method for non-destructive measurement of the activity of 14C
Patent Information
- Application Number
- FR2022014724
- Authority / Receiving Office
- FR · FR
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2022-12-30
- Publication Date
- 2025-03-14
- Estimated Expiration
- 2042-12-30
Smart Images
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Abstract
Description
Title of the invention: method for non-destructive measurement of the activity of_-Ç Technical field
[0001] The technical field of the invention relates to a technique for non-destructively measuring the 14C activity of an object. PREVIOUS ART
[0002] Knowledge of the radiological state of the processes and equipment of a nuclear installation is essential for establishing robust dismantling scenarios and defining waste management, in particular their categorizations and outlets. In situ non-destructive nuclear measurements coupled with modeling techniques make it possible to establish a radiological inventory of standing equipment, processes and civil engineering structures.
[0003] 14C is an activation product found in graphite-moderated reactors. It can be formed in the reactor according to the following three neutron activation reactions: 14N (n,p) 14C, 13C (n,y) 14C and 17O (n,a) 14C. The average cross-section for each of these neutron reactions in the thermal neutron range is respectively 1.81 barns, 0.0009 bams and 0.235 bams. 13C is naturally present in the graphite matrix at a level of 1.07%. 14N and 17O are present as impurities in graphite but are also present in the air at levels of 77.25% and 0.8% respectively, air being able to constitute the heat transfer fluid of certain graphite reactors. Therefore, they are likely to have been absorbed onto the free surfaces of the graphite.
[0004] In order to assess the activity levels of 14C in a reactor core, one can rely on calculations of activation levels by numerical modeling. But this requires having a complete history of the neutron fluxes to which the graphite has been exposed, as well as knowledge of the composition of the graphite, including impurities. For this reason, activation calculations must be adjusted by sample analyses.
[0005] 14C is a pure, low-energy [3] emitter, the maximum emitted energy being 156 keV. Its in situ radiological characterization is known to be difficult. Its activity level is generally estimated from destructive analyses carried out in the laboratory on samples taken in the field. These destructive laboratory measurements are, however, accompanied by certain disadvantages: questions about the representativeness of the samples taken, cost and analysis time.
[0006] The inventor proposes a method for estimating the 14C activity of activated objects,
[0007]
[0008]
[0009]
[0010]
[0011] for example, graphite taken from the core of a reactor. The method can also be implemented in the field, through activated graphite, for example by placing a detector in a channel made through the graphite. This is a method based on a device that is inexpensive and simple to implement. Statement of the invention A first subject of the invention is a method for estimating the 14C activity of an object, using a detector configured to form pulses under the effect of exposure to particles [3, the detector being connected to a spectrometric measurement circuit, configured to establish a spectrum, the spectrum corresponding to a histogram of the amplitude of the pulses formed during a measurement period, the method comprising the following steps: - a) arrangement of the detector facing the object and acquisition of a measurement spectrum; - b) interposition of a screen between the detector and the object and acquisition of a background spectrum, the screen being configured to absorb particles [3 emitted by the object - c) correction of the measurement spectrum, using the background spectrum, to obtain a corrected spectrum; - d) from the corrected spectrum, determination of a first spectral value in a first energy band extending below a first maximum energy, the first maximum energy being less than or equal to 156 keV; - e) possible correction of the first spectral value; - f) application of a transfer function to the first spectral value determined during step d), or to the corrected spectral value resulting from step e), so as to estimate a 14C activity of the object. The first maximum energy can be less than or equal to 130 keV or 120 keV or 100 keV. The first energy band may extend above a first minimum energy, the first minimum energy being greater than or equal to 10 keV or 20 keV or 30 keV. Step e) may include - determination of a second spectral value, in a second energy band extending between a second minimum energy and a second maximum energy, the second minimum energy being greater than 156 keV; - subtraction of the second spectral value from the first spectral value, to form the first corrected spectral value. In step a), the distance between the detector and the object can be - greater than 1 mm or 5 mm; - and / or less than 20 mm or 10 mm.
[0012] The screen used during step b) may be an aluminum screen whose thickness is greater than or equal to 3 mm or 4 mm.
[0013] The detector may comprise a detector material of organic scintillator type or of semiconductor type.
[0014] A second object of the invention is a measuring device, configured to determine a 14C activity of an object, the device comprising: - a detector comprising a detector material configured to form pulses upon exposure to particles [3; - a spectrometry circuit, connected to the detector, and configured to form a histogram of the amplitude of the pulses detected during an acquisition period; - a processing unit, connected to the spectrometry circuit, and configured to implement steps c) to f) of a method according to the first subject of the invention.
[0015] The detector material may be an organic scintillator. The thickness of the detector material may be between 3 mm and 10 mm. The detector material may be a semiconductor.
[0016] The device may comprise a movable screen, configured to move from a closed configuration, in which the screen is interposed between the detector material and the object, to an open configuration in which the screen leaves a space free between the detector material and the object.
[0017] The invention will be better understood upon reading the description of the exemplary embodiments presented in the remainder of the description, in conjunction with the figures listed below. FIGURES
[0018] [Fig.l] shows a diagram of a measuring device.
[0019] [Fig.2A] shows a spectrum detected facing an object, the detector being in the open configuration.
[0020] [Fig.2B] shows a spectrum detected facing an object, the detector being in the closed configuration.
[0021] [Fig.2C] shows a subtraction of the spectrum measured in [Fig.2B] from the spectrum measured in [Fig.2A]. The subtraction produces a spectrum [3.
[0022] [Fig.3] shows a modeling of a gamma contribution of a spectrum according to the open configuration and according to the closed configuration.
[0023] [Fig.4A] shows a modeling of a spectrum [3 of a sample, as well as the components of the radioelements forming the activity of the sample.
[0024] [Fig.4B] shows schematically the geometry of the detector modeled according to the closed configuration.
[0025] [Fig.4C] shows schematically the geometry of the detector modeled according to the open configuration.
[0026] [Fig.5] represents a difference between spectra of a 14C source respectively measured and modeled.
[0027] [Fig.6] shows an evolution of the energy deposited in the detector material as a function of the graphite thickness.
[0028] [Fig.7] represents mass activities measured on samples, as a function of the position of the samples in a fuel channel of a reactor.
[0029] [Fig.8] represents the main steps of the invention. PRESENTATION OF SPECIAL EMBODIMENTS
[0030] [Fig. 1] represents a measuring device allowing measurement of the 14C activity of an object 2. The device comprises a scintillator detector 10, comprising a scintillator material 11, preferably organic, preferably based on Polyvinyltoluene (PVT). Under the effect of interactions between ionizing radiation and the scintillator material, light pulses are formed. These light pulses are converted into electrical pulses by one or more photodetectors 12. The electrical pulses are then processed by a spectrometry circuit. The spectrometry circuit 13 is configured to form an amplitude histogram of the pulses detected by the organic scintillator during an acquisition period.
[0031] By implementing an energy calibration function, resulting from an energy calibration, it is usual to perform a correspondence between the amplitude of the pulses and energy values. When an ionizing particle deposits all its energy in the scintillator, the amplitude of the pulse that it generates corresponds to the energy of the particle before the interaction in the detector material. The energy calibration of the detector, that is to say the correspondence between the value of the amplitudes of the pulses and the energy, was carried out using a source of 207Bi and / or 137Cs, which emit electrons, at discrete energy values, by internal conversion.
[0032] The use of an organic scintillator detector is suitable for carrying out charged particle spectrometry, of type [3. An organic scintillator is also sensitive to ionizing photons of type X or y. However, the materials forming an organic scintillator have a low atomic number, which makes them not very conducive to the formation of photoelectric interactions. Thus, an organic scintillator is considered not suitable for applications of type X or y spectrometry.
[0033] The scintillator detector is covered with an optically sealed envelope 14, of low thickness, for example an aluminized PET (Polyethylene Terephthalate) film, 18 pm thick to ensure sealing against ambient light. The low thickness helps to minimize radiation attenuation [3. In the rest of the text, the term particle [3 designates a particle [3.
[0034] The thickness e of the scintillator material is for example 4 mm. The diameter of the scintillator material of the example is 76 mm. An organic scintillator material has the advantage of being relatively insensitive to gamma radiation, due to a low atomic number. In addition, this type of scintillator limits the phenomenon of backscattering of the particles [3. Another advantage is a certain stability of the response of the scintillator material with respect to thermal variations. The response, in terms of light intensity produced, when exposed to the same radiation, is stable from 0° to 50°C, which is appropriate for field use conditions.
[0035] The measuring device comprises a processing unit 20, configured to implement spectrum processing steps described below. The processing unit 20 is programmed to execute instructions coded in a memory, connected to the processing unit by wired or wireless connection. The processing unit 20 may in particular comprise a microprocessor.
[0036] According to a variant, the detector may comprise a semiconductor material, suitable for spectrometry [3. It may for example be a silicon type semiconductor, for example a planar silicon.
[0037] The advantage of an organic scintillator material is the ability to be manufactured in different dimensions and shapes. When the object is a sample being taken, the shape can be adapted to the shape of the sample.
[0038] The detector 10 comprises a movable screen 15, acting as a shutter, configured to be arranged: - in a closed position, between the detector 10 and the object to be measured 2; - or in an open position, freeing the space between the detector 10 and the object 2, as shown in [Fig.l].
[0039] In this example, the screen 15 is movable in translation in a plane parallel to the detector material 11. The screen is for example formed of aluminum, the thickness preferably being equal to 4 mm. Details relating to the thickness of the screen 15 are given in connection with [Fig. 3]. The screen 15 can be connected to the detector material by a slide system, allowing translation of the screen relative to the detector material.
[0040] The invention is based on detection, by the detector 10, of particles [3 emitted by 14C, knowing that the object analyzed is likely to contain other emitting radioelements [3, for example 3H, 152Eu, 154Eu, 36C1,60Co, 137Cs, 155Eu. The mass activity of 14C in activated graphite is generally of the order of 103 to 105 Bq.g *. The mass activity of the other emitters [3 is generally less than 1000 Bq.g *. Thus, on activated graphite, the mass activity of 14C is generally 2 to 100 times higher than the activity of other emitting radioelements [3.
[0041] A [3] spectrum on an activated graphite sample generally comprises: - a [3] component linked to the continuous emission spectra [3] for the following emitters: 152Eu, 154Eu, 155Eu, 137Cs, 60Co, 14C, 36C1 and 3H; - a component [3 linked to the discrete spectra of internal conversion electron emission for the following non-pure beta emitters: 152Eu, 154Eu, 155Eu, 137Cs, 60Co; - a y component related to the discrete gamma photon emission spectra for the following non-pure beta emitters: 152Eu, 154Eu, 155Eu, 137Cs, 60Co and 36C1.
[0042] Figures 2A, 2B and 2C show the effect of the blocking of the detector by the screen 15 on the spectrum [3 measured by the detector. Figures 2A, 2B and 2C represent spectra measured experimentally facing an activated graphite sample. [Fig.2A] represents a spectrum without a screen: screen in the open position, as shown in [Fig.l]. This is the measurement spectrum Sp. [Fig.2B] represents a spectrum with a screen (screen in the closed position). This is a background spectrum Spbdf. The latter is considered to be representative of a contribution y in the measurement spectrum Sp. The spectra were measured according to the same acquisition duration of 120 seconds. The screen was a 4 mm thick aluminum plate.
[0043] In [Fig.3], two simulated y spectra are shown, resulting from the detection of y photons in the detector in the open and closed configurations respectively. The contribution of the [3 particles in each spectrum has been ignored. The two modeled spectra are very close to each other. This means that the subtraction of the background spectrum Spbdf measured in the closed configuration (see [Fig.2B]) allows a good correction of the influence of the y photons in the Sp spectrum measured in the open configuration.
[0044] The measurement spectrum Sp is considered to be representative of the particles [3 and y emitted by the object as well as by the natural background noise y. The background spectrum Spbdf is considered to be representative of the natural background noise y and of the particles y emitted by the object.
[0045] [Fig.2C] shows the subtraction of the background spectrum Spbdf ([Fig.2B]) from the measurement spectrum Sp ([Fig.2A]). The spectrum Sp[3 shown in [Fig.2C], resulting from the subtraction, is a corrected spectrum, considered to be solely representative of the particles [3 emitted by the object.
[0046] Table 1 represents an example of an activity spectrum, obtained by sampling, from a graphite sample with a mass equal to 5 grams. The activity of 36C1 was not taken into account, due to heterogeneous results observed in different samples.
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[0048] Table 1
[0049] [Fig.4A] represents spectra [3 simulated with the MCNP6 calculation code, in a detector material with a diameter of 76 mm and a thickness of 4 mm. Figures 4B and 4C represent the measurement geometry modeled by the calculation code, according to the closed and open configurations respectively. The simulated object was a graphite sample, whose activity spectrum is that described in Table 1. It was a cylindrical sample with a diameter of 14.7 mm and a height of 20.2 mm, whose base was placed at a distance of 25 mm from the detector. The activity of the sample was assumed to be homogeneous.
[0050] The total spectrum [3 detected as well as the component of each radioelement in the spectrum were simulated. Thus, [Fig.3] allows the respective components of 154Eu, 152Eu, 155Eu, 137Cs, 60Co, 3H, and 14C to be observed.
[0051] It is observed that the 14C component in the spectrum is predominant up to 156 keV, which is the maximum emission energy of 14C particles. Beyond 156 keV, the predominant components are those of 154Eu and 137Cs.
[0052] In a first energy band AE1, extending to energies below 156 keV, the contribution of 14C is predominant. The first energy band AE1 extends between a first minimum energy El min and a first maximum energy Elmax. Thus, if we consider a first energy band AE1 = [Elmin - Elmax] between 30 keV and 100 keV, the spectral value, i.e. the integral of the spectrum in the energy band, is due, at 98%, to 14C. Thus, from a first spectral value NI, defined in the first energy band AE1, extending between a few keV and 156 keV, it is possible to estimate the 14C activity of the measured object. By spectral value is meant a sum of all the counts or count rates in the energy band considered. Preferably, the first energy band AE1 extends beyond 10 keV, or 20 keV, and below 150 keV, or 120 keV, or 100 keV. The inventor believes that a first energy band AE1 ranging between 30 keV and 100 keV is optimal.
[0053] Taking into account the first energy band [30 keV - 100 keV], in the spectrum shown in [Fig.4A], the cumulative counting rate is 54.8 counts per second for the entire spectrum, of which 53.9 counts per second come from 14C, or approximately 98%.
[0054] In order to take into account the minority contribution of radioelements other than 14 C, it may be envisaged to define a second energy band AE2, extending between a second minimum energy E2min and a second maximum energy E2max. The second minimum energy E2min is greater than 156 keV. A second spectral value N2 is taken into account, in the second energy band AE2. Advantage is taken of the fact that the spectrum [3 of radioelements emitting [3 different from 14C, called “other radioelements”, is relatively flat between the first energy band AE1 and the second energy band AE2. Thus, the contribution of the other radioelements, in the first spectral value, can be deduced from the second spectral value N2, in the second energy band AE2. As a first approximation, it can be estimated that the contribution of the other radioelements, in the first energy band AE1, corresponds to N2.We can then correct the first spectral value, so that the first corrected spectral value is equal to N1 - N2.
[0055] From the first spectral value NI, or from the first corrected spectral value N1-N2, a transfer function FT can be applied, so as to estimate the activity of 14C, according to the expressions:
[0056] A= (l) or A=^ (r)
[0057] Where A is the activity, expressed for example in Bq / g.
[0058] The determination of the transfer function FT can be carried out by modeling. [Fig.5] shows a comparison between a spectrum measured with the previously described detector, and a modeled spectrum. The detector was exposed to a point source of 14C. The comparison of the measured and modeled spectra shows a difference of less than 10%. The spectral modeling can therefore be considered reliable.
[0059] According to a variant, the contribution of the other radioelements can be evaluated by taking into account the activity of the object in radioelements emitting [3 different from 14C. This can be an activity determined a priori or an activity measured, for example by gamma spectrometry. From the activity [3 of said other radioelements, the contribution of the latter can be evaluated by modeling.
[0060] [Fig.6] represents a quantity of energy (y-axis - unit MeV.Bq '.g), deposited by particles [3 emitted by a homogeneous distribution of 14C in a depth of concrete varying between 0 and 2 mm. Beyond a depth of 300 pm, the curve of [Fig.6] flattens, which means that the particles [3, emitted beyond this depth, do not contribute to the measured spectrum. Thus, the measured spectrum corresponds only to the superficial part of the analyzed object, closest to the detector. The total activity of the object can be extrapolated by taking into account assumptions regarding the distribution of activity in the object, for example a homogeneous distribution.
[0061] When the object is a large sample, it is possible to place a collimator in front of the detector material, so as to reduce the surface area of the detector material exposed to the radiation [3. The detector can be scanned along the sample. This can make it possible to determine an activation profile along the sample.
[0062] The measurement method described above was implemented on graphite samples taken from a fuel channel in the core of a graphite-moderated gas-cooled reactor. The graphite samples were cylindrical: 15 mm in diameter and 20 mm in height. Each sample was positioned at a distance of approximately 5 mm from the sample, so as to limit the risks of contamination by contact between the detector (or screen) and the sample. The distance was optimized so as not to reduce the detection efficiency.
[0063] In a first series of measurements, a front face of the sample was arranged facing the detector. In a second series of measurements, a rear face of the sample was arranged facing the detector. In a third series of measurements, the side wall of the sample, connecting the front face and the rear face, was arranged facing the detector.
[0064] A homogeneous activity of the core was assumed. A transfer function FT was established for each series of measurements, equal to 1.05 103 s '.Bq '.g in the first and second series of measurements and 1.08 103 s '.Bq '.g in the third series of measurements. The density of graphite was assumed to be equal to 1.5. The density was determined on the basis of mass measurements and the dimensions of the samples. The measurement uncertainty associated with the transfer function was ± 6%.
[0065] In each measurement, a spectrum in open configuration and a spectrum in closed configuration were acquired, then a corrected spectrum was obtained by subtracting the spectrum in closed configuration from the spectrum in open configuration. On each corrected spectrum, the following were taken into account: - a first spectral value NI, corresponding to the cumulative counting rates in the first energy band AE1: 30 keV - 100 keV; - a second spectral value N2, corresponding to the cumulative counting rates in the first energy band: AE2: 156 keV - 256 keV.
[0066] The activity of 14C was estimated according to the expression (1')
[0067] [Fig.7] represents the estimates of 14C activity (y-axis - unit Ba), depending on the respective positions of each sample relative to the central axis of the reactor: abscissa axis - unit cm. In [Fig.7], the designations “Face 1”, “Face 2” and “Face 3” correspond respectively to the front face, rear face and side wall of the cylinder.
[0068] The profile obtained is consistent with the simulated neutron flux profile along the fuel channel from which the samples were taken.
[0069] [Fig.8] summarizes the main steps of a method according to the invention.
[0070] Step 100: arrangement of a detector facing the object to be characterized
[0071] Step 110: acquisition of a spectrum, in the open configuration: this is the measured spectrum Sp.
[0072] Step 120: acquisition of a background spectrum Spbdf, in the closed configuration.
[0073] Step 130: subtraction of the background spectrum resulting from step 120 from the spectrum resulting from step 110. The spectrum resulting from this step is a [3 Sp [3.
[0074] Step 140: determination of a first spectral value NI in the first energy band AE1 of the spectrum Sp[3.
[0075] Step 150: optionally, determination of a second spectral value N2, in the second energy band AE2 of the spectrum Sp[3, then subtraction of the second spectral value from the first spectral value, so as to obtain a corrected spectral value (NI - N2).
[0076] Step 160: application of the transfer function so as to estimate the 14C activity noted A[14C] of the object.
[0077] Steps 130 to 160 can be implemented by the processing unit 20.
[0078] The invention may be implemented to characterize the activity of 14C, for the purposes of radiological inventory and waste classification.
Claims
Claims
1. Method for estimating the 14C activity of an object, using a detector (1) configured to form pulses under the effect of exposure to particles [3, the detector being connected to a spectrometric measurement circuit, configured to establish a spectrum, the spectrum corresponding to a histogram of the amplitude of the pulses formed during a measurement period, the method comprising the following steps: - a) arrangement of the detector facing the object and acquisition of a measurement spectrum; - b) interposition of a screen (15) between the detector and the object and acquisition of a background spectrum, the screen being configured to absorb the particles [3 emitted by the object - c) correction of the measurement spectrum, using the background spectrum, to obtain a corrected spectrum;- d) from the corrected spectrum, determination of a first spectral value (NI) in a first energy band (AE1) extending below a first maximum energy (Elmax), the first maximum energy being less than or equal to 156 keV; - e) possible correction of the first spectral value; - f) application of a transfer function to the first spectral value determined during step d), or to the corrected spectral value resulting from step e), so as to estimate a 14C activity of the object.;
2. The method of claim 1, wherein the first maximum energy (Elmax) is less than or equal to 130 keV or 120 keV or 100 keV.
3. A method according to any preceding claim, wherein the first energy band (AE1) extends above a first minimum energy (Elmin), the first minimum energy being greater than or equal to 10 keV or 20 keV or 30 keV.
4. Method according to any one of the preceding claims, in which step e) comprises the sub-steps: - determination of a second spectral value (N2), in a second energy band (AE2) extending between a second minimum energy (E2min) and a second maximum energy (E2max), the second minimum energy being greater than 156 keV; - subtraction of the second spectral value from the first spectral value, to form the first corrected spectral value.
5. Method according to any one of the preceding claims, wherein during step a), the distance between the detector and the object is: - greater than 1 mm or 5 mm; - and / or less than 20 mm or 10 mm.
6. A method according to any one of the preceding claims, wherein the screen used in step b) is an aluminum screen whose thickness is greater than or equal to 3 mm or 4 mm.
7. A method according to any preceding claim, wherein the detector comprises an organic scintillator type detector material.
8. A method according to any preceding claim, wherein the detector comprises a semiconductor type detector material.
9. Measuring device (1), configured to determine a 14C activity of an object (2), the device comprising: - a detector (10) comprising a detector material (11) configured to form pulses under the effect of exposure to particles [3; - a spectrometry circuit (13), connected to the detector, and configured to form a histogram of the amplitude of the pulses detected during an acquisition period; - a processing unit (20), connected to the spectrometry circuit, and configured to implement steps c) to f) of a method according to any one of the preceding claims.
10. A device according to claim 9, wherein the detector material is an organic scintillator
11. Device according to claim 10, wherein the thickness of the detector material is between 3 mm and 10 mm.
12. A device according to claim 9, wherein the detector material is a semiconductor.
13. Device according to any one of claims 9 to 12, wherein the device comprises a movable screen (15), configured to pass from a closed configuration, in which the screen is interposed between the detector material and the object, to an open configuration in which the screen leaves a space free between the detector material and the object.