non-destructive method for measuring uranium enrichment

Abstract

A method for estimating the 235U enrichment of an object, using a detector connected to a spectrometric measurement circuit configured to acquire a spectrum, comprising: a) positioning the detector facing the object and acquiring a measurement spectrum; b) from the measurement spectrum, possibly constructing a corrected spectrum; c) from the measurement spectrum or the corrected spectrum, determining a first spectral value in a first energy band; d) from the measurement spectrum or the corrected spectrum, determining a second spectral value in a second energy band; e) calculating a ratio between the first spectral value and the second spectral value; f) applying a calibration function to the ratio determined in step e) to estimate the 235U enrichment. Fig. 13A

Description

Title of the invention: method for non-destructive measurement of uranium enrichment Technical field

[0001] The technical field of the invention relates to a technique for non-destructive measurement of the enrichment of uranium in its isotope 235 PRIOR ART

[0002] Currently, the reference method for non-destructively evaluating the enrichment of uranium in its isotope 235 is gamma spectrometry.

[0003] 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.

[0004] Uranium-235 and some uranium-238 descendants 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.

[0005] Furthermore, when the enrichment in 235U is high, and / or for small quantities of uranium, the acquisition time can be long, due to the low intensity of the gamma emissions of the descendant radioelements of 238U.

[0006] The inventor proposes an alternative method for estimating uranium enrichment. The method can be deployed on civil engineering structures, equipment or collected samples. It is a method based on a device that is inexpensive and simple to implement. Statement of the invention

[0007] A first subject of the invention is a method for estimating a 235U enrichment of an object, using a detector, the detector being configured to form pulses when it is exposed to particles [3, the detector being connected to a spectrometric measurement circuit, configured to establish a spectrum, the spectrum corresponding to a histogram of the energy 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, the measurement spectrum being representative of an energy distribution of particles [3 emitted by 238U descendants and by 235U descendants; - b) from the measurement spectrum, possible formation of a corrected spectrum; - c) from the measurement spectrum or the corrected spectrum, determination of a

[0008]

[0009]

[0010]

[0011]

[0012]

[0013]

[0014]

[0015] 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; - d) from the measurement spectrum or the corrected spectrum, determination of a second spectral value in a second energy band extending below a second maximum energy, the second maximum energy being less than or equal to 300 keV; - e) calculating a ratio between the first spectral value and the second spectral value; - f) application of a calibration function to the ratio determined during step e), so as to estimate the enrichment in 235U. The process may include the following 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 particles [3 emitted by the object; - b2) correction of the measurement spectrum, using the background spectrum, for obtain a corrected spectrum The screen used during sub-step bl) may be an aluminum screen whose thickness is greater than or equal to 3 mm or 4 mm. According to one possibility, in step c), the first energy band extends up to a first maximum energy, the maximum energy being greater than or equal to 1000 keV or 1500 keV or 2000 keV. According to one possibility, in 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. When the object is a naturally radioactive object, the process may include: - (i) taking into account a natural activity of the object; - (ii) modeling of a spectrum [3 of the natural activity of the object (SpRN) detected by the detector; - step b) involves a correction of the measured spectrum using the spectrum [3 of the natural activity of the object, to obtain the corrected spectrum. According to one possibility, step (i) results from a gamma spectrometry measurement carried out on the object. The detector may comprise an organic scintillator material. The detector may comprise a semiconductor material. A second object of the invention is a device for estimating the 235U enrichment of an object, the device comprising: - A detector (10) comprising a detector material (11) configured to form impulses under the effect of exposure to particles [3; - a spectrometry circuit (13), connected to the detector, and configured to form a histogram of the amplitude of the pulses detected by the detector during an acquisition period; - a processing unit (20), connected to the spectrometry circuit, and configured to implement steps b) to f) of a method according to the first subject of the invention

[0016] The detector may comprise an organic scintillator material, the thickness of the organic scintillator material preferably being between 3 mm and 10 mm. The detector may comprise a semiconductor material.

[0017] The detector may be of an envelope configured to absorb particles with an energy greater than or equal to 4.5 MeV or 5 MeV.

[0018] 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.

[0019] The invention will be better understood upon reading the description of the exemplary embodiments presented in the remainder of the description, in conjunction with the figures listed below. FIGURES

[0020] [Fig.l] shows a diagram of a measuring device.

[0021] [Fig.2] shows an evolution of the energy deposited in the detector material of the device by particles [3 as a function of the thickness of the latter.

[0022] [Fig. 3] shows an evolution of the energy deposited in the detector material by particles [3 as a function of the thickness of an aluminum screen interposed between the detector material and the object

[0023] [Fig.4A] shows a spectrum of the energy of particles [3 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 surface-based.

[0024] [Fig.4B] shows a spectrum of the energy of particles [3 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.

[0025] [Fig. 5] shows spectra of the particle energy [3 detected by the detector material in the presence of an aluminum screen, without the aluminum screen. Also shown is a corrected spectrum resulting from the subtraction of the modeled spectra with and without the screen.

[0026] [Fig.6] shows a spectrum of the energy of particles [3 detected by the detector material due to the natural radioactivity of the object, as well as spectra of the energy of particles [3 detected by the detector material with and without subtraction of the natural activity spectrum.

[0027] [Fig.7] shows simulations of spectra of the energy of particles [3 detected by the detector material in different configurations.

[0028] [Fig.8A] shows spectral simulations of the energy of particles [3 detected by the detector material resulting from different radioelements, for a low enrichment of 235U.

[0029] [Fig.8B] shows simulations of spectra of the energy of particles [3 detected by the detector material resulting from different radioelements, for a strong enrichment of 235U.

[0030] [Fig.9] shows a spectrum of the energy of particles a detected by the detector material.

[0031] [Fig. 10] shows a spectrum of the energy of particles [3 detected by the detector material.

[0032] [Fig. 11] shows the evolution of a transfer function, allowing a spectral value to be converted into an activity level, and this for different depths of contamination.

[0033] Figures 12A, 12B and 12C show experimental maps carried out with a device according to the invention.

[0034] [Fig.13A] shows the evolution of a ratio between a first spectral value and a second spectral value as a function of the enrichment in 235U, taking into account surface contamination.

[0035] [Fig.l3B] shows the evolution of a ratio between a first spectral value and a second spectral value as a function of the enrichment in 235U, taking into account volume contamination.

[0036] [Fig. 14] shows the main steps of a method for determining U activity using the invention. PRESENTATION OF SPECIAL EMBODIMENTS

[0037] [Fig.l] 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.

[0038] By implementing an energy calibration function, resulting from a In energy calibration, it is usual to match the amplitude of the pulses to the 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.

[0039] The use of an organic scintillator detector is suitable for carrying out charged particle spectrometry, of type [3. An organic scintillator is also sensitive to ionizing photons of type X or y. However, the materials forming an organic scintillator have a low atomic number, which makes them not very conducive to the formation of photoelectric interactions. Thus, an organic scintillator is considered not suitable for applications of type X or y spectrometry.

[0040] The scintillator detector is covered with an optically sealed envelope 14, of low thickness, for example an aluminized PET (Polyethylene Terephthalate) film, 18 μm thick to ensure sealing against ambient light. The low thickness makes it possible to minimize the attenuation of radiation [3. In the remainder of the text, the term particle [3 designates a particle [3 . The thickness of the envelope can reach 30 μm to stop alpha particles of energy 4.5 MeV or 5 MeV.

[0041] The thickness e of the scintillator detector is 4 mm. Details regarding the thickness of the scintillator will be given in connection with [Fig.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 phenomenon of particle backscattering [3. Another advantage is a certain stability of the response of the scintillator material with respect to thermal variations. The response, in terms of light intensity produced, when exposed to the same radiation, is stable from 0° to 50°C, which is appropriate for field use conditions.

[0042] The measuring device comprises a processing unit 20, configured to implement spectrum processing steps described below. The processing unit 20 is programmed to execute instructions coded in a memory, connected to the processing unit by wired or wireless connection. The processing unit 20 may in particular comprise a microprocessor.

[0043] According to a variant, the detector may comprise a semiconductor material, suitable for spectrometry [3. It may for example be a silicon type semiconductor, for example a planar silicon.

[0044] The advantage of an organic scintillator material is the ability to be manufactured in different dimensions and shapes. When the object is a sample being taken, the shape can be adapted to the shape of the sample.

[0045] The detector 10 comprises a movable screen 15, acting as a shutter, configured to be arranged: - in a closed position, between the detector 10 and the object to be measured 2; - or in an open position, freeing the space between the detector 10 and the object 2, as shown in [Fig.l].

[0046] In this example, the device is mainly dedicated to the radiological control of walls. In order to be able to be deployed along large surfaces, the surface of the detector material, in a plane perpendicular to its thickness, is 50 cm x 50 cm. This makes it possible to address a large surface during each measurement, while being sufficiently compact and light to be easily handled.

[0047] 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.

[0048] 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.

[0049] The invention is based on a detection, by the detector 10, of particles [3 emitted by descendant isotopes of Uranium isotopes. This concerns in particular 234mPa, which is in radioactive equilibrium with 238U, and emits particles [3 up to a maximum energy E[3max equal to 2269 keV. It is also possible to exploit the detection of particles [3 emitted by 231Th (descendant of 235U), of which E[3max = 308 keV or 234Th (descendant of 238U), with E[3max = 198 keV.

[0050] [Fig.2] represents an amount of energy (y-axis - unit MeV.Bq 1 .g)), deposited by particles [3 emitted by a homogeneous distribution of 234mPa in a depth of concrete of 2 mm. The x-axis corresponds to the thickness (unit mm) of the scintillator material 10. The depth of 2 mm corresponds to the maximum path of the particles [3 of energy 2269 keV (maximum emission energy of 234mPa) in the concrete. [Fig.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 allows to absorb 97% of the emission energy [3 of 234mPa. The simulation was performed by the MCNP6 (Monte Carlo N Particles) computer code, which is a reference code in the field of modeling interactions between ionizing radiation (|3, y, neutrons) and matter.

[0051] [Fig.3] represents a quantity of energy (ordinate axis - unit MeV.Bq 1 .g)), deposited by particles [3 emitted by a homogeneous distribution of 234mPa in a depth of concrete of 2 mm. The abscissa axis corresponds to the thickness (unit mm) of the aluminum screen 15. [Fig.3] was obtained by simulation, considering a distance d=7.5 mm between the detector material and the concrete. We observe that beyond a thickness e of 4 mm of aluminum, the gain in terms of absorption of particles [3 is negligible. A thickness of 4 mm is therefore sufficient to effectively seal the detector material against particles [3 emitted by 234 mPa.

[0052] Figures 4A and 4B address the question 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. [Fig.4A] shows simulations of spectra of the energy deposited by particles [3 in the scintillator material, as described in connection with [Fig.1]. A homogeneous surface distribution of 234mPa on a wall was considered 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 [Fig.4A], the abscissa axis corresponds to the energy (MeV) and the ordinate axis corresponds to the counting rate (s '.Bq '.cm2). 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.

[0053] [Fig.4B] shows simulations of the same type as those described in connection with [Fig.4A]. In [Fig.4B], a homogeneous volume distribution of 234m Pa over a thickness of 2 mm of concrete was considered. The same distances d of the detector from the wall were taken into account as in [Fig.4A]. The difference between the spectra acquired when considering distances of 2.5 mm and 10 mm is negligible.

[0054] 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 a loss of efficiency, that is to say a reduction in the number of particles [3 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.

[0055] [Fig. 5] shows the effect of blocking the detector by the screen 15 on the spectrum measured by the detector. [Fig. 5] represents spectra measured experimentally facing a wall contaminated with uranium. A spectrum without a screen (screen in the open position, as shown in [Fig. 1]) and a spectrum with a screen (screen in the closed position) 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's environment. Subtracting the spectrum with a screen Spbdf from the spectrum without a screen makes it possible to obtain a spectrum corrected for the background noise y.The corrected Sp[3 spectrum, or raw [3 spectrum, is considered to be only representative of the [3 particles emitted by the object.

[0056] Among the natural radioelements potentially present in the object, we can cite for example 40K, as well as descendants of 232Th and 238U. Such radioelements are for example present in concrete objects. The background noise can also result from artificial radioactivity present in the detector environment.

[0057] When the object contains natural radioactivity, the [3 raw Sp[3 spectrum contains a natural component due to the [3 particles emitted by the natural radioelements. This natural component can, in a first approach, be neglected because it is generally quite weak. Table 1 shows the different [3 emitting radionuclides of natural and artificial origin likely to be encountered in a concrete wall potentially contaminated with uranium.

[0058] [Tables 1] Beta emitters Max energy (keV) intensity {%| Origin 433 11 il 21 33.2 S Natural Decay chain of T235ü Natural & Attire;el 22? A: 33 Natural 22 3 L a .s A .-X 'X 1 Natural 221 P b 133? 31.2 S Natural 2O7T; 1423 39.73 Natural Decay chain of A j 7 Nsm e St ■Irt z ; W 2i4u' PC 22 9'3 393 Wu-td & m-fuc e* Z • 14 P :J 1213 3.2 Natures 12 " 13.37 Natural 21CTb si 5 L.3 Natural 210 b: 103 Natural Decay chain of 232Th 228Ra 12; Natural 228Ac 2-cc » Natural 212Pb 369.9 13.3 Natural 212 Si 2232.1 3331 Natural 2037; 1801.3 10.2 Natural

[0059] Table 1

[0060] [Fig.6] shows the corrected [3 Sp[3 spectrum, as described in connection with [Fig.5] (raw [3 spectrum), as well as a [3 spectrum, also corrected for the background noise y, measured in front of an uncontaminated concrete wall (natural background noise spectrum or RN background noise spectrum, noted SpRN). The uncontaminated concrete wall was located in the same installation as the contaminated wall, on which the corrected [3 spectrum was formed. Thus, the [3 RN background noise spectrum, noted SpRN, can be considered as representative of a natural [3 contribution in the corrected [3 spectrum. [Fig.6] shows a net [3 spectrum, noted Sp[3', obtained by subtracting the natural background noise [3 spectrum SpRN from the raw [3 spectrum. The net [3 spectrum Sp[3' is thus representative of the artificial [3 activity in the measured wall.

[0061] [Fig.7] represents spectra [3 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 232Th, 238U and 40K measured by gamma spectrometry on a wall considered to be uncontaminated. The measurement by gamma spectrometry made it possible to estimate activity levels of 15 Bq / Kg for 232Th, 22 Bq / Kg for 238U, and 485 Bq / Kg for 40K. These mass activities make it possible to simulate the natural component in the spectrum [3 measured by the detector. In [Fig.7], we have represented: the component due to 232Th in the spectrum [3; the component due to 238U in the spectrum [3; the component due to 40K in the spectrum [3; the component due to 235U in the spectrum [3, assuming natural enrichment of uranium: the mass of 235U corresponds to 0.72% of the mass of uranium;

[0062] In [Fig.7], a simulation of the spectrum [3 resulting from 234m Pa has also been shown, taking into account a homogeneous surface activity of 0.4 Bq.cm2 and an enrichment of 1% by mass in 235U (UB spectrum), and taking into account an enrichment of 8.16% by mass in 235U (UH spectrum).

[0063] 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.cm3.

[0064] We observe that: the natural component of the spectrum [3 is dominated by 40K, up to the energy 1311 keV, which corresponds to the maximum energy of the particles [3 emitted by 40 K; the measurement of the spectrum [3 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 spectrum [3 measured by the detector, taking into account mass activities, established a priori or measured, of the main natural emitting radioelements [3.

[0065] 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, makes it possible to eliminate the y component of the natural activity.

[0066] Figures 8A and 8B show simulations of spectra [3 measured by a detector as described in [Fig.l], 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 in 235U 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 234mPa, 234Th and 231Th are represented. 234mPa and 234Th are descendants of 238U, while 231Th is a descendant of 235U. 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 '.Bq '.cm2) and the x-axis corresponds to an energy level (MeV).

[0067] Figures 8A and 8B show that whatever the level of enrichment, the spectrum is dominated by the contribution of 234mPa, the latter being weaker at high enrichment ([Fig.8B]) than at low enrichment ([Fig.8A]). It is observed 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 234mPa. By spectral value, we mean a value measured from the spectrum. It may in particular be 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 noted NI.

[0068] 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 [3 per 234mPa. For example, the first energy range AE1 is [300 keV; 2500 keV]. Such an energy range makes it possible to collect the entire useful spectral content of the activity [3 of 234mPa. The first maximum energy Elmax may 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 narrower energy range within this interval may be suitable. An energy range extending from 300 keV, or from a first minimum Elmin value 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 = +^: the range considered is then [Elmin, + °°], with 300 keV < Elmin<1000 keV.

[0069] The first spectral value NI, in the first energy range AE1 = [Elmin, Elmax] previously defined, is then intended to be processed by a function of transfer, so as to be able to estimate a level of activity in 238U. The establishment of the transfer function is described later, in connection with [Fig. 11]. Taking into account the enrichment q then makes it possible to convert the activity in 238U into activity in U.

[0070] Figures 8A and 8B show another interesting aspect of the invention: a second energy range AE2 can be defined, between 0 keV and 300 keV, in which the spectrum [3 is influenced by 231Th, this radioelement descending from 235U. Thus, a second energy range AE2=[E2min, E2max] can be defined, 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, in particular at high enrichment.

[0071] A ratio N1 / N2 of the first and second spectral values ​​depends on the enrichment p. Thus, the experimental measurement of this ratio can be used to estimate the enrichment p of uranium.

[0072] When the enrichment p of uranium in 235U is unknown, the first spectral value NI makes it possible to estimate 238U, 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 ​​makes it possible to estimate the enrichment p. The activity in 238U can then be used to estimate the activity in U. The enrichment in 235U can also be determined experimentally by gamma spectrometry.

[0073] Uranium contains a-emitting isotopes, primarily 234U or 238U. In order to estimate the contribution of a particles in the measured spectrum [3, a 244Cm source, with an activity of 2800 Bq, was placed 3 mm from the detector material. 244Cm emits a particles with an energy greater than 5700 keV, which is much higher than the energies of the a particles emitted by U isotopes, for example 4775 keV for 234 U. [Fig. 9] shows the spectrum measured over an acquisition period of 900s. It can be seen that the contribution of the a particles is limited to low energies, less than 270 keV. Thus, the spectral value, in the first energy range, can be considered as unaffected by the a particles emitted by the uranium isotopes.

[0074] The a particles detected by the detector may be likely to influence the spectral value N2 in the second energy band AE2. 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 [3.

[0075] In [Fig.9], the few shots observed at an energy greater than 270 keV cor respond to y photons after subtraction of background noise, resulting from statistical fluctuations.

[0076] [Fig. 10] shows a spectrum [3 measured experimentally on a concrete wall contaminated with 1% enriched uranium. On this spectrum, the first energy range was materialized, between 300 keV and 2700 keV. The detector was an EJ200 detector (Eljen Technology), with a sensitive surface area of ​​2430 cm2 (493 mm on each side). The energy calibration of the spectrum, i.e. the correspondence between the value of the pulse amplitudes and the energy, was carried out using a 207Bi source, which emits electrons, at discrete energy values, by internal conversion.

[0077] In order to convert the spectral value, in the first energy range, into an activity value of 238U, it is necessary to take into account a transfer function. The transfer function can be obtained by modeling spectra [3 corresponding to known activities in 238U, and therefore in 234mPa. Different transfer functions can be determined, corresponding respectively to different hypotheses of distribution of the contamination in the analyzed object. The 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.

[0078] [Fig. 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 238U activity was distributed according to different depths. A unit activity of 238U was considered (1 Bq / g or 1 Bq / cm2 in the hypothesis of surface contamination). The detector model was validated in the laboratory by comparing spectra [3 respectively measured and modeled by exposing the detector to a standard source of 207Bi.

[0079] In the case of surface contamination, activity distributed over the first 10 pm of the wall was modeled. The transfer function is 1050s '.Bq '.cm2 ± 15%.

[0080] In the case of volumetric contamination, the maximum path, in the concrete, of the particles [3 emitted by 234mPa, which can be estimated at 2 mm, was taken into account. Given this low thickness, the contamination gradient was considered to be homogeneous.

[0081] In Figure 11, the y-axis corresponds to the value of the transfer function FT (unit s '.Bq '.cm2) and the x-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 IBq.g '.

[0082] If NI corresponds to the counting rate in the first energy band AE1, the activity A[238t7], in Bq.g 1 or in Bq.cm2, is obtained according to the expression:

[0083] A[238L / ] = (1)

[0084] The detector previously described was implemented on a gaseous diffusion uranium enrichment facility. The detection limit was such that:

[0085] j T\ _ k^+2xSD (2) LU - FTxTs

[0086] With

[0087] = 1.96: risk level of type $ for a 95% confidence interval;

[0088] Ts; acquisition duration, in seconds;

[0089] FT = transfer function, expressed in Bq.g 1 or in Bq.cm2

[0090] SD is the detection threshold, such that 100911

[0092] = 1.96: type a risk level for a 95% confidence interval;

[0093] n: ratio of the respective acquisition times of the background noise and the measurement

[0094] ; counting rate measured in the first energy band AE1, in a background noise measurement, on a wall considered uncontaminated. Unit counts per second.

[0095] Depending on the unit of the transfer function, the detection limit is expressed in Bq.g 1 or in Bq.cm2

[0096] 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 noise measurement. A contribution of natural activity in the spectrum was also taken into account, described in connection with [Fig.7]. The activities of the natural isotopes taken into account were 15 Bq / Kg for 232Th, 22 Bq / Kg for 238U, and 485 Bq / Kg for 40K. Each measurement includes a measurement with the aluminum screen and a measurement without the aluminum screen, of the same durations.

[0097] [Tables2] LD in Bq.cm-2 ., , ,, FT Ts ]s) Radionuclide ROI r60f (sq , , „ , (s ABqhcnF) go 180 300 600 1S00 >300-

[0098] Table 2

[0099] [Tables3] LD in Bq.g1 FT Ts [s) Radionuclide ROI rgoFts1) [s\Bq-.g[ 60 180 300 600 1800 Î3Q0- i 48 + 8% 149115% 3.4.10--2,4.10-2 1.5,10'2 1.1.10'2 6.1. 10' ■ z500]keV .........

[0100] Table 3

[0101] It is observed that the method makes it possible to obtain low detection limits, in reasonable acquisition times. The detection limits presented in Tables 2 and 3 concern 238U via the detection of the spectrum [3 of 234mPa. The more the enrichment increases, the more the quantity of 238U decreases relative to the total quantity of uranium. Also, if one wishes to maintain a low detection limit in uranium, the acquisition time must be increased.

[0102] Maps were made on walls of the gaseous diffusion uranium enrichment facility. [Fig.l2A] shows measured count rates (cps = counts per second) on surface elements of dimensions 50 cm x 50 cm. [Fig.l2B] shows the corresponding 238U activity levels. Since the 235U enrichment is known, U activity levels were estimated, these being shown in [Fig.l2C]. When the enrichment is not known, it can be determined by standard methods, for example by gamma spectrometry.

[0103] As previously described, in connection with Figures 8A and 8B, the enrichment q 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 taken into account extending from E2min=150 keV to E2max=300 keV. The second minimum limit E2min of 150 keV was determined so as to limit the influence of gamma radiation: between 150 keV and 300 keV, it was found that the contribution of gamma radiation in the spectrum [3 is stable, and can be easily subtracted by taking into account the measurement with the aluminum screen. Considering a second minimum energy E2min lower than 150 keV, the contribution of gamma radiation in the spectrum [3 can fluctuate more, due to the higher sensitivity of the plastic scintillator material to low energy photons.

[0104] [Fig.l3A] 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.

[0105] [Fig.l3B] 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.

[0106] In Figures 13A and 13B, the y-axis corresponds to the N1 / N2 ratio and the x-axis corresponds to the 235U enrichment, expressed in %. Figures 13A and 13B were established on the basis of models with the MCNP6 calculation code.

[0107] A correlation is observed between the enrichment and the N1 / N2 ratio. Thus, by applying an enrichment calibration function, as described in connection with Figures 13A or 13B, the 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.

[0108] It is specified that the use of spectrometry [3 to estimate an enrichment rate q can constitute an independent aspect of the invention.

[0109] 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 γ photons, which makes it possible to obtain more precise measurements: more precise quantification of the 238U activity or more exact enrichment value.

[0110] Furthermore, experience has shown that in the first energy band AEl , extending between 300 keV up 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.

[0111] [Fig. 14] summarizes the main steps of a method according to the invention.

[0112] Step 100: arrangement of a detector facing the object to be characterized

[0113] Step 110: acquisition of a spectrum, in the open configuration: this is the measured spectrum Sp.

[0114] Step 120: acquisition of a background noise spectrum Spbdf, in the closed configuration, or taking into account a spectrum acquired in the closed configuration.

[0115] 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 [3 raw Sp[3 spectrum.

[0116] Step 140: taking into account the natural activity [3 of the object. This step includes the following sub-steps: - 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. - Sub-step 142: estimation of a natural [3 SpRN] spectrum (or [3 RN] spectrum resulting from step 141.

[0117] Alternatively, step 140 includes a sub-step 143 of measuring a raw spectrum [3 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 spectrum [3

[0118] Step 140 is preferred, but optional.

[0119] Step 150: correction of the raw [3 spectrum, resulting from step 130, taking into account the natural [3 spectrum resulting from step 140. This step is optional. Step 150 makes it possible to obtain a net [3 spectrum (or corrected [3 spectrum) noted Sp[3'

[0120] Step 160: determination of a first spectral value NI in the first energy band AE1, and of a second spectral value N2, in the second energy band AE2 from the spectrum [3 net Sp[3' resulting from step 150 or from the spectrum [3 raw resulting from step 130.

[0121] Step 170: application of the enrichment calibration function to a ratio between the first spectral value and the second spectral value, so as to estimate the enrichment rate q in mass of 235U.

[0122] Steps 130 to 170 can be implemented by the processing unit 20.

[0123] The invention may be implemented on fuel enrichment or manufacturing installations.

Claims

Claims

1. Method for estimating a 235U enrichment of an object, using a detector, the detector being configured to form pulses when exposed to particles [3, the detector being connected to a spectrometric measurement circuit, configured to establish a spectrum, the spectrum corresponding to a histogram of the energy of the pulses formed during a measurement period, the method comprising the following steps: - a) arranging the detector facing the object and acquiring a measurement spectrum (Sp), the measurement spectrum being representative of an energy distribution of particles [3 emitted by 238U descendants and by 235U descendants; - b) from the measurement spectrum, optionally forming a corrected spectrum (Sp[3 ; Sp[3');- 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) from the measurement spectrum or the corrected spectrum, determination of a second spectral value in a second energy band (AE2) extending below a second maximum energy (E2max), the second maximum energy being less than or equal to 300 keV - e) calculation of a ratio between the first spectral value and the second spectral value; - f) application of a calibration function to the ratio determined during step e), so as to estimate the enrichment in 235U.;

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 particles [3 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 bl) is an aluminum screen whose thickness is greater than or equal to 3 mm or 4 mm.

4. A method according to claim any one of the preceding claims, wherein in step c), the first energy band extends to a first 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, wherein during step a), the distance between the detector and the object is: - greater than 1 mm or 5 mm; - and / or less than 20 mm or 10 mm.

6. Method according to any one of the preceding claims, in which the object is a naturally radioactive object, the method comprising: - (i) taking into account a natural activity of the object; - (ii) modeling a spectrum [3 of the natural activity of the object (SpRN) detected by the detector; - step b) comprises a correction of the measured spectrum using the spectrum [3 of the natural activity of the object, to obtain the corrected spectrum.

7. A method according to claim 6, wherein step (i) results from a gamma spectrometry measurement carried out on the object.

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

9. A method according to any one of claims 1 to 8, wherein the detector comprises a semiconductor material.

10. Device for estimating an enrichment in 235U of an object, the device comprising: - a detector (10) comprising a detector material (11) configured to form pulses under the effect of exposure to particles [3; - a spectrometry circuit (13), connected to the detector, and configured to form a histogram of the amplitude of the pulses detected by the detector during an acquisition period; - a processing unit (20), connected to the spectrometry circuit, and configured to implement steps b) to f) of a method according to any one of the preceding claims.

11. A device according to claim 10, wherein the detector comprises an organic scintillator material, the thickness of the organic scintillator material being between 3 mm and 10 mm.

12. A device according to claim 10, wherein the detector comprises a semiconductor material.

13. Device according to any one of claims 10 to 12, in which the detector is covered with an envelope (14) configured to absorb particles α with energy greater than or equal to 4.5 MeV or 5 MeV.

14. Device according to any one of claims 10 to 13, wherein the device comprises a movable screen (15), configured to pass from a closed configuration, in which the screen is interposed between the detector material (11) and the object (2), to an open configuration in which the screen leaves a space free between the detector material and the object.

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