JPH04269697A - Non-destructive inspection device for reactor fuel rod - Google Patents

Abstract

PURPOSE:To obtain a non-destructive inspection device for reactor fuel rods cable of accurately detecting effective fissile material amount even in case of change in the isotope ratio in reactor fuel rods made of mixed oxide. CONSTITUTION:It consists of a means 24 to irradiate neutrons to fuel rods, a means 23 to measure the fuel rod axial distribution of gamma ray intensity emitted from the fuel rods before the neutron irradiation, a measuring means 25 which has the same constitution as the gamma ray intensity measuring means 23 and measures the fuel rod axial distribution of delayed gamma ray emitted from the fuel rods after the neutron irradiation, a means 33 to calculate the net delayed gamma ray intensity distribution by subtracting the axial gamma ray distribution data before the neutron irradiation from that after the neutron irradiation, and a detection means 34 to detect the effective fissile material amount and the distribution change from the calculated net delayed gamma ray intensity distribution.

Description

[Detailed description of the invention]

[Object of the invention]

0002

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-destructive testing device for nuclear reactor fuel rods used in nuclear reactor fuel rod processing facilities and the like.

0003

[Prior Art] In general, non-destructive testing equipment for nuclear reactor fuel rods is used in nuclear reactor fuel rod processing facilities to determine the presence of abnormal pellets and to determine the total amount of fissile material per fuel rod. used for.

FIG. 5 shows an example of such a conventional non-destructive testing device for nuclear reactor fuel rods.
A neutron source 2 such as Cf-252 (Californium-252) is arranged, and around the neutron source 2, in order from the inside, a ZrH2 (zirconium hydride) layer 3, a Pb (
A lead) layer 4, a D2O (heavy water)/WEP (hydrated polyester) layer 5, a C (graphite) layer 6, a CH2 (paraffin or polyethylene) layer 7, and a WEP+Pb layer 8 are formed near the neutron source 2. are fuel rod through holes 9a,
9b, a He-4 neutron detector 10 is disposed at the bottom. Further, a NaI detector (sodium iodide scintillation detector) 11 is arranged on the side of the housing 1, and a W (tungsten) layer 12 is formed around this detector 11. It is covered with a Pb layer 13.

[0005] By inserting the fuel rod into the fuel rod through hole 9a, neutrons from the neutron source 2 are irradiated to induce fission in the fissile material in the fuel rod, and the NaI detector 11 detects the fission caused by the fission. Next, the fuel rod is inserted into the fuel rod through hole 9b, irradiated with neutrons, and the He-4 neutron detector 10 measures the distribution of delayed gamma ray intensity in the fuel rod axis direction. The average intensity of the area is measured, and based on the obtained results, it is possible to determine whether abnormal pellets are mixed in, and whether the fuel rod 1
Find the true total amount of fissile material.

The apparatus shown in FIG. 5 is an apparatus for measuring delayed gamma rays and fission-prompted neutrons, but it uses a He-3 neutron detector as a neutron detector, and the material of the neutron moderator is There are also devices that measure delayed neutrons instead of prompt neutrons.

However, in conventional non-destructive testing equipment for nuclear reactor fuel rods, the main isotopic compositions of the test objects are U-235 and U-238, such as the uranium oxide fuel used in current light water reactors. There is no problem with simple fuel (one type of fissile material U-235), but mixed fuel of Pu oxide and U oxide used in fast breeder reactors, new converter reactors, etc. In the case of oxides, there is a problem that the gamma ray intensity emitted by Pu itself is large, and when the Pu isotope ratio changes, the gamma ray intensity changes and measurement accuracy deteriorates.

[0008]

[Problems to be Solved by the Invention] Conventional non-destructive inspection equipment for nuclear reactor fuel rods detects gamma rays emitted from the fuel rods when measuring delayed gamma rays and delayed gamma rays emitted upon neutron irradiation.
Since the rays are measured in a superimposed manner, if the isotope ratio of the mixed oxide fuel rod, which is a mixture of uranium and plutonium, changes, the intensity of the γ-rays emitted from the fuel rod will change, resulting in delayed gamma-ray measurement. γ-ray intensity is affected, reducing measurement accuracy.

The present invention has been made in response to the above-mentioned conventional situation, and it is an object of the present invention to accurately test the effective amount of fissile material even if the isotope ratio of a mixed oxide fuel rod for a nuclear reactor changes. The purpose of the present invention is to provide a non-destructive inspection device for nuclear reactor fuel rods that can perform the following steps. [Structure of the invention]

0010

[Means for Solving the Problems] In order to solve the above-mentioned problems, the non-destructive testing device for nuclear reactor control rods according to the present invention includes means for irradiating fuel rods with neutrons, and means for measuring the fuel rod axial distribution of gamma ray intensity emitted from the fuel rod; and the same gamma ray intensity measuring means for measuring the fuel rod axial distribution of delayed gamma rays emitted from the fuel rod after neutron irradiation. and means for subtracting γ-ray axial distribution data before neutron irradiation from γ-ray axial distribution data after neutron irradiation to obtain a net delayed γ-ray intensity distribution;
It has a detection means for detecting the amount and distribution change of effective fissile material from the determined net delayed gamma ray intensity distribution.

Furthermore, in order to solve the above-mentioned problems, the non-destructive testing device for nuclear reactor control rods according to the present invention is such that the means for irradiating the fuel rods with neutrons includes a neutron source such as californium-252. , means for measuring the axial distribution of γ-ray intensity emitted from the fuel rod before neutron irradiation, and means for measuring the axial distribution of delayed γ-ray intensity emitted from the fuel rod after neutron irradiation. is a gamma ray detector having the same shape and detection performance, and both gamma ray detectors are installed at symmetrical positions with respect to the neutron source that irradiates neutrons, and the gamma ray detector is located between the neutron source and both gamma ray detectors. At least a neutron moderator, a neutron shield, and a gamma ray shield are arranged approximately symmetrically about the neutron source.

0012

[Operation] In the non-destructive inspection device for nuclear reactor fuel rods of the present invention, the gamma ray intensity distribution before and after neutron irradiation is measured by the gamma ray intensity measuring means before and after neutron irradiation, and the gamma ray intensity distribution after neutron irradiation is measured. Subtract the distribution data before neutron irradiation from the data to obtain the net delayed gamma ray intensity distribution data. Therefore, even if the isotope ratio of the fuel rod changes and the gamma ray intensity from the fuel rod changes, this value is subtracted, so it is not affected by the delayed gamma ray measurement.

Furthermore, the gamma ray intensity distribution measuring means before neutron irradiation is a measuring means having the same configuration and the same detection performance as the means for measuring the fuel rod axial distribution of delayed gamma rays emitted from the fuel rod after neutron irradiation. Therefore, accurate measurements can be made without the need for complicated corrections.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the non-destructive inspection apparatus for nuclear reactor fuel rods according to the present invention will be described below with reference to the accompanying drawings.

FIG. 1 shows an example of a non-destructive inspection apparatus for nuclear reactor fuel rods according to the present invention. This non-destructive testing device 20 includes a transmitted gamma ray detector 21 as a pellet detecting means for detecting the stack length of fuel pellets as fuel elements filled in fuel rods (not shown) and gaps between fuel pellets, and a transmitted gamma ray detector 21 as a pellet detecting means for detecting a gap between fuel pellets, and a reactor fuel A γ-ray spectrum detection unit 22 as a measurement means for measuring the isotope ratio of the rod, a delayed γ-ray background detection unit 23 as a means for measuring the axial distribution of γ-ray intensity in the fuel rod before irradiation with neutrons, A neutron irradiation section 24, which is a means for irradiating a nuclear reactor fuel rod with neutrons, and a delayed gamma ray detection section 25, which is a means for measuring the distribution of delayed gamma rays in the fuel rod axial direction after neutron irradiation, are sequentially arranged in parallel. The delayed gamma ray detection section 25 is configured with a gamma ray detector having the same configuration as the delayed gamma ray background detection section 23, has the same detection performance, and does not require complicated correction or calibration. It is now possible to measure with high accuracy without having to do anything. The reactor fuel rods are detected by each detection section 21, 22, 23 of the non-destructive inspection device 20.
, the neutron irradiation section 24 and the detection section 25 are moved and guided in the axial direction at a constant speed by a fuel rod drive system (not shown).

When a nuclear reactor fuel rod is guided here by a fuel rod drive system (not shown), the transmitted gamma ray detection section 21 as a pellet detection means detects, for example, cesium-137 (1
37Cs) The fuel rod is irradiated with gamma rays by an external gamma ray source,
A gamma ray detector measures the axial distribution of gamma ray transmission intensity over the entire length of the fuel rod. The stack length of the fuel pellets, which are the fuel elements, and the gap between the pellets can be determined based on the axial distribution shape of the γ-ray transmission intensity.

Next, the reactor fuel rod is guided to the gamma ray spectrum detection section 22 and passes through the detection section 22 . γ
The ray spectrum detection unit 22 uses a Ge detector to detect the γ rays emitted from the fuel rods, paying attention to the fact that each nuclide in the fuel element filled in the fuel rod emits γ rays with energy specific to the energy.

Next, the gamma ray energy spectrum of the gamma ray detection signal detected by the Ge detector is analyzed by a multiple wave height analyzer (not shown), and the various gamma ray intensities emitted from each nuclide are analyzed. and find the amount of each nuclide.

[0019] Each nuclide filled in a nuclear reactor fuel rod emits gamma rays with energy specific to the nuclide. An example of typical γ-ray energy of each nuclide is expressed as follows.

Plutonium-238 (238Pu)
152keV Plutonium-239 (239Pu) 414ke
V Plutonium-240 (240Pu) 642k
eV plutonium-241 (241Pu) 208
keV Americium-241 (241Am)
60keV uranium-235 (235U)
186keV uranium-238 (238U)
1001 keV Here, the γ-ray energy spectrum is schematically shown as shown in FIG.

In the γ-ray energy spectrum line A, there are many γ-rays.
When a peak of the ray count appears, and among these peak counts, the area of the peak of the γ-ray count that appears at the γ-ray energy position of the nuclide to be determined is calculated to determine the γ-ray intensity. is proportional to the amount of

That is, by dividing this γ-ray intensity by the γ-ray emission rate, γ-ray absorption rate, and γ-ray detection efficiency of the nuclide, the amount of the nuclide can be determined.

Target nuclide, for example 238Pu,
239Pu, 240Pu, 241Pu, 235U
, 238U, and 241Am, and by calculating their ratios, the isotope ratio of Pu, etc. can be calculated.

The fuel rod that has passed through the gamma ray spectrum detection section 22 is detected by a delayed gamma ray background detection section 23 which is a means for measuring the distribution of gamma ray intensity in the axial direction of the fuel rod before irradiation with neutrons.
and passes through the detection section 23. In this detection unit 23, a γ-ray detector such as a BGO (bismuth-germanium-oxygen compound) scintillation detector or a NaI-TI scintillation detector detects the distribution of the γ-ray intensity emitted from the fuel rod in the axial direction of the fuel rod. is measured.

The fuel rod that has passed through the delayed gamma ray background detection section 23 is exposed to a neutron irradiation means such as Cf-
Neutron source 26 such as 252 (Californium-252)
The fuel rods are guided to the neutron irradiation section 24 in which the fuel rods are stored, and pass through the fuel rod guide tube 27 loaded in the neutron irradiation section 24 .

The neutron irradiation unit 24, which serves as a means for irradiating fuel rods with neutrons, has an internal structure as shown in FIG.
A neutron source 26 of f-252 is installed, and this neutron source 2
6, a neutron moderator 29 made of polyethylene or the like, a γ-ray shield 30 made of Pb,
A neutron shielding body 31 made of boron-containing paraffin, etc.
γ-ray shielding bodies 32 made of Pb are sequentially arranged. A gamma ray shield 32 is lined within the housing 28 . Neutron moderator 2
9 includes ZrH2, Be (beryllium), heavy water, graphite, etc. instead of polyethylene, and the neutron shield 31
In place of boron-containing paraffin, boron-containing polyester, lithium-containing polyester, or paraffin may be used. The neutron shielding body 31 is arranged outside the neutron moderating body 29, but the neutron moderating body 29, the neutron shielding body 31
The arrangement of the γ-ray shields 30 and 32 is not limited to that shown in FIG.

The internal structure of the neutron irradiation section 24 is formed symmetrically around the Cf-252 neutron source 26, and the fuel rods passing through the fuel rod guide tube 27 in this neutron irradiation section 24 receive neutrons. Upon irradiation, the fissile materials 235U, 239Pu, and 241Pu in the fuel rod absorb neutrons, cause a fission reaction, and emit delayed gamma rays with a time delay after fission.

The fuel rod that has been irradiated with neutrons is then guided to a delayed gamma ray detection section 25, which is a means for measuring the distribution of delayed gamma rays in the axial direction of the fuel rod after neutron irradiation. This detection section 2
5 is composed of a gamma ray detector having the same configuration as the delayed gamma ray background detection section 23. Both gamma ray detectors have the same configuration and detection performance.
The fuel rod axial distribution of linear strength is measured.

FIG. 4 shows means 33 for determining the net delayed gamma ray intensity distribution, and this means converts the pre-neutron irradiation gamma ray axial distribution data measured by the delayed gamma ray background detection section 23 into delayed gamma rays. This is subtracted from the post-neutron irradiation γ-ray axial distribution data measured by the detection unit 25 to obtain net delayed γ-ray axial distribution data. This data value is calibrated by the effective fissile material amount detection means 34 using the delayed gamma ray intensity determined in advance and the calibration constant for the effective amount of fissile material to determine the amount of effective fissile material. Further, this detection means 34 detects a distribution change from a change point in the axial distribution of the amount of effective fissile material, and detects an abnormal pellet that has been mixed into the fuel rod by mistake.

[0030]

Effects of the Invention As explained above, according to the non-destructive testing device for nuclear reactor fuel rods according to the present invention, the method is the same as the means for measuring the distribution of delayed gamma ray intensity in the fuel rod axial direction after neutron irradiation. The detection means measures the fuel rod axial distribution of gamma ray intensity emitted from the fuel rod before neutron irradiation, and subtracts this data from the data after neutron irradiation to obtain the net delayed gamma ray intensity. Even if the isotopic ratio of the fuel rod changes and the gamma rays from the fuel rod change, the delayed gamma rays can be measured with high precision, and therefore the amount of effective fissile material and changes in its distribution can be measured with high precision.

Furthermore, the two gamma ray detectors before and after neutron irradiation have the same shape and detection performance, and are placed in symmetrical positions with the neutron source as the center. Structures such as a neutron moderator, neutron shield, and gamma ray shield between the two are also arranged symmetrically, so that background gamma rays caused by neutron sources such as Cf-252 can be detected between the two gamma ray detectors. The frequency of incidence on the neutron source becomes the same, and when subtracting the before and after data, background gamma rays caused by the neutron source do not remain in the net delayed gamma ray data, allowing highly accurate measurements.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of a non-destructive testing device for nuclear reactor fuel rods according to the present invention.

FIG. 2 is a diagram showing the relationship between the γ-ray energy of nuclides filled in nuclear reactor fuel rods and the γ-ray count.

FIG. 3 is a diagram showing the internal structure of a neutron irradiation section corresponding to an embodiment of the non-destructive testing device for nuclear reactor fuel rods of the present invention.

FIG. 4 is a diagram showing a method for determining the effective amount of fissile material by combining gamma rays before neutron irradiation and gamma rays after neutron irradiation.

FIG. 5 is a diagram showing a conventional non-destructive inspection device for nuclear reactor fuel rods.

[Explanation of symbols]

20 Non-destructive inspection device for nuclear reactor fuel rods 21 Transmitted gamma ray detection section 22 Gamma ray spectrum detection section 23 Delayed gamma ray background detection section (means for measuring the fuel rod axial distribution of gamma ray intensity before neutron irradiation) 24 Neutron irradiation section (neutron irradiation means) 25 Late gamma ray detection section (fuel rod axial distribution measuring means of delayed gamma rays after neutron irradiation) 26 Neutron source 27 Fuel rod guide tube 28 Housing 29 Neutron moderator 30, 32 γ-ray shield 31 Neutron shield

Claims (2)

[Claims] 1. A means for irradiating a fuel rod with neutrons, a means for measuring a fuel rod axial distribution of gamma ray intensity emitted from the fuel rod before neutron irradiation, and a means for measuring γ-ray intensity emitted from the fuel rod after neutron irradiation. A measuring means having the same configuration as the gamma ray intensity measuring means for measuring the fuel rod axial distribution of delayed gamma rays, and a measuring means having the same configuration as the gamma ray intensity measuring means described above, and a measuring means that subtracts the gamma ray axial distribution data before neutron irradiation from the gamma ray axial distribution data after neutron irradiation, and calculates the net value. A fuel rod for a nuclear reactor, characterized in that it has a means for determining the delayed gamma ray intensity distribution of the net delayed gamma ray intensity distribution, and a detection means for detecting the amount and distribution change of effective fissile material from the determined net delayed gamma ray intensity distribution. Non-destructive testing equipment. 2. The means for irradiating the fuel rod with neutrons includes a neutron source such as Californium-252, and means for measuring the axial distribution of gamma ray intensity emitted from the fuel rod before irradiating the fuel rod with neutrons. The means for measuring the fuel rod axial distribution of delayed gamma ray intensity emitted from the fuel rod after neutron irradiation is gamma ray detectors having the same shape and detection performance. A neutron source to be irradiated is provided at a symmetrical position with respect to the neutron source as the center, and at least a neutron moderator, a neutron shield, and a γ-ray shield are arranged approximately symmetrically with respect to the neutron source between the neutron source and both gamma ray detectors. A non-destructive inspection device for nuclear reactor fuel rods.