Method for non-destructive measurement of 14c activity

Abstract

Disclosed is a method for estimating the 14C activity of an object, using a detector configured to form pulses under the effect of exposure to β particles, the detector being connected to a spectrometric circuit configured to establish a spectrum, the spectrum corresponding to a histogram of the amplitude of pulses detected during a measurement period, said method comprising: a) placing the detector facing the object and acquiring a measurement spectrum; b) interposing a screen between the detector and the object and acquiring a background spectrum; c) correcting the measurement spectrum, using the background spectrum; d) on the basis of the corrected spectrum, determining a first spectral value in a first energy band; e) optionally correcting the first spectral value; f) applying a transfer function to the spectral value so as to estimate the 14C activity of the object.

Description

[0001] Method for non-destructive measurement of the activity of 14 C

[0002] TECHNICAL FIELD

[0003] The technical field of the invention relates to a technique for non-destructive measurement of the activity in 14 C of an object.

[0004] PREVIOUS ART

[0005] Knowledge of the radiological state of a nuclear facility's processes and equipment is essential for establishing robust dismantling scenarios and defining waste management, particularly waste 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.

[0006] THE 14C is an activation product found in graphite-moderated reactors. It can be formed in the reactor by the following three neutron activation reactions: 14 N (n,p) 14 C, 13 C (n,y) 14 This 17 O (n,a) 14 C. The average cross section for each of these neutron reactions in the thermal neutron range is 1.81 barns, 0.0009 barns and 0.235 barns respectively. 13 It is naturally present in the graphite matrix at a level of 1.07%. 14 N and 17 O are present as impurities in graphite but are also present in 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 on the free surfaces of the graphite.

[0007] In order to assess the activity levels of 14C in a reactor core, activation level calculations can be based on numerical modeling. However, this requires 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 recalibrated by sample analysis.

[0008] 14 C is a pure, low-energy P emitter, with a maximum emitted energy of 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 collected in the field. The Celiktas C document "A method to obtain a noiseless beta spectrum" describes the principles of laboratory p spectrometry applied to 14C, taking into account parameters relating to the rise time of the pulses in order to discriminate between radiation interactions and interactions of other types of particles.

[0009] Hyeonmin Lee's publication "Performance evaluation of a beta-spectrometer comprising a platic scintillator and multi-wire chamber using a coincidence method" describes a P spectrometer operating in coincidence with a gas sensor (CF4). Coincidence operation limits the influence of y radiation.

[0010] However, destructive laboratory measurements come with certain drawbacks: questions about the representativeness of the samples taken, cost and analysis time.

[0011] The inventor proposes a method for estimating activity in 14C of activated objects, 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.

[0012] STATEMENT OF THE INVENTION

[0013] A first object of the invention is a method for estimating the activity in 14 C of an object, using a detector configured to form pulses under the effect of exposure to particles p, 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:

[0014] - a) arrangement of the detector facing the object and acquisition of a measurement spectrum;

[0015] - b) interposition of a screen between the detector and the object and acquisition of a background spectrum, the screen being configured to absorb the particles emitted by the object

[0016] - c) correction of the measurement spectrum, using the background spectrum, to obtain a corrected spectrum;

[0017] - 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 preferably being less than or equal to 156 keV;

[0018] - 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 an activity in 14 C of the object.

[0019] The first maximum energy can be less than or equal to 130 keV or 120 keV or 100 keV.

[0020] 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.

[0021] Step e) may include

[0022] - 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;

[0023] - subtraction of the second spectral value from the first spectral value, to form the first corrected spectral value.

[0024] In step a), the distance between the detector and the object can be

[0025] - greater than 1 mm or 5 mm;

[0026] - and / or less than 20 mm or 10 mm.

[0027] The screen used in step b) may be an aluminum screen with a thickness greater than or equal to 3 mm or 4 mm.

[0028] The detector may comprise an organic scintillator type or a semiconductor type detector material.

[0029] A second object of the invention is a measuring device, configured to determine an activity in 14 C of an object, the device comprising:

[0030] - a detector comprising a detector material configured to form pulses under the effect of exposure to P particles;

[0031] - a spectrometry circuit, connected to the detector, and configured to form a histogram of the amplitude of the pulses detected during an acquisition period;

[0032] - 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.

[0033] 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. 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. 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.

[0034] FIGURES

[0035] Figure 1 shows a diagram of a measuring device.

[0036] Figure 2A shows a spectrum detected facing an object, the detector being in the open configuration.

[0037] Figure 2B shows a spectrum detected facing an object, the detector being in the closed configuration.

[0038] Figure 2C shows a subtraction of the spectrum measured in Figure 2B from the spectrum measured in Figure 2A. The subtraction results in a p spectrum.

[0039] Figure 3 shows a modeling of a gamma contribution of a spectrum according to the open configuration and according to the closed configuration.

[0040] Figure 4A shows a model of a P spectrum of a sample, as well as the components of the radioelements forming the activity of the sample.

[0041] Figure 4B schematizes the geometry of the detector modeled according to the closed configuration.

[0042] Figure 4C schematizes the geometry of the detector modeled according to the open configuration.

[0043] Figure 5 represents a difference between spectra of a source of 14 C respectively measured and modeled.

[0044] Figure 6 shows an evolution of the energy deposited in the detector material as a function of the graphite thickness.

[0045] Figure 7 represents mass activities measured on samples, as a function of the position of the samples in a fuel channel of a reactor.

[0046] Figure 8 represents the main steps of the invention.

[0047] PRESENTATION OF SPECIAL EMBODIMENTS

[0048] Figure 1 represents a measuring device allowing measurement of the activity in 14C 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. 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 it generates corresponds to the energy of the particle before the interaction in the detector material. The energy calibration of the detector, i.e. the correspondence between the value of the pulse amplitudes and the energy, was carried out using a source of. 207 Bi and / or of 137 Cs, which emit electrons, at discrete energy values, by internal conversion.

[0049] An organic scintillator detector is suitable for p-type charged particle spectrometry. An organic scintillator is also sensitive to ionizing X-ray or γ-type photons. However, the materials forming an organic scintillator have a low atomic number, which makes them unsuitable for photoelectric interactions. Thus, an organic scintillator is considered unsuitable for X-ray or γ-type spectrometry applications.

[0050] The scintillator detector is covered with an optically sealed envelope 14, of low thickness, for example an aluminized PET (Polyethylene Terephthalate) film, 18 μm thick to ensure sealing against ambient light. The low thickness minimizes the attenuation of p radiation. In the rest of the text, the term P particle denotes a p- particle.

[0051] The thickness e of the scintillator material is, for example, 4 mm. The diameter of the scintillator material in 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 p particles. Another advantage is a certain stability of the scintillator material's response 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 suitable for field use conditions.

[0052] The measuring device comprises a processing unit 20, configured to implement spectra 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. According to a variant, the detector may comprise a semiconductor material, suitable for p spectrometry. It may for example be a silicon-type semiconductor, for example planar silicon.

[0053] 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.

[0054] 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. 1.

[0055] 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.

[0056] The invention is based on detection, by the detector 10, of particles P emitted by 14 C, knowing that the object analyzed is likely to contain other P-emitting radioelements, for example 3 H,152 I, 154 I, 36 CI, 60 Co, 137 Cs, 155 Eu. The specific activity of 14 C in activated graphite is generally of the order of 10 3 at 10 5 Bq. g 1 The specific activity of other P emitters is generally less than 1000 Bq. g 1 . Thus, on activated graphite, the specific activity of 14 It is generally 2 to 100 times higher than the activity of other emitting radioelements p.

[0057] A p spectrum on an activated graphite sample generally comprises: a P component related to the continuous P emission spectra for the following emitters: 152Eu, 154Eu, 155Eu, 137Cs, 60Co, 14C, 36CI and 3H; a P component related to the discrete internal conversion electron emission spectra 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 36CI.

[0058] Figures 2A, 2B and 2C show the effect of blocking the detector by the screen 15 on the P spectrum measured by the detector. Figures 2A, 2B and 2C represent spectra measured experimentally in front of an activated graphite sample. Figure 2A represents a spectrum without a screen: screen in the open position, as shown in Figure 1. This is the Sp measurement spectrum. Figure 2B represents a spectrum with a screen (screen in the closed position). This is a Spbdf background spectrum. The latter is considered to be representative of a y contribution in the Sp measurement spectrum. The spectra were measured using the same acquisition time of 120 seconds. The screen was a 4 mm thick aluminum plate.

[0059] Figure 3 shows two simulated y-spectra resulting from the detection of y-photons in the detector in the open and closed configurations respectively (curves a and b). The contribution of P particles in each spectrum has been ignored. The two modeled spectra are very close to each other. This means that subtracting the background spectrum Spbdf measured in the closed configuration (see Figure 2B) allows a good correction of the influence of y-photons in the Sp spectrum measured in the open configuration.

[0060] The measurement spectrum Sp is considered representative of the P and y particles emitted by the object as well as the natural background noise y. The background spectrum Spbdf is considered representative of the natural background noise y and the y particles emitted by the object.

[0061] Figure 2C shows the subtraction of the background spectrum Spbdf (Figure 2B) from the measurement spectrum Sp (Figure 2A). The SpP spectrum shown in Figure 2C, resulting from the subtraction, is a corrected spectrum, considered to be solely representative of the P particles emitted by the object.

[0062] 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 36CI was not taken into account, due to heterogeneous results observed in different samples.

[0063] Table 1

[0064] Figure 4A represents P spectra simulated with the MCNP6 calculation code, in a detector material of diameter 76 mm and thickness 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 of diameter 14.7 mm and height 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.

[0065] In Figures 4B and 4C, the modeled materials are PMMA polymethyl methacrylate acrylic (light guide), PTFE polytetrafluoroethylene (reflector), and Al. The model shown in Figure 4B has an aluminum cover between the graphite sample and the EJ200 plastic scintillator (Eljen Technology with a sensitive surface area of ​​2430 cm 2and 493 mm on each side), while the model shown in Figure 4C does not have a cover.

[0066] The total detected P spectrum and the component of each radioelement in the spectrum were simulated. Thus, Figure 4A allows us to observe the respective components of 154 I, 152 I, 155 I, 137 Cs, 60 Co, 3 H, and 14 C.

[0067] We observe that the component, in the spectrum, of 14 C, is predominant up to 156 keV, which is the maximum emission energy of P particles of 14 C. Beyond 156 keV, the majority components are those of 154 I and 137 Cs.

[0068] In a first energy band AE1, extending to energies below 156 keV, the contribution of 14C is the majority. The first energy band AE1 extends between a first minimum energy Elmin and a first maximum energy Elmax. Its width is at least 5 keV or even 10 keV or even 20 keV. Thus, if we consider a first energy band AE1 = [Elmin - Elmax] between 30 keV and 100 keV, the spectral value, that is to say the integral of the spectrum in the energy band, is due, at 98%, to 14 C. 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 activity in 14C of the measured object. By spectral value is meant a sum of all the counts or counting rates in the energy band considered. Preferably, the first energy band AE1 extends above 10 keV, or 20 keV, and below 150 keV, or 120 keV, or 100 keV. The inventor believes that a first energy band AE1 extending between 30 keV and 100 keV is optimal.

[0069] Considering the first energy band [30 keV - 100 keV], in the spectrum shown in Figure 4A, the cumulative count rate is 54.8 counts per second for the entire spectrum, of which 53.9 counts per second come from 14 C, or approximately 98%. In order to take into account the minority contribution of radioelements other than 14C, it can be considered 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. We take advantage of the fact that the spectrum P of the emitting radioelements P 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, we can estimate 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 NI - N2. The width of the second spectral band is at least 10 keV, or at least 20 keV.

[0070] From the first spectral value NI, or from the first corrected spectral value N1-N2, we can apply a transfer function FT, in order to estimate the activity of 14 C, according to the expressions:

[0071] Where A is the activity, expressed for example in Bq / g.

[0072] The determination of the transfer function FT can be carried out by modeling. Figure 5 shows a comparison between a spectrum measured with the previously described detector (curve a), and a modeled spectrum (curve b). The detector was exposed to a point source of 14 C. Comparison of the measured and modeled spectra shows a deviation of less than 10%. The spectral modeling can therefore be considered reliable. In Figure 5, the y-axis is a count rate per activity (s 1 . Bq 1 ) and the x-axis is the energy (MeV).

[0073] According to a variant, the contribution of other radioelements can be evaluated by taking into account the activity of the object in emitting radioelements P different from 14C. It can be an activity determined a priori or a measured activity, for example by gamma spectrometry. From the P activity of the said other radioelements, the contribution of the latter can be evaluated by modeling. Figure 6 represents a quantity of energy (y-axis - unit MeV.Bq -1 .g), deposited by P particles emitted by a homogeneous distribution of 14 C in a concrete depth varying between 0 and 2 mm (abscissa axis unit pm). Beyond a depth of 300 pm, the curve in Figure 6 flattens out, which means that the P particles, emitted beyond this depth, do not contribute to the measured spectrum. Thus, the measured spectrum only corresponds 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 the activity in the object, for example a homogeneous distribution.

[0074] 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 of the detector material exposed to p-radiation. The detector can be scanned along the sample. This can allow an activation profile to be determined along the sample.

[0075] 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 risk of contamination by contact between the detector (or screen) and the sample. The distance was optimized so as not to reduce the detection efficiency.

[0076] In a first series of measurements, a front face of the sample was placed facing the detector. In a second series of measurements, a rear face of the sample was placed facing the detector. In a third series of measurements, the side wall of the sample, connecting the front face and the rear face, was placed facing the detector.

[0077] A homogeneous activity of the carrot was assumed. A transfer function FT was established for each series of measurements, equal to 1.05 10 -3 s 1 . Bq 1 . g in the first and second series of measurements and 1.08 10 -3 s 1 . Bq 1 . g in the third series of measurements. The density of graphite was assumed to be 1.5. The density was determined based on mass measurements and sample dimensions. The measurement uncertainty associated with the transfer function was ± 6%.

[0078] 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.

[0079] The activity of 14 C was estimated according to expression (1')

[0080] Figure 7 represents the mass activity estimates of 14 C (y-axis - unit Bq. g 1), depending on the respective positions of each sample relative to the central axis of the reactor: abscissa axis - unit cm. In Figure 7, the designations “Face 1”, “Face 2” and “Face 3” correspond respectively to the front face, rear face and side wall of the cylinder.

[0081] The obtained profile is consistent with the simulated neutron flux profile along the fuel channel from which the samples were taken.

[0082] Figure 8 summarizes the main steps of a method according to the invention.

[0083] Step 100: Arrange a detector facing the object to be characterized

[0084] Step 110: acquisition of a spectrum, in the open configuration: this is the measured spectrum Sp.

[0085] Step 120: acquisition of a Spbdf background spectrum, in the closed configuration.

[0086] 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 P Sp spectrum.

[0087] Step 140: determination of a first spectral value NI in the first energy band AE1 of the Spp spectrum.

[0088] Step 150: optionally, determination of a second spectral value N2, in the second energy band AE2 of the SpP spectrum, then subtraction of the second spectral value from the first spectral value, so as to obtain a corrected spectral value (NI - N2).

[0089] Step 160: Application of the transfer function to estimate the activity in 14 C noted A[ 14 C] of the object.

[0090] Steps 130 to 160 can be implemented by the processing unit 20. The invention can be implemented to characterize the activity of 14C, for radiological inventory and waste classification purposes.

Claims

CLAIMS 1. Process for estimating activity in 14 C of an object, using a detector (1) configured to form pulses under the effect of exposure to particles P, 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 P particles 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) applying 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 an activity in 14 C of the object.

2. Method according to claim 1, in which the first maximum energy (Elmax) is less than or equal to 130 keV or 120 keV or 100 keV.

3. Method according to any one of the preceding claims, in which 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. Method according to any one of the preceding claims, in which the screen used during step b) is an aluminum screen whose thickness is greater than or equal to 3 mm or 4 mm.

7. Method according to any one of the preceding claims, in which the detector comprises an organic scintillator type detector material.

8. Method according to any one of the preceding claims, in which the detector comprises a semiconductor type detector material.

9. Measuring device (1), configured to determine an activity in 14 C 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 P particles; - 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. Device according to claim 9, in which 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. Device according to claim 9, in which 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.

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