JP4351943B2 - Evaluation method for core structural materials - Google Patents

Description

  The present invention relates to a core structure material evaluation method for evaluating the soundness of a core structure material such as a fuel cladding tube and a channel box in a nuclear reactor during normal operation of the reactor.

  In nuclear power plants, monitoring and grasping the corrosion state of the core structure materials of the reactor, such as fuel cladding and channel boxes, during normal operation of the nuclear reactor maintains the integrity of the fuel assembly, This is important for further improving the reliability of the nuclear reactor.

Generally, Zircaloy, which is a zirconium (Zr, hereinafter simply referred to as Zr) based alloy, is used as a material for a fuel cladding tube, which is one of reactor core structural materials.
In a fuel rod in a nuclear reactor, a fuel cladding tube filled with uranium oxide pellets is subjected to coolant pressure, hydrodynamic stress, corrosive action due to reactor water, or mechanical friction due to foreign matter.

These effects are accelerated by exposure to high temperatures and radiation, and in some cases, the fuel cladding can be corroded and broken, impairing its integrity.
In general, there are chemical factors and mechanical factors as the main causes of damage to the fuel cladding tube and loss of soundness.

  Chemical factors that significantly accelerate the oxidation and corrosion of zircaloy components due to the water quality fluctuations of the reactor water, and elute the components accumulated in the fuel rods in the reactor water due to zircaloy corrosion include the conductivity of the reactor water, halogen concentration, There are dissolved oxygen, copper element, etc., and it is necessary to control the concentration of these metal impurities in order to suppress corrosion of the fuel cladding.

However, since these chemical factors are complex and a clear reaction mechanism has not been elucidated, changes in the health of the fuel cladding cannot be directly grasped.
On the other hand, as a mechanical factor, the foreign matter brought into the core at the regular inspection moves to the fuel assembly structure part due to the change in the coolant flow rate accompanying the change of the core control rod pattern. It is known that the fuel cladding tube is damaged due to mechanical action (fretting).

  In any case, the fuel cladding tube can be evaluated and judged by capturing changes caused by the components accumulated in the fuel rods being released into the reactor water after the fuel cladding tube is damaged. The signs of damage cannot be evaluated in advance, and any anomalies occurring in the fuel cladding cannot be detected until the damage has occurred.

  For this reason, in order to evaluate the soundness of the fuel cladding tube and other core structure materials, means for monitoring the chemical state of the reactor water have been considered, but the chemical state of the reactor water and the soundness of the fuel cladding tube are also considered. However, since this mechanism has not been elucidated, the soundness of the fuel cladding cannot be fully evaluated even with this means.

  In a healthy state where there is no damage to the fuel cladding, the main source is uranium impurities contained in the cladding, and krypton (Kr, hereinafter simply referred to as Kr) and xenon (Xe, hereinafter simply in the offgas). Radioactive elements with different half-lives of five iodine isotopes are detected in the reactor water and the seven radionuclides (referred to as non-patent document 1).

As these fission products, Zr, niobium (Nb, hereinafter simply abbreviated as Nb), tin (Sn), molybdenum (Mo), and the like are simultaneously generated and released into the reactor water in addition to iodine, Kr, and Xe.
The concentrations and isotopic compositions of these radioactive elements show almost constant values during normal operation of a reactor with a healthy fuel cladding.

  However, as mentioned above, if the fuel cladding tube or other core structural materials are damaged due to hydrodynamic stress, corrosive action, or mechanical friction, the fuel concentration increases with the concentration of these radioactive elements. The isotope composition having a long half-life accumulated in the rod is released into the reactor water, so that the iodine isotope composition in the reactor water also changes.

Conventionally, the soundness of a fuel cladding tube is evaluated and determined by detecting changes in the concentration of radioactive elements and iodine isotope composition (see, for example, Non-Patent Documents 2 and 3).
In addition to Non-Patent Documents 2 and 3 above, many reports have been published on the method for evaluating the presence or absence of damage to the fuel cladding tube. All of these are disclosed in iodine isotopes in reactor water or off-gas. This is a method of estimating from the ratio of radioactivity intensity of Kr and Xe.

This method utilizes the fact that when the fuel cladding is damaged, the radioactivity intensity of each nuclide is detected as the half-life is shorter.
That is, the smaller the decay constant λ (0.693 / half-life) of the nuclide, the smaller its strength.

  However, when the fuel cladding tube is broken, the components with a long half-life (small λ) accumulated in the fuel rod are released to the outside, so that the radioactivity intensity of each iodine isotope is almost the same. It is derived from showing a constant value.

  On the other hand, as another method for detecting abnormalities in core structure materials such as fuel cladding tubes and channel boxes, it is generated by the reaction between protons generated by the reaction of reactor water and neutrons and zircaloy, which is the material component of the fuel cladding tube A method for measuring the change in the ratio of Nb released into the reactor water has been proposed (see, for example, Patent Document 1).

In this method, the amount of Nb produced in zircaloy is quantitatively and steadily measured, and the occurrence of abnormality in the fuel cladding tube is detected by the presence or absence of a change in concentration in comparison with the normal time.
However, since the radionuclide concentration measurement in the reactor water varies depending on the operation state of the plant, sampling and measurement conditions, the reproduction accuracy of the concentration measurement value is low.

On the other hand, damage caused by mechanical action such as fretting caused by foreign matter entering the core cannot be determined by the above method, and cannot be foreseen.
Furthermore, in a plant with many fuel components adhering to the surface of the fuel cladding tube, it becomes difficult to detect Nb due to an increase in the background of Zr generated by fission.
JP-A-5-150083 An Estimation Method for Off-gas Source in a Boiling Water Reactor with Nondefective Fuel, M. TAKAHASHI, Nucl. Technol. 135, 230 (2001), American Nuclear Society Fundamental Aspects of Defective Nuclear Fuel Behavior and Fission Product Release, BJ LEWIS, J. Nucl.Material, 160, 201-217 (1988), North-Holland, Amsterdam Failed Fuel Detection in Reactor Coolant Using Radioiodine Measurement, PK GOPALAKRISHNAN, et.al., Nucl. Technol. 111, 105 (1995), American Nuclear Society

  As described above, the conventional evaluation method for monitoring the soundness of the core structure material is a method for analyzing the various data of the plant after the failure of the fuel cladding tube and elucidating the cause. In the method, no action can be taken until the fuel cladding tube breaks, even if an initial event leading to a major failure occurs in the fuel cladding tube.

  If the fuel cladding tube breaks, radioactive materials and nuclear fuel materials may be released into the furnace and further out of the furnace, which is a safety problem. For this reason, it is important from the standpoint of plant safety management to grasp the signs of damage before the fuel cladding tube breaks.

  The present invention has been made to solve the above-mentioned problems, and can continuously and accurately monitor the soundness of the core structure material during the operation of the reactor. An object of the present invention is to provide a method for evaluating a core structure material with improved performance.

  In order to achieve the above object, the invention according to claim 1 measures and measures the radiation of radioactive products generated from components eluted from the core structure material into the reactor water and present in the reactor water and having different origins. It is characterized in that the change in the amount of zirconium eluted from the core structural material is monitored by detecting the change over time in the ratio of the radiation product from the value of the radiation.

  According to the method for evaluating a core structure material of the present invention, the soundness of the core structure material can be continuously and accurately evaluated during the operation of the reactor, and the reliability of the reactor can be improved. it can.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a diagram schematically showing a part of a fuel rod of a nuclear reactor. In FIG. 1A, 1 is a pellet of a fuel rod, 2 is a fuel cladding tube in which the pellet 1 is enclosed. 1 and the fuel cladding tube 2 constitute a reactor fuel rod 3.
The fuel cladding 2 is formed of Zircaloy, which is an alloy containing Zr as a main component, and Nb and Fe are added to its components.

FIG. 1 (b) is an enlarged view of part A of FIG. 1 (a). As shown in FIG. 1 (b), there is no damage to the fuel cladding tube 2 in the reactor water. Fission products such as iodine, Kr, and Xe are released from the surface of the fuel cladding 2.
Zr, Nb, etc. are also generated by fission at the same time. Therefore, although they are not usually measured, they exist in the reactor water together with fission products such as iodine.

The concentration in the reactor water of each of these fission products is composed of 1 to 4 cycles of irradiated fuel body and is an equilibrium core, and therefore shows a substantially constant value during a normal reactor operation cycle.
On the other hand, since the fuel cladding 2 is an alloy containing Zr as a main component and Nb and iron (Fe) are added, these fuel cladding material components (hereinafter simply referred to as cladding) are activated by neutrons in the core. As a result, a part of the isotope is eluted into the reactor water.

Therefore, the Zr-Nb isotopes in reactor water (hereinafter simply referred to as Zr-Nb) are Nb-98m isotopes (hereinafter simply referred to as Nb-98m) having a mass number of 98 produced only by the fission origin as shown in FIG. Zr-95 isotope with mass number 95 (hereinafter simply referred to as Zr-95) and 97 Zr-97 isotope (hereinafter simply referred to as Zr) produced by both fission and activation sources -97)) coexist.
The characteristics of these three radionuclides are shown in Table 1. In the actual measurement of radioactivity in the reactor water, as shown in Table 1, γ-rays of Zr or Nb and Zr-Nb in radiation equilibrium are measured.

When the cladding tube material does not elute into the reactor water, the measurable Nb is Nb-95, Nb-97, Nb-98m, and the radioactivity intensity is the element residence time in the core as shown in Fig. 3 ( Day).
Further, the radioactivity intensity of Zr when the cladding tube material does not elute into the reactor water varies depending on the residence time (days) of the elements in the core as shown in FIG.

However, since the isotope composition ratio of Nb-95 and Nb-97 generated from the cladding tube during the operation cycle is loaded in the core for a long time as a cladding tube, it shows a substantially constant value as shown in FIG.
As shown in FIG. 6, the radioactivity intensity of Zr in the case of only the covering tube material changes in substantially the same manner as Nb in FIG.

When only the components from the coated tube material are used, the ratio of Zr-95 / Zr-97 and Nb-95 / Nb-97 is about 0.7, and Nb-98m is not detected.
In the case of only fission components, the ratio of Zr-95 / Zr-97 and Nb-95 / Nb-97 is about 0.1, and when the fission component contributes 1/2, this ratio is about 0.4. become.

The detection of Nb-98m is evidence of the existence of fission components, and there are Zr-Nb-97 and Zr-Nb-95 of the same fission origin.
In an actual plant, each of these values is a unique value and is a substantially constant value. However, when an abnormality such as breakage occurs in the cladding tube 2, these values fluctuate in the direction of increasing the cladding tube material component.
In addition, as the contribution from the cladding tube increases, the values of Zr-95 and Zr-97 change to larger values than Nb-98m.

  That is, paying attention to the fact that the generation ratio of fission origin and activation origin Zr is different, the ratio of the Zr and Nb present in the reactor water is used to judge the condition of the cladding tube as a sign of abnormality during the reactor operation. Can evaluate the soundness.

  In the above embodiment, when the isotope is measured, the chemical behavior is the same. In particular, in the case of Zr-Nb, the chemical behavior of both is very similar. Peaks detected on the same γ-ray spectrum can be compared with each other. For this reason, since no factors other than the time correction from sampling to measurement and the efficiency correction of the detector are included, it is possible to detect minute fluctuations in the isotope ratio.

In Zr and Nb, isotopes of fission origin and activation origin have different production ratios. Nb element is considered to behave almost the same as Zr in the reactor water. Nb-98m is produced only by fission origin. On the other hand, both Nb-95 and Nb-97 are produced by the radioactive decay of Zr produced by the fission origin and the activation origin of the material.
Thus, even if it is the same element, the generation origin of an isotope is different.

Zr-95 and Zr-97 are produced both from the fission origin and the activation origin of Zr material. The ratio of the number of Zr-95 and Zr-97 produced is about 1: 1 for fission origin, but about 1: 1.5 for activation origin.
However, Zr produced by the fission origin is released into the reactor water, and part of it adheres to the cladding tube surface due to the boiling phenomenon, desorbs to the reactor water, and is gradually removed by the reactor water purification system.

In general BWRs, the average residence time in the core neutron irradiation region of volatile iodine is about 3 hours, considering the isotope ratio of radioactive iodine.
On the other hand, tellurium (Te), which is a non-volatile iodine parent nuclide, has a residence time of about 140 hours in the core neutron irradiation region.

The Co residence time, which is thought to behave in the same way as Zr and Nb as non-volatile elements in the reactor water, is the ratio of ⁶⁰ Co to the total Co elements ( ⁶⁰ Co / Co: Bq / g or less is called the activation ratio) As shown in FIG. 7 using a comparison of the results of combustion calculation of a general core, the reactor water is about 10 to 200 days, and the components adhering to the surface of the fuel cladding tube are about 200 to 200 days. About 600 days.

Figure 7 shows the activation rate (Bq / g) of ⁶⁰ Co calculated from the amount of ⁶⁰ Co produced when Co is neutron-irradiated at an output of 30 MW / t using the neutron spectrum in the cladding tube, with the irradiation time as the horizontal axis. It is shown.
If the residence time of Zr—Nb is the same as described above, the fission-origin Zr—Nb changes as shown in FIGS. 3 and 4 depending on the difference in the residence time of the core.

  Fig. 3 shows the irradiation time on the horizontal axis and the radioactivity intensity generated on the vertical axis. Nb-, which has a longer half-life as Nb produced in the fission stays in the reactor water system, becomes longer. It can be seen that 95 accumulates and the radioactivity intensity increases.

  On the other hand, Nb-97 and Nb-98m have a short half-life, so there is almost no influence of irradiation time (corresponding to residence time), and they show almost constant values. Since the core residence time on the horizontal axis is in the range of about 10 to 200 days, the isotope ratio in the reactor water shown in the figure shows values in this range.

FIG. 4 shows the irradiation time on the horizontal axis and the radioactivity intensity generated on the vertical axis for Zr as in FIG. 3, and the Zr-generated isotopes are Zr-97 and Zr-95.
On the other hand, the radioactive isotope composition of the cladding tube material stays in the core for a long time and receives neutron irradiation, so that the radioactivity of the Zr component is almost in an equilibrium state as shown in FIGS. Yes, it is clear that the Zr-Nb components from the fission origin and the activation from the cladding material have distinct isotopic compositions in the reactor water.

FIG. 5 shows the irradiation time on the horizontal axis and the radioactivity intensity generated on the vertical axis, and shows the change in radioactivity intensity of Nb generated from the activation of neutrons in the cladding tube material.
The feature of Nb generation from the activation origin of the cladding tube is that only Nb-95 and Nb-97 are generated and Nb-98m is not generated.

Similarly, FIG. 6 shows changes in the radioactivity intensity of Zr isotopes generated by neutron activation of the core tube.
As shown in FIGS. 5 and 6, the radioactive intensity of Zr—Nb due to the activation origin of the coated tube material is almost constant because the coated tube material itself is loaded in the core for a long time.

Of the radioactivity of Zr-Nb released into the reactor water by fission, Nb-98m and Zr-Nb-97 are almost saturated due to their short half-life, and Zr-Nb-95 is about 1/10 of the saturated production. Of strength.
That is, the Nb-98m, Nb-97, and Nb-95 radioactivity intensity ratios produced by nuclear fission and present in the reactor water are in the ratio of about 1: 50: 5, and these isotope ratios usually differ from plant to plant. ing.

On the other hand, activation of the cladding tube material has a sufficient time for activation because the base material is loaded in the furnace. For this reason, the radioactivity intensity of Zr-95 and Zr-97 produced by the activation origin of the cladding tube material is in a saturated state, and the intensity ratio is about 1: 1.5. The radiation balance of Nb-95 and Nb-97 is equivalent to the behavior in the furnace, so the Nb-95 and Nb-97 activity intensity ratio due to the activation of the cladding tube material is about 1: 1.5.
In other words, the composition of Zr and Nb isotopes in the reactor water during normal operation shows a constant value for components originating from fission and those originating from activation.

However, when the cladding tube material is eluted due to abnormal elution or corrosion of the cladding tube material and friction of foreign matters, the ratio of these isotopes varies.
Thus, paying attention to the fact that the fission origin and the activation origin Zr-Nb have different isotope composition ratios, Zr and Nb present in the reactor water are steadily measured, and the fluctuations in this isotope ratio cover the By evaluating the Zr state of the pipe material, it is possible to grasp signs of damage before the fuel cladding pipe is damaged.

Next, a second embodiment of the present invention will be described.
In this embodiment, by constantly measuring the ratio of Nb-98m of fission origin shown in FIGS. 3 and 5 to Nb-97 of material activation and fission origin, the cladding tube material can be detected from the variation. The elution rate change in the reactor water is grasped, and signs of abnormality are detected before the cladding tube breaks.

  In this method, since a short half-life isotope is measured, the separation time is limited, and rapid chemical separation is required. In addition, Nb-97 needs to evaluate the effects of decay from Zr-97, so time management from sampling to measurement is required.

However, in the present embodiment, since Nb having a relatively short half-life is a measurement target, the state of the coated tube material at the time of sampling can be evaluated.
In addition, since the ratio of Nb-98m produced only from the fission origin is evaluated, a highly accurate evaluation is possible.

Next, a third embodiment of the present invention will be described.
In this embodiment, the ratio of Nb-98m of fission origin and Nb-95 of material and fission origin shown in FIG. 3 and FIG. The change of elution rate is grasped, and abnormalities are detected before the cladding tube breaks.

Also in this embodiment, rapid chemical separation is required to measure the Nb-98m isotope originating from fission as in the second embodiment.
However, since Nb-95 has a long half-life, it is less affected by the decay from Zr and less affected by the decay of Zr that occurs during the time from sampling to measurement, enabling highly accurate evaluation.

Next, a fourth embodiment of the present invention will be described.
In the present embodiment, the ratio of the radioactivity intensity ratio between Zr-95 and Zr-97, which is composed of impurity uranium in the fuel cladding 2 shown in FIGS. By measuring the fluctuation, the change in the elution amount of the Zr isotope from the cladding tube material is directly measured.

For this reason, as compared with the method of measuring the Nb isotope, the decay correction is only the elapsed time from the sampling to the measurement, and since the half-life is long, the rapidity is not so required.
In addition, since it is a Zr isotope measurement, the state of Zr can be directly evaluated without chemical influence in the furnace.

Next, a fifth embodiment of the present invention will be described.
In this embodiment, the fluctuation of the radioactivity intensity ratio between Nb-95 and Nb-97, which is composed of impurity uranium in the cladding tube 2 shown in FIGS. Is measured to evaluate the change in the amount of Zr released from the coated tube material.

For this reason, when the chemical behaviors of Zr and Nb are different, the condition of the coated tube material in the furnace can be evaluated together with the direct measurement of Zr.
When the Zr component elutes and exists as a compound such as an element ion or an oxide, it is highly likely that the component is generated from the elution from the fuel cladding 2 or the fission origin of impurity uranium in the cladding.

  In this case, the Zr isotope composition can be determined by the difference in the isotope ratio. The presence or absence of the possibility that the pipe material has moved into the reactor water can be determined.

Next, a sixth embodiment of the present invention will be described.
In ordinary reactor water, there are a large amount of radionuclides such as Co-60, Co-58, and Mn-54 along with iron eluted from stainless steel.

Co-60, Co-58, and Mn-54 have a long half-life and are present in large quantities compared to Zr and Nb. Therefore, when measuring γ-rays of Zr and Nb, these interfering nuclides are removed. It becomes necessary to do.
In addition, Zr and Nb to be measured have a short half-life compared to these interfering nuclides, so a rapid chemical separation operation is required.

  In the present embodiment, Zr and Nb eluted in the reactor water are rapidly separated from interfering nuclides, and each element is added to the anion exchange resin according to the concentration of hydrofluoric acid (HF) shown in FIG. This is a method using the difference in adsorption coefficient.

In ordinary reactor water, there are a large amount of radionuclides such as Co-60, Co-58, and Mn-54 along with iron eluted from stainless steel.
Zr and Nb adsorb to the anion exchange resin at a ratio of 100: 1 at a hydrofluoric acid concentration of about 8M, but most of the interfering elements such as Fe, Mn and Co are not adsorbed to the anion exchange resin. .

  Utilizing this difference, the reactor water clad component collected by the chemical operation procedure shown in FIG. 9 is dissolved in 7 to 10 M hydrofluoric acid (S1), and only Zr and Nb are passed through the anion exchange resin column. Are absorbed and collected (S2), and other interfering nuclides Co-60, Co-58, Mn-54, etc. are quickly removed (S3), and then γ rays of Zr and Nb are measured (S4).

Next, a seventh embodiment of the present invention will be described.
In this embodiment, it is a method for determining by which mechanism the Zr component eluted from the coated tube material is supplied.
Zr components present in general reactor water are thought to be mainly released by chemical elution of zircaloy.

However, as shown in FIG. 10, the presence state of Zr in the reactor water is that foreign matter 4 is mixed between the fuel rod 3 and the spacer 6 in the fuel assembly 5 and is caused by the vibration of the foreign matter 4 generated by the flow of the coolant. Mechanical action may occur, scraping the coated tube material and causing damage from the surface.
When the scraped metal component moves into the reactor water, it is highly likely that it is present in the form of particles.

  In this case, the Zr component released into the reactor water by chemical action such as dissolution due to changes in the reactor water environment is separated and measured by the chemical separation method shown in FIGS. The difference in the release form of can be grasped.

  In the measurement in the present embodiment, first, the reactor water clad component is dissolved in hydrochloric acid or nitric acid containing hydrochloric acid (S21), suction filtered on the filter (S22), the filtrate is taken out (S23) and evaporated to dryness ( S24), Co, Mn, and soluble Zr component are separated by the procedure shown in FIG. 9 (S25), and then both Zr component insoluble and soluble Zr component in particulate hydrochloric acid or nitric acid are easily separated. And measure (S26).

  Zr supplied into the reactor water by elution of zircaloy, or Zr and Nb components supplied from the fission origin are once released into the reactor water in an ionic state, and are present in the state of being incorporated into the oxide of Fe. Can be dissolved in nitric acid.

  On the other hand, since the Zr component that is supplied by mechanical action and exists as particles can be dissolved only by hydrofluoric acid, Zr components in different forms can be separated by the difference in solubility.

Next, an eighth embodiment of the present invention will be described.
In the present embodiment, the isotope ratio and the chemical state of Zr in the reactor water are dissolved in hydrochloric acid or nitric acid containing hydrochloric acid at the beginning of the reactor water cladding component, and Co, Mn, and soluble Zr by filtration. After the components are separated, both the Zr component that is insoluble in the particulate hydrochloric acid or nitric acid and the soluble Zr component are separated and measured.

  Zr supplied in reactor water by elution of zircaloy or Zr and Nb components supplied by fission origin exist in ionic or oxide form, and therefore can be dissolved in hydrochloric acid or nitric acid containing hydrochloric acid.

  On the other hand, since the Zr component that is supplied by mechanical action and exists as particles can be dissolved only by hydrofluoric acid, Zr components in different forms can be separated by the difference in solubility.

  In the present embodiment, among the measured Zr-Nb isotope ratios, the Zr component present as an insoluble component in particulate reactor water, or the soluble Zr component and the isotope ratio in the total Zr Providing a method for determining the cause of a change from the normal time and how to deal with it.

If a Zr component due to mechanical factors is observed, it is highly likely that the fuel cladding tube has been released into the reactor water by mechanical action such as fretting due to foreign matter. The possibility of being supplied to the structural part of the aggregate is shown.
Since the supply of such foreign matter is often due to a change in flow rate caused by a change in the core pattern, the control rod pattern is changed again and the change in the Zr component is measured.

When the change of the Zr component returns to the original state, the control rod pattern is gradually changed again while measuring the change of the Zr component.
Also, the flow rate change is evaluated by thermal flow analysis, and the location where the flow rate change becomes large is grasped. By this method, the location of the foreign matter supplied by the pattern change is specified.

Next, a ninth embodiment of the present invention will be described.
In the present embodiment, among the measured Zr-Nb isotope ratios, the Zr component present as an insoluble component in particulate reactor water, or the soluble Zr component and the isotope ratio in the total Zr The change over time is intensively monitored after the control rod pattern is changed.

In general, damage to the fuel cladding tube 2 due to the inclusion of foreign matter often occurs when the flow rate of the coolant locally varies with the control rod pattern change.
For this reason, it is possible to quickly detect the abnormality by intensively measuring after changing the control rod.

Normal measurement is performed once a week at approximately the same frequency as iodine concentration measurement in the reactor water. However, if the control rod pattern is changed, the fuel assembly output and the coolant flow rate are balanced immediately after that. Repeat the measurement for a few days to become ready.
By this intensive measurement after changing the control rod pattern, the presence or absence of Zr—Nb concentration fluctuation in the reactor water can be grasped quickly and with high sensitivity.

Next, a tenth embodiment of the present invention will be described.
In the present embodiment, when an abnormality is recognized in the fuel cladding tube 2 and then the fuel assembly 5 including the fuel rod 3 in which the abnormality has occurred by the fuel inspection at the regular inspection is specified, this fuel assembly The position where 5 is loaded in the next operation cycle is preferentially arranged in the periphery of the core.

  If a change in the Zr isotope ratio due to mechanical factors is observed, and no foreign matter is found in the appearance inspection of the fuel assembly 5 after the end of the operation cycle, the fuel assembly 5 in which the abnormality is specified does not break. There is a possibility that the fuel cladding tube 2 has been mechanically damaged.

Such a surface abnormality that has not led to damage of the fuel cladding tube 2 results in damage because the mechanical influence due to heat and coolant flow rate changes in the subsequent operation is greater than that of the normal fuel cladding tube 2. Cheap.
For this reason, the fuel assembly 5 in which an abnormality has been detected is preferentially disposed in the periphery of the core where the coolant flow rate and output are lower than in the center of the core in the next operation cycle.

  By this method, the coolant flow rate and output of the fuel assembly can be kept low, the possibility of damage can be reduced, and the fuel assembly can be used effectively.

  In the above description of the embodiment, the example of the fuel cladding tube has been described as the core structure material. However, the present invention can also be applied to other structural materials such as a channel box.

It is a schematic explanatory drawing which shows the production | generation process of the fission product in a nuclear reactor with no damage of a fuel cladding tube, (a) is a schematic inclination figure of a fuel cladding tube, (b) is the A section enlarged view of (a). . Schematic explanatory drawing which shows the production | generation process of Zr isotope. The characteristic figure which shows the change with respect to the core residence time of the radioactive intensity of the Nb isotope by the fission origin. The characteristic figure which shows the change with respect to the core residence time of the radioactive intensity of the Zr isotope by the fission origin. The characteristic figure which shows the change with respect to the core residence time of the radioactive intensity of the Nb isotope by the activation origin of a cladding tube material. The characteristic figure which shows the change with respect to the core residence time of the radioactive intensity of the Zr isotope by the activation origin of a cladding tube material. The characteristic view which shows the activation rate (Bq / g) of Co by irradiation. The characteristic view which shows the difference in the adsorption rate of the element to an anion exchange resin with respect to hydrofluoric acid concentration. The block diagram which shows the chemical separation method of all the Zr components in a reactor water. The schematic explanatory drawing which shows the production | generation process of the particulate Zr component by the vibration of the foreign material mixed in the fuel assembly part. The block diagram which shows the separation method of the particulate Zr component in a reactor water, and other Zr components.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Pellet, 2 ... Fuel cladding tube, 3 ... Fuel rod, 4 ... Foreign material, 5 ... Fuel assembly, 6 ... Spacer.

Claims (10)

  Measure the radiation of radioactive products generated from the components eluted from the core structure material into the reactor water and present in the reactor water, and detect changes in the ratio of radiation products over time from the measured radiation values. Thus, a change in the amount of zirconium elution from the core structure material is monitored.   Niobium-98m isotope produced by fission origin due to impurity uranium in the core structure material and niobium-97 isotope produced by activation origin of the core structure material and fission origin by impurity uranium in the core structure material 2. The method for evaluating a core structure material according to claim 1, wherein a change in the elution amount of zirconium from the core structure material is monitored by detecting a change with time of the ratio to the body.   By detecting the change over time in the ratio of zirconium-95 isotope and zirconium-97 isotope produced by activation origin of the core structure material and fission origin by impurity uranium in the core structure material, 2. The method for evaluating a core structural material according to claim 1, wherein a change in the amount of zirconium eluted from the structural material is monitored.   Niobium-98m isotope produced by fission origin due to impurity uranium in the core structure material, niobium-95 isotope produced by activation origin of the core structure material and fission origin by impurity uranium in the core structure material 2. The method for evaluating a core structure material according to claim 1, wherein a change in the elution amount of zirconium from the core structure material is monitored by detecting a change with time of the ratio to the body.   By detecting the change over time in the ratio of niobium-95 and niobium-97 isotopes produced by the activation origin of the core structure material and the fission origin by the impurity uranium in the core structure material, 2. The method for evaluating a core structural material according to claim 1, wherein a change in the amount of zirconium eluted from the structural material is monitored.   The step of dissolving the reactor water cladding component in hydrochloric acid or nitric acid containing hydrochloric acid, the step of filtering this solution to separate cobalt, manganese and soluble zirconium-niobium components, and the insoluble in particulate hydrochloric acid or nitric acid 6. The method for evaluating a core structure material according to claim 1, further comprising a step of separating the zirconium-niobium component and the soluble zirconium-niobium component.   A reactor water clad component containing iron, cobalt and manganese and a zirconium-niobium component is dissolved in hydrofluoric acid, and zirconium which is insoluble in hydrofluoric acid by passing the solution through a filter-like anion exchange resin. -A step of simultaneously collecting a niobium component and zirconium-niobium component dissolved in hydrofluoric acid in an ion exchange filter, and a step of separating the zirconium-niobium component from iron, cobalt and manganese. The method for evaluating a core structure material according to any one of claims 1 to 5.   The core structural material is in the form of fine particles released into the reactor water due to the mechanical action of foreign matter, and is dissolved in the reactor water by chemical action such as dissolution of zirconium-niobium components that are insoluble in the reactor water and changes in the reactor water environment. The core according to any one of claims 1 to 7, wherein a change in a discharge form of the zirconium-niobium component from the core structural material is monitored by separating and measuring the formed zirconium-niobium component. Evaluation method for structural materials.   After changing the control rod pattern, among the zirconium-niobium component ratio, the zirconium-niobium component present as particulate insoluble components in the reactor water, or the soluble zirconium-niobium component, and the isotopes in all zirconium-niobium components 9. The fuel assembly that has caused the change is determined by measuring a change in the body ratio over time, and a coping method for the plant operation method is determined. The evaluation method of the core structure material described. The core structure according to claim 9, wherein a position where the fuel assembly including the fuel rod in which an abnormality is recognized in the fuel cladding tube is loaded at the start of the next operation cycle is preferentially disposed in the periphery of the core. Material evaluation method.

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