JP2001242101A - Method and apparatus for nondestructive measurement of density of number of atoms in - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method, capable of nondestructively measuring the num ber density of thermal neutron absorbing atoms, even when thermal neutron absorbing atoms are distributed in thickness and number density so as to be in capable of transmission of thermal neutrons. SOLUTION: In the method for measuring the number density of atoms large in the cross-sectional area absorbing thermal neutrons, contained in the object to be measured by irradiating an object 25 to be measured with neutrons 8 and measuring neutron-capturing gamma rays 26 by a gammer-ray detector 5, the neutrons 8 applied to the object 25 to be measured are set to epithermal neutrons and energy of such a degree that the neutrons become thermal neutrons, when they pass through the surface 25 on the neutron-emitting side of the object to be measured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for nondestructively measuring the number density of thermal neutron-absorbed atoms by irradiating neutrons and measuring neutron capture gamma rays.

[0002]

2. Description of the Related Art Irradiation of a non-measurement object with radiation, measurement of attenuation due to absorption of a specific element after transmission, and measurement of radiation due to a nuclear reaction with the specific element, etc., make the non-destructive measurement of the element. There are a variety of ways to evaluate the quantity. Japanese Patent Application Laid-Open No. 5-28887, which is a conventional example, discloses that an object to be measured 1 (for example, a nuclear fuel pellet) containing boron ¹⁰ B is irradiated with a thermal neutron 3 from a neutron source 2 and a neutron capture gamma ray 4 is gamma ray as shown in FIG. By measuring with the detector 5, boron
^A method for performing a nondestructive measurement of the number density of ¹⁰ B is shown. Here, the neutron capture gamma ray means not only the gamma ray due to the (n, γ) reaction but also all the emitted gamma rays due to the neutron absorption. Neutrons are classified according to their speed, that is, kinetic energy. Larger neutrons are called fast neutrons, epithermal neutrons, and thermal neutrons. Each energy range is roughly ~ 6ke
V, 6 keV-0.6 eV, 0.6 eV-. In the prior art non-destructively measuring the amount of boron ¹⁰ B zirconium diboride coating 6 to be the subject matter that has been subjected to nuclear fuel pellet 1 by a sputtering coating process. This method takes advantage of the fact that boron ¹⁰ B has a very high thermal neutron absorption cross section. Here, the neutron absorption cross section is a value representing the probability of absorbing neutrons of the atom. When the nucleus of boron ¹⁰ B absorbs neutrons, the boron ¹⁰ B emits alpha particles and simultaneously emits gamma rays of characteristic energy. Therefore, by irradiating the object to be measured with thermal neutrons and counting characteristic gamma rays, boron ¹⁰ B
Can be measured.

[0003]

However, the above conventional example has the following problems. The above prior art is valid only for the object to be measured such as by applying a thin boron ¹⁰ B by sputtering. As shown in FIG. 3, when the thermal neutron absorbing atoms 7 are distributed with a certain degree of thinness and number density, the irradiated thermal neutrons 3 are only partially absorbed, and the absorption ratio and the captured gamma dose 4 are determined by the thermal neutrons. Correlates with the density of the absorbed atoms 7. However, when the thermal neutron-absorbing atoms 7 have a certain thickness and number density and are distributed as shown in FIG. 4, even if the irradiated thermal neutrons 3 penetrate a little deeper from the surface, they are eventually absorbed by the thermal neutron-absorbing atoms 7. Therefore, the absorption ratio and the captured gamma dose 3 do not correlate with the density of the thermal neutron absorption atom 11, and the density of the atom cannot be measured. The neutron permeability (shown by 3A in FIG. 3)
The distribution thickness and the number density of the atoms which affect the above are different depending on the atoms.

An object of the present invention is to provide a method and a method for non-destructively measuring the number density of thermal neutron-absorbing atoms even when the thermal neutron-absorbing atoms are distributed with a thickness and a number density such that thermal neutrons cannot pass therethrough. It is to provide a device.

[0005]

According to the present invention, an object to be measured is irradiated with epithermal neutrons having energy enough to become thermal neutrons when passing through the surface on the neutron emission side of the object to be measured. A non-destructive method for measuring the atomic number density, which non-destructively measures the number density, is disclosed.

Further, the present invention irradiates the object with epithermal neutrons having energy enough to become thermal neutrons when passing through the surface on the neutron emission side of the object to be measured. A non-destructive atomic number density measurement method for non-destructively measuring the atomic number density by utilizing the correlation between the count value and the number density of the thermal neutron-absorbed atoms is disclosed.

[0007] The present invention further child is boron ¹⁰ B, the object side thereof forms the control rods of the nuclear reactor, a stainless steel tube having an inner diameter of 3~5mm encapsulating boron ¹⁰ B, of the epithermal neutron A non-destructive method for measuring atomic number density, wherein the energy range is from 10 eV to 100 eV, is disclosed.

Further, the present invention discloses a non-destructive method for measuring atomic number density, characterized in that a neutron source of neutrons is Californium 252 and a moderator is placed between the neutron source and an object to be measured.

Further, according to the present invention, there is provided an epithermal neutron generating means having such an energy as to become a thermal neutron when passing through a neutron emission side surface of an object to be measured, and counting the number of neutron capture gamma rays by the epithermal neutron. γ-ray detector, means for detecting the number density of the atoms by utilizing the correlation between the count value of the γ-ray detector and the number density of thermal neutron-absorbing atoms, non-destructive measurement of the atomic number density An apparatus is disclosed.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The concept of the present invention will be described below with reference to FIG. In the present invention, a neutron is used as a radiation source in an epithermal region (having an energy of about 0.6 eV to 6 keV, preferably 10 eV to 100 eV). Further, it is assumed that the measured object 7A has a certain thickness or more. Since the thermal neutron absorption atoms 7 in the measured object 7A have a small absorption cross section for neutrons in the epithermal region, the incident epithermal neutrons 8
Proceeds without being absorbed in the region E ₁ to the surface close to the exit side, is reduced at the same time, the time to proceed to the area E ₂ of the exit-side surface as this is a (the thickness to be reduced to a thermal neutron It refers to thickness such as with the E ₁ and E _2). In this region, since the distance to the emission side surface of the device under test 10 is short, the same permeability as that in which thermal neutrons pass through a thin neutron absorbing atomic region as shown in FIG. 3 (indicated as passing neutrons 3A and 12). And the neutrons are only partially absorbed and emit captured gamma rays 11. At this time, the absorption ratio and the captured gamma dose correlate with the density of the thermal neutron absorbing atoms 7. Therefore, if the emitted gamma rays are counted, the number density of the thermal neutron absorbing atoms 7 in the measured object 10 can be measured. However, how much the energy of the neutrons to be irradiated is set depends on each element, the number density, and the thickness of the neutrons. Therefore, it is necessary to optimize the neutrons to become thermal neutrons near the neutron emission surface. As described above, even when the thermal neutron-absorbing atoms are distributed with a thickness and a number density such that thermal neutrons cannot pass therethrough, non-destructive measurement of the number density of the thermal neutron-absorbing atoms becomes possible.

The present invention will be described in detail based on embodiments. Here the pick is a method of measuring the number density of boron ¹⁰ B included in the form a control rod used to control the reactor was filled with boron ¹⁰ B tube. The control rod used in the boiling water reactor is schematically shown in FIG. 6, and a stainless steel tube 21 in which B ₄ C is sealed is placed in a stainless steel sheath 20. Boron ¹⁰ B of the isotopes of boron sufficient to have a large thermal neutron absorption cross section, is used to control material thermal reactor. However, the cross-sectional area is small for neutrons in the epithermal and high-speed regions,
High permeability.

[0012] shows the absorption cross-section of boron ¹⁰ B in FIG. The present invention measures atoms having a large thermal neutron absorption cross section, but it is sufficient that the absorption cross section for the thermal energy region is larger in the other epithermal and fast energy regions as shown in FIG. In addition, even when the absorption cross section is large in the epithermal region, irradiating neutrons having energy slightly higher than the energy of the peak of the cross section, and passing through the neutron emission side surface of the object to be measured, The effect of the present invention can be obtained by having the above energy. The natural isotope composition ratio of boron ¹⁰ B is about 20%. In some cases, natural B ₄ C is used as it is, and in other cases, the composition ratio of boron ¹⁰ B is increased, and non-destructive boron ¹⁰ B is used. It is necessary to provide a method for confirming the composition ratio by measuring the number density.

In this embodiment, a stainless steel tube 21 in which B ₄ C is sealed is irradiated with epithermal neutrons, and boron is removed.
The number density of boron ¹⁰ B is determined by counting the gamma rays emitted as a result of ¹⁰ B neutron absorption. Boron ¹⁰
It is known that when B absorbs neutrons, neutron-induced gamma rays corresponding to the following reactions are observed.

Embedded image

Embedded image ⁷ * Li in the above formula represents the excited state of the lithium, ⁴ the He is ⁷ Li represents helium represents a stable state of the lithium. Gamma rays are emitted as a result of the decay of lithium from an excited state to a stable state. Reaction Scheme 1 above shows that when boron ¹⁰ B in B ₄ C absorbs neutrons (n), the boron ¹⁰ B is converted to excited lithium and helium. Equation 2 above shows that as the lithium in the excited state decays from its excited state to a stable state, it emits gamma rays having an energy level of about 478 keV. Therefore, about 478 keV
The presence of a gamma ray having an energy level of ¹⁰ B indicates the presence of boron ¹⁰ B in B ₄ C. Measuring the number of gamma rays emitted at an energy level of 478 keV provides an indication of the amount of boron ¹⁰ B in B4C. By neutron irradiation, never stainless steel is destroyed, also this radiation is essentially there nondestructive also against boron ¹⁰ B. The reason for this is that in this method of quantifying non-destructive boron ¹⁰ B, only one or less of the ^{10 10} boron atoms is used.

Next, the theoretical description of the measuring device will be described with reference to FIG. The neutrons 8 irradiated from the epithermal neutron generator 22 are B ₄ C 25 sealed in a stainless steel tube 21.
Inside, and initially in the epithermal region,
Small cross-sectional area of ¹⁰ B, not absorbed. Then, it decelerates at the same time as it advances, and decelerates to thermal neutrons in the B ₄ C surface region 25A on the emission side. Here, due to the short distance to the emission side surface, the neutrons are only partially absorbed by boron ¹⁰ B and emit a capture gamma ray 26 of about 478 keV. Rate and gamma dose of neutrons absorbed at this time is proportional to the number density of boron ¹⁰ B, the number density instruction information can be obtained by counting by a gamma ray measurement instrument 28.

Next, the injection side B ₄ C used in the present invention is used.
A method for obtaining the energy of epithermal neutrons that is reduced to thermal neutrons in the surface region will be described. Here, a method using simulation will be described as an example. FIG. 8 shows a simulation system. The DUT 21 is a stainless steel tube having an inner diameter of about 4 mm filled with B ₄ C. At this time, a count value of transmitted neutrons at an arbitrary point P on the transmission side when the neutron 8 is irradiated is obtained. Here, the neutron energy is changed from 0 eV to ¹⁰ MeV, and the isotope composition ratio of boron ¹⁰ B of B ₄ C is set to 20%.
FIG. 9 shows the results when the values were set to 50%. Because neutrons when the energy of the irradiated neutrons 10keV~ and high is transmitted without being hardly absorbed, the count value of the transmission neutrons is large, no difference due to the difference in density of boron ¹⁰ B. If the energy of the irradiated neutrons conversely the 0~1eV are neutrons mostly absorbed Again no difference due to the difference in density of boron ¹⁰ B. On the other hand, the energy of the irradiation neutron is 1 to 10
In the case of 0 eV, the number density of boron ¹⁰ B is 50%
The count value is smaller for the composition ratio. This is reduced to thermal neutrons in the injection-side B ₄ C surface region as described in FIG.
The proportion of absorbed neutrons is proportional to the number density of boron ¹⁰ B. FIG. 10 shows the difference ratio between the results of the composition ratios of 20% and 50%. This shows that the optimum neutron energy range is 10 eV to 100 eV.

Next, a measuring apparatus is shown in FIG. The neutron source 30 is made of carbonium 252 ( ²⁵² Cf). High-speed neutrons generated from the neutron source are decelerated to epithermal neutrons by the moderator 31, and are irradiated on the stainless steel tube 21 containing B ₄ C. This neutron
Gamma rays emitted by absorption of ¹⁰ B are counted by the gamma ray detector 34. The periphery of the γ-ray detector 34 is shielded by a lead shield 33 to prevent intrusion of background γ-rays. 32 is a lead collimator.

FIGS. 12 and 13 show examples of measurement by the above measuring apparatus. FIG. 12 shows an example in which the boron ¹⁰ B composition ratio is 20%,
FIG. 13 shows an example of 50%. The scale is omitted for both,
Made on the same scale for comparison. The peak counts N ₁ and N ₂ , respectively, were due to 478 keV captured gamma rays. The larger the composition ratio, that is, the number density,
It can be seen that there is a correlation that the gamma ray count value increases. Therefore, by obtaining the gamma ray count by more devices, it can be converted to the number density of boron ¹⁰ B by calculation as shown in FIG. 14. The graph of FIG.
The case of changing the number density of ¹⁰ B creates, by obtaining a number of ways or measured data.

The present invention is effective for other elements whose neutron absorption cross section increases in the thermal region. However, the energy level of the neutrons to be irradiated depends on the individual elements, the number density, and the thickness of the neutrons. Therefore, it is best to use the method described above to generate thermal neutrons near the neutron emission surface. Need to be The energy of the captured gamma rays also differs depending on the element.

[0019]

As described in detail above, according to the present invention,
Even if the thermal neutron-absorbing atoms have a thickness and a number density such that the irradiated thermal neutrons cannot penetrate, the neutron absorption ratio and the captured gamma dose should be correlated with the number density by irradiating with epithermal neutrons. And the non-destructive measurement of the thermal neutron absorption atom number density by counting gamma rays.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is an explanatory diagram of a conventional measuring method.

FIG. 3 is an explanatory diagram showing neutron absorption when a thermal neutron absorbing atom is irradiated with thermal neutrons.

FIG. 4 is an explanatory diagram showing neutron absorption when a thermal neutron absorbing atom is irradiated with thermal neutrons.

FIG. 5 is a schematic view of a control rod.

6 is a diagram showing the absorption cross-section of boron ¹⁰ B.

FIG. 7 is an explanatory diagram of the measuring method of the present invention.

FIG. 8 is a diagram showing a simulation system of the present invention.

FIG. 9 is a diagram showing transmission neutron form values depending on the composition ratio.

FIG. 10 is a diagram illustrating a difference ratio of transmitted neutrons when neutron energy is changed.

FIG. 11 is a diagram showing an example of a measuring apparatus according to the present invention.

12 is a diagram showing experimental results in the case of boron ¹⁰ B composition ratio of 20%.

FIG. 13 is a view showing an experimental result when the boron ¹⁰ B composition ratio is 50%.

FIG. 14 is a diagram showing the correlation between the γ count value of boron ¹⁰ B and the number density.

[Description of Signs] 1 Nuclear fuel pellet 2 Neutron source 3 Thermal neutron 4 Neutron capture gamma ray 5 Gamma ray detector 6 Zirconium diboride coating 7 Thermal neutron absorption atom 8 Epithermal neutron 10 Measured object 11, 26 Neutron capture gamma ray 12 Transmitted neutron Reference Signs List 20 control rod 21 stainless steel tube in which B ₄ C is sealed 22 epithermal neutron irradiation device 25 B ₄ C 25 A region where neutrons become thermal neutrons E ₁ region where neutrons are epithermal E ₂ region where neutrons become thermal neutrons N _{1 ,} N ₂ characteristic gamma ray peak P Neutron content measurement point

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G21C 17/10 G21C 17/10 A (72) Inventor Akira Koizumi 3-1-1 Kochicho, Hitachi City, Ibaraki Prefecture No. Within the Nuclear Power Division, Hitachi, Ltd. (72) Inventor Tetsushi Hino 7-2-1, Omika-cho, Hitachi City, Ibaraki Prefecture F-term in the Electric Power and Electricity Research Laboratory, Hitachi, Ltd. 2G001 AA04 AA09 CA02 DA02 DA03 FA01 GA01 JA15 KA01 LA20 MA06 NA15 SA03 2G075 AA03 BA20 CA39 DA07 DA18 EA03 FA06 FA18 FB05 FB08 FC04 FC19 GA19 GA21 2G088 EE21 EE25 EE29 FF04 FF15 GG21 JJ01 JJ11 JJ29 KK01 KK24 LL08 LL28

Claims (5)

[Claims] An object to be measured is irradiated with epithermal neutrons having energy enough to become thermal neutrons when passing through the surface on the neutron emission side of the object to measure the number density of neutron-absorbing atoms in a non-destructive manner. A non-destructive method for measuring the atomic number density to be measured. 2. A method for irradiating an object with epithermal neutrons having energy enough to become thermal neutrons when passing through a surface on a neutron emission side of the object to be measured, and calculating a count value of neutron capture gamma rays by the epithermal neutrons. A non-destructive measurement method of the atomic number density, wherein the number density of the thermal neutron-absorbing atoms is non-destructively measured by utilizing the correlation. 3. The method according to claim 1, wherein the atoms are boron ¹⁰ B, and the object to be measured is a stainless steel tube enclosing boron ¹⁰ B and having an inner diameter of 3 to 5 mm, which forms a control rod of a nuclear reactor. The non-destructive method for measuring the atomic number density according to claim 2, wherein the value is 10 eV to 100 eV. 4. The method according to claim 3, wherein a neutron source of the neutron is Californium 252, and a moderator is provided between the neutron source and the object to be measured. 5. An epithermal neutron generating means having an energy sufficient to become a thermal neutron when passing through a surface on a neutron emission side of an object to be measured, and gamma ray detection for counting the number of neutron capture gamma rays by the epithermal neutron. A non-destructive atomic number density measuring apparatus comprising: a detector; and means for detecting the atomic number density by utilizing the correlation between the count value of the γ-ray detector and the thermal neutron absorption atom number density.