JP4723966B2 - Np content inspection method and Np content inspection device - Google Patents

Description

The present invention relates to an Np content inspection method and an Np content inspection device for inspecting whether or not a nuclear fuel contains a predetermined amount of ²³⁷ Np.

  When nuclear fuel is manufactured, it is important to confirm the concentration and distribution of fissile material so that the nuclear fuel has predetermined characteristics in the nuclear reactor. Therefore, during nuclear fuel production, in order to confirm the content of non-destructive fissionable materials, especially enriched uranium, neutrons are irradiated to the nuclear fuel by an external neutron source, and fission neutrons and gamma rays generated from the nuclear fuel are measured. The way to be done.

In addition to such a fissile material, a nuclear fuel for obtaining an effect of increasing resistance to nuclear diffusion, for example, by adding neptunium ( ²³⁷ Np) having a mass number of 237 to a nuclear fuel in advance is disclosed in Patent Document 1. And the like.

Pu extracted by reprocessing spent nuclear fuel may be diverted to a nuclear bomb and is subject to strong international regulations. The higher the ratio of ²³⁹ Pu and ²⁴¹ Pu with a large fission cross section among Pu isotopes, the higher the risk of being diverted to a nuclear bomb. Conversely, ²³⁸ Pu, ²⁴⁰ Pu, and ²⁴² Pu have a relatively small fission cross-section, and neutrons are always emitted by spontaneous fission, so these components hinder the production of nuclear bombs. This is because the spontaneous neutrons emitted by the Pu even-numbered nuclear component induce a fission chain reaction from an early point in time, thereby reducing the magnitude of the explosion. Further, ²³⁸ Pu has a very high calorific value due to α decay, so that it is difficult to handle it outside of a special facility and plays a role of protecting against unauthorized access for the purpose of theft.

In this way, even nuclear components of Pu composition, especially ²³⁸ Pu, has the property of preventing diversion to nuclear bombs, so spent fuel with a high ²³⁸ Pu composition has high resistance to nuclear proliferation and contributes to the peaceful use of nuclear energy. It can be an effective means to do this. Making spent fuel taken out of nuclear reactors into spent fuel with such high proliferation resistance by adding minor actinides represented by neptunium, americium and curium in advance during the production of nuclear fuel Is being considered.
JP-A-7-244182 Katoh et al., “Measurement of Thermal Neutron Capture Cross Section and Resonance Integral of the 237Np (n, γ) 238Np Reaction”, Journal of NUCLEAR SCIENCE and TECHNOLOGY, Japan Atomic Energy Society, 2000, Vol. 40, No. 8, p.559-568

In order to increase the proliferation resistance of spent fuel, when adding ²³⁷ Np to the nuclear fuel before use, a means for nondestructively confirming that the manufactured nuclear fuel surely contains ²³⁷ Np It is extremely important to actually operate such a fuel. However, the Np content of nuclear fuel to which ²³⁷ Np has been added in advance has not been confirmed at the time of production in a non-destructive manner. In addition, in the case of nuclear fuel using recovered uranium obtained by reprocessing spent fuel as a base material, since the base material contains various radionuclides, simply measuring the gamma ray spectrum of ²³⁷ Np, ²³⁷ Np content may not be reliably confirmed.

In order to solve the above-described problems, an object of the present invention is to enable non-destructive confirmation that nuclear fuel contains a predetermined amount of ²³⁷ Np.

To achieve the above object, the present invention provides an Np-containing inspection method for inspecting whether a solid nuclear fuel contains a predetermined amount of ²³⁷ Np before the nuclear fuel is used in a nuclear reactor. Of the gamma rays emitted from the nuclear fuel, a gamma ray measurement first step for measuring a count rate of gamma rays emitted from ²³³ Pa which is a daughter nuclide of ²³⁷ Np , and the count rate measured in the first step of gamma ray measurement is the atomic rate. fuel is greater than a predetermined positive reference value can be determined that contain a predetermined amount of ^{237 Np,} the nuclear fuel and the determining compares a first step containing a predetermined amount of ^{237 Np,} the comparison first when the nuclear fuel did not determination of containing a predetermined amount of ^{237 Np} in step, an irradiation step of irradiating neutrons to the nuclear fuel, after the irradiation step, the nuclear fuel Of the gamma ray et radiation, gamma ray measurement second step of measuring the count rate of gamma rays emitted from the ^{238 Np,} the nuclear fuel the gamma ray measurement count rates measured in the second step is contains a predetermined amount of ^{237 Np} If you have a greater than a predetermined judgment reference value can be determined, the nuclear fuel is determined to contain a predetermined amount of ^{237 Np,} the nuclear fuel when the count rate is less than the determination reference value ²³⁷ A second comparison step for determining that Np is not contained in a predetermined amount.

Further, the present invention relates to an Np content inspection method for inspecting whether or not a solid nuclear fuel contains a predetermined amount of ²³⁷ Np before the nuclear fuel is used in a nuclear reactor. that the gamma ray measuring first step of measuring the count rate of gamma rays emitted from ^{233 Pa,} the daughter nuclide of ²³⁷ Np of gamma rays, the daughter nuclide of ²³⁷ Np of the gamma rays emitted from the nuclear fuel ²³³ Pa reference gamma ray measurement and the first step, the gamma ray measuring first step count rate measured by the reference gamma ray measurement first ratio count rate measured in step the atomic measuring the count rate of gamma rays other than gamma rays emitted from the fuel is greater than a predetermined positive reference ratio that can be determined that the predetermined amount of ^{237 Np,} the nuclear fuel contains a predetermined amount of ^{237 Np} A comparison first step that determines that the nuclear fuel contains a predetermined amount of ²³⁷ Np in the comparison first step, and an irradiation step that irradiates the nuclear fuel with neutrons; After the irradiation step, among the gamma rays emitted from the nuclear fuel, a second step of gamma ray measurement for measuring the counting rate of gamma rays emitted from ²³⁸ Np, and after the irradiation step, emitted from the nuclear fuel. a reference gamma ray measurement second step of measuring the count rate of gamma rays other than gamma rays emitted from ^{233 Pa} and ^{238 Np} daughter nuclides of ²³⁷ Np of gamma rays, the count rate measured by the gamma ray measuring second step The ratio to the counting rate measured in the second step of the reference gamma ray measurement is a predetermined criterion ratio at which the nuclear fuel can be determined to contain a predetermined amount of ²³⁷ Np . Ri said nuclear fuel is greater is determined that the predetermined amount of ^{237 Np,} ratio the judgment for count rate measured by the reference gamma ray measurement second step count rate measured by the gamma ray measuring second step A comparison second step for determining that the nuclear fuel does not contain a predetermined amount of ²³⁷ Np when the ratio is smaller than a reference ratio.

The present invention also provides a gamma ray detector, a neutron detector, and a neutron detector in an Np-containing inspection device that inspects whether a solid nuclear fuel contains a predetermined amount of ²³⁷ Np before the nuclear fuel is used in a nuclear reactor. is released from the nuclear fuel that is not irradiated with a predetermined amount of ²³⁷ Np is ^{237 Np} count rate the nuclear fuel of the gamma rays emitted from ^{233 Pa,} the daughter nuclide of the gamma rays detected by the gamma ray detectors When the nuclear fuel is larger than a predetermined positive reference value that can be determined to be contained, it is determined that the nuclear fuel contains a predetermined amount of ²³⁷ Np. The nuclear fuel is emitted from the nuclear fuel irradiated with neutrons, and is detected by the gamma ray detector. Of the detected gamma rays, the count rate of the gamma rays emitted from ²³⁸ Np can be determined that the nuclear fuel contains a predetermined amount of ²³⁷ Np. When it is larger than a predetermined criterion value, it is determined that the nuclear fuel contains a predetermined amount of ²³⁷ Np, and ^{238 of the} gamma rays emitted from the nuclear fuel irradiated with neutrons and detected by the gamma ray detector. A determination device for determining that the nuclear fuel does not contain a predetermined amount of ²³⁷ Np when the count rate of gamma rays emitted from Np is smaller than the determination reference value, and a signal output from the gamma ray detector And a transmission means for transmitting to the determination device.

The present invention also provides a gamma ray detector, a neutron detector, and a neutron detector in an Np-containing inspection device that inspects whether a solid nuclear fuel contains a predetermined amount of ²³⁷ Np before the nuclear fuel is used in a nuclear reactor. Of gamma rays emitted from the nuclear fuel having a count rate of gamma rays emitted from ²³³ Pa, which is a daughter nuclide of ²³⁷ Np , among the gamma rays detected by the gamma ray detector. of ²³⁷ Np ratio the nuclear fuel for count rate of gamma rays other than gamma rays emitted from ^{233 Pa,} the daughter nuclide of the is greater than a predetermined positive reference ratio that can be determined that the predetermined amount of ^{237 Np} The nuclear fuel is determined to contain a predetermined amount of ²³⁷ Np, and is emitted from the nuclear fuel irradiated with neutrons, and the gamma ray detector Of the gamma rays detected by ²³⁸ Np, the count rate of gamma rays emitted from ²³⁸ Np is ²³³ Pa which is the daughter nuclide of ²³⁷ Np among the gamma rays emitted from the neutron-irradiated nuclear fuel and emitted from the nuclear fuel. the nuclear fuel own the ^{237 Np} is larger than a predetermined determination reference ratio can be determined that the ratio count rate of gamma rays other than gamma rays emitted said nuclear fuel contains a predetermined amount of ^{237 Np} and from ^{238 Np} The neutron-irradiated count rate of gamma rays emitted from ²³⁸ Np out of the gamma rays emitted from the nuclear fuel irradiated with neutrons and detected by the gamma-ray detector released from the nuclear fuel, the daughter nuclide of ²³⁷ Np of the gamma rays emitted from the nuclear fuel ²³ A determining device and the ratio count rate of gamma rays other than gamma rays the nuclear fuel when the criterion ratio smaller does not contain a predetermined amount of ^{237 Np} emitted from Pa and ^{238 Np,} the gamma-ray detector And a transmission means for transmitting a signal output from the determination device to the determination device.

According to the present invention, it can be confirmed nondestructively that the nuclear fuel contains a predetermined amount of ²³⁷ Np.

  An embodiment of Np content inspection of nuclear fuel according to the present invention will be described with reference to the drawings. In addition, the same code | symbol is attached | subjected to the same or similar structure, and the overlapping description is abbreviate | omitted.

[Embodiment 1]
FIG. 2 is a block configuration diagram of the Np-containing inspection apparatus according to the first embodiment of the present invention. FIG. 2 also shows the nuclear fuel to be inspected.

  The Np-containing inspection apparatus includes a gamma ray detector 1, a determination apparatus 6, and a cable 7. The gamma ray detector 1 and the determination device 6 are connected by a cable 7. The gamma ray detector 1 is located in the vicinity of the nuclear fuel 8 and detects gamma rays 9 emitted from the nuclear fuel 8. In general, the nuclear fuel 8 forms a fuel assembly by bundling a plurality of cylindrical cladding tubes containing nuclear fuel materials into a prismatic shape. The gamma ray detector 1 may be disposed at a plurality of positions in the longitudinal direction of the nuclear fuel 8, or the nuclear fuel 8 and the gamma ray detector 1 can move relative to each other to measure a plurality of positions. You may keep it. Further, before assembling the fuel assembly, the Np content inspection may be performed when the fuel rod in which the nuclear fuel material is contained in the cladding tube is completed.

Next, an Np content inspection method for inspecting whether or not a predetermined amount of ²³⁷ Np is contained in the nuclear fuel using this Np content inspection device will be described.

  FIG. 1 is a flowchart of Np content inspection according to the first embodiment of the present invention.

  The spectrum of gamma rays generated from the nuclear fuel 8 before being irradiated with neutrons, such as before being loaded into the nuclear reactor, is measured by the gamma ray detector 1 (step S11). By using a Ge semiconductor detector or the like as the gamma ray detector 1, a gamma ray spectrum, that is, a gamma ray count rate for each minute energy range can be measured.

  The gamma ray detector 1 is moved with respect to the nuclear fuel 8 to measure the gamma ray count rate at a plurality of positions, or the plurality of gamma ray detectors 1 are used to include the nuclear fuel material in the range of the nuclear fuel 8 (fuel). You may measure in the several position of an effective part). Further, the gamma ray count rate may be averaged for each region or for the entire fuel effective portion.

The determination device 6 sets C237 as the counting rate of 312 keV gamma rays emitted by ²³³ Pa, which is a daughter nuclide of ²³⁷ Np, among the measured gamma rays (step S12). C237 may be a gamma ray counting rate of any energy as long as it is a gamma ray emitted from ²³⁷ Np or its daughter nuclide, ²³³ Pa.

Next, the determination device 6 compares C237 with the positive reference value and the negative reference value (step S13). If C237 is greater than or equal to the positive reference value, it is determined that the nuclear fuel 8 contains a predetermined amount of ²³⁷ Np. When C237 is less than the negative reference value, it is determined that the nuclear fuel does not contain a predetermined amount of ²³⁷ Np. This affirmative reference value is a reference that can be determined that a predetermined amount of ²³⁷ Np is contained in the nuclear fuel 8 to be inspected even in consideration of measurement errors, and the negative reference value is inspected even in consideration of measurement errors. This is a standard that can be determined that the target nuclear fuel 8 does not contain a predetermined amount of ²³⁷ Np. The positive reference value and the negative reference value are determined in advance by measurement or calculation using a standard gamma ray source. In determining the positive reference value and the negative reference value, the measurement accuracy of the gamma ray detector 1, the shape of the nuclear fuel 8 to be inspected, the concentration of the nuclear fuel material, and the like are taken into consideration.

If C237 is less than the positive reference value and greater than or equal to the negative reference value, the nuclear fuel to be inspected is irradiated with neutrons (step S14). Since the half life of ²³⁸ Np produced by neutron irradiation of ²³⁷ Np is about 2.1 days, irradiation for about 1 to 4 days is practical.

  3A and 3B are diagrams showing an irradiation apparatus that irradiates nuclear fuel with neutrons, where FIG. 3A is a plan view and FIG. 3B is a vertical cross-sectional view taken along line AA in FIG.

  The irradiation device 10 includes an irradiation base 11, a neutron source 12, and a neutron moderator 14. A neutron source guide tube 13 is disposed in the center of the irradiation stand 11 so that the neutron source 12 can be included. Moreover, the irradiation stand 11 has a space for arranging the nuclear fuel 8 so as to surround the neutron source guide tube 13. The neutron source 12 is introduced into the neutron source guide tube 13. The neutron moderator 14 is disposed between the space in which the fuel assembly 8 is disposed and the neutron source guide tube 13.

  As described above, the Np content inspection may be performed at the time when the fuel rod in which the nuclear fuel material is contained in the cladding tube is completed before assembling the fuel assembly. In such a case, the irradiation device 10 is assembled so as to have a space in which the fuel rod can be placed.

  The neutron source guide tube 13 may be made of a material having a small neutron moderating effect such as aluminum. The fast neutrons emitted from the neutron source 12 are decelerated in the neutron moderator 14 and are converted into thermal neutrons. At this time, if the neutron moderator 14 between the neutron source 12 and the nuclear fuel 8 is small, thermalization is not sufficiently performed. Conversely, when there are too many neutron moderators 14, the amount of thermal neutrons irradiated to the nuclear fuel 8 decreases. For this reason, although it depends on the material of the neutron moderator 14, the distance between the neutron source 12 and the nuclear fuel 8 is preferably 2 to 5 cm.

  Furthermore, neutrons are scattered in each direction inside the neutron moderator 14, but if the area around the nuclear fuel 8 is air, the probability of leaking out of the irradiation device 10 increases. For this reason, by arranging the neutron moderator 14 around the fuel assembly to be irradiated, thermal neutrons are scattered and reflected also from the outer peripheral portion, so that the irradiation efficiency is increased. Further, in order to prevent exposure of neutrons around the irradiation device 10, a shielding material (not shown) may be arranged around the irradiation stand 11.

By placing the nuclear fuel 8 to be inspected on the irradiation device 10, neutrons are irradiated. The neutron capture cross section of ²³⁷ Np has a large resonance peak when the neutron energy is around 0.5 eV. For this reason, a part of ²³⁷ Np contained in the nuclear fuel 8 captures neutrons decelerated by the neutron moderator 14 and becomes ²³⁸ Np. For the neutron moderator 14, polyethylene, water, paraffin, graphite or the like can be used. As the neutron source 12, ²⁵² Cf or the like can be used.

  In general, uranium fuel that does not contain plutonium as a fissile material is handled in the air, and the Np-containing inspection measurement according to the present invention can also be performed in the air. On the other hand, when handling a plutonium fuel containing plutonium as a fissile material, the nuclear fuel 8 may be handled in water from the viewpoint of radiation exposure protection. When the nuclear fuel 8 is handled in water, the neutron detector 1 and the irradiation device 10 are arranged in water. FIG. 4 shows the irradiation apparatus 10 in such a case.

  4A and 4B are diagrams showing an irradiation apparatus that irradiates nuclear fuel with neutrons in water, where FIG. 4A is a plan view, and FIG. 4B is a vertical cross-sectional view taken along line BB in FIG.

  When irradiating the nuclear fuel 8 with neutrons in water, the entire irradiation device 10 is submerged in water. When neutrons are irradiated to the nuclear fuel 8 in the air, the neutron moderator 14 is disposed in the vicinity of the neutron source 12 (FIG. 3), but when neutrons are irradiated to the nuclear fuel 8 in water, Since the water 15 decelerates neutrons, it is not necessary to separately arrange the neutron moderator 14 in the irradiation device 10.

  By inserting and installing the fuel assembly 8 in such an irradiation apparatus 10 and irradiating it for a predetermined time, and sequentially taking out and performing gamma ray spectrum measurement, neutron irradiation can be efficiently performed.

  After the neutron irradiation in step S14, the spectrum of gamma rays emitted from the nuclear fuel 8 is again measured by the gamma ray detector 1 (step S15).

The determination device 6 sets C238 as the sum of the count rates of 984 keV and 1029 keV gamma rays emitted by ²³⁸ Np among the measured gamma rays (step S16). Note that the counting rate of either 984 keV or 1029 keV may be C238. C238 may be a gamma ray counting rate of any energy as long as it is a gamma ray emitted by ²³⁸ Np.

Next, the determination device 6 compares C238 with the determination reference value (step S17). When C238 is equal to or greater than the determination reference value, it is determined that the nuclear fuel 8 is an Np-containing fuel containing a predetermined amount of ²³⁷ Np. When C238 is less than the determination reference value, the nuclear fuel 8 is a predetermined amount of ²³⁷ Np. It is determined that the fuel does not contain Np. The determination reference value is a reference with which it can be determined that a predetermined amount of ²³⁷ Np is contained in the nuclear fuel 8 to be inspected after neutron irradiation in step S14. As with the positive reference value and the negative reference value, the determination reference value is determined in advance by measurement or calculation using a standard gamma ray source.

In the first embodiment, by comparing the C237 in step S1 3 with negative reference value, without irradiating neutrons nuclear fuel 8, nuclear fuel 8 is allowed to be determined not to contain ^{237 Np.} However, in step S1 ^7, since it can be determined that it does not contain the ²³⁷ Np, may only compare the positive reference value in step S1 3.

FIG. 5 shows a gamma ray spectrum obtained by measuring gamma rays emitted from ²³⁸ Np and the like generated by irradiating ²³⁷ Np with neutrons, as shown in Non-Patent Document 1. The horizontal axis is a channel corresponding to the energy of gamma rays, and the vertical axis is the count of gamma rays in each channel at a predetermined time. As shown in FIG. 5, ²³⁸ Np emits a number of energy levels of gamma rays, of which 984 keV and 1029 keV have a high emission rate. Therefore, if the count rate of one or both of the 984 keV and 1029 keV gamma rays is C238, the measurement accuracy of C238 is increased.

  As described above, it is possible to determine whether or not a predetermined amount of 237Np is contained in the manufactured nuclear fuel by the Np content inspection measurement of the first embodiment.

[Embodiment 2]
FIG. 6 is a flowchart of the Np content inspection according to the second embodiment of the present invention.

In the second embodiment, gamma rays emitted from ²³⁵ U, ²³⁸ U, and the like contained in the nuclear fuel 8 are selected as a reference, and the count rate of gamma rays emitted from ²³³ Pa, which is a daughter nuclide of ²³⁷ Np and ²³⁷ Np, is selected. This is a method of measuring the ratio. In the second embodiment, Step S12, Step S13, Step S16 and Step S17 in the Np content inspection in Embodiment 1 are replaced with Step S22, Step S23, Step S26 and Step S27, respectively. Further, the operation of the determination device 6 of the first embodiment is changed corresponding to this replacement.

In step S22, the determination device 6 sets A as the count rate of 312 keV gamma rays emitted from ²³³ Pa, which is the daughter nuclide of ²³⁷ Np, and B as the count rate of 186 keV gamma rays emitted from ²³⁵ U, that is, A / B is R237. Note that A may be a gamma ray counting rate of any energy as long as it is a gamma ray emitted from ²³⁷ Np or its daughter nuclide, ²³³ Pa, and B may be a ²³⁷ Np contained in the nuclear fuel 8. Any gamma ray counting rate of any energy may be used as long as it is a gamma ray emitted from other substances. For example, as B, a count rate of 1001 keV gamma rays emitted by ²³⁸ U daughter nuclide ²³⁴ Pa may be used.

  The determination device 6 determines that the nuclear fuel 8 does not contain a predetermined amount of 237Np when R237 is equal to or greater than the positive reference ratio.

  When R237 is less than the positive reference ratio and is equal to or greater than the negative reference ratio, neutrons are irradiated to the nuclear fuel 8 to be inspected using the irradiation apparatus 10 as in the first embodiment (step S14).

In step S26, the determination device 6 determines that the sum of the count rates of 984 keV gamma rays and 1029 keV gamma rays emitted by ²³⁸ Np is C from the gamma ray spectrum measured in step S12, and 1001 keV gamma rays emitted by ²³⁸ U daughter nuclide ²³⁴ Pa. The count rate of D is D, and the ratio, that is, C / D is R238. Note that C may be a gamma ray counting rate of any energy as long as it is emitted by ²³⁸ Np, and D is emitted from substances other than ²³⁷ Np and ²³⁸ Np contained in nuclear fuel 8. The gamma ray count rate of any energy may be used.

In step S27, the determination device 6 compares R238 obtained in step S26 with the determination reference ratio. If R238 is equal to or greater than the determination reference ratio, the determination is made that the nuclear fuel 8 contains a predetermined amount of ²³⁷ Np. If R238 is less than the determination reference ratio, it is determined that the nuclear fuel 8 does not contain a predetermined amount of ²³⁷ Np.

  The affirmative reference ratio, negative reference ratio, and determination reference ratio can be calculated in the same manner as the positive reference value, negative reference value, and determination reference value of the first embodiment.

FIG. 7 shows an example in which a gamma ray spectrum emitted from a nuclear fuel (uranium fuel) composed of ²³⁸ U and ²³⁵ U is measured in a state where neutrons are not irradiated. In uranium fuel, ²³⁸ U and ²³⁵ U occupy most of the composition, and from Fig. 7, it can be seen that 186 U gamma rays of ²³⁵ U and 1001 keV gamma rays of ²³⁴ Pa which is a daughter nuclide of ²³⁸ U can be measured with high accuracy. The In addition, the ²³⁵ U 186 keV gamma ray has the advantage that it is relatively close in energy to the 312 keV gamma ray and can be easily compared. ^{The 238} U daughter nuclide, ²³⁴ Pa, 1001 keV gamma ray is slightly different from the 312 keV gamma ray, but ²³⁸ U is a base material for uranium fuel and is suitable for the standard of quantity. ^In the comparison of ²³⁸ Np 984 keV gamma rays and 1029 keV gamma rays, a peak of 1001 keV gamma rays is observed between them.

  Since the Ge semiconductor detector has an energy resolution of about 1.3 keV for a 1.33 MeV gamma ray, it is easy to discriminate and measure the peaks of 984 keV, 1001 keV and 1029 keV gamma rays.

For plutonium fuel mixed with plutonium as a fission material, a ratio between the count rate of 312 keV gamma ray of ²³³ Pa and the count rate of 375 keV gamma ray of ²³⁹ Pu may be used as R237. ²³⁹ Pu emits a large number of energy gamma rays, and those having a large emission rate when the energy is 100 keV or higher include 129 keV, 375 keV, and 414 keV. Since the concentration of ²³⁹ Pu which is a fission material is high in plutonium fuel, the above-mentioned 129 keV, 375 keV, and 414 keV gamma rays can be used as a reference gamma ray that takes a ratio of ²³³ Pa to 312 keV gamma rays. Among them, 375 keV gamma rays having an energy close to 312 keV and a large emission rate are most suitable.

Further, when R238 is obtained, a count rate of ²³⁹ Pu 375 keV gamma rays may be used as D. A mixed oxide fuel (MOX fuel) in which uranium oxide and plutonium oxide are mixed can be considered as a usage form of plutonium fuel. At this time, since ²³⁸ U is contained in a large amount, 1001 keV gamma rays can be used as a reference gamma ray. When taking the form of rock fuel or the like not containing uranium, ²³⁹ Pu of 375 keV gamma rays can be used as the reference gamma rays.

  As in C237 and C238 of the first embodiment, in order to measure the absolute value of the gamma ray count rate and compare it with the reference value, the relationship between the gamma ray count rate and the measurement sensitivity inherent to the detector and the measurement system is grasped. It is important to determine the reference value. That is, the gamma ray count rate at which it can be determined that 237 Np is significantly contained in the nuclear fuel 8 depends on the sensitivity of the detector and the measurement system. Accordingly, the reference values for C237 and C238 can be determined by measurement or calculation using a standard radiation source, but it is necessary to determine them according to the detector sensitivity and measurement system.

  On the other hand, the method of measuring the ratio as in the second embodiment has an advantage that the dependency of the measurement system is reduced. That is, when the detector to be used is determined based on the energy of the two gamma rays taking the ratio, the ratio of the detector sensitivity is fixed. Also, the measurement efficiency ratio of the measurement system is less dependent on the system than when measuring the absolute value. By measuring the ratio, it is possible to reduce the uncertainty of the measurement value due to fluctuations in the positional relationship such as the distance between the measurement target and the detector, the amount of the measurement target, and the like in the measurement system.

[Embodiment 3]
FIG. 8 is a flowchart of Np content inspection according to the third embodiment of the present invention.

In the third embodiment, when the Np content inspection is performed on the plutonium fuel, measures are taken against the fact that the ²³³ Pa 312 keV gamma ray peak obtained by the gamma ray spectrum measurement is mixed with the ²³⁹ Pu 312 keV gamma ray as an interference peak. The step S22 in the Np content inspection in the second embodiment is replaced with the step S32 in the third embodiment. Further, the operation of the determination device 6 of the second embodiment is changed corresponding to this replacement.

  In step S32, the determination device 6 sets the count rate of 312 keV gamma rays to A, the count rate of gamma rays of 375 keV to B, and (A−αB) / B to R237. Here, α is a constant and is 0.02 (2%).

²³⁹ Pu emits 312 keV gamma rays with a release rate of 2.58 × 10 ⁻⁵ % per decay. ²³⁹ Pu emits 375 keV gamma rays, and the emission rate is 1.55 × 10 ⁻³ % per decay. That ^is, gamma rays 312keV ^{to 239} Pu is ^released, equivalent to 1.7% of gamma rays 375keV ^{to 239} Pu is released. If the count rate of ²³³ Pa 312 keV gamma rays is relatively small, the ²³⁹ Pu 312 keV disturbance may affect the measured value.

The count rate of 312 keV gamma rays emitted from ²³⁹ Pu can be estimated from the count rate of 375 keV gamma rays and the ratio of the emission rate described above. When converting from the ratio of the emission rate to the ratio of the gamma ray count rate, some correction of energy dependence is necessary. However, since the energy is close here, it is assumed that the dependence is small, and the count rate of 312 keV gamma rays can be assumed to be approximately 2% of 375 keV gamma rays. By multiplying this ratio by α and the count rate of 375 keV gamma rays and subtracting this from the count rate of gamma ray peaks of 312 keV, the count rate of 312 keV gamma rays emitted by ²³³ Pa can be obtained more accurately.

Thus, in the third embodiment, ²³⁹ Pu is contained in nuclear fuel like MOX fuel, and the influence of gamma rays emitted by ²³⁹ Pu cannot be ignored in the measurement of the count rate of gamma rays emitted by ²³⁷ Np. In this case, the influence can be reduced and the inspection accuracy of the Np-containing inspection can be increased.

  The above description is merely an example, and the present invention is not limited to the above-described embodiments, and can be implemented in various forms. For example, the measurement of gamma rays may be performed in a pellet state before assembling the fuel rod. When measuring the enrichment of uranium in the fuel, a gamma ray detector or the like may be shared and an Np content inspection may be performed at the same time.

  Further, in the determination when comparing the count rate of gamma rays and the ratio of the count rate with the reference, the operation when it is equal to the reference may be the same as when it is larger than the reference or smaller than the reference. That is, “more than”, “less than”, “greater than”, and “less than” in the comparison with the reference may be replaced with “greater than”, “less than”, “more than”, and “less than”, respectively.

The flowchart of the Np containing test | inspection of Embodiment 1 which concerns on this invention. The block block diagram of the Np containing test | inspection apparatus of Embodiment 1 which concerns on this invention. It is the figure which showed the irradiation apparatus of Embodiment 1 which concerns on this invention, (a) is a top view, (b) is the AA arrow longitudinal cross-sectional view of (a). It is the figure which showed the other irradiation apparatus of Embodiment 1 which concerns on this invention, (a) is a top view, (b) is a BB arrow longitudinal cross-sectional view of (a). An example of a gamma ray spectrum emitted from ²³⁸ Np or the like generated by irradiating ²³⁷ Np with neutrons. The flowchart of the Np containing test | inspection of Embodiment 2 which concerns on this invention. An example of a gamma ray spectrum emitted from uranium fuel. The flowchart of the Np containing test | inspection of Embodiment 3 which concerns on this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Gamma ray detector, 6 ... Judgment device, 7 ... Cable, 8 ... Nuclear fuel, 9 ... Gamma ray, 10 ... Irradiation device, 11 ... Irradiation stand, 12 ... Neutron source, 13 ... Neutron source guide tube, 14 ... Neutron deceleration Material, 15 ... water

Claims (15)

In an Np content inspection method for inspecting whether a solid nuclear fuel contains a predetermined amount of ²³⁷ Np before the nuclear fuel is used in a nuclear reactor ,
A gamma ray measurement first step of measuring a counting rate of gamma rays emitted from ²³³ Pa which is a daughter nuclide of ²³⁷ Np among the gamma rays emitted from the nuclear fuel;
The nuclear fuel contains a predetermined amount of ²³⁷ Np when the counting rate measured in the first step of gamma ray measurement is larger than a predetermined positive reference value at which it can be determined that the nuclear fuel contains a predetermined amount of ²³⁷ Np. A first comparison process for determining that
When said nuclear fuel in comparison the first step did not determination of containing a predetermined amount of ^{237 Np,} and irradiation process of irradiating neutrons to the nuclear fuel,
A second step of gamma ray measurement for measuring a counting rate of gamma rays emitted from ²³⁸ Np among the gamma rays emitted from the nuclear fuel after the irradiation step;
The nuclear fuel contains a predetermined amount of ²³⁷ Np when the counting rate measured in the second step of gamma ray measurement is larger than a predetermined criterion value at which the nuclear fuel can be determined to contain a predetermined amount of ²³⁷ Np. A second comparison step for determining that the nuclear fuel does not contain a predetermined amount of ²³⁷ Np when the counting rate is smaller than the determination reference value;
The Np content inspection method characterized by having. In the first comparison step, the nuclear fuel is further measured when the counting rate measured in the first gamma ray measurement first step is smaller than a predetermined negative reference value at which it can be determined that the nuclear fuel does not contain a predetermined amount of ²³⁷ Np. 2. The Np content inspection method according to claim 1, wherein it is determined that ²³⁷ Np is not contained in a predetermined amount.   3. The Np-containing inspection method according to claim 1, wherein the gamma ray measured in the first step of gamma ray measurement is a gamma ray of 312 keV.   4. The Np-containing inspection method according to claim 1, wherein the gamma ray measured in the second step of gamma ray measurement is at least one kind of gamma ray of 984 keV and 1029 keV. In an Np content inspection method for inspecting whether a solid nuclear fuel contains a predetermined amount of ²³⁷ Np before the nuclear fuel is used in a nuclear reactor ,
A gamma ray measurement first step of measuring a counting rate of gamma rays emitted from ²³³ Pa which is a daughter nuclide of ²³⁷ Np among the gamma rays emitted from the nuclear fuel;
A reference gamma ray measurement first step of measuring a counting rate of gamma rays other than gamma rays emitted from ²³³ Pa which is a daughter nuclide of ²³⁷ Np among the gamma rays emitted from the nuclear fuel;
When the ratio of the count rate measured in the first step of gamma ray measurement to the count rate measured in the first step of gamma ray measurement is greater than a predetermined positive reference ratio at which it can be determined that the nuclear fuel contains a predetermined amount of ²³⁷ Np And a first comparison step for determining that the nuclear fuel contains a predetermined amount of ²³⁷ Np,
When said nuclear fuel in comparison the first step did not determination of containing a predetermined amount of ^{237 Np,} and irradiation process of irradiating neutrons to the nuclear fuel,
A second step of gamma ray measurement for measuring a counting rate of gamma rays emitted from ²³⁸ Np among the gamma rays emitted from the nuclear fuel after the irradiation step;
A reference gamma ray measurement second step of measuring a counting rate of gamma rays other than gamma rays emitted from ²³³ Pa and ²³⁸ Np, which are daughter nuclides of ²³⁷ Np, among the gamma rays emitted from the nuclear fuel after the irradiation step;
Greater than a predetermined determination reference ratio can be determined that the ratio count rate measured by the reference gamma ray measurement second step count rate measured by the gamma ray measuring second step the nuclear fuel contains a predetermined amount of ^{237 Np} when the nuclear fuel is determined to contain a predetermined amount of ^{237 Np} in the gamma ray measuring second step count rate measured by the reference ratio gamma count rate measured by the measuring second step the determination reference ratio A second comparison step for determining that the nuclear fuel does not contain a predetermined amount of ²³⁷ Np when smaller than
The Np content inspection method characterized by having. The comparison first step further determines that the ratio of the count rate measured in the first gamma ray measurement first step to the count rate measured in the reference gamma ray measurement first step indicates that the nuclear fuel does not contain a predetermined amount of ²³⁷ Np. 6. The Np content inspection method according to claim 5, wherein when the fuel is smaller than a predetermined negative reference ratio, the nuclear fuel is determined not to contain a predetermined amount of ²³⁷ Np.   7. The Np-containing inspection method according to claim 5, wherein the gamma ray measured in the first step of gamma ray measurement is a gamma ray count rate of 312 keV.   The Np-containing inspection method according to any one of claims 5 to 7, wherein the gamma ray measured in the second step of gamma ray measurement is at least one kind of gamma ray of 984 keV and 1029 keV.   9. The Np-containing inspection method according to claim 5, wherein the gamma ray measured in the first step of measuring the reference gamma ray is at least one kind of gamma ray of 186 keV and 375 keV.   10. The Np-containing inspection according to claim 5, wherein the gamma ray measured in the second step of measuring the reference gamma ray is at least one kind of gamma rays of 375 keV, 948 kev, 1001 keV, and 1029 keV. Method.   In the first step of gamma ray measurement, a corrected gamma ray count rate is obtained by subtracting a 375 keV gamma ray count rate multiplied by a predetermined count from the measured 312 keV gamma ray count rate. The Np content inspection method according to any one of claims 5 to 10, wherein   The irradiating step irradiates the nuclear fuel by decelerating neutrons emitted from a neutron source with a neutron moderator disposed between the nuclear fuel and the neutron source. The Np content inspection method according to claim 1 or 11. In an Np content inspection apparatus for inspecting whether or not a solid nuclear fuel contains a predetermined amount of ²³⁷ Np before the nuclear fuel is used in a nuclear reactor ,
A gamma ray detector;
Is released from the nuclear fuel that has not been irradiated with neutrons, the nuclear fuel is the count rate of gamma rays emitted from ^{233 Pa,} the daughter nuclide of ²³⁷ Np of the gamma rays detected by the gamma ray detectors own the ^{237 Np} The nuclear fuel is determined to contain a predetermined amount of ²³⁷ Np when it is larger than a predetermined positive reference value that can be determined to contain a fixed amount, and is discharged from the nuclear fuel irradiated with neutrons, and the gamma ray detector When the counting rate of gamma rays emitted from ²³⁸ Np among the gamma rays detected by is greater than a predetermined criterion value at which it can be determined that the nuclear fuel contains a predetermined amount of ²³⁷ Np, the nuclear fuel decreases to ²³⁷ Np. It is determined that it contains a predetermined amount, released from the nuclear fuel irradiated with neutrons, and detected by the gamma ray detector. A determination device that determines that the nuclear fuel does not contain a predetermined amount of ²³⁷ Np when the count rate of gamma rays emitted from ²³⁸ Np is less than the determination reference value,
A transmission means for transmitting a signal output from the gamma ray detector to the determination device;
An Np-containing inspection device characterized by comprising: In an Np content inspection apparatus for inspecting whether or not a solid nuclear fuel contains a predetermined amount of ²³⁷ Np before the nuclear fuel is used in a nuclear reactor ,
A gamma ray detector;
Gamma rays emitted from the nuclear fuel having a count rate of gamma rays emitted from ²³³ Pa which is a daughter nuclide of ²³⁷ Np among gamma rays detected by the gamma ray detector and emitted from the nuclear fuel not irradiated with neutrons Of the ²³⁷ Np daughter nuclides, and the ratio to the count rate of gamma rays other than gamma rays emitted from ²³³ Pa is greater than a predetermined positive reference ratio at which the nuclear fuel can be determined to contain a predetermined amount of ²³⁷ Np. It is determined that the nuclear fuel contains a predetermined amount of ²³⁷ Np, and the counting rate of gamma rays emitted from ²³⁸ Np out of the gamma rays emitted from the nuclear fuel irradiated with neutrons and detected by the gamma ray detector Of the gamma rays emitted from the nuclear fuel irradiated with neutrons and emitted from the nuclear fuel ^{When the} ratio of the ²³⁷ Np daughter nuclides ²³³ Pa and ²³⁸ Np to the count rate of gamma rays other than gamma rays is larger than a predetermined criterion ratio at which it can be determined that the nuclear fuel contains a predetermined amount of ²³⁷ Np It is determined that the nuclear fuel contains a predetermined amount of ²³⁷ Np, and the number of gamma rays emitted from ²³⁸ Np among the gamma rays emitted from the nuclear fuel irradiated with neutrons and detected by the gamma ray detector is counted. Ratio of the gamma rays emitted from the nuclear fuel irradiated with neutrons to the count rate of gamma rays other than gamma rays emitted from ²³³ Pa and ²³⁸ Np which are ²³⁷ Np daughter nuclides out of the nuclear fuel determine if There the nuclear fuel when the criterion ratio smaller than not contains a predetermined amount of ^{237 Np} A determination device to determine,
A transmission means for transmitting a signal output from the gamma ray detector to the determination device;
An Np-containing inspection device characterized by comprising: A space for disposing a neutron source, a space for disposing the nuclear fuel, and an irradiation stand including a space for disposing a neutron moderator between the space for disposing the neutron source and the space for disposing the nuclear fuel,
15. The Np-containing inspection apparatus according to claim 13 or 14, characterized by comprising:

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