JP5330081B2 - Method and apparatus for measuring flammable poison concentration - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve the measurement of the quantity of the burnable poison contained in fuels loaded into a reactor even if its concentration is low. <P>SOLUTION: A test fuel is irradiated with epithermal neutrons (S22), and the intensity of the gamma rays emitted from the test fuel is measured (S23). The test fuel is also irradiated with thermal neutrons (S24), and the intensity of the gamma rays emitted from the test fuel is measured (S25). A delayed gamma-ray intensity ratio is found by dividing the gamma-ray intensity measured in S25 by the gamma-ray intensity measured in S23 (S26). The concentration of the burnable poison in the test fuel is calculated by using the delayed gamma-ray intensity ratio and a calibration curve derived in advance (S28). <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a flammable poison concentration measuring method and apparatus for measuring the concentration of a flammable poison contained in a fuel loaded in a nuclear reactor.

  Generally, in a commercial nuclear reactor using uranium as a fuel, the enrichment of uranium 235 is limited to 5 wt% (wt%) or less. This is because various facilities and equipment that handle uranium are designed on the condition that the enrichment of uranium 235 is 5 wt% or less. That is, in these facilities and equipment, subcriticality can be ensured (critical safety can be ensured) on the condition that the enrichment of uranium 235 is 5 wt% or less.

  In order to improve the economic efficiency of nuclear power generation and suppress the amount of spent fuel generated, it has been recognized that it is essential to increase the enrichment of uranium 235 to 5 wt% or more. In this case, when trying to use a conventional fuel production facility, it is impossible to guarantee that the subcriticality is secured with a certain margin in the production process (critical safety cannot be secured).

  When the enrichment of uranium 235 is increased to 5 wt% or more, in order to ensure critical safety, for example, it is necessary to reduce the fuel handling amount. In this case, the production efficiency is greatly reduced, resulting in a loss of economy. Alternatively, it is necessary to modify a conventional manufacturing facility so that a fuel in which the enrichment of uranium 235 is higher than 5 wt% (referred to as “over 5% fuel”) can be handled. This modification is likely to be quite large.

Therefore, in order to make it possible to use conventional fuel production facilities almost as they are in the production of over 5% fuel, low concentrations of combustible poisons such as gadolinia (Gd ₂ O ₃ ) and erbia (Er ₂ O ₃ ) are used as fuel. A method of adding has been proposed (see, for example, Patent Document 1).

  The concentration of flammable poisons (the minimum required concentration of flammable poisons) necessary to make the conventional fuel production facility almost usable as it is in the production of over 5% fuel is that of uranium powder with U235 enrichment of 5 wt%. This is the minimum concentration of flammable poison that is equal to or less than the maximum value of the effective neutron multiplication factor at the critical safety limit value. For example, when 0.1 wt% gadolinia is uniformly added, the U235 enrichment is equal to or smaller than the maximum value of the effective neutron multiplication factor at the critical safety limit value in the case of uranium powder of 5 wt%. The required minimum concentration is 0.1 wt%. This evaluation is performed, for example, under the evaluation conditions assuming the severest case in terms of critical safety defined in Non-Patent Document 2.

  Fuels to which a flammable poison is added are widely known mainly for the purpose of suppressing an excessive change in the reactivity of the core during operation of the reactor and controlling the power distribution appropriately. For this reason, the addition concentration of the combustible poison used conventionally is high. For example, in a gadolinia used in a boiling water reactor (BWR), at least 1 wt%, usually 3 to 10 wt%, is added to uranium oxide fuel pellets.

  On the other hand, in the process of producing over 5% fuel, the concentration of flammable poisons required to ensure critical safety is at most 0.1 wt%, usually around 0.01 wt% (low concentration) in the case of Gadolinia It is. In the case of such an addition concentration, when irradiation is started after loading in the reactor, especially in the case of gadolinia, it burns and disappears in a short period of time, so there is no need for special considerations in terms of core design. The core can be designed with thought. In the case of Elvia, the neutron absorption cross section is much smaller than that of Gadolinia, so it is completely burned out, that is, the neutron absorption effect does not completely disappear, but the core is still designed with almost the conventional design concept. Can do. When the boron compound is applied to the surface of the fuel pellet, it becomes an intermediate characteristic between Gadolinia and Elvia.

  The content of flammable poisons in the fuel rods affects the behavior of the fuel rods during irradiation in the reactor. For this reason, it is necessary to measure the amount of the flammable poison actually stored in the fuel rod in the manufacturing process in a non-destructive manner. Patent Document 2 discloses a method for obtaining a Gd concentration in a fuel rod. In this method, first, the count rate of radiation generated when a standard fuel rod having a known gadolinia concentration is irradiated with thermal neutrons is measured, and the relationship and relationship between the gadolinia concentration and the radiation count rate are obtained. Thereafter, the count rate of radiation generated when the fuel rod to be inspected is irradiated with thermal neutrons is obtained, and the gadolinia concentration is obtained based on the count rate and the relationship between the gadolinia concentration and the radiation count rate.

Japanese Patent Laid-Open No. 2004-177241 JP-A-54-93795

Watanabe and three others, “Combustible Toxic Credit for Rationalization of Criticality Safety in Next Generation Light Water Reactor Fuel Cycle”, Proceedings of Autumn Meeting 2007 of the Atomic Energy Society of Japan, p. 738, Japan Atomic Energy Society, 2007 Science and Technology Agency, Nuclear Safety Bureau, Nuclear Fuel Regulations Section, "Kai Safety Handbook", October 31, 1988, Nikka Shobo

  In a fuel in which the enrichment of uranium 235 is higher than 5 wt%, a flammable poison such as gadolinia may be added at a low concentration. In this case, the addition concentration of the flammable poison is a low concentration of at most 0.1 wt%, usually about 0.01 wt%. However, the method for measuring the gadolinia concentration in uranium oxide disclosed in Patent Document 2 has not been studied for such a low concentration combustible poison. In addition, there is no known method for measuring such a low-concentration combustible poison with the accuracy required for production.

  Accordingly, an object of the present invention is to make it possible to measure the amount of combustible poison contained in the fuel loaded in a nuclear reactor even when the concentration is low.

  In order to achieve the above object, the present invention provides a method for measuring the concentration of a flammable poison added to a test fuel containing uranium, wherein the test fuel is irradiated with an epithermal neutron. A neutron irradiation step, an epithermal fission delayed gamma ray measurement step for measuring gamma ray intensity emitted from the test fuel irradiated with epithermal neutrons after the epithermal neutron irradiation step, and thermal neutrons on the test fuel. A thermal neutron irradiation step for irradiating; a thermal fission delayed gamma ray measuring step for measuring gamma ray intensity emitted from the test fuel irradiated with thermal neutrons after the thermal neutron irradiation step; and Delayed gamma ray intensity obtained by dividing the thermal fission delayed gamma ray intensity measured in the thermal fission delayed gamma ray measurement step by the epithermal fission delayed gamma ray intensity measured in the measurement step. A delayed gamma ray intensity ratio calculating step, a calibration curve derivation step for deriving a calibration curve of the delayed gamma ray intensity ratio and the combustible poison concentration, and the test fuel from the calibration curve and the delayed gamma ray intensity ratio And a combustible poison concentration calculating step for calculating the combustible poison concentration.

The present invention also relates to a combustible poison concentration measuring apparatus for measuring the concentration of a combustible poison added to a test fuel containing uranium, a drive mechanism for moving the test fuel along the fuel passage, and the fuel passage. An epithermal neutron irradiating unit for irradiating the test fuel moving through the epithermal neutron, and the test fuel moving in the fuel passage provided downstream of the epithermal neutron irradiating unit in the moving direction of the test fuel. A thermal outer fission delayed gamma ray measurement unit for measuring the emitted gamma ray intensity, a thermal neutron irradiation unit for irradiating the test fuel moving in the fuel passage with thermal neutrons, and movement of the test fuel in the thermal neutron irradiation unit provided downstream direction, the thermal fission delayed gamma ray measurement unit for measuring the gamma ray intensity emitted from said test fuel for moving the fuel passage, the heat outside fission delayed gamma rays A delayed gamma ray intensity ratio calculating means for obtaining a delayed gamma ray intensity ratio obtained by dividing the thermal fission delayed gamma ray intensity measured by the thermofission delayed gamma ray measurement unit by the epithermal fission delayed gamma ray intensity measured by the fixed part; A calibration curve deriving means for deriving a calibration curve of the delayed gamma ray intensity ratio and the combustible poison concentration, and a combustible poison for calculating the burnable poison concentration of the test fuel from the calibration curve and the delayed gamma ray intensity ratio Density calculating means

  According to the present invention, the amount of the flammable poison contained in the fuel loaded in the nuclear reactor can be measured even at a low concentration.

It is a flowchart in 1st Embodiment of the combustible poison concentration measuring method which concerns on this invention. It is a flowchart of the fuel manufacturing method using 1st Embodiment of the combustible poison concentration measuring method which concerns on this invention. Metal amount is a UO ₂ powder of 33kg and a sphere is a graph showing the effective neutron multiplication factor of the presence of water reflecting the thickness of 30 cm. It is a graph showing the relationship between the enrichment gadolinia concentration and uranium enrichment to be added to UO ₂ powder of more than 5 wt% in the first embodiment of the burnable poison concentration measuring method according to the present invention. It is a graph which shows the example of the change of late gamma ray intensity with respect to the change of uranium-235 enrichment. It is a graph which shows the example of the change of late gamma ray intensity ratio with respect to the change of uranium-235 enrichment. It is a graph which shows the example of the change of late gamma ray intensity ratio with respect to the change of a combustible poison concentration. It is a longitudinal cross-sectional view of the measuring apparatus used for 1st Embodiment of the combustible poison concentration measuring method which concerns on this invention. It is a flowchart of the derivation method of the calibration curve in 1st Embodiment of the combustible poison concentration measuring method which concerns on this invention. It is a longitudinal cross-sectional view of the multiple calibration fuel used for 1st Embodiment of the combustible poison concentration measuring method which concerns on this invention. It is a graph which shows the example of the change of the normalized delayed gamma ray intensity with respect to the change of uranium-235 enrichment. It is a flowchart of the measuring method of the enrichment of the uranium 235 in 2nd Embodiment of the combustible poison concentration measuring method which concerns on this invention. It is a flowchart of the measuring method of the enrichment of the uranium 235 in 3rd Embodiment of the combustible poison concentration measuring method which concerns on this invention. It is a graph which shows the gamma ray spectrum which uranium 235 discharge | releases. It is a longitudinal cross-sectional view of the measuring apparatus used for 3rd Embodiment of the combustible poison concentration measuring method which concerns on this invention. It is a flowchart of the measuring method of the enrichment of the uranium 235 in 4th Embodiment of the combustible poison concentration measuring method which concerns on this invention. It is a flowchart in 5th Embodiment of the combustible poison concentration measuring method which concerns on this invention. It is a graph which shows the example of the change with respect to the change of the combustible poison density | concentration of the gamma ray intensity ratio with respect to a reference fuel. It is a longitudinal cross-sectional view of the measuring apparatus used for 5th Embodiment of the combustible poison concentration measuring method which concerns on this invention. It is a flowchart of the derivation method of the calibration curve in 5th Embodiment of the combustible poison concentration measuring method which concerns on this invention. It is a graph which shows the example of the combustible poison density | concentration with the delayed gamma ray intensity after thermal neutron irradiation with respect to a reference fuel, and a relationship.

  An embodiment of a flammable poison concentration measuring method 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.

[First Embodiment]
FIG. 2 is a flowchart of a fuel production method using the first embodiment of the method for measuring the concentration of combustible poisons according to the present invention.

  In this fuel manufacturing method, a fuel having a U235 enrichment exceeding 5 wt% (a fuel exceeding 5%) is manufactured. This over 5% fuel has a trace amount that can ensure the criticality safety even if this fuel is handled at the fuel processing facility where the criticality safety is ensured when the U235 enrichment is less than 5wt%. Combustible poison is added. In other words, this over 5% fuel contains the smallest flammable poison that is equal to or less than the maximum value of the effective neutron multiplication factor at the critical safety limit for uranium powder with U235 enrichment of 5 wt%. A flammable poison having an added concentration (minimum required concentration of the flammable poison) is added. Here, for example, gadolinia is added as the flammable poison.

First, the necessary minimum concentration (P _min ) of the flammable poison is derived for each enrichment of U235 (S1). For example, based on a critical safety design manual such as Non-Patent Document 2, the required minimum concentration of the flammable poison can be obtained as a dependency amount of the U235 enrichment.

In the criticality safety handbook (Non-patent Document 2), “complete submersion” is assumed as the severest case in terms of criticality safety. This “completely submerged” state is a state in which water penetrates into the gaps between the nuclear fuel materials and is surrounded by water. In such a “completely submerged” state, the “minimum estimation” is used as the limit value in “mass management” or “geometry management” in the entire uranium concentration range for a homogeneous UO ₂ / H ₂ O system. "Critical value" and "Minimum estimated critical lower limit value" are defined. “Mass management” is management that does not handle reactor fuel exceeding a certain mass, which is a critical safety limitation. “Shape and dimension management” refers to management that does not handle reactor fuel that exceeds a certain shape and dimension.

For enrichment 5 wt% in the step (referred to UO ₂ powder process) handling UO ₂ powder, the "minimum estimated criticality value" is 36.7Kg, "minimum estimated criticality lower limit" is the 33.0 kg. “Estimated critical value” is a value that is determined to be critical if the mass, shape, and dimensions are those values, and “estimated critical lower limit value” is subcritical if the mass, shape, and dimensions are less than those values. It is a value to be judged. Considering UO ₂ powder, pellets, fuel rods and fuel assemblies as the form of fuel handled in the facility, UO ₂ powder is the most severe.

  Therefore, when the uranium mass was 33 kg and water with a reflection thickness of 30 cm was present, the change in the effective neutron multiplication factor when the uranium concentration changed was evaluated by calculation using the neutron transport calculation code. . This evaluation was performed on uranium having a concentration of U235 in the range of 5 wt% to 10 wt%.

  FIG. 3 is a graph showing the relationship between uranium concentration and neutron effective multiplication factor.

When the concentration of U235 is 5 wt%, the effective neutron multiplication factor is maximized at a uranium concentration of about 0.9 g / cm ³ . This maximum value is represented by a one-dot chain line A. As the enrichment of U235 increases, the neutron effective multiplication factor increases. However, when 53 ppm, 110 ppm, 170 ppm and 305 ppm of gadolinia are added to UO ₂ powders with U235 enrichment of 6, 7, 8, and 10 wt%, respectively, the effective neutron multiplication factor is lower than the one-dot chain line A, that is, It can be seen that the concentration is less than or equal to that of uranium with a concentration of 5 wt%.

That is, the required minimum concentrations (P _min ) of flammable poisons for UO ₂ powders with U235 enrichment of 6, 7, 8 and 10 wt% are 53 ppm, 110 ppm, 170 ppm and 305 ppm, respectively. These necessary minimum densities are values when there is no calculation error. In consideration of calculation error, a value larger than these values may be set as the necessary minimum density.

FIG. 4 is a graph showing the relationship between gadolinia concentration and uranium enrichment to be added to UO ₂ powder whose enrichment exceeds 5 wt% in the present embodiment.

As shown in FIG. 3, the required minimum concentrations (P _min ) of flammable poisons for UO ₂ powders with U235 enrichment of 6, 7, 8 and 10 wt% are 53 ppm, 110 ppm, 170 ppm and 305 ppm, respectively. When the U235 enrichment is 5 wt%, the necessary minimum concentration (P _min ) of the flammable poison is 0. From these, it can be seen that the necessary minimum concentration of the flammable poison draws a curve that increases monotonically as the U235 concentration increases through 0 at 5 wt% and increases.

After deriving the necessary minimum concentration (P _min ) of the combustible poison for each enrichment of U235, the combustible poison concentration (P _T ) of the test fuel is measured (S2). It is determined whether the measured combustible poison concentration (P _T ) of the test fuel is equal to or higher than the necessary minimum concentration (P _min ) in the U235 enrichment of the test fuel (S3).

When the flammable poison concentration (P _T ) of the test fuel is less than the minimum required concentration (P _min ) for the U235 enrichment of the test fuel, when the fuel having a U235 enrichment of 5 wt% or less is handled as the test fuel Even if this fuel is handled in a fuel processing facility where critical safety is ensured, it cannot be said that a small amount of flammable poison is added to the extent that critical safety can be ensured. Therefore, in this case, the dispensing of the test fuel to the downstream process is stopped and appropriate measures are taken.

When the flammable poison concentration (P _T ) of the test fuel is equal to or higher than the necessary minimum concentration (P _min ) of the U235 enrichment of the test fuel, when the fuel having a U235 enrichment of 5 wt% or less is handled as the test fuel It can be said that a small amount of flammable poison is added so that the criticality safety can be ensured even when the fuel is handled in the fuel processing facility where the criticality safety is ensured. Therefore, this test fuel is discharged to the downstream process to produce fuel.

  FIG. 1 is a flowchart of the combustible poison concentration measuring method of the present embodiment.

In this flammable poison concentration measuring method, first, a calibration curve of the delayed gamma ray intensity ratio (γ ^t / γ ^e ) and the flammable poison concentration is obtained for each U235 enrichment (S21). The delayed gamma ray intensity ratio (γ ^t / γ ^e ) is the ratio of the thermal fission delayed gamma ray intensity (γ ^t ) to the epithermal fission delayed gamma ray intensity (γ ^e ).

  Here, the fission that occurs when the fuel is irradiated with epithermal neutrons is called epithermal fission, and the delayed gamma rays emitted by the resulting fission products are called epithermal fission delayed gamma rays. Also, the fission that occurs when the fuel is irradiated with thermal neutrons is called thermal fission, and the delayed gamma rays emitted from the resulting fission products are called thermal fission delayed gamma rays. Delayed gamma rays are gamma rays emitted by fission products generated as a result of fission.

Next, the test fuel is irradiated with epithermal neutrons (S22), and then gamma rays are measured (S23). The gamma rays measured at this time are delayed gamma rays emitted from fission products resulting from fission that occurs when the test fuel is irradiated with epithermal neutrons. That is, in S22 and S23, the thermal outer fission delayed gamma ray intensity (γ _T ^e ) of the test fuel is measured.

Further, the test fuel is irradiated with thermal neutrons (S24), and then gamma rays are measured (S25). The gamma rays measured at this time are delayed gamma rays emitted by fission products resulting from fission that occurs when the test fuel is irradiated with thermal neutrons. In other words, the thermal fission delayed gamma ray intensity (γ _T ^t ) of the test fuel is measured in S24 and S25.

Thereafter, a delayed gamma ray intensity ratio (γ ^t / γ ^e ) _T of the test fuel is calculated (S26). That is, the thermal fission delayed gamma ray intensity (γ _T ^t ) of the test fuel is divided by the epithermal fission delayed gamma ray intensity (γ _T ^e ) of the test fuel to obtain a delayed gamma ray intensity ratio (γ ^t / γ of the test fuel. ^e ) _T is calculated.

Also, the U235 enrichment of the test fuel is specified (S27). For the U235 enrichment, for example, management data when receiving UO ₂ powder obtained in the upstream process can be used.

Next, a delayed gamma ray intensity ratio (γ ^t / γ ^{e) _T} test fuels obtained in S26, delayed gamma ray intensity ratio of each U235 enrichment obtained in S21 (γ ^t / γ ^e) and burnable poison Using the calibration curve with the concentration, the flammable poison concentration (P _T ) of the test fuel is calculated (S28).

  FIG. 5 is a graph showing an example of changes in delayed gamma ray intensity with respect to changes in uranium 235 enrichment. This graph shows three cases where the flammable poison concentration is P1 <P2 <P3.

Combustible poisons have little effect on epithermal neutrons. Therefore, the intensity of delayed gamma rays emitted from fission products generated by irradiating epithermal neutrons, that is, the epithermal fission delayed gamma ray intensity (γ ^e ) is hardly affected by the concentration of flammable poisons. . Moreover, the shielding effect of epithermal neutrons by uranium is small. For these reasons, the epithermal fission delayed gamma ray intensity (γ ^e ) varies almost linearly with U235 enrichment.

The intensity of delayed gamma rays emitted from fission products generated by irradiation with thermal neutrons, ie, thermal fission delayed gamma ray intensity (γ ^t ), tends to increase with increasing U235 enrichment. However, flammable poisons have a significant effect on thermal neutrons. As a result, the thermal fission delayed gamma ray intensity (γ ^t ) tends to saturate as the U235 enrichment increases. In addition, the thermal fission delayed gamma ray intensity (γ ^t ) decreases as the flammable poison concentration increases.

  FIG. 6 is a graph showing an example of the change in the delayed gamma ray intensity ratio with respect to the change in uranium 235 enrichment. This graph shows three cases where the flammable poison concentration is P1 <P2 <P3.

As shown in FIG. 5, the epithermal fission delayed gamma ray intensity (γ ^e ) is hardly affected by the flammable poison concentration, whereas the thermal fission delayed gamma ray intensity (γ ^t ) increases the U235 enrichment. On the other hand, it has a saturation tendency. Therefore, the delayed gamma ray intensity ratio (γ ^t / γ ^e ) between thermal fission and epithermal fission decreases with U235 enrichment. Further, the delayed gamma ray intensity ratio (γ ^t / γ ^e ) becomes smaller as the combustible poison concentration is higher.

  FIG. 7 is a graph showing an example of the change in the delayed gamma ray intensity ratio with respect to the change in the combustible poison concentration. This graph shows three cases where the U235 enrichment satisfies ε1 <ε2 <ε3.

This figure shows the delayed gamma ray intensity ratio (γ ^t / γ ^e ) between the thermal fission and the outer epithelial fission shown in FIG. 6 as a flammable poison concentration dependent amount. The delayed gamma ray intensity ratio (γ ^t / γ ^e ) between thermal fission and epithermal fission decreases with the flammable poison concentration. Further, the delayed gamma ray intensity ratio (γ ^t / γ ^e ) becomes smaller as the U235 enrichment is higher.

  In the flammable poison concentration measurement method of the present embodiment, attention is paid to the fact that the flammable poison hardly affects the epithermal neutrons, but significantly affects the thermal neutrons. That is, the ratio of the intensity of delayed gamma rays between the case where the nuclear fuel material is irradiated with thermal neutrons and the case where the nuclear fuel materials are irradiated with epithermal neutrons changes with the change in the concentration of the flammable poison in the nuclear fuel material. Therefore, in the present embodiment, the concentration of the flammable poison in the nuclear fuel material is determined based on the ratio of the intensity of delayed gamma rays when the nuclear fuel material is irradiated with thermal neutrons and when irradiated with epithermal neutrons. .

  FIG. 8 is a longitudinal sectional view of a measuring apparatus used in the method for measuring the concentration of combustible poisons according to the present embodiment.

  In the present embodiment, the fuel 12 containing the fuel pellets obtained by sintering the nuclear fuel material inside the cladding tube is measured, and the concentration of the combustible poison inside is measured. A fuel passage 11 is formed in the combustible poison concentration measuring device. The fuel 12 moves inside the fuel passage 11 by the fuel drive mechanism 10. The fuel passage 11 is a pipe through which the fuel 12 can pass, for example. Around the fuel passage 11, an epithermal neutron irradiation unit 30, an epithermal fission delayed gamma ray measuring unit 40, a thermal neutron irradiation unit 50, and a thermonuclear fission delayed gamma ray measuring unit 60 are provided as fuel 12 by the fuel driving mechanism 10. Are arranged in order along the moving direction.

  The epithermal neutron irradiation unit 30 irradiates the fuel 12 with epithermal neutrons. The epithermal fission delayed gamma ray measurement unit 40 measures the intensity of gamma rays emitted from the fission product generated in the fuel 12 when the fuel 12 is irradiated with epithermal neutrons. The thermal neutron irradiation unit 50 irradiates the fuel 12 with thermal neutrons. The thermal fission delayed gamma ray measurement unit 60 measures the intensity of gamma rays emitted from the fission product generated in the fuel 12 when the fuel 12 is irradiated with thermal neutrons. Using such a flammable poison concentration measuring apparatus, the fuel 12 is moved along the fuel passage 11 while irradiating the fuel 12 with epithermal neutron irradiation, measuring epithermal fission delayed gamma rays, and irradiating the fuel 12 with thermal neutrons. Then, thermal fission delayed gamma rays are measured sequentially.

  In the epithermal neutron irradiation section 30, a neutron source 31, a graphite neutron moderator 32, a polyethylene neutron moderator 34, a nickel reflector 33, a cadmium thermal neutron absorber 36, and a boron-added polyethylene 37 are disposed. The cadmium thermal neutron absorber 36 is provided so as to surround the fuel passage 11. The neutron source 31, the graphite neutron moderator 32, the polyethylene neutron moderator 34, and the nickel reflector 33 are provided inside a region surrounded by the cadmium thermal neutron absorber 36. The boron-added polyethylene 37 is provided so as to surround the fuel passage 11 on both sides of the cadmium thermal neutron absorber 36 in the moving direction of the fuel 12 by the fuel driving mechanism 10.

  In the region surrounded by the cadmium thermal neutron absorber 36, a polyethylene neutron moderator 34 is disposed at a position closest to the fuel passage 11. An epithermal neutron irradiation window 35 in which the polyethylene neutron moderator 34 is removed is formed in a region surrounding the fuel passage 11 having a predetermined length along the fuel passage 11.

  The neutron source 31 is disposed inside an epithermal neutron irradiation window 35 that is a gap sandwiched between polyethylene neutron moderators 31 and emits fast neutrons. As the neutron source 31, the radioactive isotope source Californium 252 can be used. Californium 252 is small and has a relatively high neutron intensity, and thus is highly convenient.

  The graphite neutron moderator 32 is graphite disposed so as to surround the fuel passage 11 at a position farther from the fuel passage 11 than the neutron source 31. Fast neutrons emitted from the neutron source 31 are decelerated in the graphite neutron moderator 32 to the energy of epithermal neutrons. The nickel reflector 33 is nickel disposed so as to surround the fuel passage 11 at a position farther from the fuel passage 11 than the graphite neutron moderator 32.

  In such an epithermal neutron irradiation unit 30, a part of fast neutrons emitted from the neutron source 31 is decelerated to the energy of epithermal neutrons in the graphite neutron moderator 32. Further, neutrons flying in a direction away from the fuel passage 11 than the graphite neutron moderator 32 are reflected by the nickel reflector 33 and move in a direction approaching the fuel passage 11.

  Neutrons are decelerated to the energy of thermal neutrons by the polyethylene neutron moderator 34 on both sides of the epithermal neutron irradiation window 35 with respect to the moving direction of the fuel 12. Instead of polyethylene, a neutron moderator containing many hydrogen atoms such as paraffin can be used. Neutrons decelerated to thermal neutron energy are absorbed by the cadmium thermal neutron absorber 36. In this way, the fuel 12 moving in the fuel passage 11 is irradiated with epithermal neutrons directed from the epithermal neutron irradiation window 35 toward the fuel 12. On the other hand, the fuel 12 is hardly irradiated with thermal neutrons in the epithermal neutron irradiation unit 30.

  When neutrons collide with hydrogen atoms, their masses are almost the same, so energy may be lost at a stretch, and the ratio of epithermal neutrons decreases. On the other hand, if the mass of the nucleus receiving the collision is larger than that of the neutron, energy will not be lost at a stretch, so the proportion of epithermal neutrons will be relatively high and the efficiency of epithermal neutron irradiation will be high. . For this reason, in order to increase the ratio of epithermal neutrons, graphite is suitable as a moderator as in this embodiment.

  Although heavy water can be used as a moderator for obtaining epithermal neutrons, it is expensive. Moreover, since heavy water is a liquid, there are drawbacks such as inconvenience in handling.

  Further, a hydrogen-containing substance having a small hydrogen content can also be used in place of the moderator for obtaining epithermal neutrons. However, in this case, it is necessary to select an appropriate amount of hydrogen.

  A nickel reflector 33 having a large neutron scattering effect is disposed outside the graphite neutron moderator 32. This has the role of a reflector for reflecting the neutrons flying toward the outside inward again. The presence of this reflector suppresses the amount of neutron leakage. As a reflector, metals other than nickel, such as stainless steel and iron, can also be used.

  The polyethylene neutron moderator 34 may be further provided in a layer having a thickness of about 1 cm between the nickel reflector 33 and the graphite neutron moderator 32. This polyethylene layer also suppresses neutron leakage from the graphite neutron moderator 32. However, since it may be counterproductive to the case where it is effective in selecting the thickness, careful design is required according to the material, configuration, and dimensions of the epithermal neutron irradiation part, including the thickness of graphite and the thickness of nickel. It is.

  When fast neutrons emitted from the neutron source 31 are decelerated by graphite or polyethylene to become thermal neutrons, they are absorbed by the cadmium thermal neutron absorber 36 surrounding the fuel passage 11. For this reason, irradiation with thermal neutrons is avoided.

  Neutrons that have emerged from graphite as epithermal neutrons are thermalized by polyethylene disposed at positions away from the epithermal neutron irradiation window 35 and are absorbed by cadmium. Therefore, the epithermal neutrons in such a movement path are not irradiated to the fuel 12.

  The epithermal neutrons emerging from the graphite neutron moderator 32 are irradiated to the fuel through the gaps in the epithermal neutron irradiation window 35. Since cadmium hardly absorbs epithermal neutrons, the irradiation position of epithermal neutrons is almost limited to the position of the epithermal neutron irradiation window 35. That is, the polyethylene neutron moderator 34 around the epithermal neutron irradiation window 35 functions as an epithermal neutron collimator.

  The epithermal fission delayed gamma ray measurement unit 40 includes a lead gamma ray shield 42 and a gamma ray detector 41 surrounding the fuel passage 11. In the lead gamma ray shielding body 42, a gap 43 extending from the fuel passage 11 to the gamma ray detector 41 is formed, and the lead gamma ray shielding body 42 also functions as a collimator.

  When the fuel 12 moves along the fuel passage 11, gamma rays emitted from the fuel 12 pass through a gap 43 formed in the lead gamma ray shield 42 and are detected by the gamma ray detector 41. Since the gamma rays measured here have passed a certain amount of time during the movement of the fuel 12 from the epithermal neutron irradiation section 30 to the epithermal fission delayed gamma ray measurement section 40, the epithermal neutron irradiation section 30 This is a delayed gamma ray emitted from a fission product generated by epithermal fission inside the fuel 12 irradiated with epithermal neutrons.

  The gap 43 formed in the lead gamma ray shielding body 42 is a collimator for limiting the fuel measurement site. When the space (width) of the collimator is narrowed, the position resolution of the measurement site is improved. However, if the width of the collimator is narrowed, the measured gamma ray count rate decreases, so the accuracy of the measured value decreases. Therefore, the width of the collimator takes into account the sensitivity of the gamma ray detector 41, the intensity of the neutron source 31, the counting rate that changes according to the fuel 12 to be measured, and the position resolution required for the fuel 12 to be measured. Design.

  Further, a boron-added polyethylene 37 is disposed between the cadmium thermal neutron absorber 36 and the epithermal fission delayed gamma ray measurement unit 40 of the epithermal neutron irradiation unit 30. Boron absorbs neutrons and emits relatively low energy capture gamma rays. The layer of the lead gamma ray shielding body 42 of the epithermal fission delayed gamma ray measurement unit 40 also has a function of shielding this gamma ray.

  Further, cadmium may be disposed between the boron-added polyethylene 37 of the epithermal neutron irradiation unit 30 and the lead gamma ray shield 42 of the epithermal fission delayed gamma ray measurement unit 40. When cadmium is arranged between the epithermal neutron irradiation unit 30 and the epithermal fission delayed gamma ray measurement unit 40, thermal neutrons leaking from the epithermal neutron irradiation unit 30 are absorbed by the cadmium and measured by the gamma ray detector 41. Accuracy is improved.

  The detection of gamma rays in the epithermal fission delayed gamma ray measurement unit 40 does not require high energy resolution. For this reason, a NaI detector or a BGO detector is suitable as the gamma ray detector 41. In order to reduce the size of the apparatus, a CdTe detector, a SiC detector, or the like can be used.

  In the thermal neutron irradiation unit 50, a neutron source 51, a polyethylene neutron moderator 54, a cadmium thermal neutron absorber 56, and a boron-added polyethylene 57 are arranged. The polyethylene neutron moderator 34 is a rectangular parallelepiped having an outer dimension of about 30 to 40 cm, for example, and surrounds the fuel passage 11 with polyethylene. The neutron source 51 is disposed inside the polyethylene neutron moderator 34. The boron-added polyethylene 57 is provided so as to surround the fuel passage 11 on both sides in the moving direction of the fuel 12 by the fuel driving mechanism 10 with respect to the polyethylene neutron moderator 54. The cadmium thermal neutron absorber 56 is provided so as to surround the polyethylene neutron moderator 54 and the boron-added polyethylene 57, and a thermal neutron irradiation window 55 is formed at a portion facing the fuel passage 11.

  As the neutron source 51, californium 252 which is a radioactive isotope source can be used. Fast neutrons emitted from the neutron source 51 are decelerated to the energy of thermal neutrons by the polyethylene neutron moderator 54. Instead of polyethylene, a neutron moderator containing many hydrogen atoms such as paraffin can be used.

  Thermal neutrons decelerated by the polyethylene neutron moderator 54 are almost absorbed by the cadmium thermal neutron absorber 56 at portions other than the thermal neutron irradiation window 55. However, the thermal neutrons that have passed through the thermal neutron irradiation window 55 are irradiated to the fuel 12 that passes through the fuel passage 11. Therefore, the irradiation position of the thermal neutron on the fuel 12 is limited to the portion facing the thermal neutron irradiation window 55. When the thermal neutron irradiation window 55 is narrowed, the resolution of the irradiation position is improved. However, if cadmium is present in the vicinity, the thermal neutron flux decreases and the statistical accuracy of gamma ray measurement decreases. In addition, when measuring by moving a fuel rod continuously at a constant speed, it is not necessary to limit the irradiation position with cadmium.

  The thermal fission delayed gamma ray measurement unit 60 includes a lead gamma ray shielding body 62 and a gamma ray detector 61 surrounding the fuel passage 11. A gap 63 extending from the fuel passage 11 to the gamma ray detector 61 is formed in the lead gamma ray shielding body 62, and the lead gamma ray shielding body 62 also functions as a collimator.

  When the fuel 12 moves along the fuel passage 11, gamma rays emitted from the fuel 12 pass through a gap 63 formed in the lead gamma ray shielding body 62 and are detected by the gamma ray detector 61. Since the gamma rays measured here have passed a certain amount of time during the movement of the fuel 12 from the thermal neutron irradiation unit 50 to the thermal fission delayed gamma ray measurement unit 60, Delayed gamma rays emitted from fission products produced by thermal fission inside irradiated fuel 12.

  Further, boron-added polyethylene 57 is disposed between the polyethylene neutron moderator 54 and the thermal fission delayed gamma ray measurement unit 60 of the thermal neutron irradiation unit 50. Boron absorbs neutrons and emits relatively low energy capture gamma rays. The layer of the lead gamma ray shielding body 62 of the thermal fission delayed gamma ray measurement unit 60 also has a function of shielding this gamma ray.

  Further, a cadmium thermal neutron absorber 56 is disposed between the boron-added polyethylene 57 of the thermal neutron irradiation unit 50 and the lead gamma ray shield 62 of the thermal fission delayed gamma ray measurement unit 60. When cadmium is arranged between the thermal neutron irradiation unit 50 and the thermal fission delayed gamma ray measurement unit 60, thermal neutrons leaking from the thermal neutron irradiation unit 50 are absorbed by the cadmium, and the measurement accuracy at the gamma ray detector 61 is improved. To do.

  In the detection of gamma rays by the thermal fission delayed gamma ray measurement unit 60, high energy resolution is not required. For this reason, a NaI detector or a BGO detector is suitable as the gamma ray detector 61. In order to reduce the size of the apparatus, a CdTe detector, a SiC detector, or the like can be used.

  It is practical to use cadmium (Cd) to effectively irradiate epithermal neutrons, excluding thermal neutrons. In this embodiment, since the ratio of delayed gamma rays when there is no Cd, that is, when mainly irradiating thermal neutrons, and when Cd is present, that is, when mainly irradiating epithermal neutrons is used, this method is used as the Cd ratio. It can be called the law.

Next, a method for deriving a calibration curve between the delayed gamma ray intensity ratio (γ ^t / γ ^e ) and the combustible poison concentration for each U235 enrichment (S21) will be described.

  FIG. 9 is a flowchart of the calibration curve derivation method in the present embodiment.

In the derivation of the calibration curve, the delayed gamma ray intensity (γ _c ^e ) after epithermal neutron irradiation is obtained by calculation using the U235 enrichment and the combustible poison enrichment as parameters (S41). Similarly, the delayed gamma ray intensity (γ _c ^t ) after thermal neutron irradiation is obtained by calculation (S42).

Next, a delayed gamma ray intensity ratio (γ ^t / γ ^e ) _c is obtained. In other words, the relative change in delayed gamma ray intensity ratio (γ ^{t /} γ ^e) _c with respect to a change in the burnable poison concentration, delayed gamma ray intensity ratio of the thermal fission and the epithermal fission (γ ^{t /} γ ^e) _c As the calculated value of the correlation between the flammable poison concentration and the flammable poison concentration, it is obtained for each U235 enrichment (S43).

Further, using the measuring apparatus of the present embodiment, calibrated fuel with known U235 enrichment and flammable poison concentration is irradiated with epithermal neutrons (S44), and delayed gamma ray intensity after epithermal neutron irradiation (Γ _m ^e ) is measured (S45). Similarly, using the measurement apparatus of the present embodiment, thermal neutrons are irradiated to calibration fuel with known U235 enrichment and flammable poison concentration (S46), and delayed gamma ray intensity ( (γ _m ^t ) is measured (S47).

Next, a delayed gamma ray intensity ratio (γ ^t / γ ^e ) _m is obtained (S48). Based on the actually measured delayed gamma ray intensity ratio (γ ^t / γ ^e ) _m , the correlation between the delayed gamma ray intensity ratio (γ ^t / γ ^e ) _c obtained in S43 and the flammable poison concentration is normalized ( S49).

  FIG. 10 is a longitudinal sectional view of a multiple calibration fuel used in the method for measuring the concentration of combustible poisons according to the present embodiment.

In the multiple calibration fuel, a plurality of types of calibration fuel pellets 13 having different concentrations of U235 and flammable poisons are separated by 3 to 10 cm, for example, by spacers 15, and a cladding tube 14 made of the same material and shape as the test fuel is used. It is what was stored in. The U235 enrichment of the calibration fuel pellet 13 covers the enrichment range assumed for the test fuel. Moreover, it is preferable that the combustible poison density | concentration of the calibration fuel pellet 13 is the required minimum density | concentration ( _Pmin ) of the combustible poison corresponding to each U235 enrichment.

  FIG. 11 is a graph showing an example of changes in normalized delayed gamma ray intensity with respect to changes in uranium 235 enrichment.

The change in the delayed gamma ray intensity ratio with respect to the change in the combustible poison concentration determined by the calculation is normalized by the delayed gamma ray intensity ratio (γ ^t / γ ^e ) _m obtained by actual measurement using multiple calibration fuels. The Specifically, it is obtained by calculation so as to pass through a delayed gamma ray intensity ratio (γ ^t / γ ^e ) _m obtained by actual measurement using multiple calibration fuels indicated by white circles (◯) in FIG. The change of the delayed gamma ray intensity ratio (see FIG. 7) with respect to the change of the concentration of the flammable poison is moved. A curve indicated by a broken line in FIG. 11 indicates a delayed gamma ray intensity ratio (γ ^t / γ ^e ) _m in a fuel to which a flammable poison having a minimum necessary concentration (P _min ) corresponding to each U235 enrichment is added. .

In this way, a calibration curve of the delayed gamma ray intensity ratio (γ ^t / γ ^e ) for each U235 enrichment and the flammable poison concentration, normalized by the actual measurement value, is completed. In this calibration curve derivation method, the calibration curve obtained by calculation using the actual measurement values is standardized, so that a calibration curve corresponding to a given measurement system can be obtained without repeating many actual measurements.

Thus, in this Embodiment, the quantity of the combustible poison contained in the fuel with which a nuclear reactor is loaded can be measured. In this method for measuring the concentration of flammable poisons, the concentration of flammable poisons is determined using the delayed gamma ray intensity ratio (γ ^t / γ ^e ).

  The radiation count rate of gamma rays and the like emitted after thermal neutron irradiation is affected by the measurement system, measurement time, elapsed time from irradiation to measurement, and the like. Therefore, when the absolute value of the radiation count rate is used, if the concentration of the flammable poison is low and the measurement error is not negligible with respect to the change in the radiation count rate due to the concentration of the flammable poison, the flammable poison is substantially reduced. Concentration cannot be measured.

However, in the present embodiment, the concentration of the flammable poison is obtained using the delayed gamma ray intensity ratio (γ ^t / γ ^e ) between thermal fission and outer epithelial fission. The influence of the measurement system on the delayed gamma ray intensity after thermal neutron irradiation is considered to be equivalent to the effect on the delayed gamma ray intensity after epithermal neutron irradiation, so the delayed gamma ray intensity ratio (γ ^t / By using γ ^e ), the influence of these measurement systems and the like can be reduced. As a result, in the present embodiment, the concentration of the low concentration combustible poison can also be measured. In other words, in this embodiment, a low concentration added to ensure critical safety even when a fuel having a U235 enrichment of over 5% is handled in a facility that handles a fuel having a U235 enrichment of 5% or less. It can be measured even at low concentrations of flammable poisons, that is, at most 0.1 wt%, generally about 0.01 wt%.

By measuring such a low concentration of flammable poison and confirming that it is above the minimum required concentration (P _min ) of the flammable poison, the conventional U235 enrichment is 5 wt% in the subsequent fuel production process. In the following, it can be handled as it is in a facility where critical safety is ensured. Therefore, according to the present embodiment, it becomes possible to manufacture or handle more than 5% fuel without greatly modifying the existing fuel handling facility.

Further, in the present embodiment, in a given measurement system, delayed gamma ray intensity (γ ^t ) and thermal neutron irradiation after thermal neutron irradiation are applied to a calibration fuel with known U235 enrichment and combustible poison concentration. The later delayed gamma ray intensity (γ ^e ) is measured, and the calibration curve is normalized using the intensity. For this reason, the difference between the system used for the calculation in deriving the calibration curve and the actual measurement system can be corrected, and the measurement accuracy is improved.

  Further, during the calibration, a plurality of fuels having different U235 concentrations and combustible poison concentrations are stored separately, and the fuel passage 11 is moved to complete the calibration by one movement of the calibration fuel. Can do. For this reason, the time required for calibration is shortened.

[Second Embodiment]
FIG. 12 is a flowchart of the method for measuring the enrichment of uranium 235 in the second embodiment of the method for measuring the concentration of combustible poisons according to the present invention.

First, in a given measurement system, epithermal neutrons are irradiated to a concentration calibration fuel whose U235 concentration is known (S44). Next, the delayed gamma ray intensity (γ _m ^e ) after irradiation with epithermal neutrons is measured (S45). Thereafter, a concentration calibration curve of the U235 concentration with respect to the epithermal fission delayed gamma ray intensity (γ _m ^e ) is derived from the relationship between the U235 concentration and the measured delayed gamma ray intensity (γ _m ^e ) (S51).

Next, in the given measurement system, the test fuel is irradiated with epithermal neutrons (S22), and then the delayed gamma ray intensity (γ _T ^e ) is measured (S23). Using the enrichment calibration curve and the measured delayed gamma ray intensity, the U235 enrichment of the test fuel is determined (S52).

  Except for specifying the U235 enrichment of the test fuel, it is the same as in the first embodiment. For irradiating calibration fuel and test fuel with epithermal neutrons and subsequent delayed gamma ray measurement, the epithermal neutron irradiation section 30 and epithermal fission delayed gamma ray measurement of the measurement apparatus (FIG. 8) in the first embodiment. Part 40 can be used.

In addition, exothermal neutron irradiation (S44) to the calibration fuel and subsequent delayed gamma ray measurement (S45) are for deriving a calibration curve between the delayed gamma ray intensity ratio (γ ^t / γ ^e ) and the flammable poison concentration. Also need to do. Therefore, it is not necessary to carry out specially for deriving the concentration calibration curve of the U235 concentration with respect to the epithermal fission delayed gamma ray intensity.

  Exothermic neutron irradiation (S22) to the test fuel and subsequent delayed gamma ray measurement (S23) must also be performed for the measurement of the flammable poison concentration. Therefore, there is no need to do anything special to determine the U235 enrichment of the test fuel.

  In the present embodiment, the U235 enrichment of the test fuel is determined by measurement. Again, paying attention to the fact that flammable poisons have little effect on epithermal neutrons or much less than thermal neutrons, the test fuel is determined from the delayed gamma ray intensity after epithermal neutron irradiation. U235 enrichment can be determined. That is, it is used that fission caused by epithermal neutrons is less affected by flammable poisons and is approximately proportional to U235 enrichment if the weight, size, and shape of the fuel are the same.

  Moreover, the U235 enrichment of uranium handled at a fuel processing facility is generally limited to a plurality of discrete values. In such a case, the U235 enrichment measurement is not required to have such a high accuracy, and it is only necessary to distinguish between the discrete values. In this embodiment, the U235 enrichment is evaluated using the absolute value of the delayed gamma ray intensity, but at least a measurement accuracy that can distinguish between the discrete U235 enrichment values is obtained. .

[Third Embodiment]
FIG. 13 is a flowchart of the method for measuring the enrichment of uranium 235 in the third embodiment of the method for measuring the concentration of combustible poisons according to the present invention.

  First, the peak of spontaneous gamma rays released from U235 contained in the fuel from a concentration calibration fuel having a known U235 enrichment is detected and measured (S61). Thereafter, a concentration calibration curve of the U235 concentration with respect to the spontaneous gamma ray intensity is derived from the relationship between the U235 concentration and the measured spontaneous gamma ray intensity (S62).

  The calibration fuel is the same as the test fuel in shape, size, and density. The intensity of the measured spontaneous gamma ray is proportional to the U235 enrichment.

  FIG. 14 is a graph showing a gamma ray spectrum emitted by uranium 235.

  Uranium 235 undergoes alpha decay with a half-life of about 700 million years to produce thorium 231 (Th231). When this Th 231 shifts from the excited state to the ground state, gamma rays are emitted. As shown in FIG. 14, among these gamma rays, for example, those having an energy of 186 keV are the strongest and easy to measure. Further, gamma rays of 144 keV, 164 keV, and 207 keV may be measured.

  Next, the spontaneous gamma ray intensity of the test fuel is measured with the same measurement system as in S61 (S63). Using the enrichment calibration curve derived in S62 and the measured spontaneous gamma ray intensity, the U235 enrichment of the test fuel is determined (S64). Thus, in this embodiment, the U235 enrichment of the test fuel can be obtained without irradiating with neutrons. Except for specifying the U235 enrichment of the test fuel, it is the same as in the first embodiment.

  FIG. 15 is a longitudinal sectional view of a measuring apparatus used in the method for measuring the concentration of combustible poisons according to the present embodiment.

  This measuring apparatus is obtained by adding a spontaneous gamma ray measuring unit 20 to the measuring apparatus (see FIG. 8) in the first embodiment. The spontaneous gamma ray measurement unit 20 is upstream of the epithermal neutron irradiation unit 30 in the moving direction of the fuel 12 passing through the fuel passage 11 by the fuel driving mechanism 10 before being irradiated with epithermal neutrons or thermal neutrons. Provide.

  The spontaneous gamma ray measurement unit 20 includes a lead gamma ray shielding body 22 and a gamma ray detector 21 that surround the fuel passage 11. In the lead gamma ray shielding body 22, a gap 23 extending from the fuel passage 11 to the gamma ray detector 21 is formed, and the lead gamma ray shielding body 22 also functions as a collimator. When the fuel 12 moves along the fuel passage 11, gamma rays emitted from the fuel 12 pass through a gap 23 formed in the lead gamma ray shielding body 22 and are detected by the gamma ray detector 21.

  By using such a measuring apparatus, the U235 enrichment can be obtained from both the measurement of epithermal fission delayed gamma rays (see the second embodiment) and the measurement of spontaneous gamma rays. For this reason, the measurement precision of U235 enrichment improves.

  The gamma ray detector 21 of the spontaneous gamma ray measurement unit 20 is preferably a Ge detector with good energy resolution. In order to measure the gamma ray peak, it is necessary to use a detector capable of gamma ray spectrum measurement. However, since the target gamma ray energy is low, a detector such as CdZnTe, BGO, or NaI that can downsize the device can be used. .

[Fourth Embodiment]
FIG. 16 is a flowchart of a method for measuring the enrichment of uranium 235 in the fourth embodiment of the method for measuring the concentration of combustible poisons according to the present invention.

First, in a given measurement system such as the measurement apparatus of the first embodiment (see FIG. 8), the calibrated fuel with a known U235 enrichment is irradiated with epithermal neutrons (S44). Next, the delayed gamma ray intensity (γ ^e ) emitted from the calibration fuel irradiated with epithermal neutrons is measured (S45).

Further, in the given measurement system, the reference fuel is irradiated with epithermal neutrons (S71). Next, the delayed gamma ray intensity (γ ₅ ^e ) emitted from the reference fuel irradiated with epithermal neutrons is measured (S72). Here, uranium that does not contain a flammable poison and has a U235 enrichment of less than 5 wt% and close to 5 wt% is used as the reference fuel.

Thereafter, the delayed gamma ray intensity ratio (γ ^e / γ ₅ ^e ) after the epithermal neutron irradiation of the concentration calibration fuel and the reference fuel is calculated (S73). A concentration calibration curve of U235 concentration with respect to the delayed gamma ray intensity ratio (γ ^e / γ ₅ ^e ) is derived (S74).

Since the delayed gamma ray intensity ratio (γ ^e / γ ₅ ^e ) is a relative value, factors such as detection sensitivity of the measurement system are offset. Therefore, even the concentration calibration curve of U235 concentration with respect to the delayed gamma ray intensity ratio (γ ^e / γ ₅ ^e ) derived by calculation is practical. Therefore, the concentration calibration curve of U235 concentration relative to the delayed gamma ray intensity ratio (γ ^e / γ ₅ ^e ) may be obtained by calculation.

Next, the reference fuel is irradiated with epithermal neutrons using the given measurement system (S75), and the delayed gamma ray intensity (γ ₅ ^e ) is measured (S76). Note that the results of neutron irradiation to these reference fuels and the subsequent gamma ray measurement may be used as long as they are performed when the concentration calibration curve is derived.

In addition, the test fuel is irradiated with epithermal neutrons with the given measurement system (S22), and the delayed gamma ray intensity (γ _T ^e ) is measured (S23). In addition, it is necessary to irradiate these test fuels with neutrons and then measure gamma rays to measure the concentration of combustible poisons. Therefore, there is no need to do anything special to determine the U235 enrichment of the test fuel.

Then, the delayed gamma ray intensity (γ _T ^e ) of the test fuel after the epithermal neutron irradiation is divided by the delayed gamma ray intensity (γ ₅ ^e ) of the reference fuel after the epithermal neutron irradiation, and a delayed gamma ray with respect to the reference fuel. An intensity ratio (γ ^e / γ ₅ ^e ) is obtained (S77). Using this concentration calibration curve of U235 enrichment against delayed gamma ray intensity ratio (γ ^e / γ ₅ ^e ) and delayed gamma ray intensity ratio (γ ^e / γ ₅ ^e ) relative to this reference fuel, the U235 enrichment of the test fuel Is obtained (S78).

  Except for specifying the U235 enrichment of the test fuel, it is the same as in the first embodiment. For irradiating the calibration fuel, the reference fuel, and the test fuel with epithermal neutrons and subsequent delayed gamma ray measurement, the epithermal neutron irradiation section 30 and the epithermal fission delay of the measurement apparatus (FIG. 8) in the first embodiment are used. A gamma ray measurement unit 40 can be used.

[Fifth Embodiment]
FIG. 17 is a flowchart in the fifth embodiment of the combustible poison concentration measuring method according to the present invention.

In this flammable poison concentration measuring method, first, a calibration curve of the delayed gamma ray intensity ratio (γ / γ ₅ ) ^t with respect to the reference fuel and the flammable poison concentration is obtained for each U235 enrichment (S71). The delayed gamma ray intensity ratio (γ / γ ₅ ) ^t with respect to the reference fuel is a ratio of the thermal fission delayed gamma ray intensity (γ ^t ) to the thermal fission delayed gamma ray intensity (γ ₅ ^t ) of the reference fuel. Here, uranium that does not contain a flammable poison and has a U235 enrichment of less than 5 wt% and close to 5 wt% is used as the reference fuel.

Next, the reference fuel is irradiated with thermal neutrons (S72), and then gamma rays are measured (S73). The gamma rays measured at this time are delayed gamma rays emitted from fission products resulting from fission that occurs when the reference fuel is irradiated with thermal neutrons. That is, in S72 and S73, the thermal fission delayed gamma ray intensity (γ _T ^t ) of the reference fuel is measured.

Further, the test fuel is irradiated with thermal neutrons (S74), and then gamma rays are measured (S75). The gamma rays measured at this time are delayed gamma rays emitted by fission products resulting from fission that occurs when the test fuel is irradiated with thermal neutrons. That is, in S74 and S75, the thermal fission delayed gamma ray intensity (γ _T ^t ) of the test fuel is measured.

Thereafter, a delayed gamma ray intensity ratio (γ _T / γ ₅ ) ^t of the test fuel with respect to the reference fuel is calculated (S76). That is, by dividing the thermal fission delayed gamma ray intensity (γ _T ^t ) of the test fuel by the thermal fission delayed gamma ray intensity (γ ₅ ^t ) of the reference fuel, the ratio of the delayed gamma ray intensity of the test fuel to the reference fuel (γ _T / Γ ₅ ) ^t is calculated.

Further, the U235 enrichment of the test fuel is specified (S77). For the U235 enrichment, for example, management data when receiving UO ₂ powder obtained in the upstream process can be used.

Next, a delayed gamma ray intensity ratio (γ _{^T /} γ ₅₎ ^t with respect to the reference fuel test fuels obtained in S66, delayed gamma ray intensity ratio with respect to the reference fuel per U235 enrichment obtained in S71 (γ / γ ₅ ) Using the calibration curve of ^t and the flammable poison concentration, the flammable poison concentration (P _T ) of the test fuel is calculated (S78).

  FIG. 18 is a graph showing an example of a change in a gamma ray intensity ratio with respect to a reference fuel with respect to a change in combustible poison concentration.

As shown in FIG. 5, the intensity of delayed gamma rays emitted from fission products generated by thermal neutron irradiation, that is, thermal fission delayed gamma ray intensity (γ ^t ) increases the concentration of combustible poisons. It becomes small with it. On the other hand, the thermal fission delayed gamma ray intensity (γ ₅ ^t ) of the reference fuel is constant. Therefore, as shown in FIG. 18, the delayed gamma ray intensity ratio with respect to the reference fuel decreases as the concentration of the combustible poison in the fuel increases.

Further, the thermal fission delayed gamma ray intensity (γ ^t ) increases as the U235 enrichment increases. Therefore, as shown in FIG. 18, when the flammable poison concentration is the same, the higher the U235 enrichment, the larger the delayed gamma ray intensity ratio with respect to the reference fuel.

  That is, in this embodiment, the flammable poison concentration is obtained by comparing the influence of the flammable poison with respect to the thermal neutrons with the reference fuel excluding the influence of the flammable poison. That is, as the concentration of the flammable poison in the nuclear fuel material changes, the ratio of the delayed gamma ray intensity when the nuclear fuel material is irradiated with thermal neutrons to that when the reference fuel is irradiated with thermal neutrons changes. Therefore, the concentration of the flammable poison in the nuclear fuel material is determined based on the ratio of the intensity of delayed gamma rays when the test fuel is irradiated with thermal neutrons and when the reference fuel is irradiated with thermal neutrons.

  FIG. 19 is a longitudinal sectional view of a measuring apparatus used in the method for measuring the concentration of combustible poisons according to the present embodiment.

  The measurement apparatus of the present embodiment is obtained by removing the epithermal neutron irradiation unit 30 and the epithermal fission delayed gamma ray measurement unit 40 from the measurement apparatus of the first embodiment (see FIG. 8). As described above, in the present embodiment, although it is necessary to use the reference fuel, fewer devices / members are required than in the first embodiment. For this reason, a combustible poison density | concentration can be measured with a cheaper apparatus.

  FIG. 20 is a flowchart of a calibration curve derivation method in the present embodiment.

First, in a given measurement system such as the measurement apparatus shown in FIG. 19, thermal neutrons are irradiated to a calibration fuel having a known flammable poison concentration in a plurality of cases where the U235 enrichment is different (S91). Next, the delayed gamma ray intensity (γ ^t ) emitted from the calibration fuel irradiated with thermal neutrons is measured (S92).

Further, in the given measurement system, the reference fuel is irradiated with thermal neutrons (S93). Next, the delayed gamma ray intensity (γ ₅ ^t ) emitted from the calibration fuel irradiated with thermal neutrons is measured (S94). Thereafter, the calibration fuel and the reference fuel delayed gamma ray intensity ratio after thermal neutron irradiation (γ ^{_t /} γ ₅ ^t) is calculated, and delayed gamma ray intensity ratio with respect to the reference fuel (γ ^{_t /} γ ₅ ^t) and the calibration fuel A calibration curve with the flammable poison concentration is derived (S95).

Since the function indicating the relationship between the delayed gamma ray intensity ratio (γ ^t / γ ₅ ^t ) with respect to the reference fuel and the flammable poison concentration is a relative value, factors such as detection sensitivity of the measurement system are offset. For this reason, even a function indicating a calibration curve of the delayed gamma ray intensity ratio (γ ^t / γ ₅ ^t ) derived by calculation and the flammable poison concentration is practical. Therefore, a calibration curve between the delayed gamma ray intensity ratio (γ ^t / γ ₅ ^t ) and the flammable poison concentration may be obtained by calculation.

  FIG. 21 is a graph showing an example of the relationship between the flammable poison concentration and the delayed gamma ray intensity after thermal neutron irradiation on the reference fuel.

This graph is obtained by neutron transport calculation. The delayed gamma ray intensity after thermal neutron irradiation is proportional to the fission rate. The vertical axis of this graph represents the delayed gamma ray intensity (γ ^t / γ ₅ ^t ) after thermal neutron irradiation with respect to the reference fuel. That is, the curve indicated by the solid line for each U235 enrichment is obtained by standardizing the fission rate ratio in the case of the reference fuel. Here, the U235 enrichment of the reference fuel is 4.9 wt%.

As shown in FIG. 4 and FIG. 5, the minimum required concentration (P _min ) of gadolinia is zero at a U235 enrichment of 4.9 wt%. Further, the necessary minimum concentration (P _min ) of gadolinia is 53 ppm when the U235 concentration is 6 wt%, 110 ppm when the U235 concentration is 7 wt%, and 170 ppm when the U235 concentration is 8 wt%.

Therefore, connecting the points at the required minimum concentration (P _min ) of gadolinia of delayed gamma ray intensity (γ ^t / γ ₅ ^t ) after thermal neutron irradiation with respect to the reference fuel at each U235 enrichment, the broken line in FIG. It becomes like this. If the additive concentration of gadolinia increases, the delayed gamma ray intensity (γ ^t / γ ₅ ^t ) after thermal neutron irradiation to the reference fuel decreases, so if there is no calculation error, critical safety is shown on the right side of the curve indicated by the broken line. Is secured.

That is, when the value of delayed gamma ray intensity (γ ^t / γ ₅ ^t ) after thermal neutron irradiation with respect to the reference fuel shown on the vertical axis of FIG. 21 is measured and given U235 enrichment, the flammable poison concentration is determined. . For example, in consideration of the error, whether or not critical safety can be ensured is determined based on whether the value is larger or smaller than the necessary minimum concentration (P _min ) of the flammable poison.

[Other embodiments]
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, in each of the above-described embodiments, gadolinium is used as the flammable poison. However, the present invention can also be applied to a case where erbium, a mixture of gadolinium and erbium, or a boron compound is used.

  Moreover, the combustible poison density | concentration may be measured in the state of a fuel pellet, and may be measured in the state of UO2 powder. It can also be applied when a flammable poison is applied to the surface of the fuel pellet.

  Furthermore, it can be implemented by combining the features of the embodiments.

DESCRIPTION OF SYMBOLS 10 ... Fuel drive mechanism, 11 ... Fuel passage, 12 ... Fuel, 13 ... Calibration fuel pellet, 15 ... Spacer, 20 ... Spontaneous gamma ray measurement part, 21 ... Gamma ray detector, 22 ... Lead gamma ray shield, 23 ... Air gap, 30 ... external neutron irradiation part, 31 ... neutron source, 32 ... graphite neutron moderator, 33 ... nickel reflector, 34 ... polyethylene neutron moderator, 35 ... external neutron irradiation window, 36 ... cadmium thermal neutron absorber, 37 ... Boron-added polyethylene, 40 ... thermal outer fission delayed gamma ray measurement unit, 41 ... gamma ray detector, 42 ... lead gamma ray shield, 43 ... gap, 50 ... thermal neutron irradiation unit, 51 ... neutron source, 54 ... polyethylene neutron moderator 55 ... Thermal neutron irradiation window, 56 ... Cadmium thermal neutron absorber, 57 ... Boron-added polyethylene, 60 ... Thermal fission delayed gamma ray measurement , 61 ... gamma ray detector, 62 ... lead gamma shield, 63 ... gap

Claims (11)

In a method for measuring the concentration of a flammable poison that measures the concentration of a flammable poison added to a test fuel containing uranium,
An epithermal neutron irradiation step of irradiating the test fuel with epithermal neutrons;
An epithermal fission delayed gamma ray measurement step for measuring gamma ray intensity emitted from the test fuel irradiated with epithermal neutrons after the epithermal neutron irradiation step;
A thermal neutron irradiation step of irradiating the test fuel with thermal neutrons;
A thermal fission delayed gamma ray measurement step for measuring gamma ray intensity emitted from the test fuel irradiated with thermal neutrons after the thermal neutron irradiation step;
The delay for obtaining a delayed gamma ray intensity ratio obtained by dividing the thermal fission delayed gamma ray intensity measured in the thermofission delayed gamma ray measurement step by the epithermal fission delayed gamma ray intensity measured in the epithermal fission delayed gamma ray measurement step. Gamma ray intensity ratio calculation step,
A calibration curve deriving step for deriving a calibration curve of the delayed gamma ray intensity ratio and the flammable poison concentration;
A flammable poison concentration calculating step of calculating a flammable poison concentration of the test fuel from the calibration curve and the delayed gamma ray intensity ratio;
A method for measuring the concentration of a flammable poison, comprising: The calibration curve derivation process
A correlation calculation step for obtaining a correlation between the delayed gamma ray intensity ratio and the combustible poison concentration for each predetermined U235 enrichment by calculation using the U235 enrichment and the combustible poison concentration as parameters;
A calibration fuel epithermal neutron irradiation step in which calibrated fuel having a known U235 enrichment and flammable poison concentration is irradiated with epithermal neutrons in the same system as the epithermal neutron irradiation step;
After the calibration fuel epithermal neutron irradiation step, the calibration fuel epithermal fission delay is measured by the same system as the epithermal fission delay gamma ray measurement step after measuring the gamma ray intensity emitted from the calibration fuel irradiated with epithermal neutrons. Gamma ray measurement process;
A calibration fuel thermal neutron irradiation step of irradiating the calibration fuel with thermal neutrons in the same system as the thermal neutron irradiation step;
After the calibration fuel thermal neutron irradiation step, a calibration fuel thermal fission delayed gamma ray measurement step for measuring gamma ray intensity emitted from the calibration fuel irradiated with thermal neutrons in the same system as the thermal fission delay gamma ray measurement step; ,
The calibrated fuel thermal fission delayed gamma ray intensity measured in the calibration fuel thermal outer fission delayed gamma ray measurement step is divided by the calibration fuel thermal outer fission delayed gamma ray intensity measured in the calibration fuel thermal fission delayed gamma ray measurement step. Obtaining a calibration fuel delayed gamma ray intensity ratio;
A normalization step of deriving the calibration curve by normalizing the correlation obtained in the correlation calculation step with the calibration fuel delayed gamma ray intensity ratio;
The flammable poison concentration measuring method according to claim 1, further comprising: A concentration calibration fuel spontaneous gamma ray measurement step for measuring the spontaneous gamma ray intensity emitted from the concentration calibration fuel with known U235 enrichment;
Deriving a concentration calibration curve between the spontaneous gamma ray intensity and the U235 enrichment based on the gamma ray intensity measured in the enrichment calibration fuel spontaneous gamma ray measurement step and the U235 enrichment of the enrichment calibration fuel;
A spontaneous gamma ray measuring step for measuring the spontaneous gamma ray intensity emitted from the test fuel;
Determining the U235 enrichment of the test fuel based on the gamma ray intensity measured in the spontaneous gamma ray measurement step and the enrichment calibration curve;
The flammable poison concentration measuring method according to claim 1 or 2, characterized by comprising: An enrichment calibration fuel epithermal neutron irradiation step in which epithermal neutrons are irradiated to the enrichment calibration fuel with known U235 enrichment in the same system as the epithermal neutron irradiation step;
Concentration calibration for measuring gamma ray intensity emitted from the enrichment calibration fuel irradiated with epithermal neutrons after the enrichment calibration fuel epithermal neutron irradiation step in the same system as the epithermal fission delayed gamma ray measurement step Fuel thermal outer fission delayed gamma ray measurement process,
An enrichment calibration curve of the epithermal fission delayed gamma ray intensity and the U235 enrichment based on the gamma ray intensity measured in the enrichment calibration fuel thermal outer fission delayed gamma ray measurement step and the U235 enrichment of the enrichment calibration fuel. Deriving
Determining the U235 enrichment of the test fuel based on the gamma ray intensity measured in the epithermal fission delayed gamma ray measurement step and the enrichment calibration curve;
The flammable poison concentration measuring method according to claim 1 or 2, characterized by comprising: After irradiating an enrichment calibration fuel with known U235 enrichment with epithermal neutrons, it does not contain a flammable poison for the gamma ray intensity emitted from the enrichment calibration fuel, and it is exothermic to a reference fuel with an enrichment of 5 wt% or less. Deriving an enrichment calibration curve of U235 enrichment for the ratio of gamma ray intensity emitted from the reference fuel after irradiation with neutrons;
A test fuel epithermal neutron irradiation step of irradiating the epithermal neutrons in the test fuel,
After the test fuel epithermal neutron irradiation step, slow the thermal outer fission gamma ray intensity emitted from said test fuel irradiated with epithermal neutrons onset gamma ray measurement step of measuring in the same system as the test fuel heat outside fission late Gamma ray measurement process;
Based on the ratio of the gamma ray intensity measured in the test fuel epithermal fission delayed gamma ray measurement step to the gamma ray intensity measured in the epithermal fission delayed gamma ray measurement step and the enrichment calibration curve, U235 of the test fuel. A step of determining the concentration;
The flammable poison concentration measuring method according to claim 1 or 2, characterized by comprising: In a flammable poison concentration measuring device for measuring the concentration of a flammable poison added to a test fuel containing uranium,
A drive mechanism for moving the test fuel along a fuel passage;
An epithermal neutron irradiation unit for irradiating the test fuel moving in the fuel passage with epithermal neutrons;
An epithermal fission delayed gamma ray measurement unit that is provided downstream of the epithermal neutron irradiation unit in the direction of movement of the test fuel and measures gamma ray intensity emitted from the test fuel moving in the fuel passage;
A thermal neutron irradiation unit for irradiating the test fuel moving in the fuel passage with thermal neutrons;
A thermal fission delayed gamma ray measurement unit that is provided downstream of the thermal neutron irradiation unit in the moving direction of the test fuel and measures the gamma ray intensity emitted from the test fuel that moves through the fuel passage;
A delayed gamma ray intensity ratio obtained by dividing the epithermal fission delayed gamma ray intensity measured by the epithermal fission delayed gamma ray measurement unit by the thermal fission delayed gamma ray measurement unit measured by the thermofission delayed gamma ray measurement unit. Gamma ray intensity ratio calculating means,
A calibration curve deriving means for deriving a calibration curve of the delayed gamma ray intensity ratio and the flammable poison concentration;
A flammable poison concentration calculating means for calculating a flammable poison concentration of the test fuel from the calibration curve and the delayed gamma ray intensity ratio;
A combustible poison concentration measuring apparatus characterized by comprising: The epithermal neutron irradiation part is
Epithermal neutron irradiation part neutron source,
A graphite neutron moderator that decelerates fast neutrons emitted from the neutron source to the energy of epithermal neutrons;
A thermal neutron absorber surrounding the neutron source and the graphite neutron moderator;
The combustible poison concentration measuring device according to claim 6, comprising:   The epithermal neutron irradiator includes a neutron moderator surrounded by the thermal neutron absorber, surrounding a gap extending between the graphite neutron moderator and the fuel passage, and decelerating neutrons to thermal neutron energy. The combustible poison concentration measuring apparatus according to claim 7. The thermal neutron irradiation unit is
A thermal neutron irradiation unit neutron source,
A neutron moderator that decelerates fast neutrons emitted from the neutron source of the thermal neutron irradiation unit to the energy of thermal neutrons;
A thermal neutron absorber surrounding the neutron source and the neutron moderator so as to form a gap between the neutron moderator and the fuel passage;
The combustible poison concentration measuring device according to any one of claims 6 to 8, characterized by comprising:   A spontaneous gamma ray measuring unit provided on the upstream side in the moving direction of the test fuel of the epithermal neutron irradiation unit and the thermal neutron irradiation unit and measuring a spontaneous gamma ray emitted from the test fuel moving in the fuel passage; The combustible poison concentration measuring device according to any one of claims 6 to 9, characterized by comprising:   11. The fuel cell according to claim 6, wherein a plurality of types of fuel having different U235 enrichment and combustible poison concentration are arranged at intervals and the multiple calibration fuel is formed so as to be movable in the fuel passage. The combustible poison concentration measuring apparatus according to any one of the above items.

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