neutrons is disposed outside the storage container 4. An apparatus that generates high-energy X-rays, such as the X-ray generator 2, has recently been developed in the medical field and the like, and an X-ray generator of about 10 MeV can be easily manufactured at a low cost. Has been reduced.

  Here, in order to generate neutrons from the target 1 by photonuclear reaction, the energy of X-rays needs to be about 2 MeV. Although it is desirable that it is as high as possible in order to pass through the γ-ray shield efficiently, if it exceeds 5 to 6 MeV, the probability that uranium will generate neutrons by photonuclear reaction increases, and if it is close to 10 MeV, the iron of the structural material is also neutron Is likely to occur. Therefore, it is sufficient that the X-ray energy intensity exceeds the detection level of the neutron detectors 3a and 3b installed outside the storage container 4, and about 2 to 5 MeV is appropriate. Under such conditions, The activation of the storage container 4 is no problem. Also, the neutron intensity that can be detected by adjusting the X-ray generation intensity can be adjusted.

  Gamma rays are also generated from the spent fuel 7, but the energy is about 1 MeV at most, and the photonuclear reaction requires gamma rays or X-rays of about 2 MeV or more. The probability that the target 1 generates neutrons with gamma rays is very small.

  Further, the neutron shielding of the storage container 4 has little effect on X-rays, and the γ-ray shielding assumes γ-rays of about 1 MeV generated from the spent fuel 7 at most. The inside of the container 4 can be sufficiently reached. Moreover, even if the storage container 4 is irradiated with X-rays, the material is not activated like neutrons, and the environmental dose is not increased.

  In the spent fuel subcriticality measuring apparatus configured as described above, in order to measure the subcriticality, the X-ray generator 2 arranged outside the storage container 4 is moved to the target 1 which is a neutron generating substance. An X-ray of about 2 to 5 MeV is incident. At the target 1, neutrons are generated by the photonuclear reaction, and the generated neutrons are multiplied in the spent fuel 7 and reach the neutron detectors 3a and 3b to be detected. Based on the count rates detected by the neutron detectors 3a and 3b, the subcriticality can be calculated by the subcriticality measuring means 20 using the correlation previously obtained by calculation.

  Although the apparatus is complicated, if the X-ray generator 2 generates X-rays on the pulse, it is possible to estimate the subcriticality using a pulse neutron method (see Non-Patent Document 1, etc.). is there. That is, by generating X-rays in pulses, neutrons are generated in pulses from the target, the time decay constant of the number of neutrons is evaluated from the time response appearing in the neutron detector, and the subcriticality is calculated from the time decay constant. Can be calculated.

  As described above, by incorporating a substance that causes a photonuclear reaction in a storage container in advance, the subcriticality can be evaluated without the presence of a neutron source during transportation. That is, only when subcriticality is measured periodically during storage, etc., X-rays are generated and collided with the target to irradiate the spent fuel and multiply it, and it is installed outside the storage container. By using the neutron detector, the subcriticality can be evaluated without activating the storage container.

[Second Embodiment]
A second embodiment of the spent fuel subcriticality measuring apparatus and measuring method according to the present invention will be described with reference to FIGS. FIG. 1 is common to the first embodiment. FIG. 2 shows a schematic configuration in the vertical direction of the subcriticality measuring apparatus for spent fuel according to the second embodiment.

  In FIG. 2, the length in the axial direction of the target 1 disposed in the spent fuel 7 is at least as long as the effective fuel portion of the spent fuel. Further, the X-ray generator 2 is configured such that the axial position can be changed by the X-ray generator axial drive device 8.

  With this configuration, the collision position between the X-ray and the target 1 can be changed, and the neutron generation position can be moved to an arbitrary position in the axial direction. Thereby, the subcriticality of the spent fuel that is long in the axial direction can be measured at a plurality of heights, and the reliability of the subcritical measurement can be improved.

[Third Embodiment]
FIG. 3 is a schematic view showing the configuration of a third embodiment of the spent fuel subcriticality measuring apparatus according to the present invention.

  In FIG. 3, a target 1 made of a neutron generating material that generates neutrons by a photonuclear reaction is disposed in the center of the spent fuel 7 stored in the spent fuel storage pool 9, and the neutrons are disposed outside the side surface of the spent fuel 7. The neutron detectors 3a and 3b are arranged for detecting. In addition, the X-ray generator 2 is disposed outside the spent fuel storage pool 9.

  In the third embodiment, the spent fuel storage container 4 in the first embodiment is replaced with a storage pool. Other basic configurations and respective components are the same as those in the first embodiment. It is the same.

  Therefore, also in the spent fuel storage pool of the present embodiment, the subcriticality of the spent fuel can be measured as in the first embodiment.

[Fourth Embodiment]
A spent fuel subcriticality measuring apparatus and method according to a fourth embodiment of the present invention will be described with reference to FIGS. 3 and 4. FIG. FIG. 3 is common to the third embodiment. In FIG. 4, a drive device 10 that moves the target 1 in the spent fuel storage pool 9 is provided above the spent fuel storage pool 9.

  According to this configuration, the neutron source can be moved to an arbitrary position in the spent fuel storage pool 9 by moving the target 1 by the driving device 10, and more detailed subcriticality measurement is possible. It has the effect of becoming.

[Fifth Embodiment]
FIG. 5 is a schematic view showing a basic configuration example as a fifth embodiment of the spent fuel integrity evaluation apparatus according to the present invention.

  In FIG. 5, an FP gas pool space 11 is provided above the spent fuel 7 in the storage container 4, and the target 1 that is a neutron generating substance is disposed in the FP gas pool space 11. In addition, the X-ray generator 2 and the FP leakage detection neutron detector 12 are disposed outside the storage container 4.

  In such a configuration, when the spent fuel 7 is damaged, Xe which is a fissile material (hereinafter referred to as FP) leaks and is FP gas in the FP gas pool space 11 provided in the upper part of the storage container 4. Xe stays. Since Xe has a large neutron absorption cross section, neutrons generated by the photonuclear reaction caused by X-ray irradiation from the X-ray generator 2 are absorbed by Xe, and the indicated value of the neutron detector 12 for FP leakage detection decreases. By monitoring this indicated value, it is possible to determine the presence or absence of fuel damage. By determining whether or not the spent fuel 7 is damaged, the soundness of the spent fuel can be evaluated.

  In addition, the X-ray generator used here can be used also for the nondestructive inspection of a container material. In addition to detecting neutrons from the inside, not only can the subcriticality be determined, but also the integrity of the neutron-absorbing material, neutron shielding material, and fission product (FP) gas that leaks when fuel is damaged. By detecting Xe having a large area, particularly Xe-131, it is also effective for evaluating fuel integrity.

[Sixth Embodiment]
FIG. 6 is a schematic diagram showing a basic configuration example as a sixth embodiment of the spent fuel health evaluation apparatus of the present invention. A target 1 that is a neutron-generating substance is disposed in the spent fuel 7, and a FP gas collection unit 13 with a shielding container that accumulates FP gas is provided above the spent fuel 7. An FP leakage detection γ-ray detector 14 is provided at a position outside the storage container 4 that is substantially the same height as the FP gas collection unit 13. Moreover, the X-ray generator 2 and the drive mechanism 8 are provided outside the storage container 4 so as to be driven in the vertical direction.

  As shown in FIG. 7, the gas collection unit 13 is covered with a shielding material 13 a that shields γ rays, and an opening 13 c that collects FP gas is provided on the spent fuel 7 side. Further, only the side 13b facing the detector is thinned so that strong γ rays from the fuel can be shielded and background γ rays can be removed.

  According to such a configuration, when X-rays are caused to collide with the target 1 made of a neutron generating material by the X-ray generator 2, neutrons are generated, and the neutrons irradiate the spent fuel 7 with a low output for a short period. . This irradiation is much lower than in the reactor, so there is no constant heat increase due to heat generation or long half-life nuclides, but the spent fuel 7 undergoes a low level of fission and a short half-life. FP can be generated.

  If the spent fuel 7 is damaged at this time, a gas with a short half-life FP, for example, a rare gas represented by Xe-135 (half-life 9.1 h) leaks from the spent fuel 7. It becomes a strong gamma ray source. As described above, since the radioactive substance having a short half-life has a strong radioactivity even if the amount is small, the presence or absence of fuel breakage can be determined by counting the γ-rays generated therefrom.

  With the above configuration, it is possible to determine whether the spent fuel is damaged or not and to evaluate the soundness.

1: Target 2: X-ray generator 3a, 3b: Neutron detector 4: Storage container 5: γ-ray shield 6: Neutron shield 7: Spent fuel 8: X-ray generator axial drive 9: Spent Fuel storage pool 10: Drive device 11: FP gas pool space 12: FP leak detection neutron detector 13: FP gas collector 14: FP leak detection γ-ray detector 20: Subcriticality measuring means

Claims (6)

A gas pool space is provided above the spent fuel in the storage container that stores the spent fuel.
A target made of a substance that generates neutrons by photonuclear reaction is disposed inside the gas pool space,
Arranging an X-ray generator and at least one neutron detector outside the storage container;
The neutron detector counts neutrons generated when the X-rays emitted by the X-ray generator collide with the target,
Detecting an abnormality of spent fuel based on the count value by the neutron detector,
A method for evaluating the integrity of spent fuel, characterized by Provide a gas collection container with gamma ray shielding above the spent fuel in the storage container that stores the spent fuel,
Place a target made of a substance that generates neutrons by photonuclear reaction at a height below the level of the spent fuel,
Arranging an X-ray generator and at least one gamma ray detector outside the storage container;
By irradiating the spent fuel with neutrons generated when the X-rays emitted by the X-ray generator collide with the target, the spent fuel is caused to undergo fission,
Collecting fission products generated by fission in the gas collection container,
The gamma rays generated from the fission products are counted with the gamma ray detector,
Detecting spent fuel anomalies based on the counted gamma rays;
A method for evaluating the integrity of spent fuel, characterized by   The method for evaluating the integrity of spent fuel according to claim 1 or 2, wherein a substance containing at least one of beryllium or heavy water is used as a substance that generates neutrons by a photonuclear reaction. A gas pool space provided above the spent fuel in the storage container storing the spent fuel;
A target made of a substance that generates neutrons by a photonuclear reaction provided in the gas pool space;
An X-ray generator provided outside the storage container;
At least one neutron detector provided outside the storage container;
Means for detecting an abnormality of spent fuel by counting neutrons generated when the X-rays generated by the X-ray generator collide with the target with the neutron detector;
A device for evaluating the integrity of spent fuel, comprising: A gas collection container provided above the fuel assembly in the storage container for storing spent fuel and subjected to gamma ray shielding for collecting fission products generated by fission;
A target made of a substance that generates neutrons by photonuclear reaction provided at a height equal to or lower than the height of the spent fuel;
An X-ray generator provided outside the storage container;
At least one gamma ray detector provided outside the storage container;
By irradiating the spent fuel with neutrons generated when the X-rays generated by the X-ray generator collide with the target, the spent fuel is caused to undergo fission, and the generated gamma rays are detected by the gamma ray detector. Means for counting;
Means for detecting an abnormality of spent fuel based on a count value by the gamma ray detector;
A device for evaluating the integrity of spent fuel, comprising:   The gas collection container has an opening facing the spent fuel, and a portion of the gas collection container facing the gamma ray detector of the gamma ray shielding is configured to be thin. The spent fuel integrity evaluation apparatus according to claim 5.

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