JPS6128954B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention non-destructively measures the content of fissile materials such as uranium and plutonium in the vitrified high-level liquid waste generated in nuclear material handling facilities such as nuclear fuel reprocessing plants in a vitrified canister. The present invention relates to a method and apparatus capable of performing the same. At spent nuclear fuel reprocessing plants, useful fissile materials such as uranium and plutonium are recovered and reused, but unnecessary fission products are treated and disposed of as waste. The waste from this reprocessing plant has extremely high radioactivity levels, and it is thought to contain trace amounts of uranium,
There are various difficulties involved in quantifying fissile materials such as plutonium by radiation measurements.
In particular, the vitrified material, which is the most downstream product of the reprocessing process, is stored in a canister (for example, approximately 30 cm in diameter and approximately 30 cm in height).
Cylindrical stainless steel container (150cm, approximately 10mm thick)
A method for non-destructive measurement of the filled state has not yet been established. Fissile material emits alpha rays, gamma rays, and neutron rays, but since alpha rays are absorbed within the canister and are not emitted to the outside at all, it is impossible to detect alpha rays and estimate the amount of fissile material. It's impossible. Neutron beams can be detected outside the canister, but if neutron poisons such as gadolinium are mixed in the waste, errors will increase due to the influence of gadolinium, making it difficult to accurately measure neutron beams. It becomes difficult to estimate the amount of fissile material. In the case of gamma rays, fission products, which make up the majority of waste, also emit relatively high-energy gamma rays, so in such a high-level, high-energy gamma-ray atmosphere, a trace amount of fissile material It is extremely difficult to measure the low-level, relatively low-energy gamma rays emitted by ordinary gamma-ray detectors due to detection sensitivity. For example, semiconductor detectors using Ge (germanium), which is the mainstay of today's gamma-ray spectroscopy, have high resolution and relatively good detection sensitivity, but when the intensity of high-energy gamma rays is strong, Line detection sensitivity decreases. This is because, as shown in FIG. 1, while the γ-ray passes through the Ge semiconductor crystal, the influence of Compton scattering forms a plateau region called the Compton tail C on the low energy side of the γ-ray spectrum. In the figure, P
is the photoelectric peak. As the number of gamma ray photons with different energies increases, the Compton tails are superimposed, increasing the continuum spectrum on the low energy side. Since the gamma ray energy is determined from the photoelectric peak, on the low energy side the photoelectric peak is superimposed on the Compton tail. Now, as shown in Figure 2, the γ-ray energy is E
Assume that the count value Ac of the Compton tail portion and the count value Ap of the photoelectric peak of low-energy γ-rays are superimposed in the range from to E+ΔE. The standard deviation σ value, which is commonly used as a statistical error in the count value, is given by the square root of the count value, so if the Compton-Tail count value is Ac, the σ value will be √, and if the condition Ap≦√ is met, the photovoltaic Identification of peak Ap becomes impossible. On the other hand, the vitrification of high-level waste liquid contains a large amount of fission products that emit high-energy γ-rays, and on the low-energy side, the Compton tail suddenly rises as shown in Figure 3. It shows a spectrum like this. The low energy γ of less than 500 KeV emitted by fissile material under these conditions
When measuring lines, there is a limit to detection sensitivity due to the condition Ap≦√. Therefore, the γ-ray intensity of fissile material is much weaker than that of fission products.
It is impossible to detect with Ge semiconductor detectors. The present invention has been made in view of this problem, and provides a method for measuring low-level, low-energy γ-rays in an atmosphere of high-level, high-energy γ-rays produced by an object to be measured such as a vitrified body of high-level waste liquid. The gamma rays focused by the collimator are separated by a crystal and detected by a gamma ray detector, and the object to be measured is moved relative to the collimator, thereby making it possible to scan the entire surface of the object to be measured. By doing so, the present invention aims to provide a method and apparatus that can non-destructively measure fissile material contained in the vitrified high-level liquid waste generated at nuclear fuel reprocessing plants, etc. while the canister is still filled. . Gamma rays emitted from fissile materials have a relatively low energy of less than 500 KeV. The gamma ray energies of typical nuclides are shown in the table below.

[Table] For Pu-241, the amount of plutonium present in the canister can be estimated by measuring 208KeV gamma rays, and for Pu-239, it is 129KeV,
For 375KeV, 431.8KeV, and Pu-240
By measuring 160.3KeV gamma rays, the abundance of each nuclide type can be estimated. Low-energy gamma rays are analyzed and measured using a crystal monochromator, making use of the electromagnetic properties of gamma rays. The relationship between the wavelength λ of the γ-ray and the incident energy E of the γ-ray is expressed by the following equation. λ=hc/E where h is Planck's constant (6.62×10 ^-27 erg.
sec), c represents the speed of light. Further, the γ-rays having the wavelength λ are scattered by the crystal monochromator by an angle θ given by Bragg's law as shown below. 2d·sinθ=nλ Here, d is the distance between lattice planes, and n is the order of scattering. An example of the spectrum measured by this crystal monochromator is shown in Figure 4, and it shows that a spectrum is clearer on the low energy side than on the high energy side, making it suitable for measuring gamma rays below about 500 KeV. There is. Hereinafter, one embodiment of the present invention will be described based on the drawings. FIG. 5 is a schematic diagram of a curved crystal monochromator, which is a gamma-ray crystal spectrometer, viewed from above, and a canister 1 filled with vitrified material is placed on a turntable that is driven vertically and rotationally. 2 is a shielding wall, and a collimator 3 is attached to an opening of this shielding wall 2. Reference numeral 4 denotes a curved crystal which separates the γ-rays incident through the collimator 3.
A gamma ray detector 5 detects gamma rays separated by a curved crystal, and is movable according to the spectroscopic angle θ. Next, the operation of the curved crystal monochromator configured as described above will be explained. First, the vitrified material-filled canister 1 is placed on a turntable and set at a predetermined position. The gamma rays emitted from the canister 1 are focused by a collimator 3 and separated into spectra by a curved crystal 4. wavelength λ
γ-rays are scattered by the angle θ shown by the crystal part Bragg's law, so the angle θ from the traveling direction of the incident γ-rays is
The gamma ray detector 5 is set in a direction shifted by the same amount, and detects the separated gamma rays of wavelength λ. After measuring gamma rays in the wavelength range to be detected, the turntable is driven up and down and/or rotationally to change the surface position of the canister, and the same measurement is performed again. In this way, the entire surface of the canister is scanned to perform spectroscopic measurement of gamma rays. FIG. 6 shows a double monochromator as a modification of the present invention, and the same parts as in FIG. 5 are indicated by the same symbols. In this method, γ-rays are scattered twice in the crystal part, and when the scattering angle is small, the incident γ-rays are
Suitable for reducing the influence of radiation or interference scattering. As is clear from the above description, according to the present invention, low-level, low-energy γ-rays can be detected with high sensitivity in an atmosphere of high-level, high-energy γ-rays. It becomes possible to non-destructively measure the content of fissile materials such as uranium and plutonium in a canister filled with vitrified material.

[Brief explanation of the drawing]

Figures 1, 2, and 3 are graphs showing an example of a γ-ray spectrum obtained by a Ge semiconductor detector, Figure 4 is a graph showing an example of a γ-ray spectrum obtained by a crystal spectrometer, and Figure 5 is a graph showing an example of a γ-ray spectrum obtained by a crystal spectrometer. FIG. 6 is a schematic diagram of a curved crystal monochromator showing one embodiment, and FIG. 6 is a schematic diagram of a double monochromator showing a modification of the present invention. 1... Canister, 2... Shielding wall, 3... Collimator, 4... Curved crystal, 5... γ-ray detector.

Claims (1)

[Claims] 1. A method for measuring low-level, low-energy γ-rays in an atmosphere of high-level, high-energy γ-rays, in which γ-rays focused by a collimator are separated into spectra by a crystal and detected by a γ-ray detector; . A method for measuring low energy gamma rays, characterized in that the entire surface of the object to be measured can be scanned by moving the object to be measured relative to the collimator. 2. In an apparatus for non-destructively measuring the amount of fissile materials such as uranium and plutonium contained in the vitrified material of high-level waste liquid when the container is filled with the vitrified material, the apparatus includes a collimator, and a device after passing through the collimator. a γ-crystal spectrometer consisting of a crystal for dispersing γ-rays and a γ-ray detector for detecting the separated γ-rays; and a rotating device for placing an object to be measured placed in front of the collimator. A method for measuring low-energy γ-rays, comprising: a support base that can be raised and lowered; and a drive mechanism for rotating and driving the support base up and down.