JPH0670675B2 - Uranium enrichment measuring method and measuring device - Google Patents

Description

The present invention relates to a method for nondestructively measuring ²³⁵ U enrichment of uranium solution, and more particularly to a method capable of continuously measuring ²³⁵ U in in-line monitoring.

Generally, in a nuclear fuel manufacturing plant or a nuclear fuel reprocessing plant, it is possible to monitor whether or not the ²³⁵ U enrichment of the fuel or material handled at each processing stage is within a predetermined range.
It is essential for process and quality control.

Conventionally, the measurement of uranium enrichment includes a measurement method using a mass spectrum and a radiation measurement method used as a nondestructive measurement method. The former generally has high measurement accuracy, but it is destructive, so it is not suitable for in-line monitoring that assumes nondestructive measurement. The latter includes a passive method that uses spontaneous decay radiation generated from uranium to be measured, and an active method that requires an external irradiation source such as neutron irradiation.
There is a method, for example, the method disclosed in JP-A-50-80880.

The conventional passive method is disclosed in JP-A-48-8596 and 48-8597.
No. 48-8598, etc., but these are all methods that require calibration with a reference sample with a known uranium enrichment, so they are suitable when the uranium concentration changes during in-line monitoring. There is no way.
Also, LKKull "CATALOG OF OF NUCLEAR MATERIAL SA
FEGUARDS INSTRUMENTS "BNL-17165,1972; H.Ebeve et a
The methods described in l., Journal of Nuclear Materials 81 , 203-214 (1979) are applicable to specific cases such as when the concentration of uranium is sufficiently high or when measuring the concentration of UO ₂ powder storage tank. . In these methods, when the uranium enrichment is a low concentration of several hundred g / l or less, the
Accurate measurement becomes difficult because the measurement error due to the line self-absorption increases.

Since the active method requires an external irradiation source, it has the disadvantage that the device has to be large-scaled, but it measures specific radiation due to nuclear reaction between irradiation radiation and the nuclide to be measured. By doing so, it is possible to perform a measurement that cannot be performed by the passive method.

A method using a relatively small external irradiation source of the conventional active method is disclosed in JP-A-54-140094. It targets solid uranium, such as uranium oxide powder, and requires calibration with a reference sample of known uranium enrichment. Therefore, it is not suitable for in-line monitoring like the aforementioned passive method.

Therefore, an object of the present invention is to provide a destructive measurement method of uranium enrichment suitable for in-line monitoring by the active method.

By the way, the conditions necessary for in-line monitoring of solutions with changes in uranium concentration are that attenuation of ²³⁵ U-specific γ-rays is required, calibration by reference sample is not required, and uranium concentration and ²³⁵ U concentration can be measured simultaneously. .

The method of the present invention is a measuring method belonging to the active method, which does not require a reference sample and can simultaneously measure the uranium concentration and the ²³⁵ U concentration, and also can measure the low uranium concentration. The method of the present invention is particularly useful when the solution to be measured contains substantially only elements having small atomic numbers such as N, F, O, and H as elements other than uranium. However, in the present specification, the “element having a small atomic number” means an element having an atomic number of 17 (C1) or less. Further, "substantially free" means that the amount of an element having a large atomic number (the larger the atomic number, the larger the absorption of γ-rays), the error due to the γ-ray absorption of the element is unacceptable for the purpose of measurement. It means not too much.

The method of the present invention is briefly described as follows: (1) The intensity of γ-rays peculiar to ²³⁵ U emitted from a uranium-containing solution to be measured in which elements other than uranium are substantially elements having a small atomic number is measured by γ-ray spectrometry. (2) On the other hand, the solution is irradiated with γ-rays from an external source that emits γ-rays of specific energy different from the γ-rays peculiar to ²³⁵ U, and The intensity is measured by γ-ray spectrometry to calculate the uranium concentration in the solution from the measured value, and from the obtained uranium concentration, the γ-ray peculiar to the ²³⁵ U by the solution and the γ-ray transmitting substance other than the solution. (3) Calculate the ²³⁵ U concentration using the measured ²³⁵ U-specific γ-ray intensity and the correction factor; and (4) Calculate the uranium concentration and ²³⁵ seeking ²³⁵ U enrichment than U concentration Do that, a method of measuring the uranium enrichment without using a known reference sample.

The present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a longitudinal sectional view showing an outline of an example of a uranium enrichment measuring apparatus used for carrying out the method of the present invention. In Figure 1, the pipe 1 is
A uranium-containing solution 2 is a pipe through which a γ-ray detector 3 is arranged outside the pipe 1 in order to perform nondestructive measurement of the uranium enrichment of the uranium-containing solution 2 by the method of the present invention. Further, an external γ-ray source 4 is arranged at a position facing the γ-ray detector 3 across the pipe 1. Source 4
Appropriate collimators 5 and 6 are provided between the pipe 1 and the pipe 1, and between the pipe 1 and the γ-ray detector 3, respectively. In addition, the entire measuring section is surrounded by the shield 7 to prevent intrusion of radiation from the outside. The shielding material 7 is not always necessary when the surrounding radiation background is sufficiently low, and can be integrated with the collimators 5 and 6. In the example of FIG. 1, the pipe 1 is the measurement container in the present invention, but the type of the measurement container may of course be a simple container for sampling the uranium-containing solution (measurement liquid). In the case of performing in-line monitoring, as shown in FIG. 1, generally, in a certain processing system, the pipe or the like through which the uranium-containing solution flows will be the measuring container.

The γ-ray detector 3 is a γ-ray sensor of the so-called γ-ray spectrometer leading to multi-channel pulse height analyzer through a preamplifier (not shown), ²³⁵ U specific γ-rays emitted from the target solution 2 Is measured as a count value N by this γ-ray spectrometer. ^As γ-rays peculiar to ²³⁵ U, those with energy of 186 KeV or 144 KeV are suitable.

For the count value N, the following equation I is established.

N = cVa γKεt (I) where c: concentration of ²³⁵ U (g / cm ³ ) V: effective volume of the measured solution (cm ³ ) a: specific activity of ²³⁵ U (dps / g) γ: ²³⁵ Emission rate of γ-rays of specific energy ^* per decay of U k: Attenuation of γ-rays of specific energy by the measured liquid and other γ-ray transmitting materials (mainly measuring containers) and correction factors for geometric effects ε: Counting efficiency of the γ-ray detector for γ-rays with specific energy t: Counting time (sec) * As the γ-rays with specific energy, 186 KeV as described above
Or 144 KeV is suitable. In the right side of the formula I, a and γ are uniquely determined, and V and ε are constant if the measuring device is determined.
On the other hand, k changes depending on the composition of the liquid to be measured, the concentrations of the various elements contained, and the material of the measuring container. When the uranium concentration in the measured solution is sufficiently high at approximately 1000 g / l or higher, k can be considered to be constant because it is hardly affected by fluctuations in the uranium concentration, and ²³⁵ U can be immediately determined from the measured N value ^. Concentration c
Can be asked. However, when the concentration of uranium in the liquid to be measured is as low as several hundred g / l or less, and when the concentration of various elements, especially the element with a large atomic number such as uranium, changes, k should be regarded as constant because it fluctuates considerably. I can't. However, most of the elements to be measured other than uranium have a small atomic number such as N, F, O, and H, and when the concentration of the element with a large atomic number is low, the Since the attenuation coefficient is sufficiently small and almost the same as that of uranium, the γ-ray attenuation effect of the measured solution is approximately dependent on the total uranium concentration. After all, if it is possible to know the total uranium concentration, it is possible to obtain k, and it is also possible to obtain the concentration c of ²³⁵ U from the measurement of N. If the total uranium concentration _co and the ²³⁵ U concentration c are known,
The uranium enrichment is obtained. The present inventor pays attention to this point, uses an external γ-ray source to obtain the total uranium enrichment _co, and determines the intensity of the transmitted γ-rays that permeate the liquid to be measured, that is, from the count value No by the γ-ray spectrometer I decided to ask for _o .

Return to FIG. 1 again. The measured liquid 2 is irradiated with γ-rays of specific energy from the external γ-ray source 4, the intensity of the transmitted γ-rays is detected by the γ-ray detector 3, and the count value is obtained by the γ-ray spectrometer.
When measuring No, the specific energy γ-rays to be used are those that exceed the energy of the ²³⁵ U-specific γ-rays to be measured, and if other γ-ray emitting nuclides are contained in the measured liquid. It does not overlap with the gamma-ray energy, and is about 1 MeV
It is preferably within the following range and has a long half-life. For example, ¹³³ Ba: 336 KeV, ¹³⁷ C _s : 662 KeV, etc.

For No, the following formula II holds.

In _{N o = s o γ o k} o 'k o "ε o t ... (II) here, s _o: an external source strength of the γ-ray source (dps) γ _o: line source 1 decay per the specific energy Emission rate of γ-rays k _o ′: Correction factor for geometrical effects k _o ″: γ of specific energy depending on the measured liquid and measuring container
Correction factor epsilon _o for the attenuation of the _line: counting efficiency t of gamma ray detectors for gamma-rays of a particular energy: in counting time (sec) formula II, the right side of the _{s o, γ o, k o} ', ε o is a constant value Given as. The following equation III holds for k _o ″.

k _o ″ = (attenuation rate of measured liquid) x (attenuation rate of measurement container) Where _co : uranium concentration (g / cm ³ ) μ _o : mass attenuation coefficient of γ-ray of specific energy with respect to uranium (cm ² / g) l _o : γ-ray passage length (cm) in the measured solution (See Fig. 1) c _i : Concentration of element i other than uranium in the measured liquid (g / cm ³ ) μ _i : Mass attenuation coefficient of γ-ray of specific energy with respect to element i (cm ² / g) ω _j : Weight ratio of the constituent element j of the measurement container ρ: Density of the measurement container (g / cm ³ ) l _i : γ-ray passing thickness (cm) on one side of the measurement container wall (see FIG. 1) l ₂ : Measurement container wall Γ-ray passing thickness (cm) on the opposite side (see FIG. 1) In the first term on the right side of the formula III, since the element i is an element having a small atomic number, μ _i is sufficiently smaller than μ _o . the composition of the test solution, the concentration is also varied in the first term (test solution attenuation rate) of connexion determined by exclusively the uranium enrichment c _o. The second item (attenuation rate of measurement container) is constant. Therefore, the correction term k _o ″ can be regarded as being proportional to the reciprocal of the exponential function of the uranium concentration c _o . Therefore, when the counting rate No at a constant counting time t is measured, the uranium concentration can be calculated from the formulas II and III. c _o can be calculated.

Once the value of c _o is found, it can be obtained by known numerical analysis (for example, R.
E. Malenfant “QAD: A SERIES OF POINTKERNEL GENERAL-P
URPOSES SHIELDING PROGRAMS Refer to LA-3573 (1967)) The correction term k of formula I is calculated, and ²³⁵ U is calculated from the value of k and the measured value of N.
The concentration c of is calculated by the formula I. After all, uranium enrichment E
Is the expression Is obtained by

The γ-ray detector used in the present invention is capable of detecting γ-rays of specific energy of ²³⁵ U from a target liquid to be measured and γ-rays of specific energy from an external radiation source, and has sufficient energy resolution. As long as it has it, any can be used, and examples thereof include a Ge (Li) γ-ray detector and a Geγ-ray detector. The γ of energy specific to ²³⁵ U from the measured liquid
The γ-ray spectrometer for measuring the X-rays and the γ-ray spectrometer for measuring the γ-rays from the external γ-ray source that have passed through the liquid to be measured may be the same. , May be provided separately.

The solution to be measured is a nitric acid solution of uranium or a hydrofluoric acid solution, but it is preferable that the element other than uranium has a low concentration of an element having a large atomic number, for example, a metal element.
If the concentration of these elements becomes high, the error becomes large due to interference. The method of the present invention is most effective when most or all of the elements other than uranium are elements having a small atomic number such as N, F, O, and H, as described above. The solution to be measured has a uranium concentration of several tens g / l to several hundreds g / l. It is rare that the concentration exceeds this concentration when the measurement target is a liquid, and in that case, another method can be used.

In the embodiment shown in the drawings, the measurement container is a uranium-containing solution transport pipe in the processing system, and continuous monitoring is possible in the middle of the flow path. Of course, the measurement can be performed even when the liquid to be measured is flowing in the pipe or when the flow is stopped. Alternatively, it is also possible to carry out the method of the present invention by sampling a required amount of the liquid to be measured in a measuring container.

The γ-rays incident on the γ-ray detector are amplified according to a known method, analyzed by a multichannel wave height analyzer, and processed by a computer that gives known values to the above equations I, II and III. It is convenient to carry out the above so that the concentration E can be directly obtained.

According to the present invention, it is possible to measure the concentration of ²³⁵ U by the simple method without the need for a large-scale device such as neutron irradiation for the destruction and rapid measurement of the ²³⁵ U concentration. Moreover, it became possible to easily measure the total uranium concentration and the ²³⁵ U concentration in parallel at one time. Therefore, even if the uranium concentration fluctuates up to the range of low uranium concentration that could not be measured in the past, in-line tracking is possible. The method of the present invention exerts a great effect in the process, quality control and criticality safety control of the system containing ²³⁵ U in the future.

Example 1 Using the apparatus shown in FIG. 2, the measurement was performed by the method of the present invention. The pipe 8 is made of SUS304 stainless steel with an inner diameter of 28 mm, a thickness of 3 mm and a nominal diameter of 25A. A Ge (Li) detector is used as the γ-ray detector 9, and 49.8 μc _i ¹³³ Ba is used as the external γ-ray source 10.
(336 KeV) was used. The collimator 11 is made of lead and serves not only to collimate γ-rays but also as a shield. The γ-ray detector 9 is connected to a preamplifier 12, and the signal obtained from the preamplifier 12 is sent via a cable to a multichannel wave height analyzer (not shown) to display a count value and a computer. Data processing by. The Dewar bottle 13 contains liquid nitrogen and cools the gamma ray detector 9 to maintain its function. 8-13 described above
Each element of is firmly fixed to the mount 14.

As sample liquid, three kinds of uranyl fluoride solutions with known enrichment shown in Table 1 were used, and the flow rate in the looped pipe was 10 l /
The measurement was performed while circulating in minutes.

Table 2 shows the measurement results and the calculation results of the enrichment.

Example 2 A SUS304 stainless steel pipe having an inner diameter of 53 mm, a thickness of 3.5 mm, and a nominal diameter of 50 A was used as a measuring container, and 53.8 μc _i ¹³⁷ was used as an external γ-ray source.
The concentration of three known uranyl nitrate solutions shown in Table 3 was measured with the same device configuration as in Example 1 except that C _s (662 KeV) was used. However, the sample solution was measured in a stationary state without being circulated. Table 4 shows the measurement results and the obtained degree of enrichment.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional view showing an outline of an example of an apparatus for measuring uranium enrichment according to the present invention, and FIG. 2 is a side view of another example of the apparatus. 1: Piping (measurement vessel) 2: Liquid to be measured 3: γ-ray detector 4: External γ-ray source 5,6: Collimator 7: Shield 8: Measurement pipe (solution containing uranium) 9: Ge γ-ray detector 10 : External γ-ray source 11: Lead collimator (shield) 12: Preamplifier (connected to multichannel wave height analyzer) 13: Dewar bottle 14: Stand

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Jun Shimizu 4-4 Muramatsu, Tokai-mura, Naka-gun, Ibaraki Prefecture Inside the Tokai Plant, Reactor and Nuclear Fuel Development Corp. (56) Reference JP 54-140094 (JP, JP, A) Japanese Patent Publication Sho 50-20866 (JP, B1)

Claims (3)

[Claims] (1) The intensity of γ-rays peculiar to ²³⁵ U emitted from a solution to be measured containing uranium, which substantially contains only elements having a small atomic number other than uranium, is measured by γ-ray spectrometry. (2) On the other hand, the solution is irradiated with γ-rays from an external source that emits γ-rays of specific energy different from the γ-rays peculiar to the ²³⁵ U, and the intensity of γ-rays of specific energy transmitted through the solution is changed. The uranium concentration in the solution was calculated from the measured values by γ-ray spectrometry, and the uranium concentration peculiar to the ²³⁵ U was attenuated by the solution and the γ-ray permeable substance other than the solution from the obtained uranium concentration. And a correction factor for the geometric effect is calculated; (3) ²³⁵ U concentration is calculated using the measured ²³⁵ U-specific γ-ray intensity and the correction factor; (4) Obtained uranium concentration and ²³⁵ U concentration Do more to determine ²³⁵ U enrichment , A method for measuring uranium enrichment without using a known reference sample. 2. A measuring vessel in which a uranium-containing solution to be measured containing substantially only a small atomic number element other than uranium is circulated, and ²³⁵ U-specific γ-rays arranged outside the vessel. Is for measuring the intensity of γ-rays that emit different γ-rays, ²³⁵ U specific γ-rays emitted from the inside of the container and γ-rays emitted from the external γ-ray source and transmitted through the solution in the container. A device for measuring uranium enrichment without using a known reference sample, which comprises a γ-ray spectrometer arranged outside the container. 3. The device according to claim 2, wherein the γ-ray spectrometer is emitted from the inside of the measuring container.
²³⁵ U An instrument consisting of a spectrometer for γ-ray measurement unique to ²³⁵ U and a spectrometer for γ-ray measurement derived from an external γ-ray source.