JPH0194284A - Quantification of radioactivity in radioactive waste packed in container - Google Patents

Abstract

PURPOSE:To quantify the radioactivity in the waste packed in a container in a non- destructive manner, by measuring emitted gamma-rays by the gamma-ray detector arranged outside the container. CONSTITUTION:A cylindrical container is divided into a plurality of slices having a predetermined thickness in the direction vertical to the center line thereof and a region dividing system for determining the radioactivity distribution thereof is set to a plurality of circles centering around the center line of the slices. From a density value preliminarily given at every circle, the detection efficiency of gamma-rays emitted from the radioactivity in the circle of a plurality of gamma-ray detectors when radioactivity is present in the circle is calculated at every gamma-ray detector with respect to each circle to be set to a detection efficiency data group. The gamma-rays emitted from the container during a time when the container rotates n-times (n: integer) without rising and falling are measured by a plurality of the gamma-ray detectors and integrated at every detector to be set to a measured data group. Radioactive quantity at every circle is operated from both data groups and the radioactive quantities of the slices at every circle are totalized to set radioactive quantity per a slice. This work is repeated at every slice and the radioactive quantities at every slices are totalized to set radioactive quantity per one container.

Description

[Detailed Description of the Invention] [Field of Industrial Application] This invention detects radioactivity in radioactive waste packed in containers (such as drums) generated at nuclear facilities, and detects the waste by using a gamma ray detector installed outside the container. This relates to a non-destructive method of quantifying gamma rays emitted from radioactivity in objects.

[Conventional technology]

A commonly used method is to quantify the radioactivity in containerized radioactive waste by measuring the gamma rays emitted from the container using a gamma ray detector installed outside the container, but this method is time-consuming for the following reasons. ■ If the radioactivity distribution in the container is uneven and unknown, the distance from the gamma ray emission point to the gamma ray detector may be different, and even if the amount of radioactivity is the same, the gamma ray detector may The responses will be different. Furthermore, if a collimator is employed between the gamma ray detector and the container, the response of the gamma ray detector will differ depending on the relative positional relationship between the collimator opening and the gamma ray emission point.

■ When the density distribution inside the container is uneven and unknown Even if the position of the gamma ray emission point is known, if the density distribution of the surrounding filling material in the container is different, the gamma ray flux that passes through the outside of the container will be affected by the absorption effect. They are different because of their differences. Therefore, the gamma ray detector response also differs. As a method to reduce quantitative errors associated with these non-uniformities, CT' (Computer Tomography)
) There is a law.

This method is disclosed, for example, in Japanese Patent Application Laid-Open No. 107188/1988, in which the container is divided into a plurality of slices Sl to Sn in the axial direction, as shown in FIG. As shown in Figure 2 (a) and (b), the area is divided into sectors or rectangles, the density distribution is first determined for each division unit, and the density distribution obtained is known and the radiation is calculated for each division unit. Find capacity. Add up this amount to determine the amount of radioactivity per slice, and repeat the same process for all Kleine.This method will be explained in detail below, but for the sake of simplicity, the density distribution will not be obtained by some method. Based on FIG. 8, the explanation will be limited to the method of determining the radioactivity distribution per null rice.

In FIG. 8, (1) represents a horizontal cross section of the container. Here, in the case of rectangular division, the case where the container is divided into six in both the i direction and the j direction in the figure is shown. (2) is a gamma ray detector installed outside the container; the figure shows an example where six units are installed. (3) is a horizontal collimator for gamma rays installed between the gamma ray detector and the container. be. The collimator in the vertical direction is not necessary for the explanation and is therefore omitted from the figure. Five sets of gamma ray detectors (2) and horizontal collimators (3) are arranged so that different parts of the cross section of the container can be seen through the collimator openings. The container is installed on a rotating/elevating mechanism and can be rotated around the central axis of the container (the center of the circle in FIG. 8), but this mechanism is not clearly shown in the figure.

Next, the measurement method required when such a division method is adopted and the reason thereof will be explained. When using the division method as shown in Figure 8, the amount of radioactivity also needs to be determined for each divided area. mutually independent gamma ray detectors (
Without measurement data from 2), it is not possible to determine the amount of radioactivity for each divided area. In the example shown in Figure 8, the number of gamma ray detectors (2) is five, so it is necessary to obtain at least eight or more mutually independent data from one gamma ray detector. Arrow the container to (4)
Every time the container is rotated by Δθ in the direction, the gamma ray detector (2) measures gamma rays from the container for a certain period of time T.6 Normally Δθ
are taken at equal intervals, so in this case 860' +8 ==
It becomes 45°.

The set of measurement data obtained by the gamma ray detector (2) in this manner is assumed to be R(k, w). Here, k is a number for distinguishing between multiple gamma ray detectors (2) (in this example, 1 to 5), and w is a number for distinguishing the rotational position of the container (for example, in this example, w= 1-8), therefore, R
(k, w) is the measurement data (for example, counting rate) when the container is rotated by wXΔθ and the gamma ray detector measures for 1 second.

Next, a method for determining the amount of radioactivity for each divided region from R(k, w) will be described. We decided to specify the divided areas using the divided area numbers in the i direction and j direction in Fig. 8, and this is expressed as (i・j
) is the amount of radioactivity in the 0-division area (1*j) written as Q(
i, j), measurement data cella) R(k, w)
Therefore, the task is to find Q(Lj) for the entire area within the circular cross section of FIG.

Q(i, j) is obtained by repeatedly calculating and modifying its value so as to minimize the following squared error -.

−=Σ Σ(R(k, w) −Res (k, w) )
”w=1 k−1 Res(k, w); For a certain Q(i, j) distribution, this is the calculated response value of the gamma ray detector when the container is rotated by W×Δθ. Next Find it using the formula.

f(klw;++j) ;Detection efficiency, divided area (i・j
) The response of the gamma-ray detector when the container is rotated by w×Δθ when a unit amount of radioactivity is present only at ).

The iterative calculation method is a commonly used method, so it will be omitted here.

[Problem that the invention seeks to solve]

The conventional method for quantifying radioactivity in containerized radioactive waste using the CT method is performed using the measurement and analysis methods described above. Therefore, regardless of the uniformity of the density distribution, the measurement method remains the same, and there are the following problems.

As the number of region divisions increases, the number of measurement points inevitably increases.

(2) Therefore, in order to perform measurements with a small number of gamma ray detectors, it is necessary to rotate the container every Δθ and perform measurements, which complicates the rotational drive system and its control software, which reduces both reliability and economic efficiency.

(2) If the measurement time per measurement point is constant, the increase in the number of measurement points increases the measurement time per container, reducing the throughput of the measuring device.

■ Conversely, if the measurement time per container is kept constant, the number of measurement points increases, which reduces the measurement time per measurement point, increases statistical errors in the measurement data, and deteriorates quantitative accuracy. ■ The amount of data handled increases. Due to this increase, the analysis time increases and the capacity of the computer used also increases.

This invention was made to solve the above-mentioned problems.If the density distribution inside the container satisfies certain conditions, the number of measurement points can be significantly reduced, and the rotation drive system and its control software can also be simplified. The purpose of this study is to obtain a method for quantifying radioactivity in containerized radioactive waste that can shorten measurement time or increase the accuracy of quantification, shorten the time required for analysis, and reduce computer capacity. shall be.

[Failure to solve the problem]

The method for quantifying radioactivity in radioactive waste packed in a container according to the present invention includes installing a plurality of gamma ray detectors outside the radioactive waste packed in a cylindrical container, and positioning the container relative to the gamma ray detector. Rotate the center line of the container as an axis,
By measuring the gamma rays emitted from the radioactive waste in the container by moving it up and down along the center line with the gamma ray detector, the density distribution information of the radioactive waste provided is used to detect the radioactivity in the container. In the method of quantifying radioactivity by determining the radioactivity distribution of waste, ■ Divide the above cylindrical container into multiple slices of a predetermined thickness perpendicular to the center line, ■ Ascertain the radioactivity distribution of the above-mentioned sphuis. The area division format for this purpose is a plurality of rings (including the center circle) formed by a plurality of concentric circles centered on the center line of the slice. (shall be equal to or smaller than the number), ■ Based on the density value for each ring given in advance, if radioactivity exists in the ring, the radioactivity emitted from the plurality of gamma ray detectors within the ring. The detection efficiency for gamma rays detected is calculated in advance for each ring and each gamma ray detector, and this is used as a detection efficiency data group. : Integer) Measure the gamma rays emitted from the radioactive waste inside the container while it rotates, integrate the responses of the gamma ray detectors for each detector during that time, and make this a measurement data group, ■ The above detection efficiency data group Calculate the amount of radioactivity for each ring from the measured data group, ■ Add up the amount of radioactivity for each ring of the slice to obtain the amount of radioactivity per slice, and ■ Raise and lower the cylindrical container to collect multiple pieces. The above operations are repeated for each slice divided into 2. The amount of radioactivity for each slice of the container is summed to obtain the amount of radioactivity per container.

[Effect]

This invention deals with the case where the density distribution on the plane perpendicular to the center line of the cylindrical container is distributed in a concentric ring shape, or as a special case, it is distributed in a saw shape. In this case, the detection efficiency of gamma rays from the radioactivity within the annular ring does not approximately depend on the position within the annular ring, as long as it is considered as the integrated response value of the gamma ray detector during one rotation of the container. Therefore, by making the area division shape a ring and combining this with gamma ray measurement while the container rotates once or an integral multiple thereof, the number of divided areas and the number of measurement points can be significantly reduced.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. 1st
In Figures a+b, the configurations of devices (1) to (3) are the same as the conventional one. The difference is that in the analysis, the region division shape of the container slice (1) is divided into annular shapes (8 divisions from ■ to ■ in Figure 1) by concentric circles centered on point 0 (central axis), and the container (1) rotates continuously at a constant speed n times at the same height, and multiple (in this case 5
The gamma ray detector (2) on the collimator (stand) continuously detects gamma rays from the container (1) while the container (1) rotates n times.
3).

Now, for example, in the circular area ■, there are points Pt, P, , P,
Let us consider the response of each gamma ray detector (2) when radioactivity is present. Here, Pl and Pl have the same distance from the container center axis O, and their length is a. R3 has a distance from the center axis 0 of the container, which is different from a, and is set to b. (For example a
) b. ) The responses of the kth (k=1 to 5) gamma ray detectors to the radioactivity located at P, , P, are naturally different because their geometrical arrangements are different from each other.

However, when R8 and P are each rotated once around point 0, the integral values of the responses of the gamma ray detector are P, ,
They are equal for P. This is because P, , P are equal in distance from the common point 0. For R3 a
~b, strictly speaking, each integral value of the response of the gamma ray detector when rotated as described above is P, , P.

Although this is different from the case of , if the difference between a and b is small, the difference between both responses is also small. Now, regarding the area of ■, if the difference in the size of the radius of the circles with radii R1 and R2 forming the boundary of the annular area of ■ is small, the difference in the radial position within the area of ■ is correspondingly small. Differences in the response of gamma ray detectors to radioactivity due to radiation are also reduced. (Figure 1 shows an example of division into 8 annular areas, but this is equivalent to the conventional 6 x 6 area in Figure 8 in terms of the length of the area in the radial direction. (This is also true in terms of the difference in the response of the gamma ray detector due to the difference in the position of the radioactivity.) From the above,
By appropriately dividing the annular region, the response of the gamma ray detector when the container is rotated once is independent of the distribution within each annular ring, as long as the amount of radioactivity within each annular region is the same. It is almost constant. Assuming that the density value for each ring is known, the response (efficiency) of the gamma ray detector when the container rotates once when a unit amount of radioactivity is present in the mth ring is f (k, m) teeth,
It can be determined by simple calculation.

Here, k is the number (1 to 5) of the gamma ray detector (2).

m is the circular region number (■ to ■).

When the density distribution fζ in the plane perpendicular to the center line of the cylindrical container is distributed in a concentric ring shape, the following effects can be obtained.

(1) Even if the number of divided regions is significantly reduced, the error in quantifying radioactivity due to radioactivity distribution can be made equal to or lower than that of the conventional CT method shown in FIG.

(2) As the number of divided regions is reduced, the number of required measurement points can also be reduced proportionally. From now on■ By keeping the measurement time per container constant, it is possible to take a longer measurement time per measurement than with conventional CT methods, which reduces statistical errors and increases sensitivity when measuring weak radioactivity. can be achieved.

@ If the measurement time per measurement point is kept constant, conventional CT
Compared to the method, the measurement time per container can be significantly shortened.

(3) Conventional CT when the container rotation control system is continuous rotation
◇Also, the measurement/rotation system control S/W can be simplified and costs can be reduced.

(4) The computer load required to determine the amount of radioactivity for each area is reduced, as is the calculation processing time and required memory capacity. This contributes to not only reducing conuts but also improving measurement processing capacity.

In the above example, when a unit amount of radioactivity exists in the mth annular region, the response f (k m) of the gamma ray detector (2) when the container rotates once is If the density of each ring is known, it is determined by a simple calculation, but in other words, if the density of each ring changes, that is, if the container or slice changes, it will be necessary to calculate it each time. However, if the density can be considered uniform within each slice, calculate the value of f(k, m) above for several density values in advance, and then calculate the density separately for each slice or container. The value of f(k, m) for each value may be interpolated/extrapolated using the density value to obtain a simpler calculation. In other words, in the case where the density can be considered uniform for each slice, the detection efficiency data group is: ■ The ring division conditions are determined in advance, and the gamma ray detection efficiency is calculated by dividing the density into a parameter for each ring and each gamma ray detector. Calculate in advance and use it as a data tape μ, ■ Interpolate or extrapolate the gamma ray detection efficiency in the data table above from the density information for each slice of containerized radioactive waste, and calculate the gamma ray detection efficiency for a given density using the above circular ring. Furthermore, in the above example, the amount of radioactivity obtained per slice was calculated by summing the amount of radioactivity obtained for each annular region, but Regarding the amount of radioactivity of radionuclides that were not targeted for detection, although the absolute amount is different, the relative distribution of radioactivity may be considered to be the same as the distribution already determined. In such a case, a gamma ray detector 1 configured to be able to measure gamma rays from multiple nuclides, separate from the plurality of gamma ray detectors, is used.
A table is placed around the container in such a way that gamma rays from the container can be seen, and the detection necessary to determine the amount of the target radionuclide from the count rate value for gamma rays from the target radionuclide obtained from this detector. The efficiency (response of the detector to a unit amount of radioactivity in the container) is calculated based on this relative distribution.

By doing this, once the distribution of the representative nuclide is understood, the detection efficiency can be calculated for that and all nuclides that can be considered to have the same distribution, and as a result, it becomes possible to quantify multiple nuclides. The quantification method based on the annular region division of the present invention also makes it possible to quantify multiple nuclides using such a method.

Note that the number of rings is equal to or smaller than the number of gamma ray detectors. In other words, if the number of toruses is equal to the number of gamma-ray detectors, the detection efficiency is determined by simultaneous equations, and if the number of toruses is smaller than the number of gamma-ray detectors, it is calculated repeatedly using the method of least squares, for example. What is required is a generally known throughput. [Effects of the Invention] As described above, according to this invention, ■ A cylindrical container is divided into a plurality of sufinus of a predetermined thickness perpendicular to the center line. ■ A region division format for understanding the radioactivity distribution of the slice is a plurality of rings (including the center circle) formed by a plurality of concentric circles centered on the center line of the slice. However, the number of rings shall be equal to or smaller than the number of gamma ray detectors), ■ Based on the density value for each ring given in advance, calculate the number of rings when radioactivity is present in the ring. The detection efficiency of the gamma ray detectors for gamma rays emitted from the radioactivity within the ring is calculated in advance for each ring and each gamma ray detector, and this is set as a detection efficiency data group. The gamma rays emitted from the radioactive waste inside the container are measured while the container rotates n times (n: an integer) without going up and down, and the responses of the gamma ray detectors during that time are integrated for each detector. Using this as a measurement data group, ■ Calculate the amount of radioactivity for each ring from the detection efficiency data group and measurement data group, and ■ Add up the radioactivity l for each ring of the Sufinu to calculate the slice force. The amount of radioactivity is determined by repeating the above steps for each slice divided into multiple slices by raising and lowering the cylindrical container, and then calculating the amount of radioactivity per container by summing up the amount of radioactivity for each slice of the container. Therefore, if the density distribution in the plane perpendicular to the center line of the cylindrical container is distributed in a concentric ring shape, the number of measurement points can be significantly reduced, the rotation drive system and its control software can be simplified, and the measurement time can be reduced. This has the effect of shortening the time required for analysis and increasing the precision of quantification, reducing the time required for analysis and reducing computer capacity.

[Brief explanation of the drawing]

Figures 1a and b are a perspective view and a plan view, respectively, illustrating a method for quantifying radioactivity in radioactive waste packed in a container according to an embodiment of the present invention, and Figures 2a and b are diagrams illustrating a method for quantifying radioactivity in radioactive waste packed in a container according to an embodiment of the present invention. FIG. 8 is a perspective view illustrating the state of region division in the radioactivity quantification method, and FIG. 8 is a plan view illustrating the radioactivity quantification method by region division as shown in FIG. In FIG. 1, (2) is a gamma ray detector, 81 to Sn Hanurice. In each figure, the same reference numerals indicate the same or corresponding parts.

Claims (2)

[Claims] (1) A plurality of gamma ray detectors are installed outside the radioactive waste packed in a cylindrical container, and the container is rotated about the center line of the container relative to the gamma ray detector, and the center line The gamma rays emitted from the radioactive waste in the container are measured by the gamma ray detector, and the density distribution information of the radioactive waste provided is used to determine the density of the radioactive waste in the container. In the method of quantifying radioactivity by determining the radioactivity distribution, [1] the cylindrical container is divided into a plurality of slices of a predetermined thickness perpendicular to the center line, and [2] the radioactivity distribution of the slices is determined. The area division format for understanding is a plurality of rings (including the center circle) formed by a plurality of concentric circles centered on the center line of the slice (however, the number of rings is determined by the gamma ray detection described above). [3] From the density value for each ring given in advance,
If radioactivity exists in the ring, the detection efficiency of the plurality of gamma ray detectors for gamma rays emitted from the radioactivity in the ring is calculated in advance for each ring and each gamma ray detector. As a detection efficiency data group, [4] The gamma rays emitted from the radioactive waste inside the container are measured by the plurality of gamma ray detectors while the container rotates n times (n: an integer) without going up and down, and The responses of the gamma ray detectors are integrated for each detector to form a measurement data group, [5] the amount of radioactivity for each ring is calculated from the detection efficiency data group and the measurement data group, [6] Add up the amount of radioactivity for each ring of the slice to obtain the amount of radioactivity per slice, [7] Repeat the above operation for each slice divided into a plurality of slices by raising and lowering the cylindrical container, [8] The above. A method for quantifying radioactivity in radioactive waste packed in a container, characterized in that the amount of radioactivity in each slice of the container is summed to determine the amount of radioactivity per container. (2) In the case where the density can be considered uniform for each slice, the detection efficiency data group is
Calculate the gamma ray detection efficiency using density as a parameter for each gamma ray detector in advance and create a data table. [2] Interpolate or extrapolate the gamma ray detection efficiency in the data table from the density information for each slice of containerized radioactive waste. A method for quantifying radioactivity in containerized radioactive waste according to claim 1, characterized in that gamma ray detection efficiency for a predetermined density is determined for each ring and each gamma ray detector.