JP2703409B2 - Radioactivity measurement method - Google Patents

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 method for measuring radioactivity which is most suitable for measuring the radioactivity of a sample such as waste during operation or accompanying dismantling of a nuclear reactor, particularly a non-homogeneous large sample.

[0003]

2. Description of the Related Art Conventionally, as a radioactivity measuring apparatus for measuring the radioactivity of a particularly large sample, various 200-liter drum inspection apparatuses have been generally known. As shown in an example in FIG. 7, a 200-liter drum 1 is mounted on a turntable 2 and rotated, and a radiation detector 3 such as a Ge detector disposed on the side of the drum can 1 is used as a detector. Alternatively, the radioactivity is measured by placing the turntable 2 on the lifting platform 4 and relatively moving it, and cannot be applied to large-sized contaminated equipment before dismantling, and in each case, one drum can Was used to determine the average radioactivity concentration.

[0004]

However, for wastes and the like generated in the future due to the dismantling of a nuclear reactor, large-sized equipment (sample) that cannot be accommodated in the drum 1 may be included. It is anticipated that it may be necessary to evaluate the radioactivity of such samples without dismantling the sample from the viewpoint of speeding up measurement, reducing processing costs, or reducing exposure. It is expected to be.

[0005] In such a case, when treating and disposing of waste after measurement, from the viewpoint of safety evaluation, not the average concentration of the entire equipment (sample body) but the local concentration of radioactivity per weight is regarded as a problem. In some cases, conventional techniques cannot cope with this point of view.

Further, as the size of the sample to be measured increases, the influence of the heterogeneity of the measurement on the evaluation accuracy is expected to increase, and the development of a new radioactivity measurement method is required. It was the current situation.

[0007] In view of the above, the present invention enables the evaluation of the radioactivity concentration in a fragmented state without decomposing a non-homogeneous and large sample, and the improvement of the measurement accuracy. It is intended to provide a measuring method. [Configuration of the invention]

[0008]

In order to achieve the above object, a method for measuring radioactivity according to the present invention comprises dividing a sample containing radioactivity emitting two or more types of γ-rays into a plurality of blocks. Γ obtained by measuring γ-rays emitted from the radioactivity contained in the block at a plurality of measurement positions with one radiation detector
A function having the photopeak count rate of the radioactivity contained in each block based on the line spectrum, the amount of radioactivity contained in the block, the distance from one of the plurality of measurement positions, and the density of each block. The photoelectric peak count rate obtained by summing the photoelectric peak count rates of the respective blocks in a plurality of blocks is represented as the photoelectric peak count rate of the sample at the one measurement position, and the The amount of radioactivity and the density are obtained by measuring a photoelectric peak count rate and comparing the measured function with the function.

When only one kind of γ-ray is emitted from the radioactivity contained in the sample, the γ-ray spectrum obtained by simultaneously measuring with a plurality of radiation detectors is used. The radioactivity and the density of the sample can be determined in advance.

[0010]

According to the present invention constructed as described above, the radioactivity concentration for each unit of measurement can be accurately measured without physically decomposing the sample to be measured and also for a non-homogeneous sample. can do.

[0011]

Embodiments of the present invention will be described below with reference to the drawings.

FIGS. 1 and 2 show an example of a method for measuring radioactivity when one radiation detector 3 is used and N measurement points (measurement positions) are taken.

That is, the γ-rays in the radioactivity of the test object (waste) 5 to be measured, which is assumed to be a rectangular parallelepiped, is transmitted to the detector 3 by a radiation detector 3 such as a Ge detector capable of measuring energy. While moving relatively, and dividing into N (N = 5) sections for measurement. Assuming that the radioactivity of each section is A _i and the density is ρ _i , the photoelectric peak count rate C _nk at each measurement point is given by the following equation 1.

[0014]

(Equation 1) Here, n means the measurement position (maximum N), i means the measurement division position (maximum I), and k means the number (maximum K) of emitted γ-rays due to the same decay. E _k is the k-th emitted γ-ray energy, μ (E _k ) is the mass absorption coefficient for γ-ray energy, and there is no significant difference among substances. When each block (division position) in the passage distance from the measurement position n to the division position i is represented as j separately from i, the passage distance for each block from the measurement position n to the division position i is represented as t _ijn. , And r _in
Similarly, it is the distance from the measurement position to the division position i.

Here, when the object to be measured is a nuclide such as Co-60 or Eu-152 which emits a plurality of γ-rays, the value of k is about 2 to 5 each.

In this case, the above equation 1 is an N × K simultaneous equation, and the relationship between each photoelectric peak count rate C _i and the measurement point n is as shown in FIG.

As shown in FIG. 3, when the radiation detector 3 similar to the above is disposed on both sides of the sample body 5 and the radioactivity is measured while moving the sample body 5, the above equation is obtained. Is the sum of several m (maximum M) of the radiation detectors 3,

(Equation 2) Holds for each measurement point and detector over N × K × M.

The photoelectric peak count rate C _nkm is detected as an energy peak of γ-rays due to the photoelectric effect, as shown in FIG. FIG. 4 also shows scattered radiation due to Compton scattering, which is one of the interaction phenomena between γ-rays and a substance.

A similar formula is _applied to the scattered radiation count rate B _nkm for each emitted radiation shown in FIG.

(Equation 3)

The total photopeak count rate C _n when using a radiation detector 3 that cannot detect energy, such as a plastic scintillation detector.
Is

(Equation 4) Equation 4 holds with the product M × N of the number M of the detectors 3 and the measurement points N, and is obtained as data.

The evaluation of the radioactivity concentration is performed by obtaining M + N unknowns, A _i and ρ _i , respectively, using the above data and converting them into the radioactivity concentration a _i by the following equation (5). It is desirable that I be as large as possible in order to improve the measurement accuracy. As a result, the number of detectors M
In addition, since the product of the measurement points N is equal to or greater than the unknown 2 × I, the measurement time and the apparatus cost increase. Therefore, in the operation of these equations, in order to reduce the number of measurement points or detectors as _much as possible, it is necessary to use a plurality of _pieces of information obtained by the same measurement, that is, the scattered radiation count rate B _nkm and the photoelectric peak count rate C _nkm in combination. desirable. In this case, it is necessary to combine Equation 2 or Equation 3 depending on the type of radionuclide contained in a plurality of test specimens. Assuming that one detector (M = 1) and the measurement point N = I, if the nuclide to be measured emits only one gamma ray such as Cs-137, the expression 1 Since a solution cannot be obtained only by using the above, the solution is obtained by the information (relational expression) of Expressions 2 and 3 when a plurality of radiation detectors 3 are used. In addition, Eu-
For those having a very large amount of measured γ-rays, such as 152, it is difficult to set the scattered radiation area.
And the information of Equation 2 is preferably applied.

[0022]

[Number 5] _{a i = A i / ρ i} V i ...... ( Equation 5) where, is the volume of V _i is the measuring section.

Further, as for t _ijnm and r _inm, it is _assumed that the radioactivity exists in each measurement section at one point and exists at the same position, and is given as constants in advance.
The condition where this element is not very useful for measurement is when the distance between the measurement container and the detection institution is relatively large.

As a method of solving the above series of nonlinear equations, for example, there is a method based on a repetitive operation by the least square method.

FIG. 5 shows an example of a radioactivity measuring apparatus suitable for the above radioactivity measurement.

That is, G is provided on both sides of the rectangular
A radiation detector 3 such as an e-detector is disposed, and a sample body 5 such as waste is set on a measurement cart 6. The measurement cart 6 is configured to repeatedly move and stop in a step-like manner between the radiation detectors 3 via a driving device and a position detector 7, and each stop position is used as a measurement point and the radiation detector 3 A γ-ray spectrum is measured. Then, the calculator calculates the photoelectric peak count rate and the diffuse ray (scattered ray) count rate from the γ-ray spectrum information at each measurement point,
According to the above procedure, the radioactivity concentration is calculated for each measurement section.

As the radiation detector 3, in addition to a Ge detector as a detector capable of measuring energy, a NaI detector is a representative of which energy measurement is not possible, and a plastic scintillation detector is representative. No.

As an example, a Co-60 point source is installed at each position from the center position to the surface of a 100 cm cubic test specimen (metal having a bulk density of 1) by the above-mentioned apparatus, and 5 points in the lateral direction of the test specimen. FIG. 6 shows an analysis result obtained by measuring a γ-ray spectrum with a Ge detector. Co-60 is
Since two gamma rays with almost the same intensity are emitted, a total of 1
Zero data was obtained, and a total of 10 data of the density and the amount of radioactivity divided into 5 were solved using Equation 1, and compared with actual measured values as the total value of radioactivity. as a result,
Simply, from the photoelectric peak count rate of each measurement point, it was found that the accuracy was improved by a source position closer to one digit compared to the measurement data obtained when the radioactivity was uniformly distributed throughout the sample.

[0029]

Since the present invention has the above configuration,
With respect to a large test sample having non-uniform contamination, it is possible to measure the radiographic concentration subdivided without actually dividing the sample, thereby enabling accurate evaluation of local contamination. Further, there is an effect that the measurement accuracy of a large measurement sample having non-uniform contamination can be improved at the same time.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of counting rate measurement when one detector is used.

FIG. 2 is a graph showing a relationship between a counting rate and measurement points corresponding to FIG.

FIG. 3 is a plan view of counting rate measurement when two detectors are used.

FIG. 4 is a graph showing a relationship between a counting rate and γ-ray energy.

FIG. 5 is a perspective view of a measuring apparatus optimal for use in the measuring method of the present invention.

FIG. 6 is a graph showing an example of an analysis result by the method of the present invention.

FIG. 7 is a perspective view showing an example of a conventional radioactivity measuring device.

[Explanation of symbols]

 3 Radiation detector 5 Sample body (measurement target, waste)

Claims (2)

(57) [Claims] 1. A sample containing radioactivity that emits two or more types of γ-rays is divided into a plurality of blocks, and γ-rays emitted from the radioactivity contained in each block are converted into one unit at a plurality of measurement positions. Based on the γ-ray spectrum obtained by measuring with the radiation detector, the photoelectric peak count rate of the radioactivity contained in each block,
Expressed by a function having the amount of radioactivity contained in the block, the distance from one measurement position of the plurality of measurement positions, and the density of each block, the photoelectric peak count rate of each block was summed over the plurality of blocks. The photoelectric peak count rate is expressed as the photoelectric peak count rate of the sample at the one measurement position, and the photoelectric peak count rate of the sample at a plurality of measurement positions is measured and compared with the function to obtain the emission peak count. A method for measuring radioactivity, comprising determining the capacity and the density. 2. The method according to claim 1, wherein the sample containing radioactivity emitting one kind of γ-ray is divided into a plurality of blocks, and the γ-ray emitted from the radioactivity contained in each block is divided into a plurality of radiations at a plurality of measurement positions. Based on the γ-ray spectrum obtained by simultaneous measurement with the detector, the photoelectric peak count rate of the radioactivity contained in each block, the amount of radioactivity contained in that block, and the distance from one measurement position of a plurality of measurement positions And, represented by a function having the density of each block, the photoelectric peak count rate of the sum of the photoelectric peak count rate of each block in a plurality of blocks as a photoelectric peak count rate of the sample at the one measurement position. Expressing, measuring the photoelectric peak count rate of the sample body at a plurality of measurement positions, and comparing the function with the function to obtain the radioactivity amount and the density. Method.