Neutron irradiation dose evaluation method

Abstract

To provide an evaluation method capable of measuring a neutron irradiation amount using a stainless steel member of an actual plant during long-term stop, and capable of accurately evaluating influence of a neutron irradiation amount.SOLUTION: A method for evaluating a neutron irradiation amount comprises: a collection step of collecting a sample from a material that has been irradiated with neutrons; a step of dissolving the sample to obtain a solution; a step of measuring a Nb element concentration and a Mo element concentration in the solution; a step of separating Nb and Mo in the solution; a step of measuring the radioactivity of 93mNb in the separated Nb and 93Mo in Mo; a step of estimating a contribution ratio of Nb and Mo to 93mNb from the measurement results of the element concentrations and the radioactivity; and a step of subtracting the radioactivity of 93mNb generated from 92Mo from the measurement value of the radioactivity of 93mNb to estimate a neutron flux at a sample collection point.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] An embodiment of the present invention relates to a method for evaluating a neutron irradiation dose. [Background technology]

[0002] As nuclear reactors age, there is a real need for technical evaluation of material degradation phenomena caused by neutron irradiation. In particular, structures such as the upper lattice plate close to the fuel assemblies loaded in the reactor need to be evaluated for the initiation and growth of cracks in the materials induced by neutron irradiation. However, the current evaluation of fast neutron flux uses analytical values, and for more accurate evaluation, actual measurements of the neutron flux of the actual structure are required.

[0003] In addition, since the 2011 earthquake, nuclear reactors have been out of operation for long periods of time, and in order to actually measure the amount of neutron irradiation, it is necessary to select a nuclide that allows the amount of radioactivity to be quantified even after more than 10 years have passed.

[0004] 93 Inelastic scattering of fast neutrons from Nb (production formula: 93 Nb(n,n') 93m Nb) 93m Nb has a relatively long half-life of 16.13 years and is an element that undergoes nuclide transduction by fast neutrons produced by the fuel loaded in a nuclear reactor, so it is used as an analysis target when measuring and evaluating fast neutron flux. Up until now, methods have been developed that enable the actual measurement of neutron irradiation doses of materials loaded in a nuclear reactor by analyzing and evaluating Nb dosimeter wires with known Nb concentrations (for example, Patent Document 1).

[0005] However, when trying to investigate the neutron irradiation dose of stainless steel used in reactor structures, the initial values ​​of the elements are unknown, so it is necessary to develop a method to estimate the initial values ​​of the target elements.

[0006] In such cases, the method for measuring the neutron irradiation dose is as follows: 93m Radioactivity of Nb and 93By estimating the element concentration before Nb transmutation occurs, the fast neutron flux during the reactor operation period can be derived.

[0007] However, in stainless steel 93 The concentration of Nb is extremely small, on the order of ppm, and therefore requires trace element analysis technology. 93m Nb is 92 Mo(n,γ) 93 Mo, 93 It is also known to be produced by orbital electron capture decay of Mo. 92 Mo is a naturally occurring isotope of Mo, which accounts for approximately 2% of stainless steel in nuclear reactors. 93 Generated from Nb 93m When measuring the amount of Nb radioactivity, 92 Generated from Mo 93m It is necessary to eliminate the effect of Nb radioactivity. 93m Nb decays 93 Generate Nb. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent registration number 3845638 Summary of the Invention [Problem to be solved by the invention]

[0009] As mentioned above, in the past, for actual structures such as nuclear reactors that have been shut down for a long time since the earthquake disaster and where it is difficult to actually measure the neutron irradiation dose, analytical values ​​were used for the neutron irradiation dose used in the aging evaluation, and the impact evaluation of the neutron irradiation dose was performed using an overly conservative evaluation method described in the maintenance standard. In addition, when evaluating the neutron irradiation dose of a nuclear reactor, it was necessary to take into account the long half-life of trace amounts of neutrons contained in stainless steel. 93 Generated from Nb 93m Nb is useful as a nuclide for evaluating fast neutron flux, but 93m Among the various mechanisms of Nb formation,93 Mo 93m There is no established method to eliminate the effect of Nb.

[0010] The present invention has been made in response to the above-mentioned conventional circumstances, and aims to provide a method for evaluating neutron exposure dose that can measure neutron exposure dose using stainless steel components of an actual plant that has been shut down for a long period of time, and that can accurately evaluate the impact of neutron exposure dose. [Means for solving the problem]

[0011] The method for evaluating a neutron irradiation dose according to the embodiment includes a collection step of collecting a sample from a material irradiated with neutrons from a nuclear power plant, a dissolution step of dissolving the sample to obtain a solution, an element concentration measurement step of measuring the Nb element concentration and the Mo element concentration in the solution, a separation step of separating Nb and Mo in the solution, and a measurement step of measuring the Nb concentration in the Nb separated in the separation step. 93m The radioactivity of Nb and the Mo separated in the separation process 93 From a radioactivity measurement step of measuring the radioactivity of Mo, a measurement result of the element concentration measurement step, and a measurement result of the radioactivity measurement step, 93m The contribution ratio of Nb and Mo to Nb was estimated, 93m From the measured value of Nb radioactivity, 92 Generated from Mo 93m and a neutron flux estimation step of subtracting the radioactivity of Nb and estimating the neutron flux at the sample collection point. Effect of the Invention

[0012] According to an embodiment of the present invention, it is possible to measure the neutron irradiation dose using stainless steel components of an actual plant that has been shut down for a long period of time, and it is possible to provide a neutron irradiation dose evaluation method that can accurately perform neutron irradiation dose impact evaluation. [Brief description of the drawings]

[0013] [Figure 1] 1A to 1C are process diagrams illustrating a procedure of a neutron irradiation dose evaluation method according to a first embodiment. [Diagram 2] FIG. 2 is a schematic process diagram showing an outline of weighing and dissolving a sample in the first embodiment. [Diagram 3] FIG. 3 is a schematic process diagram illustrating an outline of dissolution liquid separation in the first embodiment. [Figure 4] FIG. 4 is a diagram for explaining a method for correcting analytical values ​​based on actual measured values ​​of fast neutron flux in the first embodiment. [Diagram 5] FIG. 4 is a diagram for explaining a method for improving irradiation-assisted stress corrosion cracking evaluation based on the correction results of analytical values ​​in the first embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, the details of the neutron irradiation dose evaluation method according to the embodiment will be described with reference to the drawings.

[0015] The purpose of this embodiment is to establish a method for actually measuring fast neutron flux in stainless steel components that are irradiated with neutrons from fuel loaded into a reactor, using trace element concentration analysis and the radioactivity of activated nuclides, even in cases where the reactor has been shut down for a long period of time since the earthquake disaster.

[0016] It is useful for evaluating the concentration of Nb, a trace element contained in stainless steel components in nuclear reactors, and for evaluating the radiation dose of long-term shut-down nuclear reactors. 93m Measure the amount of Nb radioactivity. 93 Even in materials with Nb content on the ppm order, it is possible to measure the Nb concentration by using an inductively coupled plasma mass spectrometer or an inductively coupled plasma optical emission spectrometer. 93m Radioactivity of Nb and 93 After quantifying the Nb concentration, 92 Mo 93m The radioactivity of Nb was excluded. 93 Generated from Nb 93m The fast neutron flux is estimated from Nb, and the neutron exposure dose is calculated.

[0017] (First embodiment) FIG. 1 shows an example of the procedure of the neutron irradiation dose evaluation method using stainless steel according to the first embodiment. FIG. 2 shows a schematic process diagram of weighing and dissolving a sample. As shown in FIG. 2, first, a sample 2 is taken from the irradiated stainless steel 1. The sample 2 is weighed and placed in a Teflon (registered trademark) beaker 3, and hydrochloric acid 4, nitric acid 5, and hydrofluoric acid 6 are added and dissolved by heating (step 11 in FIG. 1). After the heating and dissolving is completed, the dissolved liquid 7 is collected in a container 8 with a known mass, and the cleaning liquid obtained by washing the Teflon (registered trademark) beaker 3 with pure water is also collected in the container and combined with the dissolved liquid 7. The mass of the dissolved liquid 7 in the container 8 with a known mass is measured.

[0018] The Nb concentration in stainless steel is measured using an inductively coupled plasma mass spectrometer or the like, using solution 9 obtained by diluting solution 7. When measuring using an inductively coupled plasma mass spectrometer, the metal element concentration is diluted to 100 ppm or less to eliminate the effects of matrix interference, etc. The solution is diluted using 2% nitric acid or the like, and the dilution ratio is accurately determined. Nb is quantified using the calibration curve method or internal standard method. The Nb concentration in stainless steel is calculated by (Nb concentration in diluted solution x dilution ratio x amount of solution / amount of sample). Similarly, elemental analysis of Mo in stainless steel is performed using the same measuring device, using diluted solution 9 (step 12 in Figure 1).

[0019] In the case of solution 7, the 93m Measure the amount of Nb radioactivity (step 15 in Figure 1). 93m To evaluate Nb 93 The amount of radioactivity of Mo is also measured (step 16 in Figure 1). The radioactivity measurement is performed after chemical separation (step 13 in Figure 1).

[0020] Figure 3 shows a schematic diagram of the chemical separation process. All or part of the solution is adjusted to 2 mol / l hydrofluoric acid and passed through the anion exchange resin. During this process, Mo and Nb are adsorbed onto the resin. Next, Mo is eluted and collected by passing a mixture of 8 mol / l hydrofluoric acid and 4 mol / l hydrochloric acid through the anion exchange resin.

[0021] After that, 1 mol / l hydrochloric acid is passed through the anion exchange resin to elute and recover Nb. After evaporating and drying the Mo recovery solution and Nb recovery solution, they are dissolved again to prepare Mo solution and Nb solution, and then evaluation of Mo and Nb recovery rate 1 is performed (step 14 in Figure 1). At this time, an inductively coupled plasma mass spectrometer or an inductively coupled plasma optical emission spectrometer is suitable for measuring Mo and Nb.

[0022] Recovery evaluation is a process to evaluate the change in the mass of elements before and after chemical separation in percentage terms. Mo solution and Nb solution are separately taken, and lanthanum and ammonia water are added to precipitate lanthanum hydroxide. At this time, in the Mo solution, Mo remains in the filtrate side, and nuclides with high doses or the same radiation energy are precipitated together with lanthanum hydroxide.

[0023] The Mo solution after lanthanum hydroxide precipitation is filtered, and the filtrate containing Mo is collected. The filtrate is subjected to Mo precipitation to obtain Mo precipitates. In this case, if there is a restriction on the minimum amount of Mo required for Mo precipitation, a specified amount of Mo is added to the Mo filtrate before the precipitation operation. In the Nb solution, Nb precipitates together with lanthanum.

[0024] Mo precipitate sample and Nb precipitate sample were used. 93m Nb and 93 Radioactivity measurement of MO (steps 15 and 16 in Figure 1) and recovery rate evaluation 2 of Nb and Mo (steps 17 and 18 in Figure 1) are carried out. Radioactivity measurement is 93m Nb and 93 A Ge semiconductor detector for measuring low-energy photons is suitable for measuring the X-rays emitted by Mo. The recovery rate evaluation 2 of the precipitate sample is evaluated, for example, by quantifying the elements in the precipitate sample using an X-ray fluorescence analyzer. Quantification is performed using a calibration curve method.

[0025] The amount of radioactivity in the sample is (Radioactivity in the precipitate sample) / ((Separation sample ratio) x (Ion exchange recovery rate) x (Measurement sample ratio) x (Sedimentation recovery rate)) It is. Here, the separation sample ratio is the ratio of the solution that was subjected to chemical separation using an anion exchange resin from the solution. The ion exchange recovery ratio is the mass ratio of elements obtained from the ratio of element concentration analysis and recovery rate evaluation 1. The measurement sample ratio is the proportion of the solution that was subjected to measurement from the recovered solution. The precipitation recovery ratio is the ratio of the mass of an element contained in the solution subjected to the precipitation operation to the mass of the element in the precipitate sample.

[0026] 93m From the measured Nb radioactivity, 92 Generated from Mo 93m By subtracting the amount of Nb radioactivity, 93 Generated from Nb 93m Only the radioactivity of Nb is derived. 92 From Mo 93m The amount of Nb produced is Nb93m , 93 The amount of Mo produced was increased by N Mo93m If the operation period (irradiation time) is t, it can be expressed by the following formula. N Nb93m =λ1λ2N1[{e -λ1t / (λ2-λ1)(λ3-λ1)}+ {e -λ2t / (λ1-λ2)(λ3-λ2)}+{e -λ3t / (λ1-λ3)(λ2-λ3)}] N Mo93 =λ1N1[{e -λ1t / (λ2-λ1)}+{e -λ2t / (λ1-λ2)}]

[0027] The constant λ in Eq. i is as follows: λ1=σ 92Mo / 93Mo ×φ th λ2=λ Mo93 λ3=λ Nb93m N1 is 92 The initial number of Mo atoms, σ 92Mo / 93Mo teeth, 92 Mo(n,γ) 93 Thermal neutron capture cross section of Mo, φ th is the thermal neutron flux, λ Mo93 teeth 93Decay constant of Mo, λ Nb93m teeth 93m This is the decay constant of Nb. N1 can be calculated using the following formula: N1 = M × C × F × N A / G where M is the amount of sample (g), C is the elemental composition in the sample (%), F is the natural abundance ratio of the element (%), and N A : Avogadro constant (mol -1 ), G: Atomic weight (amu)

[0028] In stainless steel 92 The elemental composition C of Mo can be obtained by referring to the values ​​in the mill sheet, etc. Also, taking into account the cooling period tc after the end of operation (irradiation), 93 Mo production amount N' Mo93 Since is known by actual measurement, N Mo93 The thermal neutron flux φ can be calculated using the following formula: th can be derived. N Mo93 =N' Mo93 ×e λ2tc For plants that are out of operation, 93 From Mo 93m It is also necessary to consider the Nb production reaction. 93 Mo 93m Number of Nb atoms, N' Nb93m and radioactivity A Nb93m(Mo) ' is calculated using the following formula: N' Nb93m =(λ2 / (λ3-λ2))N Mo93 (e -λ2tc -e -λ3tc )+N Nb93m e -λ3tc A Nb93m(Mo) '=N' Nb93m λ3

[0029] Fast neutron flux φ f teeth, 93 Produced by inelastic scattering of fast neutrons with Nb 93m Nb radioactivity A Nb93m(Nb) , 93m Nb radioactivity measurement result ANb93m ', when measuring radioactivity 93 Nb-derived 93m Radioactivity of Nb A Nb93m(Nb) ', when measuring radioactivity 92 Mo 93m Radioactivity of Nb A Nb93m(Mo) ', 93 The initial amount of Nb, N´ Nb93 , 93 Nb(n,n') 93m Nb inelastic scattering cross section σ Nb93 / 93m , 93m Decay constant λ of Nb 2Nb93m It is expressed by the following formula using A Nb93m(Nb) '=A Nb93m '-A Nb93m(Mo) ' A Nb93m(Nb) =A Nb93m(Nb) '×e λNb93mtC φ f =A Nb93m(Nb) / [N Nb93 σ Nb93 / 93m {1-exp(-λ Nb93m t)}] N Nb93 teeth, 93 indicates the initial amount of Nb, 93 It is calculated using the same method as N1 based on the Nb concentration.

[0030] In this way, the fast neutron flux φ f Then, the fast neutron flux φ f The neutron irradiation dose that sample 2 receives from the fuel loaded in the reactor is calculated using t and the number of years of operation (irradiation period). From the above, it is possible to evaluate the neutron irradiation dose using stainless steel with an unknown initial Nb concentration (step 20 in Figure 1).

[0031] 93m The analytical value of the fast neutron flux in the reactor internals is corrected using the measured value of the fast neutron flux obtained from Nb (step 21 in Fig. 1). The vertical axis is the fast neutron flux (n / cm 2As shown in the graph of Figure 4, where the horizontal axis is the neutron flux (Hz) and the horizontal axis is the distance from the fuel loaded in the reactor, the current evaluation of material degradation events in reactor internal structures with a certain thickness or length uses the analytical value 101 shown by the solid line of fast neutron flux, but in the current evaluation, the analytical value of the part with the highest neutron flux in the structure being evaluated (the analytical value of the part of the component being evaluated closest to the fuel) is used for the evaluation.

[0032] The fast neutron flux is actually measured, and the neutron dose evaluation of the structure is performed by correcting the entire evaluation line based on the fast neutron flux analytical value 101 used in evaluating the target component, based on the fast neutron flux measured value (example 1) 102 shown by the dashed-dotted line and the fast neutron flux measured value (example 2) 103 shown by the dashed-dotted line. This makes it possible to evaluate the neutron dose of the reactor internal structure, which was previously evaluated only by analytical values, with values ​​based on the actual measurement data. Note that the measured value (example 1) shown by the dashed-dotted line shows an example in which the fast neutron flux is higher than the analytical value shown by the solid line, and the measured value (example 2) shown by the dashed-dotted line shows an example in which the fast neutron flux is lower than the analytical value shown by the solid line. In either case, the circular regions shown by 102 and 103 can be evaluated lower than the analytical value of the part with the highest neutron flux (the analytical value of the part closest to the fuel of the evaluation target component).

[0033] As an example of material evaluation, Figure 5 shows a case where this method is applied to the evaluation of radiation-induced stress corrosion cracking of reactor internals (step 22 in Figure 1). In contrast to the conventional crack growth evaluation 201 (solid line), which applied a conservative neutron dose based on analytical values, the improved crack growth evaluation 202 (dotted line) uses neutron dose values ​​that are corrected based on actual measurements, which extends the number of years it takes for the crack depth and length caused by radiation-induced stress corrosion cracking to reach the thickness and length of the component, and as a result, the operating years are expected to be extended.

[0034] As described above, according to the first embodiment, by measuring the neutron irradiation dose using stainless steel members of an actual plant that has been shut down for a long period of time, it is possible to obtain evidence that the irradiation dose is distributed even inside the reactor, and to improve the evaluation of the impact of the neutron irradiation dose. In addition, depending on the evaluation target, it is expected that the effective operating years (EFPY) until the crack generation / propagation induced by neutron irradiation penetrates the material can be extended.

[0035] Second embodiment Next, a second embodiment will be described. In the second embodiment, in response to a long-term shutdown of a nuclear reactor, 92 Mo 93 Considering the gradually increasing proportion of Nb, 93 Nb content 92 Eliminate the influence of Mo for greater accuracy 93 The initial value of Nb is estimated, i.e., after the end of neutron irradiation. 93 Produced by the decay of Mo 93m Nb, and 93m Produced by the decay of Nb 93 Nb is 93 Evaluate the effect on estimating the initial element concentration before Nb neutron irradiation, 93 The initial value before neutron irradiation of Nb is derived using the following formula: 92 Mo 93 Calculate the amount of Nb. Note that in this case, add (λ i -λ i ) is not included. N Nb93 =λ1λ2λ3N1×Σ 4 i=1 [{1 / (λ1-λ i )…(λ4-λ i )}e -λit ] +λ2λ3N Mo93 ×Σ 4 i=2 [{1 / (λ2-λ i )…(λ4-λ i )}e -λitc ]

[0036] Here, λ4 is 93 Decay constant λ of NbNb93 However, 93 Since Nb is a stable isotope with a natural abundance of 100%, λ Nb93 = 0. From the above, 93 Obtained by measurement from the amount of Nb 93 By calculating the difference with the amount of Nb, 93 This enables more sophisticated calculation of the initial amount of Nb.

[0037] Although some embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, modifications, and combinations can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and spirit of the invention, and are included in the scope of the invention and its equivalents described in the claims. [Explanation of symbols]

[0038] 1...Irradiated stainless steel, 2...Sample, 3...Teflon (registered trademark) beaker, 4...Hydrogen acid, 5...Nitric acid, 6...Hydrofluoric acid, 7...Dissolving solution, 8...Container of known mass, 9...Diluted dissolving solution.

Claims

1. A sampling step of sampling a sample from a neutron-irradiated material of the nuclear power plant; a dissolving step of dissolving the sample to obtain a dissolution solution; an element concentration measuring step of measuring an Nb element concentration and an Mo element concentration in the solution; A separation step of separating Nb and Mo in the solution; In the Nb separated in the separation step ９３ｍ The radioactivity of Nb and the Mo separated in the separation process ９３ A radioactivity measuring step of measuring the radioactivity of Mo; From the measurement results of the element concentration measurement step and the measurement results of the radioactivity measurement step, ９３ｍ The contribution ratio of Nb and Mo to Nb is estimated, ９３ｍ From the measured value of Nb radioactivity, ９２ Generated from Mo ９３ｍ A neutron flux estimation step of subtracting the radioactivity of Nb and estimating the neutron flux at the sample collection point; A method for evaluating a neutron irradiation dose, comprising:

2. The neutron irradiation dose evaluation method according to claim 1, further comprising a neutron irradiation dose evaluation step of evaluating an actual measured value of the neutron irradiation dose at the sampling point of the sample based on the estimated result of the neutron flux estimation step.

3. 3. The method for evaluating a neutron irradiation dose according to claim 1, further comprising a step of evaluating recovery rates of Nb and Mo after the separation step.

4. 4. The method for evaluating a neutron irradiation dose according to claim 1, further comprising the steps of: preparing a Nb precipitate sample and a Mo precipitate sample after the separation step; and carrying out the radioactivity measurement step.

5. 5. The method for evaluating a neutron irradiation dose according to claim 4, further comprising a step of evaluating recovery rates of the Nb precipitate sample and the Mo precipitate sample.

6. After the neutron irradiation is completed ９３ Generated by the decay of Mo ９３ｍ Nb, and ９３ｍ Generated by the decay of Nb ９３ Nb is ９３ To evaluate the effect on estimating the initial elemental concentration before neutron irradiation of Nb, ９３ 6. The method for evaluating a neutron irradiation dose according to claim 1, further comprising the step of: deriving an initial value of Nb before the neutron irradiation.

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