JP2000193610A - Content rate measuring method of neutron absorbing material and neutron irradiator used for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure the content rate of neutron-absorbing material with simple nondestructive method. SOLUTION: A measurement sample S is a board-shaped stainless steel containing neutron absorbing material, such as boron or the like. A polyethylene reflector 18 is arranged on one side of the measurement sample S, and a neutron radiation source 15 and a neutron measuring element 17, such as a boron trifluoride counter tube or the like, are arranged on the other side of the measurement sample S. Irradiating neutrons emitted from the neutron radiation source 15 are scattered and decelerated by the reflector 18, and a part thereof is reflected to the neutron measuring element 17 side. A surface density of the neutron absorbing material is obtained by counting, by the neutron measuring element 17, thermal neutrons permeating the measurement sample S among the thermal neutrons, a part of the scattered and reflected neutrons R, and the content rate of the neutron absorbing material in the measurement sample S is measured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for measuring the content of a neutron absorbing substance such as boron and a neutron irradiator used for the method. More specifically, a neutron-absorbing substance content measuring method for measuring the content of a neutron-absorbing substance by measuring thermal neutrons transmitted through a measuring specimen by the neutron measuring element, and a neutron irradiator used for the method It is about.

[0002]

2. Description of the Related Art Conventionally, in measuring the boron concentration in a stainless steel plate, a sample was taken from a test specimen, the sample was dissolved by a wet method, and the concentration was determined by chemical analysis. Therefore, according to this conventional method,
It was necessary to destroy the measurement specimen, and the degree of dispersion of the boron concentration in the stainless steel plate could not be determined.

[0003]

DISCLOSURE OF THE INVENTION In view of such a conventional situation, an object of the present invention is to measure the content of a neutron absorbing substance capable of measuring the content of the neutron absorbing substance by a simple non-destructive method. A method and a neutron irradiator used for the method.

[0004]

In order to achieve the above object, a feature of the method for measuring the content of a neutron absorbing substance according to the present invention is that a reflective material is arranged on one of the measurement specimens and the other of the measurement specimens A neutron source and a neutron measuring element are arranged in
The neutron-absorbing substance contained in the measurement specimen is counted by counting the thermal neutrons transmitted from the measurement specimen among the thermal neutrons emitted from the neutron beam source and the scattering of which is slowed down by the reflector, by the neutron measurement element. Is to measure the rate. In the present invention, for example, the measurement test body may be made of stainless steel, the neutron absorbing substance may be made of boron, the reflective material may be made of polyethylene, and the neutron measuring element may be implemented as a boron trifluoride counter tube.

For example, neutrons generated at the time of spontaneous fission such as ²⁵² Cf are called fast neutrons having a high energy of about 2 MeV, and neutrons transmitted through or scattered by a substance collide with atoms in the substance. , And eventually becomes a thermal neutron having a low energy of about 0.5 eV or less, which is in equilibrium with room temperature. Thermal neutrons have a high probability of causing a nuclear reaction by being absorbed by various atoms.
The material serving as a measurement specimen containing a neutron absorbing substance has the purpose of suppressing such thermal neutron transmission, and in the present invention, the neutron shielding ability of the measurement specimen is a constant level by the count value of the neutron measurement element. Is determined.

According to the above-mentioned feature of the present invention, the reflector is disposed on one of the measurement specimens, and the neutron source and the neutron measurement element are disposed on the other of the measurement specimens. The high-speed neutrons emitted are transmitted through the measuring test body and then scattered and decelerated by the reflecting material, and a part of the neutrons becomes thermal neutrons and is reflected toward the neutron measuring element. According to the experiments by the inventors, neutrons generated from a neutron source are transmitted through a reflective material and then transmitted through the measured test object, as compared with the case where the neutrons are once transmitted through the reflective test material as in the present invention. It was found that the efficiency of thermal neutron generation was significantly higher when the light was reflected. Therefore, according to the above-mentioned feature of the present invention, it is possible to reduce the neutrons emitted from the neutron radiation source so that the neutrons are efficiently converted into thermal neutrons, and to transmit a large amount of thermal neutrons into the measurement specimen.

Another feature of the method for measuring the content of a neutron absorbing substance according to the present invention is that the surface density of the neutron absorbing substance and the counted value are measured using the measurement specimen whose surface density of the contained neutron absorbing substance is known. The calibration curve representing the correlation with the pre-created, the measurement value of the measurement test specimen at the time of the main measurement and the correlation with the decay and the correlation over time from the time of the reference measurement to the time of the main measurement obtained the correlation. In consideration of the above, the surface density of the neutron absorbing substance in the measurement specimen at the time of the main measurement is determined to measure the content of the neutron absorbing substance.

In verifying the validity of a measurement system including a neutron source, the ratio of the count values of two types of standard specimens having different known surface densities of the neutron absorbing substance contained therein is determined based on an error propagation. The relationship with the calibration curve may be clarified by confirming that it is within the range. In addition, for each of the two types of standard test specimens having known and different surface densities of the neutron absorbing substances contained therein, the theory at the time of the main measurement is considered in consideration of attenuation with time from the reference measurement to the main measurement. Calculate the count values and confirm that the actual measurement count value at the time of the main measurement for each of the two types of standard specimens is within a range of three times the standard deviation of the theoretical count value. However, the validity of the measurement system including the neutron source can be verified.

The feature of the neutron irradiator according to the present invention used in the method for measuring the content of the neutron absorbing substance shown in any of the above features is that the base portion 14 made of a neutron moderator and the first and second cores are provided. Meters 11, 12; a neutron shielding material 13 that covers the side and bottom of the holes for the first and second collimators 11, 12 formed in the base portion 14 and the outer surface of the base portion 14; A neutron source 15 arranged at the bottom of the meter 11 and a neutron detector 17 arranged at the bottom of the second collimator 12 are provided, and the first collimator 11 and the second collimator 12 are brought close to each other. It is in.

According to the characteristic configuration of the neutron irradiator, accidental leakage of neutrons can be prevented by surrounding the whole with a neutron shielding material. The first collimator prevents unwanted diffusion of neutrons emitted from the neutron source, and the second collimator removes thermal neutron noise by limiting the angle of incidence. Then, by bringing the first and second collimators into close proximity to each other and communicating with each other, thermal neutrons reflected from the reflector base can be efficiently introduced into the second collimator.

[0011]

As described above, according to the feature of the method for measuring the content of a neutron absorbing substance according to the present invention, neutrons emitted from a neutron source by reflection in a reflecting material are efficiently converted into thermal neutrons. Thus, it is possible to allow many thermal neutrons to penetrate into the test specimen. as a result,
The measurement can be performed in a short time, and a practical method for measuring the content of a neutron absorbing substance can be provided. Moreover, according to the present invention, for example, a test can be easily performed by using only a small amount of a neutron source without requiring a nuclear reactor or an accelerator.

Further, according to the above-mentioned other feature, the content of the neutron absorbing substance can be easily obtained by utilizing the correlation obtained in advance.

Further, it is possible to easily assure the validity of the measurement system including the neutron source by confirming that the measured value is within the range of error propagation or within the range of three times the standard deviation of the theoretical value. It has become possible.

On the other hand, according to the characteristic configuration of the neutron irradiator,
By enabling the thermal neutrons reflected from the reflector base to be efficiently introduced into the second collimator, the measurement can be performed in a shorter time.

[0015]

Next, a first embodiment of the present invention will be described with reference to FIGS. 1 and 2 are cross-sectional views showing the structure around the neutron irradiator 10 in the neutron absorbing substance content measuring device 1.

A thickness 70 supported by a table (not shown)
A flat plate-shaped measurement specimen S to be measured is placed on a thick plate-shaped reflector base 18 made of polyethylene which is a neutron reflector (absorber) of about 100 mm. The neutron irradiator 10 is moved to a plurality of locations on the measurement specimen S, so that the neutron absorbing material content at each location can be measured.

The neutron irradiator 10 forms holes for the first and second collimators 11 and 12 in a base portion 14 made of polyethylene, which is a neutron moderator, and forms side and bottom portions of these holes and the base portion 14. Surrounding the entire outer surface with a cadmium plate 13 as a neutron shielding material having a large neutron absorption cross-sectional area prevents neutrons from leaking unexpectedly. The first collimator 11 prevents unnecessary diffusion of neutrons emitted from the neutron radiation source 15 and the second collimator 1
2 removes thermal neutron noise by limiting the incident angle. The holes of the base portion 14 for forming the first and second collimators 11 and 12 are brought close to each other and communicated with each other, so that the thermal neutrons reflected from the reflector base 18 can be efficiently placed in the second collimator 12. It can be introduced. The cadmium plate 13 located between the first and second collimators 11 and 12 is formed thicker than other portions. And the measurement specimen S
For example, a plate of stainless steel (B-SUS or the like) in which boron is dispersed as a neutron absorber is used. In addition,
As a neutron moderator constituting the base portion 14, polyethylene containing a neutron absorber such as boron may be used.

The neutron source 15 attached to the bottom of the first collimator 11 is detachable. In the present embodiment, ²⁵² Cf is used as the neutron source 15.
Neutrons generated from the neutron source 15 are fast neutrons.

On the other hand, the neutron detection element 17 is disposed in the second collimator 12 by passing the cylindrical neutron detection element 17 through a through hole in the lateral direction communicating with the bottom of the second collimator 12. is there. As the neutron detecting element 17, a boron trifluoride counter tube filled with boron trifluoride (BF ₃ ) gas having high sensitivity to thermal neutrons is used.

Irradiated neutrons B emitted from the neutron source 15 are still hardly affected by the neutron absorbing substance in the measurement specimen S, and penetrate through the measurement specimen S, and a part thereof is reflected on the reflection material table 1.
8 is scattered and reflected. After that, many thermal neutrons are included in the scattered reflection neutrons R toward the measurement specimen S again, and this thermal neutron is easily absorbed by the neutron absorbing substance in the measurement specimen S. Therefore, the count value of thermal neutrons per unit time measured by the neutron detection element 17 changes depending on the presence or absence of the measurement specimen S and the surface density of the neutron absorbing material in the measurement specimen S, and thereby, the neutron shielding ability Can be determined.

FIG. 3 is a schematic block diagram showing the entire neutron absorbing substance content measuring apparatus 1 according to the present invention. The neutron absorbing substance content measuring apparatus 1 includes a preamplifier 21, a high voltage supply 22, a signal waveform. Shaper 23 and monitor 24
It has. Neutron detecting element 1 by the above process
The thermal neutrons incident on 7 emit alpha particles by a nuclear reaction, and the boron trifluoride gas is ionized along its range, and a pulse current flows. This pulse current is supplied to the preamplifier 21
And a pulse waveform is shaped by the signal waveform shaper 23, and a signal corresponding to the pulse height is displayed on the monitor 24 as a spectrum. However, in the present invention, the information of the spectrum is not used, but the total thermal neutron count is measured.

Next, a second embodiment of the present invention will be described with reference to FIGS. In the following embodiments, the same members as those in the first embodiment are denoted by the same reference numerals.

In the above embodiment, the first collimator 11 for preventing neutron diffusion from the neutron source 15 and the second collimator 12 for removing incident neutron noise are separately provided. However, in the second embodiment, the function corresponding to the first and second collimators 11 and 12 is replaced by the shared collimator 19 provided only in one place. According to the second embodiment, since the area of the measurement specimen S facing the common collimator 19 is small, it is possible to determine the distribution of the neutron shielding ability in a narrower range.

Further, other embodiments of the present invention will be listed. In each of the above embodiments, a boron trifluoride counter was used as a neutron detecting element. However, another type of detector having high sensitivity to thermal neutrons can be used as the neutron detecting element.

[0025] In the embodiments described above, the neutron source 15 ²⁵²
Cf was used, but other neutron sources, such as ²⁴¹ Am-
Be or the like may be used.

The present invention provides, in addition to the above stainless steel, carbon steel, 1 1/4 Cr-0.5Mo steel, 2 1/4 Cr-1M
The present invention can be particularly preferably applied to steel materials such as o-steel and 3.0 Cr-0.5Mo steel, and can also be applied to metal materials other than Fe such as Al.

As the reflector used in the present invention, other than polyethylene, hydrocarbons such as graphite, paraffin and polypropylene may be used.

The fast neutron is a neutron having an energy of 500 KeV or more, and the thermal neutron is 0.5 neutron.
Although neutrons having an energy of eV or less may be referred to, specific numerical values in the present specification show an example of an embodiment of the present invention, and fast neutrons and thermal neutrons referred to in the present invention are not necessarily those neutrons. However, the present invention is not limited to the specific numerical values.

[0029]

Next, an embodiment in which the neutron count value is measured and the neutron shielding ability of the test specimen S is determined will be described. First, a test piece (B-SUS material) containing 1.57% and 1.1% boron was used as a test piece, and 1.0 m
^{m t (12.18 [mg / cm} 2]) ~69.8mm
Scattered neutrons were measured for a plate thickness (boron surface density) of ^t (704.5 [mg / cm ² ]), and a calibration curve was created.
The neutron source intensity of ²⁵² Cf used for the neutron source was 33.8 μCi 9 days after the reference date, which is the calibration curve creation date. The boron surface density D [mg / cm ² ] of the plate thickness H [mm] at the boron content M [%] was determined by the following equation. The density of the B-SUS material (stainless steel) is 7.76.
[G / cm ³ ]. D = 7.76 × M × H [mg / cm ² ]

The horizontal axis represents the boron surface density, and the vertical axis represents the neutron count, and the results are shown in the graph of FIG. Measurement conditions are 3
Minutes / 3 times, the average value is shown. In the region where the boron surface density is low (up to 70 [mg / cm ² ]), the neutron absorption ratio sharply increases with the increase in the boron surface density.
After that, it becomes moderate (70-200 [mg / c
m ² ]), when a certain limit (〜200 [mg / cm ² ]) is exceeded, little change is observed. In other words, B-SUS
It can be seen that the neutron count value remains almost constant despite the increase in the plate thickness.

Further, the neutron counts N and B of the materials B and C having a boron concentration of 1.57% on the reference date, which is the date on which the calibration curve was prepared.
b and Nc were actually counted (measured). The measurement time per measurement was 3 minutes, and the measurement was performed three times, and the average value was obtained. Material B: 1.0 mm ^t Nb = 26112 [count] (3 minutes / 3 times) Material C: 1.3 mm ^t Nc = 23190 [count] (3 minutes / 3 times)

Next, the propagation of the error on the reference date is calculated using this data. Since Nb and Nc are actual count values, the standard deviation σ of each count value is the square root of each count value. The error ranges dNb and dNc of the materials B and C are obtained as three times the standard deviation of the count value, and are as follows. dNb = ± 485 (3σ of count value 26112) dNc = ± 457 (3σ of count value 23190)

Here, assuming that the range of error propagation is dx, the following equation is established. dx ² = (Nc / Nb ² ) ² dNb ² + (1 / Nb) ² dNc ² = 1 / (26112) ⁴ [(23190) ² (485) ² + (26112) ² (457) ² ] = 2. 151 × 10 ⁻¹⁸ [1.265 × 10 ¹⁴ + 1.424 × 10 ¹⁴ ] = 5.784 × 10 ⁻⁴

Therefore, dx = 0.0241. On the other hand, Nc / Nb = 23190/26112 = 0.8881
It is.

Therefore, the propagation range of the error is as follows. 0.8881 ± 0.0241 = 0.864 to 0.912

The test specimen measurement is actually started 21 days after the reference date. Before the test object measurement (BD, BE material), measure the reference test object (B, C material), confirm that the count ratio is within the range based on the propagation of error, and check the measurement system (measurement system). Verify correctness. Material B: 1.0 mm ^t : Nb = 26000 [count] (3 minutes / 3 times) Material C: 1.3 mm ^t : Nc = 22835 [count] (3 minutes / 3 times)

Therefore, the propagation of the error is as follows. Nc / Nb = 22835/26000 = 0.778 This value is 0.864 to 0.864 which is the propagation range of the error determined earlier.
The value was between 0.912 and the validity of the system was verified.

On the other hand, another method is used to verify the validity of this measurement system. 21 days from the reference date for B and C materials
The theoretical count values N'b0 and N'c0 after the day are obtained. N′b0 = Nb0exp (−0.693t / T) = 26112exp (−21 × 0.693 / 2.64 × 365) = 25720 N′c0 = 23190exp (−21 × 0.693 / 2.64 × 365) = 22842

Where T is 2.64 with a half-life of ²⁵² Cf.
The year and t are 21 days in elapsed days. Since N′b0 and N′c0 are both theoretical count values, the standard deviation σ of each theoretical count value is the square root of each theoretical count value. The verification checks whether or not the actual measured value falls within the range of 3σ, which is three times the standard deviation of the theoretical count value. N'b0-3 (N'b0) ^1/2 <N'b <N'b0 + 3 (N'b0) ^1/2 25720-3 (25720) ^1/2 <26000 <25720 + 3 (25720) ^1/2 25240 <26000 <26200 Therefore, it can be said that the measurement of the material B is valid.

Next, the material C will be examined. N'c0-3 (N'c0) ^1/2 <N'c <N'c0 + 3 (N'c0) ^1/2 22842-3 (22842) ^1/2 <22835 <22842 + 3 (22842) ^1/2 22389 <22835 <23295 Therefore, it can be said that the measurement was also valid for the C material, and from these results, the validity of the entire system was verified.

Next, an allowable count value of the BD material and the BE material which are the measurement specimens is determined. Boron surface density of BD material 34.1
Regarding the allowable count value obtained from the calibration curve in [mg / cm ² ], the boron surface density of the calibration curve prepared on the reference day is 3
The neutron count value at the time of 4.1 [mg / cm ² ] is obtained, and considering the attenuation up to the day, a value obtained by adding three times the standard deviation is set as an allowable value. On the measurement date 21 days after the reference date, 15689 [count / 3 minutes] is the allowable value.
The allowable count value obtained from the calibration curve at a boron surface density of BE of 37.9 [mg / cm ² ] of the BE material is also represented by B from the calibration curve.
Determined in the same way as D. 14800 [cou] like BD material
nt / 3 minutes].

The actual measurement was carried out three times at substantially the center of a BD material and a BE material of 200 × 300 × 4 mm ^t . The measured values of the BD material are 14332, 14398, 14393.
[Count], which is an allowable count value of 15689
It was smaller than [count]. The measured value of the BE material is 13656, 13658, 13437 [c
out] and the permissible count value of 14800 [co
unt]. Therefore, it is possible to determine that the neutron shielding ability of these BD materials and BE materials is sufficient. This completes the measurement, but the validity of the measurement system may be confirmed by comparing the count values of the B and C materials at the end of the measurement as well as at the start of the measurement.

In the above embodiment, the neutron shielding ability was determined by comparing the actual measured count value with the allowable count value. However, the actual count value is calculated as the count value on the reference date in consideration of the time-dependent decay according to the above formula, and further, the estimated boron surface density is calculated using the correlation shown in FIG. It is also possible to determine the neutron shielding ability based on whether or not

The boron content M [%] is determined by the obtained boron surface density D [mg / cm ² ] and the thickness H [mm].
And the density ρ [g / cm ³ ] of the measurement specimen S can be finally obtained by the following equation. M = D / (ρ · H) [%]

It should be noted that the reference numerals entered in the claims are merely for convenience of comparison with the drawings, and the present invention is not limited to the configuration of the accompanying drawings by these entries. Absent.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a neutron irradiator used in the present invention.

FIG. 2 is a sectional view taken along line II of FIG. 1;

FIG. 3 is a schematic block diagram of a neutron absorbing substance content measuring device used in the present invention.

FIG. 4 is a graph showing a relationship between a boron surface density and a neutron count value.

FIG. 5 is a sectional view of a neutron irradiator used in a second embodiment of the present invention.

FIG. 6 is a sectional view taken along line II-II of FIG.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 neutron absorbing substance content measuring device 10 neutron irradiator 11 first collimator 12 second collimator 13 cadmium plate 14 base unit 15 neutron radiation source 17 neutron detection element (boron trifluoride counter tube) 18 reflector base 19 common Collimator 21 Preamplifier 22 High voltage supply 23 Signal waveform shaper 24 Monitor 30 Neutron irradiator S Measurement specimen B Irradiated neutron R Scattered neutron

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Takuichi Imanaka 1-18-14 Kitahorie, Nishi-ku, Osaka-shi Non-Destructive Inspection Co., Ltd. (72) Inventor Hideo Nabeshima 1-1-1, Otemachi, Chiyoda-ku, Tokyo No. 3 Sumitomo Metal Industries Co., Ltd. F term (reference) 2G001 AA04 AA05 BA11 CA04 DA01 DA02 GA01 HA01 JA17 KA01 LA02 MA05 NA15 NA16 SA02 SA03 SA12 2G088 EE29 FF09 GG05 JJ01 JJ11 JJ29 JJ30 LL26

Claims (6)

[Claims] A reflector (1) is provided on one of the test specimens (S).
8), a neutron source (15) and a neutron measuring element (17) are arranged on the other side of the measurement specimen (S), and the reflection material (1) emitted from the neutron source (15) and
The neutron measuring element (17) counts the thermal neutrons (R) that pass through the measuring test body (S) among the thermal neutrons (R) whose scattering has been slowed down by 8), thereby measuring the measuring test body (S). A method for measuring the content of a neutron absorbing substance, which measures the content of a neutron absorbing substance. 2. A calibration curve representing the correlation between the surface density of the neutron absorbing substance and the count value is prepared in advance using the measurement specimen (S) in which the surface density of the contained neutron absorbing substance is known, The measurement value of the measurement specimen (S) at the time of the main measurement is determined, and the measurement at the time of the main measurement is performed in consideration of the attenuation over time from the reference measurement of the correlation to the main measurement and the correlation. The method for measuring the content of a neutron absorbing substance according to claim 1, wherein the content of the neutron absorbing substance is measured by determining the surface density of the neutron absorbing substance in the test body (S). 3. The relationship with the calibration curve is clarified by confirming that the ratio of the count values of two types of standard specimens having known and different surface densities of the contained neutron absorbing substances is within a range based on propagation of an error. 3. The method for measuring the content of a neutron absorbing substance according to claim 1, wherein the validity of the measurement system including the neutron source is verified. 4. The method according to claim 1, wherein each of two types of standard test specimens having different known surface densities of the neutron-absorbing substances contained therein takes into account decay with time from the reference measurement to the main measurement. In each of the two types of standard specimens, the actual measurement count value at the time of the main measurement is within a range of three times the standard deviation of the theoretical count value. 2. The method according to claim 1, wherein the confirmation verifies the validity of the measurement system including the neutron source.
4. The method for measuring the content of a neutron absorbing substance according to any one of items 1 to 3. 5. The test specimen (S) is stainless steel, the neutron absorbing substance is boron, the reflector (18) is polyethylene, and the neutron measuring element (17) is boron trifluoride. The method for measuring the content of a neutron absorbing substance according to any one of claims 1 to 4, which is a counter tube. 6. A neutron irradiator for use in the method for measuring a content rate of a neutron absorbing substance according to claim 1, wherein a base part 14 formed of a neutron moderator is provided.
A second neutron shielding material 13 for covering the side surfaces and bottom portions of the holes for the first and second collimators 11 and 12 formed in the base portion 14 and the outer surface of the base portion 14; A neutron source 15 disposed at the bottom of the first collimator 11; and a neutron detection element 17 disposed at the bottom of the second collimator 12. The first collimator 11 and the second collimator 12 A neutron irradiator in close proximity.