JPH0616101B2 - ▲ Up 2 ▼ Up 3 ▼ Up 9 Quantitative Pu measurement system by gamma ray mirror symmetry measurement - Google Patents

Description

TECHNICAL FIELD The present invention relates to a ²³⁹ Pu quantification device. More specifically, the present invention is based on a gamma-ray mirror symmetry measurement capable of measuring an analyte and a detector in a stationary state.
^{The present} invention relates to a ²³⁹ Pu quantification device.

(Background Art) When handling Pu (plutonium), metrological control is always required. That is, the amount of nuclear fuel material
The amount of movement must be clearly understood. In addition, since it is an alpha radioactive substance with a long half-life, it is necessary to display the amount of its load when transporting or storing waste.

Destructive analysis and non-destructive analysis that is externally measured are conventionally known as methods for quantifying Pu that requires such measurement control. In the latter case of nondestructive analysis, neutron rays emitted from Pu or gamma rays ( ²³⁹ Pu) are used.

Each of these has its own characteristics, and one of these methods is selected in consideration of usage.

Among these conventional methods, the method by gamma ray measurement is
²³⁹ Pu Since it can directly measure gamma rays and has high counting efficiency, it has high detection sensitivity and is effective for low-level quantification. However, the gamma ray method produces a solid angle when the volume of the sample is larger than that of the detector. And the counting efficiency decreases,
Further, there is a drawback that the measurement error becomes large.

Conventionally, in order to compensate for this drawback, segment measurement and scanning measurement (both are measured on a horizontal rotating table and a lifting table). However, when the sample is a heavy object or a container with piping, these measurement methods cannot be applied.

For this reason, it has been desired to realize a method and an apparatus capable of accurately and easily performing quantitative measurement by gamma ray measurement even in the case of a heavy sample or a container having a pipe.

(Object of the Invention) The present invention has been made in view of the above-described circumstances, improves the drawbacks of the conventional method, and makes it possible to accurately measure even a heavy substance sample while the sample and the detector remain stationary. The present invention provides a ²³⁹ Pu quantification device that can be easily measured.

DISCLOSURE OF THE INVENTION The ²³⁹ Pu quantification device of the present invention is characterized in that a pair of detectors, which are configured to simultaneously measure ²³⁹ Pu gamma rays and convert the sum of ^count values into ²³⁹ Pu weight, are installed in mirror symmetry. I am trying.

The device of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a diagram showing an outline of the device of the present invention. As shown in FIG. 1, for example, 200 waste drum pipes (JIS-Z-1600, 1.
2mm thick SUS-304) and other samples (1)
The detector (2) and the second detector (3) are installed in mirror symmetry. The distance between the side surface of the sample (1) and the detectors (2) and (3) is held by R (cm). One set of counters (A ₁ ) is connected to the first detector (2), and two sets of counters (A ₂ , B ₂ ) are connected to the second detector (3). Counter ²³⁹ Pu gamma ray (356-450 KeV)
(A ₁ ) and (A ₂ ) are simultaneously measured for a fixed time, for example, 3 minutes (not a coincidence circuit). The sum of both count values thus obtained is converted into ²³⁹ Pu weight.

In this quantification of ²³⁹ Pu, the points (a) and (a ') near the detectors (2) and (3) where Pu is scattered in the sample (1) have the highest counting efficiency. On the other hand, the center line portion has low counting efficiency, and particularly the top point (b) and the point (b ') are low. FIG. 2 shows points (a) when the center point is set to 1.0 when the distance (R) from the sample (1) to the detectors (2) and (3) is changed in the range of 20 to 80 cm. ) And the efficiency ratio at point (b), (ER) r (= Ia, b / I center). The broken line shows the value calculated geometrically, and the solid line shows the measured value. As is clear from FIG. 2, R was 50 cm or more in the actual measurement, and the point (a),
Both point (b) can be measured within ± 25%.

FIG. 3 shows the case where the center point and the comparative evaluation, (ER) _w (= I whole / I center), of the whole sample (1) were performed. In FIG. 3, the broken line is a value obtained from the integral of the rotating body, assuming that Pu is uniformly dispersed. Solid line is 11 Pu source C ²³⁹ Pu purity 95
% Or more) is arbitrarily arranged and measured. R is 5
When it is 0 cm or more, it can be evaluated with an error of ± 25%.

In other words, for the 200 drums as sample (1), if R is kept at 50 cm, the error due to solid angle is ± 25%.
It can be measured within. In addition, in the device of the present invention, in addition to the sample volume, (a) americium (
²⁴¹ Am) It is necessary to consider the effects of gamma rays, (a) calibration curve, and (c) attenuation of gamma rays depending on the contents.

First, ^regarding the effect of ²⁴¹ Am gamma rays, the Pu sample consists of its isotopes, such as nuclides such as ²³⁹ Pu, ²⁴⁰ Pu, and ²⁴¹ Pu. By weight, ²³⁹ Pu and ²⁴⁰ Pu 80-90%
And ²⁴¹ Pu is present in 1 to 15%. Of these, ²⁴¹ Pu has a half-life of 14.4 years and undergoes beta-minus decay to ²⁴¹ Am (half-life of 433 years). Therefore, ²⁴¹ Am accumulates in the Pu sample over time. This nuclide is alpha-radioactive, but emits a number of gamma rays with different energies. Figure 4 shows ²³⁹ Pu and ²⁴¹ A
3 shows a gamma ray spectrum of m. ²⁴¹ Am
The entire spectrum of the broken line in FIG.

In the device of this invention, ²³⁹ Pu (356-450 Ke
V) is measured by the counters (A ₁ ) and (A ₂ ), and the ²⁴¹ Am gamma ray in the area is also counted by each counter. In order to subtract this, the counter (B
₂ ) is provided.

The area of the counter (B ₂ ) is set, for example, as follows.

That is, using any pure ²⁴¹ Am gamma source,
A counter (A ₁ ) (A ₂ ) measures for a certain period of time (for example, 3 minutes). These values are A ₁ , A ₂ values (A ₁
A ₂ ), the area of the counter (B ₂ ) (470-790 KeV) is selected so as to have these doubled values. By doing this, the ²⁴¹ Am gamma ray count value is
The relationship of A ₁ A ₂ 1/2 B ₂ is shown regardless of the amount. Next, when counters (A ₁ ) (A ₂ ) (B ₂ ) are simultaneously used to count ²³⁹ Pu gamma rays, Net ²³⁹ Pu count value = ( A ₁ + A ₂ −B ₂ ).
As a result, ²³⁹ Pu gamma rays can be measured even in the presence of ²⁴¹ Am.

Next, regarding the calibration curve, this calibration curve is prepared by sequentially placing a known amount of ²³⁹ Pu radiation source at the center point of the drum of the sample (1), and making a calibration curve for 3 minutes on the horizontal axis of the logarithmic graph paper. By taking the ( A ₁ + A ₂ − B ₂ ) value of the count and taking the amount of ²³⁹ Pu (g) on the vertical axis, 0.02-1 per drum can
A calibration curve of 0 g ²³⁹ Pu can be prepared.

Further, the point of gamma ray attenuation can be handled as follows. Normally, ²³⁹ Pu gamma rays are shielded by the analyte and the intensity decreases. Therefore, the correction count (f) is obtained in advance using an external radiation source, and the measurement correction is performed in advance.
For example, as shown in FIG. 1, a ¹³⁷ Cs (662 KeV, 1 MBq) radiation source (4) is installed above the first detector (2) while being shielded by a lead block (5). When the shutter (6) on the front surface is opened, the specimen (1) is irradiated obliquely in a beam shape from the 8 mm diameter hole. The penetrating gamma rays enter the second detector (3) and the counter (B ₂ ) is sensitive. Attenuation ratio of sample (1) after counting for 3 minutes (1.0 in air)
To measure. Various simulated waste drums made and known quantity
After inserting the ²³⁹ Pu radiation source, first measure the attenuation ratio of the sample with an external radiation source. FIG. 5 shows the damping ratio on the horizontal axis. Next, close the shutter (6) of the same line source, and set ²³⁹ P
u Measure gamma rays. The corrected count (f) can be determined by comparison with the count value in air. In FIG. 5, a graph is created with (f) scaled on the vertical axis. That is, if the intensity Io in the air and the intensity I at the time of decay are f: (Io
/ I) Pu- ₂₃₉ , external source intensity ratio (I / Io) Cs-
₁₃₇ .

Next, an embodiment of the present invention will be shown, and the apparatus of the present invention will be described in more detail. Of course, the present invention is not limited to the following examples. Example (1) Simulated sample 180 mg of ²³⁹ Pu radiation source was inserted into a drum filled with simulated waste and quantified twice. Table 1 shows the result.

In the re-measurement, the data at the position rotated 90 degrees from the first time is shown. Reproducibility was confirmed for 10 samples.

(2) Implementation waste Pu-containing solid waste (vinyl, SUS-304, aluminum plate, etc.) 86 drums, weight 60-170 kg
Was measured twice from different sides.

Table 2 shows four results. The agreement is within ± 25% without exception.

(Effects of the Invention) According to the present invention, as described in detail above, ²³⁹ Pu gamma rays are measured in mirror symmetry to measure the stationary state, but the inside of the sample and any position are designed to have substantially the same counting efficiency. It is possible to obtain the same effect as the rotary platform measurement.

Further, the apparatus of the present invention is effective for quantifying ²³⁹ Pu in a waste drum or liquid container having a large sample volume and shape. As shown in the examples, even if the waste drum can has uneven distribution of Pu, the apparatus of the present invention shows good reproducibility even if the measurement position is changed.

The effectiveness of the mirror symmetry measurement is shown.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the device of the present invention. FIG. 2 and FIG. 3 are diagrams showing the measurement efficiency with respect to uneven distribution of ²³⁹ Pu, respectively. Figure 4 shows ²³⁹ Pu and ²⁴¹ Am
It is a figure showing the gamma ray spectrum of. Figure 5 shows
It is a figure showing correction of ²³⁹ Pu gamma ray attenuation by a ¹³⁷ Cs external radiation source. 1 ... Sample, 2, 3 ... Detector, 4 ... ¹³⁷ Cs radiation source, 5 ... Lead block, 6 ... Shutter.

Claims (1)

[Claims] 1. A gamma-ray mirror symmetry measurement, characterized in that a pair of detectors consisting of simultaneous measurement of ²³⁹ Pu gamma rays and converting the sum of ^count values into ²³⁹ Pu weight are installed in mirror symmetry ^. Pu quantification device.