Method for non-destructive measurement of 238u or u activity

Abstract

Disclosed is a method for estimating the uranium, or uranium-238, activity of an object, using a spectrometric detector configured to establish a 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, the spectrum being representative of an energy distribution of β particles emitted by descendants of 238U; b) on the basis of the measurement spectrum, optionally forming a corrected spectrum; c) on the basis of the measurement spectrum or of the corrected spectrum, determining a first spectral value in a first energy band extending from a minimum energy, the minimum energy being greater than or equal to 300 keV; d) applying a transfer function to the spectral value determined in step c), so as to estimate the uranium or 238U activity of the object.

Description

[0001] Method for non-destructive measurement of the activity of 238 U or of U

[0002] TECHNICAL FIELD

[0003] The technical field of the invention relates to a technique for non-destructive measurement of the activity in 238 U or of U.

[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] Gamma spectrometry is one of the most commonly used passive non-destructive nuclear measurement techniques for obtaining qualitative and quantitative information on gamma-emitting radionuclides. As for pure P emitters, their in situ radiological characterization is made difficult by the very short electron travel in dense media and is usually carried out in the laboratory through destructive analyses of samples collected in the field. However, these destructive laboratory measurements are accompanied by certain disadvantages: questions about the representativeness of the samples collected, cost and analysis time.

[0007] In order to address the quantification of the activity of pure P emitters, numerous non-destructive radiological characterization methods, based on beta counting or beta spectrometry techniques, have been developed by different laboratories around the world to characterize in particular 90 Sr in contaminated soils. See for example AV Chesnokov, SM Ignatov, VI Liksonov, VN Potapov, SB Shcherbak, LI Urutskoev, MV Ivanitskaya, A method for measuring in situ a specific soil activity of 90Sr, Nucl. Instrument. Methods Phys. Res. A 443 (2000) 197-200.

[0008] In many upstream facilities, it is necessary to characterize uranium activity, or even determine its isotopy. Uranium 235 and uranium 238 can be measured by gamma spectrometry. However, gamma spectrometry is quite complex to implement, and requires certain assumptions about the depth and gradient of contamination, particularly in porous objects such as concrete walls. These assumptions are not necessarily valid. Moreover, when enrichment in 235 U is high, the acquisition time can be long, due to the low intensity of gamma emissions from the descending radioelements of 238 U.

[0009] The publication Ashrafi S. "Measurement of natural radioactivity of Iranian granite samples using beta-gamma coincidence spectrometer and maximum likelihood method" describes a laboratory gamma spectrometry device applied to granite samples. Gamma spectrometry is performed in coincidence, that is, a detection of a y-radiation is only taken into account if it coincides in time with a detection of a p by another detector.

[0010] The inventor proposes an alternative method for estimating uranium activity in civil engineering structures, equipment or samples taken, for example concrete cores. This method is based on a device that is inexpensive and easy to implement.

[0011] STATEMENT OF THE INVENTION

[0012] A first subject of the invention is a method for estimating the uranium, or uranium 238, activity of an object, using a detector configured to form pulses under the effect of exposure to P particles, 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:

[0013] - a) arrangement of the detector facing the object and acquisition of a measurement spectrum, the measurement spectrum being representative of an energy distribution of P particles emitted by descendants of 238 U ;

[0014] - b) from the measurement spectrum, possible formation of a corrected spectrum;

[0015] - c) from the measurement spectrum or the corrected spectrum, determination of a first spectral value in a first energy band extending from a first minimum energy, the first minimum energy being greater than or equal to 300 keV;

[0016] - d) application of a transfer function to the first spectral value determined during step c), so as to estimate an activity in uranium or in 238 U of the object.

[0017] Step b) may comprise the sub-steps: - bl) interposition of a screen 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;

[0018] - b2) correction of the measurement spectrum, using the background spectrum, to obtain a corrected spectrum.

[0019] The screen used in sub-step bl) may be an aluminum screen with a thickness greater than or equal to 3 mm or 4 mm.

[0020] In step c), the first energy band may extend up to a first maximum energy, the first maximum energy being greater than or equal to 1000 keV or 1500 keV or 2000 keV.

[0021] The method may comprise the steps: f) determining a second spectral value, in a second energy band, extending between a second minimum energy and a second maximum energy, the second maximum energy being less than 300 keV;

[0022] - g) estimation of an enrichment in 235 U of Uranium, by comparing the first spectral value resulting from step c) and the second spectral value resulting from step f).

[0023] In step a), the distance between the detector and the object can be:

[0024] - greater than 1 mm or 5 mm;

[0025] - and / or less than 20 mm or 10 mm.

[0026] According to one embodiment, in which the object is a naturally radioactive object, the method comprises:

[0027] - (i) taking into account a natural activity of the object;

[0028] - (ii) modeling of a spectrum P of the natural activity of the object detected by the detector;

[0029] - step b) includes a correction of the measured spectrum using the spectrum P of the natural activity of the object resulting from (ii), to obtain the corrected spectrum.

[0030] Step (i) may result from a gamma spectrometry measurement carried out on the object.

[0031] The detector comprises an organic scintillator material. The detector may comprise a semiconductor material.

[0032] A second object of the invention is a measuring device, configured to determine a Uranium activity of an object, the device comprising:

[0033] - a detector comprising a detector material configured to form pulses under the effect of exposure to P particles; - a spectrometry circuit, connected to the detector, and configured to form a histogram of the amplitude of the pulses detected by the detector during an acquisition period;

[0034] - a processing unit, connected to the spectrometry circuit, and configured to implement steps b) to d) of a method according to the first subject of the invention.

[0035] According to one possibility, the detector comprises an organic scintillator material, the thickness of the organic scintillator material being preferably between 3 mm and 10 mm.

[0036] The organic scintillator detector may extend over an area of ​​at least 20 cm on each side. The detector may be covered with an envelope configured to absorb α particles with an energy greater than or equal to 4.5 MeV or 5 MeV.

[0037] The detector may comprise a semiconductor material.

[0038] 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 connection with the figures listed below.

[0039] FIGURES

[0040] Figure 1 shows a diagram of a measuring device.

[0041] Figure 2 shows an evolution of the energy deposited in the detector material of the device by P particles as a function of the thickness of the latter.

[0042] Figure 3 shows an evolution of the energy deposited in the detector material by P particles as a function of the thickness of an aluminum screen interposed between the detector material and the object

[0043] Figure 4A shows a spectrum of the energy of P particles detected by the detector material as a function of different distances between the detector material and the object, with uranium contamination assumed to be surface-based.

[0044] Figure 4B shows a spectrum of the energy of P particles detected by the detector material as a function of different distances between the detector material and the object, the uranium contamination being assumed to be volumetric, according to a thickness of 2 mm.

[0045] Figure 5 shows spectra of the energy of P particles detected by the detector material in the presence of an aluminum shield, and without the aluminum shield. Also shown is a corrected spectrum resulting from the subtraction of the modeled spectra with and without the shield. Figure 6 shows a spectrum of the energy of P particles detected by the detector material due to the natural radioactivity of the object, as well as spectra of the energy of P particles detected by the detector material with and without subtraction of the spectrum of the natural activity. Figure 7 shows simulations of spectra of the energy of P particles detected by the detector material in different configurations.

[0046] Figure 8A shows spectral simulations of the energy of P particles detected by the detector material resulting from different radioelements, for a low enrichment of 235 U.

[0047] Figure 8B shows simulations of the energy spectra of P particles detected by the detector material resulting from different radioelements, for a strong enrichment of 235 U. Figure 9 shows a spectrum of the energy of particles a detected by the detector material.

[0048] Figure 10 shows a spectrum of the energy of P particles detected by the detector material. Figure 11 shows the evolution of a transfer function, allowing a spectral value to be converted into an activity level, for different contamination depths.

[0049] Figures 12A, 12B and 12C show experimental maps carried out with a device according to the invention.

[0050] Figure 13A shows the evolution of a ratio between a first spectral value and a second spectral value as a function of the enrichment in 235 U, taking into account surface contamination.

[0051] Figure 13B shows the evolution of a ratio between a first spectral value and a second spectral value as a function of the enrichment in 235 U, taking into account volume contamination.

[0052] Figure 14 shows the main steps of a method for determining U activity using the invention.

[0053] PRESENTATION OF SPECIAL EMBODIMENTS

[0054] Figure 1 represents a measuring device allowing measurement of the uranium 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.

[0055] 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.

[0056] The use of an organic scintillator detector is suitable for performing charged particle spectrometry, of the P" type. An organic scintillator is also sensitive to ionizing photons of the X or γ type. 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 X or γ spectrometry applications.

[0057] The scintillator detector is covered with an optically sealed envelope 14, of thin thickness, for example an aluminized PET (Polyethylene Terephthalate) film, 18 pm 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 designates a P- particle. The thickness of the envelope can reach 30 pm to stop alpha particles of energy 4.5 MeV or 5 MeV.

[0058] The thickness e of the scintillator detector is 4 mm. Details of the scintillator thickness will be given in connection with Figure 2. 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 backscattering phenomenon 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.

[0059] 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.

[0060] Another 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.

[0061] 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.

[0062] In this example, the device is primarily dedicated to the radiological monitoring of walls. In order to be able to be deployed along large surfaces, the surface area of ​​the detector material, in a plane perpendicular to its thickness, is 50 cm x 50 cm. This allows a large surface area to be addressed during each measurement, while being sufficiently compact and light to be easily handled.

[0063] 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.

[0064] Object 2 is an object to be controlled, likely to have a mass or surface activity of Uranium, the isotopy being known or unknown. In the examples described below, the object is a concrete wall.

[0065] The invention is based on detection, by the detector 10, of P particles emitted by descendant isotopes of Uranium isotopes. These include in particular 234m Pa, which is in radioactive equilibrium with 238U, and emits P particles up to a maximum energy EPmax equal to 2269 keV. We can also exploit the detection of P particles emitted by 231 Th (descendant of 235 U), of which EPmax = 308 keV or 234 Th (descendant of 238 U), with EPmax = 198 keV.

[0066] Figure 2 represents an amount of energy (y-axis - unit MeV.Bq -1 .g), deposited by P particles emitted by a homogeneous distribution of 234m Pa in a concrete depth of 2 mm. The x-axis corresponds to the thickness (unit mm) of the scintillator material 11. The depth of 2 mm corresponds to the maximum path of P particles with energy 2269 keV (maximum emission energy of 234mPa) in the concrete. Figure 2 was obtained by simulation, considering a distance d=7.5 mm between the detector material and the concrete. It is observed that by using a thickness e of 4 mm, the scintillator material can absorb 97% of the emission energy P of 234m Pa. The simulation was carried out by the MCNP6 (Monte Carlo N Particles) computer code, which is a reference code in the field of modeling interactions between ionizing radiation (P, y, neutrons) and matter.

[0067] Figure 3 represents an amount of energy (y-axis - unit MeV.Bq -1 .g)), deposited by P particles emitted by a homogeneous distribution of 234mPa in a concrete depth of 2 mm. The abscissa axis corresponds to the thickness (unit mm) of the aluminum screen 15. Figure 3 was obtained by simulation, considering a distance d=7.5 mm between the detector material and the concrete. It is observed that beyond a thickness £ of 4 mm of aluminum, the gain in terms of absorption of P particles is negligible. A thickness of 4 mm is therefore sufficient to effectively seal the detector material against P particles emitted by 234mp

[0068] Figures 4A and 4B address the issue of the distance between the detector and the object to be controlled. Since the controlled object is potentially contaminating, it is preferable to avoid the detector 10 being in direct contact with the object. A slight recoil is useful, so as to allow the screen 15 to move from the open position to the closed position. Figure 4A shows simulations of spectra of the energy deposited by particles P in the scintillator material, as described in connection with Figure 1. A homogeneous surface distribution of 234m Pa on a wall and several distances d of the detector from the wall were taken into account: d = 2 mm; d = 7 mm and d = 10 mm. In Figure 4A, the abscissa axis corresponds to the energy (MeV) and the ordinate axis corresponds to the counting rate (s 1 . Bq 1 . cm 2). It is observed that between 2 mm and 10 mm, the spectrum is not significantly modified. The difference between the spectra acquired considering distances of 7 mm and 10 mm is negligible.

[0069] Figure 4B shows simulations of the same type as those described in connection with Figure 4A. In Figure 4B, a homogeneous volume distribution of 234mPa on a 2 mm thickness of concrete. The same distances d of the detector from the wall were taken into account as in Figure 4A. The difference between the spectra acquired by considering distances of 2.5 mm and 10 mm is negligible. The inventor considers that the optimal distance between the object 2 and the detector 10 is between 5 mm and 10 mm. Beyond 10 mm, there is a risk of loss of efficiency, i.e. a reduction in the number of particles P detected. Below 5 mm, handling the screen becomes complex, the latter being too close to the object, with risks of contact between the screen and the object. Subsequently, the distance between the detector and the object is equal to 7.5 mm.

[0070] Figure 5 shows the effect of blocking the detector by screen 15 on the spectrum measured by the detector. Figure 5 represents spectra measured experimentally facing a uranium-contaminated wall. Y axis: counting rate (s 1) - abscissa axis: energy (MeV). A spectrum without a screen (screen in the open position, as shown in Figure 1 SpPy spectrum) and a spectrum with a screen (screen in the closed position Spbd spectrum) were measured respectively. The spectra were measured using the same acquisition time of 120 seconds. The screen was a 4 mm thick aluminum plate. The spectrum with a screen, noted Spbdf, is considered to be representative of the background noise y- The background noise y is due to the emitting radioelements y present in the object (for example natural radioelements) or in the detector environment. Subtracting the spectrum with a screen Spbdf from the spectrum without a screen SpPy allows a spectrum to be corrected for the background noise y to be obtained. The corrected spectrum SpP, or raw P spectrum, is considered to be solely representative of the P particles emitted by the object.

[0071] Among the natural radioelements potentially present in the object, we can cite for example 40K, as well as descendants of the 232 Th and of 238 U. Such radioelements are present, for example, in concrete objects. Background noise can also result from artificial radioactivity present in the detector's environment.

[0072] When the object contains natural radioactivity, the raw P spectrum SpP includes a natural component due to P particles emitted by natural radioelements. This natural component can, in a first approach, be neglected because it is generally quite weak. Table 1 shows the different P-emitting radionuclides of natural and artificial origin likely to be encountered in a concrete wall potentially contaminated with uranium.

[0073] Table 1

[0074] Figure 6 shows the corrected P spectrum Sp , as described in connection with Figure 5 (raw P spectrum), as well as a P spectrum, also corrected for the background noise y, measured facing an uncontaminated concrete wall (natural background noise spectrum or RN background noise spectrum, noted SpRN).

[0075] The uncontaminated concrete wall was located in the same facility as the contaminated wall, on which the corrected P spectrum was formed. Thus, the RN background P spectrum, denoted SpRN, can be considered representative of a natural P contribution in the corrected P spectrum. Figure 6 shows a net P spectrum, denoted SpP', obtained by subtracting the natural background P spectrum SpRN from the raw P spectrum. The net P spectrum SpP' is thus representative of the artificial P activity in the measured wall.

[0076] Figure 7 represents P spectra simulated with the MCNP6 calculation code, in a detector material as previously described (50 cm x 50 cm x 4 mm), taking into account mass activities of 232 Th, 238 U and 40 K measured by gamma spectrometry on a wall considered uncontaminated. The gamma spectrometry measurement made it possible to estimate activity levels of 15 Bq / Kg for 232 Th, 22 Bq / Kg for 238 U, and 485 Bq / Kg for 40 K. These mass activities make it possible to simulate the natural component in the P spectrum measured by the detector. In Figure 7, we have represented: the component due to 232 Th in the P spectrum; the component due to 238 U in the P spectrum; the component due to 40 K in the P spectrum; the component due to 235 U in the P spectrum, assuming natural enrichment of uranium: the mass of 235 U corresponds to 0.72% of the mass of uranium;

[0077] In Figure 7, we also show a simulation of the P spectrum resulting from 234m Pa taking into account a homogeneous surface activity of 0.4 Bq. cm -2 and an enrichment of 1% by mass in 235 U (UB spectrum), and taking into account an enrichment of 8.16% by mass in 235 U (UH spectrum).

[0078] In these simulations, the distance between the wall and the detector was considered equal to 7.5 mm, and the density of the concrete was considered equal to 2.3 g.crrr 3 .

[0079] We observe that: the natural component of the P spectrum is dominated by 40 K, up to the energy 1311 keV, which corresponds to the maximum energy of the P particles emitted by 40 K; the measurement of the spectrum P resulting from 234mPa is largely superior to the natural contribution at low enrichment as well as at high enrichment, beyond 1 MeV; it is possible to model the natural component in a P spectrum measured by the detector, taking into account mass activities, established a priori or measured, of the main natural P-emitting radioelements.

[0080] Furthermore, simulations have shown that the y component of the natural activity is not significantly absorbed by the aluminum screen. Also, subtracting the background noise spectrum, measured in the presence of the aluminum screen, allows us to eliminate the y component of the natural activity.

[0081] Figures 8A and 8B show simulations of P spectra measured by a detector as described in Figure 1, taking into account a homogeneous surface activity of uranium, on a wall element of 50 cm on each side, taking into account respectively an enrichment in235 U of 1% and 82.55% by mass. By surface activity, we mean an activity distributed over a depth of 10 pm. In each of these figures, the contributions of 234m Pa, 234 Th and 231 Th. 234m Pa and 234 Th are descendants of 238 U, while 231 Th is a descendant of 235 U. The distance between the detector and the wall was considered equal to 7.5 mm. In each of these figures, the y-axis corresponds to a counting rate (s 1 . Bq 1 . cm 2 ) and the abscissa axis corresponds to an energy level (MeV).

[0082] Figures 8A and 8B show that regardless of the enrichment level, the spectrum is dominated by the contribution of 234mPa, the latter being lower at high enrichment (Figure 8B) than at low enrichment (Figure 8A). We observe that by considering a first energy range AE1 delimited by a first minimum energy Elmin, greater than or equal to 300 keV, and by summing the spectrum over this first energy range AE1, the spectral value obtained depends only on the activity of 234m Pa. A spectral value is a value measured from the spectrum. This can include a count rate or a count in the energy range. The spectral value obtained over the first energy range AE1 is a first spectral value denoted NI.

[0083] Preferably, the first energy range AE1 extends up to a first maximum energy Elmax greater than or equal to 2269 keV, the latter corresponding to the maximum emission energy of a particle P per 234mPa. For example, the first energy range AE1 is [300 keV; 2500 keV], Such an energy range allows to collect the entire useful spectral content of the P activity of 234mPa. The first maximum energy Elmax can be equal to 2300 keV, or be less than 2000 keV or 1500 keV or 1000 keV. It is noted that the first maximum energy Elmax is preferably less than or equal to 2500 keV, or even 2300 keV. Thus, although the first optimal energy range is [300 keV; 2500 keV], a more restricted energy range, within this interval, may be suitable. An energy range extending from 300 keV, or from a first minimum value Elmin greater than or equal to 300 keV, and less than or equal to 1000 keV, or 1500 keV may be suitable. It is noted that beyond 1000 keV, the value of the spectral channels decreases. It is therefore preferable that the first minimum energy Elmin is between 300 keV and 1000 keV. It is possible not to take into account a first maximum energy: Elmax =+oo: the range considered is then [Elmin, +oo], with 300 keV < Elmin<1000 keV.

[0084] The first spectral value NI, in the first energy range AE1 = [Elmin, Elmax] previously defined, is then intended to be processed by a transfer function, so as to be able to estimate an activity level in 238 U. The establishment of the transfer function is described below, in connection with Figure 11. Taking into account the enrichment rj then makes it possible to convert the activity into 238 U in U activity. Figures 8A and 8B show another interesting aspect of the invention: we can define a second energy range AE2, between 0 keV and 300 keV, in which the spectrum P is influenced by 231 Th, this radioelement descended from 235 U. Thus, we can define a second energy range AE2=[E2min, E2max], between a second minimum energy E2min>0 and a second maximum energy E2max<300 keV, in which the spectral value N2, called the second spectral value, depends on 235U, especially at high enrichment.

[0085] A ratio N1 / N2 of the first and second spectral values ​​depends on the enrichment q. Thus, the experimental measurement of this ratio can be used to estimate the enrichment q of uranium.

[0086] When the enrichment q of uranium in 235 U is unknown, the first spectral value NI allows to estimate 238 U, using the transfer function FT. Indeed, the spectral content, in the first energy range, does not depend on the enrichment. The ratio of the first and second spectral values ​​allows the enrichment q to be estimated. The activity in 238 U can then be used to estimate the activity in U. The enrichment in 235 U can also be determined experimentally by gamma spectrometry.

[0087] Uranium contains a-emitting isotopes, primarily 234 U or 238U. In order to estimate the contribution of particles a in the measured P spectrum, a source of 244 Cm, activity 2800 Bq, 3 mm from the detector material. 244 Cm emits a-particles with energies above 5700 keV, which is much higher than the energies of a-particles emitted by U isotopes, for example 4775 keV for 234 U. Figure 9 shows the spectrum measured over an acquisition period of 900s (y-axis: number of detected pulses - x-axis: energy (keV)). It can be seen that the contribution of α particles is limited to low energies, below 270 keV. Thus, the spectral value, in the first energy range, can be considered as unaffected by α particles emitted by uranium isotopes.

[0088] The a particles detected by the detector may be likely to influence the spectral value N2 in the second energy band AE2, or even the spectral value NI in the first spectral band AE1. This is particularly the case when the detector comprises a semiconductor material, for example Si. In order to avoid an influence of the a particles, the envelope 14 is advantageously sized to stop the a particles of energy 4.5 MeV or 5 MeV. When the envelope is made of aluminized PET (Polyethylene Terephthalate), a thickness of 30 μm makes it possible to stop the a particles, while absorbing, in a manner considered negligible, the particles . In Figure 9, the few shots observed at an energy greater than 270 keV correspond to y photons after subtraction of the background noise, resulting from statistical fluctuations.

[0089] Figure 10 shows a P spectrum measured experimentally on a concrete wall contaminated with 1% enriched uranium (y-axis: number of pulses detected - x-axis: energy (keV)). On this spectrum, the first energy range, between 300 keV and 2500 keV, was materialized. The detector was an EJ200 detector (Eljen Technology), with a sensitive surface area of ​​2430 cm 2 (493 mm side). The energy calibration of the spectrum, 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 207 Bi, which emits electrons, at discrete energy values, by internal conversion.

[0090] In order to convert the spectral value, in the first energy range, into the activity value of 238U, it is necessary to take into account a transfer function. The transfer function can be obtained by modeling P spectra corresponding to known activities in 238 U, and therefore in 234m Pa. Different transfer functions can be determined, corresponding respectively to different hypotheses of contamination distribution in the analyzed object. Uranium contamination can be considered as surface (for example on metallic objects), or having diffused in the object, for example in the case of liquid contamination of porous objects, such as a floor or a concrete wall.

[0091] Figure 11 shows the result of numerical modeling of a detector as previously described, placed at a distance of 7.5 mm from a concrete wall, of density 2.3, whose uranium activity 238 U was distributed according to different depths. We considered a unit activity of 238 U (1 Bq / g or 1 Bq / cm 2in the hypothesis of surface contamination). The detector model was validated in the laboratory by comparing P spectra respectively measured and modeled by exposing the detector to a standard source of 207 Bi.

[0092] In the case of surface contamination, activity distributed over the first 10 pm of the wall was modeled. The transfer function is 1050s 1 . Bq 1 . cm 2 ± 15%.

[0093] In the case of volumetric contamination, the maximum path, in the concrete, of the P particles emitted by 234m Pa, which can be estimated at 2 mm. Given this low thickness, the contamination gradient was considered homogeneous.

[0094] In Figure 11, the y-axis corresponds to the value of the transfer function FT (unit s 1 . Bq 1 . cm 2) and the abscissa axis corresponds to the depth of contamination considered. In Figure 11, the transfer function corresponds to the first spectral value NI, in an energy range of [300 keV - 2500 keV], for a unit activity of lBq.g 1 .

[0095] If NI corresponds to the counting rate in the first energy band AE1, the activity A [238t7], in Bq.g 1 or in Bq. cm -2 , is obtained according to the expression:

[0096] The detector described above was implemented on a gaseous diffusion uranium enrichment facility. The detection limit was such that:

[0097] With

[0098] = 1.96: risk level of type / 3 for a 95% confidence interval;

[0099] T s acquisition time, in seconds;

[0100] FT = transfer function, expressed in Bq.g 1 or in Bq. cm -2

[0101] SD is the detection threshold, such that k -a = 1.96: type a risk level for a 95% confidence interval; n ratio of the respective acquisition times of the background noise and the measurement r bd f count rate measured in the first energy band AE1, in a background noise measurement, on a wall considered uncontaminated. Unit counts per second.

[0102] Depending on the unit of the transfer function, the detection limit is expressed in Bq.g 1 or in Bq. cm -2 .

[0103] Tables 2 and 3 represent detection limit values ​​expressed respectively in surface activity and in mass activity. The values ​​are calculated from expressions (2) and (3), considering a measurement duration equal to the duration of the background measurement. A contribution of natural activity in the spectrum was also taken into account, described in connection with Figure 7. The activities of the natural isotopes taken into account were 15 Bq / Kg for 232 Th, 22 Bq / Kg for 238 U, and 485 Bq / Kg for 40 K. Each measure consists of one measure with the aluminum screen and one measure without the aluminum screen, of the same durations. LD in Bq.cnr 2

[0104] R 4

[0105] Table 2

[0106] LD in Bq.g- 1

[0107] FT Ts (s)

[0108] Radionuclide ROI reop (s 1 ] (s^.Bq g) 60 180 300 600 1800

[0109] 149 ± 15% 3.4.10- 2 2.4.10- 2 l,5.10- 2 l,1.10” 2 6.1.10-'

[0110] Table 3

[0111] It is observed that the method allows to obtain low detection limits, in reasonable acquisition times. The detection limits presented in tables 2 and 3 concern 238 U via the detection of the P spectrum of 234m Pa. The higher the enrichment, the greater the quantity of 238 U decreases relative to the total amount of uranium. Therefore, if we want to maintain a low uranium detection limit, we must increase the acquisition time.

[0112] Mapping was carried out on walls of the gaseous diffusion uranium enrichment facility. Figure 12A shows measured count rates (cps = counts per second) on surface elements measuring 50 cm x 50 cm. Figure 12B shows activity levels 238Corresponding U (Bq / m 2 ). Enrichment in 235 Since U is known, activity levels in U were estimated, the latter being represented in Figure 12C (Bq / m 2 ). When the enrichment is not known, it can be determined by usual methods, for example by gamma spectrometry.

[0113] As previously described, in connection with Figures 8A and 8B, the rj enrichment in 235U can be evaluated from a ratio between the first spectral value NI and the second spectral value N2. The second energy band AE2 is between 0 keV and 250 keV. For example, a second energy band AE2 can be considered extending from E2min=150 keV to E2max=300 keV. The second minimum bound E2min of 150 keV was determined in order to limit the influence of gamma radiation: between 150 keV and 300 keV, the contribution of gamma radiation in the P spectrum was found to be stable, and can be easily subtracted by taking into account the measurement with the aluminum screen. By taking into account a second minimum energy E2min lower than 150 keV, the contribution of gamma radiation in the P spectrum can fluctuate more, due to the higher sensitivity of the plastic scintillator material to low-energy photons.

[0114] Figure 13A shows the evolution of a ratio between the first and second spectral values. The first energy band is [300 keV - 2500 keV] and the second energy band is [150 keV - 300 keV]. The contamination is assumed to be surface-based, i.e. considered to be distributed over a thickness of 10 pm.

[0115] Figure 13B shows the evolution of a ratio between the spectral values ​​(count rate) in the first energy band [300 keV - 2500 keV] and in the second energy band [150 keV - 300 keV] in the case of mass contamination, distributed over a thickness of 2 mm.

[0116] In Figures 13A and 13B, the y-axis corresponds to the N1 / N2 ratio and the x-axis corresponds to the enrichment in 235 U, expressed in %. Figures 13A and 13B were established on the basis of models with the MCNP6 calculation code.

[0117] A correlation is observed between enrichment and the N1 / N2 ratio. Thus, by applying an enrichment calibration function, as described in connection with Figures 13A or 13B, enrichment can be estimated from the N1 / N2 ratio. Figures 13A and 13B show the establishment of enrichment calibration functions, respectively for a surface activity and a volume activity.

[0118] It is specified that the use of P spectrometry to estimate an enrichment rate rj may constitute an independent aspect of the invention.

[0119] In practice, the use of a background noise spectrum, obtained according to the closed configuration, with the screen 15, is not necessary. Indeed, if one wishes to carry out a rough control, measurements carried out only according to the open configuration, i.e. without a screen, may prove sufficient to carry out a first-level control. The coupling between the open configuration and the closed configuration makes it possible to subtract the background noise, essentially due to the y photons, which makes it possible to obtain more precise measurements: quantification of the activity 238 More precise U or more accurate enrichment value.

[0120] Furthermore, experience has shown that in the first energy band AEl , extending from 300 keV to 2500 keV, the contribution of the gamma background noise due to natural radioactivity is stable, in the same installation. However, carrying out measurements with and without a screen increases the total duration of the measurement. Given the low dispersion of the contribution, it is possible not to carry out a systematic measurement in the closed configuration. It is possible to subtract, from a spectrum measured in the open configuration, a stored spectrum, carried out in the closed configuration.

[0121] Figure 14 summarizes the main steps of a method according to the invention.

[0122] Step 100: Arrange a detector facing the object to be characterized

[0123] Step 110: acquisition of a spectrum, in the open configuration: this is the measured spectrum Sp.

[0124] Step 120: acquisition of a background noise spectrum Spbdf, in the closed configuration, or taking into account a spectrum acquired in the closed configuration.

[0125] Step 130: Subtraction of the spectrum resulting from step 120 from the spectrum resulting from step 110. The spectrum resulting from this step is a raw P spectrum Sp .

[0126] Step 140: taking into account the natural activity P of the object. This step includes the following sub-steps:

[0127] Sub-step 141: taking into account a level of natural activity of the object: the level of natural activity may have been estimated by a measurement on a comparable object, considered to be uncontaminated.

[0128] Sub-step 142: estimation of a natural P SpRN spectrum (or P RN spectrum) resulting from step 141.

[0129] Alternatively, step 140 includes a sub-step 143 of measuring a raw P spectrum carried out on an object considered to be representative of the measured object, and not contaminated. This involves implementing steps 110 and 120 on the object considered to be representative, which makes it possible to obtain the natural P spectrum.

[0130] Step 140 is preferred, but optional.

[0131] Step 150: correction of the raw P spectrum, resulting from step 130, taking into account the natural P spectrum resulting from step 140. This step is optional. Step 150 makes it possible to obtain a net P spectrum (or corrected P spectrum) noted SpP'.

[0132] Step 160: determination of a first spectral value NI in the first energy band AE1 from the net P spectrum SpP' resulting from step 150 or from the raw P spectrum resulting from step 130.

[0133] Step 170: Application of the transfer function FT to the first spectral value in order to estimate an activity of the object in 238 U noted A[ 238 U] Step 180: taking into account an enrichment rate q in mass of 235 U. The enrichment is either known, as it depends on the installation, or unknown, in which case it can result from a non-destructive measurement of the gamma spectrometry type. The enrichment can also be determined from the spectrum from the corrected P spectrum resulting from step 150 or from the net P spectrum resulting from step 130. For this, step 190 is implemented.

[0134] Step 190: Step 190 consists of the following sub-steps:

[0135] Sub-step 191: determination of a second spectral value N2 in the second energy band AE2 from the net P spectrum resulting from step 150 or from the raw P spectrum resulting from step 130.

[0136] Sub-step 192: Application of the enrichment calibration function to the N1 / N2 ratio to estimate an enrichment q. The enrichment calibration function depends on hypotheses regarding the nature of the activity: surface activity (see figure 13A) or volume activity (see figure 13B).

[0137] Step 200: from the enrichment q, resulting from step 180, and the activity of 238 U resulting from step 170, determination of uranium activity A[U],

[0138] In some applications, the activity in 238 U resulting from step 170 is sufficient: steps 180 to 200 are not implemented.

[0139] Steps 130 to 200 may be implemented by the processing unit.

[0140] The invention may be implemented in fuel enrichment or manufacturing installations.

Claims

CLAIMS 1. Method for estimating the uranium, or uranium 238, activity of an object (2), using a detector (10) configured to form pulses under the effect of exposure to P particles, the detector being connected to a spectrometric measurement circuit (13), 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 (Sp), the measurement spectrum being representative of an energy distribution of P particles emitted by descendants of 238 U ; - b) from the measurement spectrum, possible formation of a corrected spectrum (SpP; Sp '); - c) from the measurement spectrum or the corrected spectrum, determination of a first spectral value (NI) in a first energy band (AE1) extending from a first minimum energy (Elmin), the first minimum energy being greater than or equal to 300 keV; - d) application of a transfer function (TF) to the first spectral value determined during step c), so as to estimate an activity in uranium or 238 U of the object.

2. Method according to claim 1, in which step b) comprises the sub-steps: - bl) 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; - b2) correction of the measurement spectrum, using the background spectrum, to obtain a corrected spectrum.

3. Method according to claim 2, in which the screen (15) used during sub-step b1) is an aluminum screen whose thickness is greater than or equal to 3 mm or 4 mm.

4. Method according to claim any one of the preceding claims, wherein during step c), the first energy band extends up to a maximum energy (Elmax), the maximum energy being greater than or equal to 1000 keV or 1500 keV or 2000 keV.

5. Method according to any one of the preceding claims, comprising the steps: f) determining a second spectral value (N2), in a second energy band (AE1), extending between a second minimum energy (E2min) and a second maximum energy (E2max), the second maximum energy being less than 300 keV; - g) estimation of an enrichment in 235U of Uranium, by comparing the first spectral value resulting from step c) and the second spectral value resulting from step f).

6. 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.

7. A method according to any preceding claim, wherein the object is a naturally radioactive object, the method comprising: - (i) taking into account a natural activity of the object; - (ii) modeling of a P spectrum of the natural activity (SpRN) of the object detected by the detector; - step b) includes a correction of the measured spectrum using the spectrum P of the natural activity of the object resulting from (ii), to obtain the corrected spectrum.

8. Method according to claim 7, in which step (i) results from a measurement by gamma spectrometry carried out on the object.

9. A method according to any preceding claim, wherein the detector comprises an organic scintillator material.

10. Method according to any one of claims 1 to 9, in which the detector comprises a semiconductor material.

11. Measuring device, configured to determine a uranium 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 P particles; - a spectrometry circuit (13), connected to the detector, and configured to form a histogram of the amplitude of the pulses detected by the detector during an acquisition period; - a processing unit (20), connected to the spectrometry circuit, and configured to implement steps b) to d) of a method according to any one of the preceding claims.

12. Device according to claim 10, in which the detector comprises an organic scintillator material (11), the thickness of the organic scintillator material being between 3 mm and 10 mm.

13. Device according to claim 11, in which the detector comprises a semiconductor material (11).

14. Device according to any one of claims 11 to 13, in which the detector is covered with an envelope (14) configured to absorb particles with an energy greater than or equal to 4.5 MeV or 5 MeV.

15. Device according to any one of claims 10 to 14, in which 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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