Apparatus and method for characterizing the depth of radioactivity of radionuclides in a solid medium

Abstract

The present invention is a method for determining the depth to which a radionuclide emitting gamma rays spreads in a solid medium, the solid medium being concrete or soil. This method is based on the measurement of the spectrum of gamma rays emitted by the radionuclide and radiated from the surface surrounding the solid medium. Two energy bands are selected in the spectrum. Based on the spectral values of each energy band, a calibration function can be applied that enables the depth to be estimated. The calibration function is established in advance based on spectral modeling.

Description

Technical Field

[0001] The technical field of the present invention is the characterization of the depth of radioactivity of radionuclides in solid media, particularly in concrete walls.

Background Art

[0002] During the operation of nuclear facilities, civil structures can be contaminated by radionuclides. When the structures are formed from concrete, especially when they are in the liquid phase, some radionuclides can move. As a result, the radioactivity due to contamination is non-uniformly distributed within the concrete structure. In general, it is known that the radioactivity is distributed along a gradient that decreases with the thickness of the structure. That is, the radioactivity is higher closer to the surface exposed to contamination.

[0003] During demolition work, it is useful to estimate the residual radioactivity levels in the concrete structures of the facility. This makes it possible to establish the scenario of the work to be carried out and predict the amount of nuclear waste generated. It is understood that this is based on estimating the depth of movement of contaminants in the concrete.

[0004] Currently, the main method for estimating such a depth is based on taking samples and performing destructive analysis in the laboratory. However, taking core samples is cumbersome. Another limitation is the statistical representativeness of the samples taken.

[0005] Non-patent document 1, a publication by Potapov V.N. et al., describes a non-destructive method that makes it possible to estimate the depth of movement of contamination in contaminated soil and concrete. This method is based on using gamma spectroscopy analysis. Estimating the depth of contamination based on non-destructive measurements is particularly advantageous. This implementation is considerably simplified with respect to sample collection. This makes it possible to increase the number of measurements and obtain a more comprehensive knowledge of the radiation state of civil engineering.

Prior Art Documents

Non-Patent Documents

[0006]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] The inventors have discovered that the above method can be improved, particularly in the calibration stage or when implemented on-site. The present invention presented below meets these needs for improvement.

Means for Solving the Problems

[0008] A first subject of the present invention is a method for estimating the depth to which a radionuclide spreads within a solid medium from a surface, wherein the solid medium is surrounded by the surface, the radionuclide emits gamma rays at at least one emission energy, and the method comprises: a) arranging a spectroscopic detector facing the surface of the solid medium, the spectroscopic detector measuring the energy spectrum of gamma rays emitted by the radionuclide and radiated from the surface, the measured energy spectrum including a large number of pulses detected for various energy values, the energy spectrum including a peak of the emission energy; b) considering a predefined first energy band and a second energy band, the first energy band including lower energy values than the second energy band, the second energy band including the peak of the emission energy; c) determining a first spectral value in the first energy band and a second spectral value in the second energy band based on the measured energy spectrum; d) To obtain the first calibrated spectral value, applying a first calibration coefficient to the first spectral value, and to obtain a second calibrated spectral value, applying a second calibration coefficient to the second spectral value, wherein each calibration coefficient corresponds to a ratio in each energy band between a predetermined radioactivity of the solid medium and an estimated value of the spectral value corresponding to the predetermined radioactivity; e) Calculating a comparison index based on a comparison between the first calibrated spectral value and the second calibrated spectral value obtained from step d); f) Applying a calibration function to the comparison index obtained from step e) so as to estimate the depth at which the radionuclide spreads in the solid medium, the calibration function having been established in advance by taking into account the radioactivity of the radionuclide that decreases as a function of depth; comprising During step d), the first calibration coefficient and the second calibration coefficient are determined by numerically modeling the spectrum detected by the spectroscopic detector in each energy band, and during the modeling, the solid medium is considered to have the predetermined radioactivity; A method.

[0009] The first spectral value and the second spectral value are, respectively, the number of pulses or the counting rate in the first energy band and the second energy band.

[0010] The predetermined radioactivity is a uniform radioactivity surface density or a uniform specific radioactivity in the analyzed medium, or a predetermined radioactivity distributed along a predetermined radioactivity gradient.

[0011] Before step c), the method comprises considering a spectrum representative of the solid medium; considering at least one predetermined energy band corresponding to the emission energy of natural radionuclides naturally present in the solid medium; estimating the radioactivity of the natural radionuclide in the solid medium based on the spectral value of the representative spectrum in the predetermined energy band; estimating the contribution of the natural radionuclide in the spectrum measured during step a); correcting the spectrum measured during step a) so as to remove at least the contribution of the natural radionuclide in the first energy band; may be included.

[0012] The spectrum representing the solid medium is the spectrum measured during step a), or a spectrum measured by a spectrometer different from the spectrometer used during step a), or a spectrum measured by the spectrometer used during step a), which is sufficient.

[0013] Before step d), the method may include determining the density of the inspected solid medium; selecting one of the first calibration coefficients determined for various densities as a function of the density of the inspected solid medium; may be included.

[0014] Before step f), the method may include determining the density of the inspected solid medium; selecting one of the calibration coefficients determined for various densities as a function of the density of the inspected solid medium; may be included.

[0015] According to one embodiment, the step of determining the density of the inspected solid medium includes a sub-step of considering the spectrum representing the solid medium; A sub-step of selecting at least two energy bands corresponding to the emission energies of the same natural radionuclide that naturally exist in the solid medium, among others; A sub-step of estimating the radioactivity of the natural radionuclide based on the spectral values of the respective representative spectra in each selected energy band, wherein each radioactivity estimation is performed taking into account the density of the solid medium; A sub-step of determining the density that minimizes the gap between the radioactivities of the radionuclides estimated based on the selected energy bands; including.

[0016] The spectrum representing the solid medium is the spectrum measured during step a), or the spectrum measured by a spectrometer different from the spectrometer used during step a), or the spectrum measured by the spectrometer used during step a), it may be.

[0017] The method is g) A step of multiplying the second calibrated spectral value obtained from step e) by a correction function to estimate the radioactivity of the solid medium, wherein the correction function is established in advance for various values of the comparison index; may include.

[0018] The comparison index may be the ratio or difference between the first calibrated spectral value and the second calibrated spectral value.

[0019] The solid medium may be concrete or soil.

[0020] A second subject of the present invention is An apparatus intended to estimate the depth to which radionuclides spread within a solid medium from a surface, wherein the solid medium is surrounded by the surface; A spectroscopic detector configured to measure the spectrum of gamma rays emitted from the surface; A processing unit programmed to perform steps b) to f) of the method according to the first embodiment and optionally step g) based on the spectrum measured by the spectroscopic detector; and a device comprising the same.

[0021] The present invention will be better understood by reading the disclosure of the examples of the embodiments presented in the remainder of the detailed description of the invention in connection with the drawings listed below.

Brief Description of the Drawings

[0022]

Figure 1

Figure 2

Figure 3

Figure 4A

Figure 4B

Figure 5A

Figure 5B

Figure 6A

Figure 6B

Figure 6C

Figure 6D

Figure 6E

Figure 7A

Figure 7B

Figure 7C

Figure 7D

DETAILED DESCRIPTION OF THE INVENTION

[0023] FIG. 1 shows an apparatus 1 configured to implement the present invention. The apparatus includes a spectroscopic detector 10 intended to be arranged opposite a surface S surrounding a solid medium M. The solid medium is a medium that may be contaminated by radionuclides. This may in particular be a medium containing concrete, soil or sand. One object of the present invention is to estimate the thickness Z of the medium that may contain most of the radioactivity. 0 By "most" is meant more than 80% or more than 90% of the radioactivity of the radionuclide.

[0024] The spectroscopic detector 10 is arranged at a distance D from the surface S of the solid medium M. This is a detector sensitive to X-rays or gamma rays. Therefore, the implementation of the present invention assumes that the radionuclide is an X-ray or gamma-ray emitter. Depending on the type of facility, the radionuclide is, but is not limited to, (for example, within a fuel reprocessing facility) 137Cs can also be (e.g., in a nuclear reactor) 60 Co.

[0025] When gamma photons interact within detector 10, detector 10 generates a pulse whose amplitude preferably linearly depends on the energy emitted by the photons during the interaction within the detector. Detector 10 is connected to an electron spectroscopy circuit 12. During a measurement period lasting from several seconds to several minutes or several tens of minutes, the pulses generated by detector 10 are counted and classified as a function of their amplitudes to form an amplitude spectrum. An energy spectrum is obtained by using an energy calibration function that establishes the relationship between the amplitude of the pulses and the energy emitted within the detector. Generally, the energy spectrum is discretized into various energy channels k, and the number of channels K is generally between several hundred and several thousand.

[0026] Thus, device 1 enables the formation of an energy spectrum of the photons emitted by medium M. If a radionuclide emitting gamma rays is present in a detectable amount within or on the wall, a spectral characteristic including one or more peaks whose energies are known is generated in the measured spectrum. The energy of each peak corresponds to the energy of the photons emitted by the radionuclide. The emission energies of the main radionuclides are known and available in a nuclear database.

[0027] In the example shown in FIG. 1, the detector is a NaI scintillation detector coupled to a scintillation photon / charge carrier converter. Other types of detectors, such as LaBr 3 scintillation detectors may also be used. Advantageously, a semiconductor, such as a Ge detector, may be implemented. This makes it possible to obtain a spectrum with a more favorable energy resolution. The use of germanium detectors will be described below in connection with FIGS. 6A - 6E.

[0028] FIG. 1 also schematically shows the radioactivity gradient Act(z) as a function of depth z.

[0029] The measured spectrum may include a background noise component corresponding to the radiation background to which detector 10 is exposed. The background noise may have several components.

[0030] The first component is the natural background noise due to the natural radioactivity of the medium being inspected. This is significant when the medium is formed from concrete or soil. The natural background noise is due to gamma-ray radioactive isotopes naturally present in the medium M. One natural gamma-ray emitter commonly encountered is 40 K, 238 U or 232 it has been found that other gamma-ray emitters due to the decay chains of Th may be encountered. Taking into account the natural background noise constitutes an advantageous aspect of the present invention described below.

[0031] The second component is the background noise induced by the vicinity of the wall being monitored. To reduce the contribution of the "vicinity" background noise, the detector is usually surrounded by a shield 14. The shield forms a collimator that limits the opening angle so as to limit the observation area of the detector 10. However, the shield does not act in an all-or-nothing manner, and some photons emitted in the vicinity of the detector may pass through the shield and be detected by the detector. This is significant when the vicinity background noise is stronger than the signal emitted by the medium M in the observation area of the detector. The shield 14 may include a removable plug 14' configured to block the observation area of the detector 10. In this case, the vicinity background noise is considered by performing two consecutive measurements with and without the plug. The measurement performed with the plug is regarded as representative of the vicinity background noise. Therefore, it can be subtracted from the measurement performed without the plug.

[0032] Device 1 includes a processing unit 20 intended to process each spectrum obtained from the spectroscopic analysis circuit 12. The processing unit 20 can include, for example, a microprocessor. The processing unit is programmed to carry out some of the steps of the method described below in connection with FIG. 2.

[0033] FIG. 2 shows the main steps for carrying out the method described below.

[0034] Step 100: Place device 1 facing the medium to be analyzed. In this example, the solid medium is a concrete wall.

[0035] Step 110: Obtain the spectrum of the ambient background noise. This step is optional. During this step, as described above, plug 14' blocks the observation area of the detector. A spectrum representative of the ambient background noise is obtained.

[0036] Step 120: Obtain the measurement spectrum. During this step, with the collimator open, the spectrum of the gamma rays detected by detector 10 is obtained. The acquisition time can be between several tens of seconds and several minutes, depending on the radioactivity of the wall and the sensitivity of detector 10.

[0037] Step 130: Take into account the ambient background noise. This step is carried out when the spectrum of the background noise obtained from step 110 is available. During this step, the spectrum of the ambient background noise is subtracted from the measurement spectrum to obtain a corrected spectrum. Preferably, the acquisition times of the measurement spectrum (step 120) and the spectrum of the ambient background noise (step 110) are the same. If that is not possible, the spectra are adjusted to correspond to the same acquisition time.

[0038] Following step 120 or possible step 130, the energy spectrum of gamma radiation emitted from the surface is available. In the remainder of the detailed description of the invention, this spectrum is designated as the target spectrum. This can be either the spectrum obtained from step 120 or the corrected spectrum obtained from step 130.

[0039] Step 140: Select an energy band.

[0040] In the course of this step, two different energy bands are selected in the target spectrum. Part of the present invention is based on different variations in the content of the target spectrum in two different energy bands as a function of the depth of contamination. Each energy band was determined in the course of the prior calibration phase 70 described below.

[0041] The first energy band ΔE 1 extends on both sides of the spectral region corresponding to the Compton edge. The first energy band ΔE 1 width depends on the energy resolution of the detector, and the maximum energy of the energy band ΔE 1 can correspond to the minimum energy of the energy band ΔE 2 . The energy band generally spreads over several hundred keV or more. One special feature of the first energy band is that the spectral value hardly changes with respect to the depth of contamination. The spectral value means the number of detected pulses, and its energy belongs to the first energy band. The number of detected pulses is usually called the "number of hits".

[0042] Figure 3 shows the spectrum obtained by a 1-inch × 1-inch volume NaI scintillation detector simulated using the particle transport calculation code (MCNP code). The detector is placed facing a concrete wall surrounded by the surface S, and the gradient of the concrete wall at depth z is modeled by a decreasing exponential function 137 and contains the radioactivity of Cs. In Figure 3, the first energy band ΔE 1extends between 290 keV and 600 keV.

[0043] The first energy band ΔE 1 is formed by interactions having two origins. - Interactions corresponding to Compton scattering in a detector of gamma photons that are not attenuated or are weakly attenuated by the concrete wall: These interactions form the first component of the spectrum of the first energy band ΔE 1 Figure 1 shows such interactions by solid arrows. - Interactions corresponding to absorption in the detector by the photoelectric effect of gamma photons scattered at scattering angles from 0° to 90° through the concrete wall: These interactions form the second component of the spectrum of the first energy band ΔE 1 Figure 1 shows such interactions by dotted arrows.

[0044] When the depth of contamination is low, the first component is dominant. When the depth of contamination is high, the second component is dominant. Thus, the spectral content of the first energy band ΔE 1 is thought to depend weakly, e.g., by only ±20%, on the depth of contamination.

[0045] The first energy band ΔE 1 is pre-defined and optimized by modeling the spectrum detected by the detector, taking into account various radioactivity gradients within the wall and various depths of contamination for the same radioactivity gradient. This aspect is described below in connection with FIGS. 6A through 6E.

[0046] The second energy band ΔE 2 includes the emission energies of the radionuclides. Under sufficient branching ratios, each emission energy of a radionuclide is known to result in the appearance of a peak, called the full energy peak, in its spectrum. The second energy band ΔE 2 corresponds to all or part of the full energy peaks. As a function of the energy resolution of the detector, the second energy band ΔE 2is more or less broad. Preferably, the second energy band ΔE 2 is centered on the emission energy of the radionuclide. Unlike the first energy band, the spectral content of the second energy band varies greatly as a function of the depth of contamination. This is due to the photoelectric absorption in the solid medium M of the photons emitted with that emission energy.

[0047] In the example shown in Figure 3, the second energy band ΔE 2 is 137 either the emission energy of Cs, or extends on either side of 662 keV. In this example, the second energy band ΔE 2 extends between 577 keV and 747 keV.

[0048] Preferably, the first energy band and the second energy band do not overlap or overlap very little. The first energy band may extend between a first lower limit and a first upper limit. The second energy band may extend between a second lower limit and a second upper limit. The second lower limit is above the first upper limit.

[0049] Following step 140, the first number of pulses N 1 in the first energy band ΔE 1 and the second number of pulses N 2 in the second energy band ΔE 2 are available. The number of pulses is generally expressed per unit time, and in this case, this is the counting rate, that is, the number of pulses detected per second.

[0050] In the remainder of the detailed description of the invention, the term spectral value refers to the number of pulses in an energy band of the spectrum, or the number of pulses per unit time. Following step 140, the first spectral value N 1 and the second spectral value N 2 are available.

[0051] Step 150: Consider natural radioactivity. This step is optional but is particularly advantageous when the contamination level of the medium M is low.

[0052] This method is intended for use with materials such as concrete and soil. This type of material tends to contain natural radionuclides, some of which are gamma emitters. The presence of the latter can induce measurement errors. In fact, the amount of natural radionuclides can affect the first spectral value N 1 To improve the measurement accuracy, it is necessary to quantify and remove the contribution of gamma-radioactive natural isotopes, especially in the first energy band ΔE 1 .

[0053] Similarly, the presence of natural radionuclides can affect the second spectral value N 2 . Therefore, it is desirable to quantify and remove the contribution of gamma-radioactive natural isotopes in the second energy band ΔE 2 .

[0054] The main natural emitters can be easily identified by their photoluminescence peaks. These are 232 the daughter nuclides of Th, 238 U. This is also 40 K.

[0055] Taking natural radioactivity into account corresponds to sub-steps 151 to 154. These steps are performed considering the spectrum Sp’ representative of the inspection volume. The representative spectrum may be the spectrum Sp obtained in step 120 so as to obtain a more accurate quantification of natural radioactivity, or, for example, a different spectrum obtained with the same detector with a longer acquisition time. The representative spectrum is performed on the inspected wall or on a wall considered to be representative of the inspected wall. Also, the representative spectrum may be obtained by different detectors.

[0056] Sub-step 151: In the representative spectrum Sp’, 232 identify the emission peaks corresponding to the daughter nuclides of Th and 238 U or 40 K.

[0057] The following table lists natural radionuclides (first column), their gamma-ray emitting daughter nuclides (second column), and their emission energies (third column - keV) and branching ratios (fourth column - %). This list is not exhaustive. Only the most intense energy lines are considered.

[0058] [Table 1]

[0059] Figure 4A shows the spectrum measured with a germanium spectroscopic detector on a clean concrete wall. This spectrum represents natural radioactivity. Peaks corresponding to the energies shown in Table 1 can be observed. The acquisition time of this spectrum was 12 hours. Such a spectrum can be considered representative of all or part of the walls of the same facility. Thereby, as described below, the average radioactivity of the main natural radionuclides constituting the concrete can be estimated. For the walls of the facility, especially the first energy band ΔE 1 To correct the spectrum measured at, the contribution of the radioactivity of each radionuclide may be considered.

[0060] Sub-step 152: 232 Th, 238 U, and 40 Estimate the specific radioactivity of K.

[0061] 232 The specific radioactivity of Th and 238 U is estimated based on the average of the specific radioactivities calculated based on the peaks of the spectra detected at each emission energy. 232 The radioactive daughter nuclides of Th and 238 U are assumed to be in radioactive equilibrium with their respective parent nuclides.

[0062] Based on each emission peak, the specific radioactivity A i of the natural radionuclide RN i is obtained by applying the following formula: [Equation] Here, N(E i ) is the count rate at the emission energy E i of the considered radionuclide RN i , and FT(i, E i , d) is the transfer function obtained by modeling and corresponds to the number of hits detected per unit time for a given radioactivity of the considered wall, for example, 1 Bq / g of specific radioactivity. The transfer function is considered in relation to natural radioactivity, taking into account the assumption of a uniform distribution within the wall. The transfer function FT(i, E i , d) depends on the density d of the concrete. The latter may be determined beforehand or may be determined experimentally as described later. Preferably, the detector modeling is pre-verified in the laboratory using a benchmark irradiation source.

[0063] The average radioactivity A m of each parent radionuclide is estimated by averaging the specific radioactivities A i of each radionuclide RN i that are the daughter nuclides of the said parent radionuclide:

Equation

[0064] 40 In the case of K, the specific radioactivity is obtained by applying Equation (1) to the emission peak at 1460 keV.

[0065] Sub-step 153: 232 Simulate the contributions of Th, 238 U, and 40 K.

[0066] Based on the specific radioactivity estimated in sub-step 152, the spectrum, particularly the first energy band ΔE1 The contribution of natural radioactivity in 1,nat is estimated. According to one possibility, the contribution of each natural radionuclide has been pre-simulated for a unit uniform radioactivity, e.g., 1 Bq / g, taking into account the measurement configuration (wall thickness, detector position relative to the wall). Then, the contribution of natural radioactivity is obtained by weighting each unit contribution of each natural radionuclide with the specific radioactivity of the radionuclide obtained in sub-step 152. Thus, the contribution N

[0067] Sub-step 154 In the process of this step, the contribution of natural radioactivity is subtracted from the first spectrum value. The value N 1 is replaced by N 1 -N 1,nat . Similarly, the value N 2 may be replaced by N 2 -N 2,nat .

[0068] Figure 4B shows 137 the spectrum of a concrete wall weakly contaminated with 137 Cs. An emission peak of 1 Cs at an energy of 661.6 keV may be observed. Figure 4B shows a first energy band ΔE 2 defined between 300 keV and 659 keV. A second energy band ΔE 137 corresponds to the emission peak of

[0069] Cs, taking into account the spectral resolution of the detector. In this example, it extends between 659.5 keV and 663.1 keV.

[0070] In the process of this step, the first spectrum value N 1 and the second spectrum value N 2 are each multiplied by a first calibration factor and a second calibration factor, respectively.

[0071] Each calibration coefficient is determined during the previous calibration step 50. During this step, a first reference value N 10 and a second reference value N 20 are determined.

[0072] Each reference value corresponds to an estimated value of the spectral value (count or count rate) detected by the detector with the same measurement configuration (same collimator, same position and direction with respect to the surface S), taking into account the reference radioactivity A 0 of the wall. For example, the reference radioactivity A 0 is a uniform and pure surface radioactivity having a predetermined value. This may be, for example, a uniform surface radioactivity equal to 1 Bq / cm 2 corresponding to the surface radioactivity called a unit. Alternatively, the reference radioactivity may be a specific radioactivity of 1 Bq / g considered over a predetermined thickness of the wall, for example 1 cm. Further, the reference radioactivity may be a specific radioactivity of 1 Bq / g considered to be distributed along a predetermined gradient to any depth.

[0073] The first calibration coefficient α and the second calibration coefficient β are

Number

Number

[0074] Preferably, each calibration coefficient is determined by numerical modeling that executes a particle transport code, for example, the aforementioned MCNP code. By determining the first calibration coefficient and the second calibration coefficient using a numerical model based on the reference radioactivity, it is possible to obtain accurate values of each coefficient, thereby improving the accuracy of the measurement.

[0075] During the process of step 140, the first spectral value N 1 is multiplied by the first calibration coefficient α, and the second spectral value N 2 is multiplied by the second calibration coefficient β. Thus, the first calibrated value

Number

Number

[0076] When the reference radioactivity is the unit surface radioactivity, assuming that there is no depth of contamination, G and A can be regarded as having the same surface radioactivity based on N 1 and N 2 and can be regarded as having the same surface radioactivity.

[0077] Figure 5A shows the variation of the first calibrated quantity G and the second calibrated quantity A as a function of the depth of contamination Z (x-axis). It can be observed that the quantity G is relatively stable and varies within a margin of ±10% up to a depth of about 15 cm.

[0078] Step 180: Calculate the ratio of the first calibrated value to the second calibrated value.

[0079] In the process of this step, the ratio of the first calibrated value G to the second calibrated value A is calculated. Thus, the ratio G / A is obtained. This ratio corresponds to the comparison index.

[0080] Step 190: Consider the calibration function.

[0081] In the process of this step, the ratio G / A obtained in step 180 is used as an argument of a pre-defined calibration function f to estimate the depth of contamination Z 0 of the inspected wall. Thus,

Number

[0082] The calibration function f takes into account an analytical form of the radioactivity gradient in the wall, for example, an exponential decay form, and the spectral values N 1 and N 2 for various depths of contamination Z 0It is defined in the process of step 70 before being estimated. Note that the term depth of contamination indicates the depth containing x% of the radioactivity, preferably x≥75%. In this example, x = 90%. For example, the radioactivity gradient of the radionuclide is specifically considered to follow the decreasing law P(z) in the analysis form:

Number

Number

[0083] In this example, the depth Z 0 corresponds to the depth at which 90% of the radioactivity of the radionuclide is estimated to be contained. Therefore,

Number

[0084] The definition of the calibration function f consists of considering different depths Z of contamination for the same contamination gradient in the analysis form and the same total radioactivity.[[]] 0 For each considered depth Z of contamination 0 , the spectrum obtained from the detector is estimated, and based on that, the spectrum values N 1 (Z 0 ) and N 2 (Z 0 ) are estimated. The calibration coefficients α and β obtained according to step 50 are used to estimate the calibrated values G(Z 0 ) and A(Z 0 ) and A(Z 0 ) for each depth Z.[[]]

[0085] The calibration function f is the quantity (G(Z 0 ) as a function of Z0 )) / (A(Z 0 corresponds to the change in )). It is obtained by numerical modeling using a calculation code that models the transport of photons in a substance, taking into account the geometry of the measurement. That is, the detector, possible collimators, the concrete wall, and the position of the detector relative to the concrete wall are modeled. The modeling may be performed using the MCNP calculation code.

[0086] Figure 5B (curve 1 - left y-axis) shows an example of a calibration function f obtained considering a decreasing activity gradient according to the exponential function as expressed in Equation (8). In Figure 5B, the x-axis corresponds to the quantity G / A, and the left y-axis corresponds to the depth Z of the activity defined by Equation (10). 0 corresponds to.

[0087] The modeling that enables simulating the detected spectrum is also based on assumptions regarding the density of the concrete. Various modelings can be performed considering various values of the density d of the material forming the solid medium. This makes it possible to obtain a calibration function for each value considered for the density.

[0088] Step 200: Estimate the total activity.

[0089] Optionally but preferably during this step, the total activity is estimated based on the first calibrated value G. As a first approximation, the total activity may be estimated directly using G. However, it is preferable to apply a correction function K corr to the function G. The correction function K corr is established in advance based on the modeling.

[0090] Thus, as a first approximation, the total activity Act tot can be estimated by the first calibrated value G: TIFF2025517035000014.tif1260.

[0091] A more accurate way to estimate the activity is the function K corrUsing [Number] is.

[0092] Function K corr is predefined in the process of step 80 based on modeling considering several radiation gradients.

[0093] Steps 50, 70, and 80 form steps for calibrating the device, enabling the definition of the first calibration coefficient α and the second calibration coefficient β (step 50), the calibration function f (step 70), and the correction function K corr (step 80). In practice, steps 50, 70, and 80 may be executed simultaneously based on the same modeling.

[0094] Figure 5B (right y-axis of curve 2) shows an example of a correction function obtained considering the decreasing activity gradient according to the exponential function described in Equation (8). In Figure 5B, the right y-axis corresponds to the value of the correction function K corr of. Function K corr can be observed to vary within a narrow range between 0.9 and 1.2.

[0095] As explained in step 190, modeling is performed considering the assumptions regarding the density of the concrete. By considering various values of the density of the concrete, various modelings can be performed, thereby obtaining a correction function for each possible value of the density of the concrete.

[0096] Estimation of density The method can include step 170 of estimating the density of the material forming the inspection volume. The density of the material forming the wall, whether it is this concrete or soil, is generally not accurately known. Here, for example, if the concrete is standard concrete (density = 2.35) or barite concrete (density = 3.35), this density can vary. To improve the accuracy of the measurement, the method may include estimating the density of the material forming the inspection medium. For this purpose, using the natural radioactivity of the material and utilizing the contrast related to the magnitude of attenuation as a function of the energy of the emitted rays in the medium, the same natural radionuclide covering a wide energy range, for example, 214 select various radiation peaks of Bi.

[0097] The specific activity of the selected natural radionuclide is estimated based on various radiation peaks according to Equation (1). The transfer function FT(i,E i ,d) depends on the density of the concrete. Based on several energy peaks E i , the specific activity A i (E i ) of the natural radionuclide i is determined, which is done using various transfer functions FT(i,E i ,d) established considering various density values of the concrete. Then, the closest density d is determined for the various specific activities of the same radionuclide.

[0098] If the density considered in the transfer function is higher than the actual density of the inspection material, the specific activity determined based on the lowest emission energy peak is considered to be greater than the specific activity determined based on the highest emission energy peak. Conversely, if the density considered in the transfer function is lower than the actual density, the specific activity determined based on the lowest emission energy peak is smaller than the specific activity determined based on the highest emission energy peak.

[0099] When various calibration functions and / or various correction functions based on various values of density are established in advance, it is possible to select the calibration function and the correction function, so the estimation of the density of the material forming the inspection medium is beneficial.

[0100] The estimation of density can also be applied to other materials that may contain natural radioactivity, such as concrete and soil.

[0101] Similar to considering natural radioactivity, the estimation of density is established based on the spectrum considered representative of the walls of the facility. The representative spectrum may be measured on the walls considered representative of the facility. And the density determined based on the representative spectrum is used to process various spectra obtained on the same wall or multiple walls of the same facility. Such a solution makes it possible to obtain a representative spectrum with a long acquisition time in order to estimate the density more accurately. As another possibility, the density is estimated based on each measured spectrum. However, this assumes that the acquisition time is long enough to accurately estimate the density and the presence of artificial radioactivity of the same level or weaker than natural radioactivity so that the characteristic lines of natural radioactivity can be identified.

[0102] Modeling The inventors modeled various measurement configurations. Figure 6A shows one modeled configuration. Each modeling is based on the description of the following arrangements (geometry): - The arrangement of the detectors, - The arrangement of the shields and collimators arranged around the detectors, - The arrangement of the inspection material: the radioactivity gradient as a function of thickness, density, and depth, - The position of the detectors relative to the inspection material, in particular, the distance from the detectors to the surface surrounding the inspection material.

[0103] In the example of Figure 6A, the detectors used are detectors based on germanium crystals cooled by a liquid nitrogen tank.

[0104] Figure 6B shows 137Spectra simulated considering the same pure surface radioactivity of Cs (curve a), or spectra spreading over 100 mm (curve b), 200 mm (curve c), and 300 mm (curve d) along the decreasing gradient are shown. The considered radioactivity gradient was an exponential gradient. In Figure 6B, concrete with a density of 2.3 g / cm 3 was considered.

[0105] This type of simulation makes it possible to define the boundaries of the first energy band ΔE 1 . In this example, the upper limit of the first energy band corresponds to the lower limit of the second energy band ΔE 2 . The lower limit of the first energy band ΔE 1 corresponds to the energy at which the spectrum obtained by simulating the surface radioactivity (curve a) intersects the spectrum obtained by simulating the depth radioactivity, for example, curve b. In this example, the lower limit of the first energy band ΔE 1 is 290 keV. Due to feedback, the lower limit of the first energy band may vary between 290 keV and 400 keV, or more, depending on the radioactive elements considered and the measurement conditions.

[0106] Figure 6C shows 137 spectra simulated considering the same pure surface radioactivity of Cs (curve a), or spectra spreading over 100 mm (curve b), 200 mm (curve c), and 300 mm (curve d) along the decreasing gradient. The considered radioactivity gradient was an exponential gradient. In Figure 6C, concrete with a density of 3.35 g / cm corresponding to barite concrete 3 was considered. It can be observed that the spectra in Figure 6C are different from those in Figure 6B when the same assumptions are made regarding the radioactivity gradient. This is due to the difference in the density considered. Considering the same criterion for determining the lower limit of the first energy band ΔE 1 , here this lower limit is equal to 310 keV.

[0107] Figures 6D and 6E each show standard concrete (d = 2.3 g / cm3 ) and barite concrete (d = 3.35 g / cm 3 ) are considered 60 It corresponds to the spectrum simulated considering the radioactivity of Co. Each figure shows the spectrum considering the pure surface radioactivity (curve a), or the spectrum spreading over 100 mm (curve b), 200 mm (curve c), and 300 mm (curve d) along the decreasing gradient. The lower limits of the first energy band are 340 keV (Figure 6D) and 360 keV (Figure 6E), respectively.

[0108] As described in connection with Figures 6B to 6E, by varying the depth of contamination Z 0 from 0 to 300 mm, a calibration function can be established.

[0109] The calibration function was established considering the following: 137 For the radioactivity of Cs, - In the case of normal concrete (d = 2.3 g / cm 3 ), α = 4.507 Bq / cm 2 , β = 6.716 Bq / cm 2 , - In the case of barite concrete (d = 3.35 g / cm 3 ), α = 4.535 Bq / cm 2 , β = 6.716 Bq / cm 2 , and 60 For the radioactivity of Co, - In the case of normal concrete (d = 2.3 g / cm 3 ), α = 1.157 Bq / cm 2 , β = 4.013 Bq / cm 2 , - In the case of barite concrete (d = 3.35 g / cm 3 ), α = 1.161 Bq / cm 2 , β = 4.013 Bq / cm 2 . As described in connection with Figures 6B to 6E, since the boundary of the first energy band ΔE 1 varies as a function of density, α varies with density.

[0110] Figures 7A and 7B each show 137 Cs and 60 the calibration functions established considering normal concrete (curve a) and barite concrete (curve b) for the radioactivity of Co. In each of these figures, the x-axis corresponds to the ratio G / A and the y-axis corresponds to the depth Z 0 corresponding. When G / A has the same value, the difference between the two depths Z 0 estimated considering the calibration functions corresponding to each density shows the influence on the estimated value of the depth Z 0 when density is ignored.

[0111] Figures 7C and 7D each show 137 Cs and 60 the correction functions established considering normal concrete (curve a) and barite concrete (curve b) for the radioactivity of Co. In each of these figures, the x-axis corresponds to the ratio G / A and the y-axis corresponds to the value of the correction function.

[0112] Figures 6B to 6E and Figures 7A to 7D show the importance of the phase of modeling the measurements in order to establish the calibration function and the correction function as accurately as possible. The modeling is based on assumptions regarding the density of the concrete or the form of the radioactivity gradient as a function of depth. As described above, the assumptions regarding density can be confirmed by the phase of estimating the density (see step 170). Regarding the radioactivity gradient, since it is premised on the analysis of samples, it is difficult to estimate accurately. The inventors consider that the exponential form of the gradient described in relation to formula (8) is appropriate. More generally, when the actual radioactivity gradient deviates from the radioactivity gradient considered in the modeling, the uncertainty of the measurement increases. However, considering the decrease in the radioactivity gradient, it is considered possible to obtain a depth Z 0 of sufficient size by this method.

[0113] Furthermore, considering the attenuation of gamma rays by concrete, this method is considered suitable for determining the depth of radioactivity from 0 cm to 20 cm or 30 cm. Beyond that, the effect of attenuation is too large. That is, the amount of gamma rays emitted at depths greater than 20 cm or 30 cm is too small. The effective region of this method is limited by the vertical asymptote of the calibration function f(G / A). It can be observed that the greater the slope of the function f(G / A), the higher the measurement uncertainty. The "limiting" depth depends on the material observed, especially its density, and the acquisition time of each spectrum. The latter is preferably a few minutes, and if it is desired to maintain compatibility with industrial measurement speeds, it is preferably shorter than 10 minutes or 15 minutes. The limiting depth also depends on the emission energy of the radionuclide aimed at evaluating the depth in the inspection medium. The limiting depth is 60 higher for 137 Co (emission energies 1173 keV and 1332 keV) than for

[0114] This invention can be implemented to monitor the structures of nuclear facilities and assist in maintenance or dismantling operations. In addition, this invention can also be deployed to perform radiation monitoring of contaminated soil.

Claims

1. A method for estimating the depth (Z) to which a radionuclide spreads within a solid medium (M) from a surface (S), wherein the solid medium is surrounded by the surface, the radionuclide emits gamma rays with at least one emission energy, and the method comprises: ０ ), the solid medium being surrounded by the surface, the radionuclide emitting gamma rays with at least one emission energy, the method comprising: a) arranging a spectroscopic detector (10) facing the surface (S) of the solid medium, the spectroscopic detector measuring the energy spectrum of gamma rays emitted by a radionuclide and radiated from the surface, the measured energy spectrum including a number of pulses detected for various energy values, the energy spectrum including a peak of the emission energy; b) considering a pre-defined first energy band and a second energy band, wherein the first energy band (ΔE １ ) includes energy values lower than those of the second energy band (ΔE ２ ), and the second energy band includes the peak of the emitted energy c) Based on the measured energy spectrum, a first spectral value (N １ ) in the first energy band and a second spectral value (N ２ ) in the second energy band are determined; d) applying a first calibration coefficient (α) to the first spectral value to obtain a first calibrated spectral value and a second calibration coefficient (β) to the second spectral value to obtain a second calibrated spectral value, each calibration coefficient corresponding to a ratio in each energy band between a predetermined radioactivity (A ０ ) of the solid medium and an estimated value of a spectral value (N １０ , N ２０ ) corresponding to the predetermined radioactivity; e) calculating a comparison index (G / A) based on a comparison between a first calibrated spectrum value (G) and a second calibrated spectrum value (A) obtained from step d); f) estimating the depth (Z ０ ) in which the radionuclide spreads in the solid medium, applying a calibration function (f) to the comparison index (G / A) obtained from step e), the calibration function being pre-established by taking into account the radioactivity of the radionuclide that decreases as a function of depth, step; comprising; the first spectrum value is considered to be independent of the depth at which the radionuclide spreads; during step d), the first calibration coefficient and the second calibration coefficient are determined by numerically modeling the spectrum detected by the spectroscopic detector in each energy band, and during the modeling, the solid medium is considered to have the predetermined radioactivity; method.

2. the first spectrum value and the second spectrum value are respectively the number of pulses or the counting rate in the first energy band and the second energy band; The method according to claim 1.

3. the predetermined radioactivity is a uniform radioactivity surface density or a uniform specific radioactivity in the analyzed medium, or a predetermined radioactivity distributed along a predetermined radioactivity gradient; The method according to claim 1 or 2.

4. before step c), considering a spectrum representative of the solid medium; considering at least one predetermined energy band corresponding to the emission energy of natural radionuclides naturally present in the solid medium; estimating the radioactivity of the natural radionuclides in the solid medium based on the spectrum value of the representative spectrum in the predetermined energy band; estimating the contribution of the natural radionuclides in the spectrum measured during step a); correcting the spectrum measured during step a) to remove the contribution of the natural radionuclides at least in the first energy band; comprising; The method according to any one of claims 1 to 3.

5. the spectrum representative of the solid medium is the spectrum measured during step a), or The spectrum measured by a spectroscopic detector different from the spectroscopic detector performed during step a), or The spectrum measured by the spectroscopic detector performed during step a) is The method according to claim 4

6. Before step d), Determining the density of the inspected solid medium; and Selecting one of the first calibration coefficients respectively determined for various densities as a function of the density of the inspected solid medium; including The method according to any one of claims 1 to 5

7. Before step f), Determining the density of the inspected solid medium; and Selecting one of the calibration coefficients respectively determined for various densities as a function of the density of the inspected solid medium; including The method according to any one of claims 1 to 6

8. The step of determining the density of the inspected solid medium A sub-step of considering a spectrum representative of the solid medium; A sub-step of selecting at least two energy bands corresponding to the emission energies of the same natural radionuclide naturally present in the solid medium; A sub-step of estimating the radioactivity of the natural radionuclide based on the spectral values of the respective representative spectra in each selected energy band, wherein each radioactivity estimation is performed taking into account the density of the solid medium; Determining a density that minimizes the gap between the radioactivities of the radionuclides estimated based on the selected energy bands; including The method according to claim 6 or 7

9. The spectrum representing the solid medium The spectrum measured during step a), or The spectrum measured by a spectroscopic detector different from the spectroscopic detector performed during step a), or The spectrum measured by the spectroscopic detector performed during step a) is The method according to claim 8

10. g) To estimate the radioactivity of the solid medium, multiplying the second calibrated spectral value (G) obtained from step e) by a correction function (K ｃｏｒｒ ) which is established in advance for various values of the comparison index (G / A), step including The method according to any one of claims 1 to 9

11. The solid medium is concrete or soil The method according to any one of claims 1 to 10

12. The first spectral band extends to several hundred keV or more The method according to any one of claims 1 to 11

13. A device (1) intended to estimate the depth (Z ０ ) at which a radionuclide spreads within a solid medium from a surface (S), the solid medium being surrounded by the surface, the device comprising: A spectroscopic detector (10) configured to measure the spectrum of gamma rays emitted from the surface; A processing unit (20) programmed to perform steps b) to f) of the method according to any one of claims 1 to 12 based on the spectrum measured by the spectroscopic detector; Comprising; Device (1).

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