Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
  • G01T1/16—Measuring radiation intensity
  • G01T1/167—Measuring radioactive content of objects, e.g. contamination
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T3/00—Measuring neutron radiation

Definitions

• the present invention relates to a method and a device for probing an object containing nuclear material.
• the latter is irradiated by means of a particle beam whose energy is sufficient to cause photofission of actinide elements contained in this nuclear material.
• Photofission is a nuclear fission, that is to say the separation of the nucleus of a heavy atom with release of a certain energy, caused by the incidence of energetic photons, notably on actinides such as uranium and plutonium. .
• the de-excitation of an impacted atom gives rise to the emission of fast neutrons, and to a much smaller quantity of delayed neutrons.
• a neutron detector makes it possible to detect the actinides present.
• the present invention finds a particularly interesting application in the field of the characterization of nuclear waste packages, in particular to determine their storage location.
• the term "package” means drums of concrete waste with a diameter of the order of a meter or more, or metal containers whose volume can reach several cubic meters.
• the probe particles used are either photons, neutrons or both.
• Photons usually of the gamma type, can be produced either by radioactive decay or by
• a beam of electrons of sufficient energy strikes a target generally in tungsten, and the passage of the electrons in the neighborhood very close to the atoms of the target gives rise to the "braking radiation" characterized by a photon gamma, of energy at most equal to the energy of the incident electron.
• Neutrons which constitute the other alternative, can be produced either directly by a neutron source, or from nuclear reactions, by bombardment of a Beryllium target constituting a converter.
• US 4 497768 (Caldwell) proposes to evaluate only the quantities of fissile or fertile material in a sample from a pulsed irradiation of gamma photons and neutrons.
• Document US Pat. No. 5,495,106 (Matsny) proposes an assembly for measuring possible contaminations in the ground from pulsed X-ray irradiation.
• US 5,838,759 proposes an assembly for measuring possible contaminations in the ground from pulsed X-ray irradiation.
• GDR Giant Dipolar Resonance
• N, Z and A conventionally refer to the total number of neutrons, protons and nucleons.
• the GDR can induce cross sections of a barn fraction, of the same order of magnitude as the standard nuclear reactions (strong interaction), for example neutronics, although it is of electromagnetic origin. It is therefore, at these photon energies, a physical phenomenon of importance.
• the photons 2 pass through the air to be sent on the package 3 to be probed. It is therefore the photons that are used as a probe.
• the detection is done through neutron detectors 4 placed around the package. Due to the importance of the "gamma flash" when sending the electron pack, the detectors 4 are blinded in two ways.
• the huge amount of gamma photons produced by Bremsstrahlung from the electron beam can create ionizations in neutron detectors.
• the conversion target having a high atomic number produces, under the effect of photon irradiation, parasitic neutrons in a quantity greater than the fast neutrons resulting from photofission.
• “Gamma flash” induces background noise several orders of magnitude higher than useful signals.
• a solution proposed by the document US4497768 would consist of firstly thermalizing the neutrons and then measuring the fast neutrons resulting from fission reactions induced by neutrons, that is to say about 0.5 ms to 2.5 ms after the flash, by surrounding the cadmium detectors to absorb slow thermalized neutrons. The immediate consequence is the generation of a very important neutron environment
• a US5495106 document proposes to use X-rays (actually gamma rays produced by Bremsstrahlung electrons impacting a target) to create photofission in the contaminants present in the soil: uranium, plutonium and Beryllium . Then he proposes to measure, up to 100 ms after the photon source, the neutrons that have been produced by these contaminants. It is therefore necessary to inhibit the detectors 4 during the "gamma flash", which limits this system to the measurement of delayed neutrons, arriving well after the flash. This limitation results in the impossibility of identifying the actinides present in the package, which constitutes a first limitation of this method.
• the package is probed globally because once created, the photons continue their journey in a straight line, without being able to be directed to a specific spot of the package. Fine localization of the waste inside the package is not possible in this system according to the prior art.
• the spectrum of these Bremsstrahlung photons ranges from zero to the maximum energy of the incident electron (rare case where the electron stops and radiates all its energy). This spectrum is for example illustrated in Figure 2 where a 50 MeV electron strikes a 5 mm thick tungsten converter. The density of photons decreases exponentially with energy, the majority of photons being emitted at low energy. In this example, each electron emits on average about forty photons of which only 0.6 are in the "useful" energy range for the GDR, that is to say between 10 and 20 MeV.
• the emitted photons interact with an actinide nucleus present in the target to be probed, those of them having the good energy will be able to excite a GDR dipolar giant resonance and possibly to fission the nucleus.
• neutrons are emitted during the de-excitation: they are the fast neutrons. But the fragments of fission are generally themselves radioactive, and in excited states. Some emit neutrons with a delay time with respect to the fission itself (thus depending on the lifetime of said fragment): these are the delayed neutrons.
• the detection of the amount of actinides is based on the measurement of the number of these delayed neutrons, after a burst of electrons has been sent by the accelerator to the target, producing a gamma flash.
• the amount of delayed neutrons produced by the photofission is proportional to the mass of actinides present, according to the linear relationship:
• the number of delayed neutrons emitted per second is proportional to the mass of fissionable elements (by photofission) and to the average current of electrons.
• the coefficient of proportionality depends on the photon efficiency, the transport and the attenuation of these photons and the cross sections of the various elements considered. It should be noted that the method according to document FR 2,764,383 (equivalent US6452992) makes it possible, by measuring the number of delayed neutrons, to deduce the total mass of the actinides in a target. It is therefore a very interesting measure because fast (from a few minutes to a few hours, once the package is in place) and non-destructive. However, it does not allow the direct identification of the different actinides.
• a second limitation of this method lies in the fact that it measures only the mass of actinides and not their activity.
• the delayed neutrons are emitted inside the package by number n * proportional to the intensity of the electron beam and the mass m 1 g actinides present in the package. After emission, they undergo repeated shocks and slowdowns, then broadcast to the outside of the package and through the ambient air before reaching a detector. This process causes the number of delayed neutrons detected to be a small fraction of the neutrons emitted, mainly because of the absorption in the package. The proportion of delayed neutrons detected will be all the more important as the distance to be traveled inside the waste package will be small.
• the coefficient of proportionality integrates the geometry of the device (solid angle) and the efficiency of the detector used (number of shots per neutron arriving on the detector). Strictly speaking, the coefficient ⁇ and the coefficient of proportionality depend on the energy of the neutron. But in practice we can admit that the spectrum in delayed neutron energy is identical to that of the classical (neutron-induced) fission of average energy around 450 keV. This eliminates the impact of the spectral distribution which may differ depending on the type of actinide present in the waste package. Therefore, there are only two unknowns left: the quantity m of radioactive material, and its location inside the package that is expressed by its distance d to a given detector. To determine these two unknowns, one uses either several detectors or several measures by for example turning the package around its central axis. In order to improve the accuracy of the measurements, both can be made together.
• shot S / S can then be written by considering A as a parameter dependent on prior measurements (calibration). It establishes well that the number S of pulses per second measured by the detectors is proportional to the mass m of actinides present in the package.
• Each detector will therefore be calibrated to have the corresponding exact value of its coefficient of proportionality A. Since the thickness of material traversed d is unknown, it is possible by using several detectors (theoretically two situated at 180 ° on either side of the package should be sufficient), deduce the two unknowns d and m. Of course, in the very likely case where several actinides located in several places emit, one would have to sum up their different contribution.
• a third limitation of this method resides in the noise which limits the sensitivity of the system to a value too high to allow to classify the package into a category. storage and determine where it should be stored.
• the maximum admissible activity (for the isotope considered) for the storage of a TFA package is 100 Bq / g.
• the radioactivity of 239 Pu is 2.284 GBq / g. It follows that the TFA classification limit for this type of package must be such that
• the neutrons inside the waste drum have a poorly controlled path, which does not allow to reach all the zones of this drum, and on the other hand the enormous The amount of neutrons (produced by the impact of the photons on the target) blinds all the detectors, to the point of making it impossible to measure fast neutrons. Only delayed neutrons can be measured effectively, and the sensitivity of measurements is greatly affected. Finally, the efficiency of the process is extremely low, a very small portion of emitted photons having sufficient energy to trigger a photofission reaction.
• the distribution throughout the useful section is a priori uniform, which is an important advantage.
• the penetration depth of the photons is low, and the vast majority of them, insufficiently energetic, stop in the very first centimeters, which does not allow to know the composition of the drum in the whole of its thickness.
• the photons produced by the impact of electrons go in a straight line (according to the direction of incidence of the electron that created them), the entire photon beam is nevertheless affected by some divergence, sufficient to blind the neutron detector.
• the method according to FR 2,764,383 (equivalent US6452992) has the following three disadvantages: firstly a measure of the mass of the actinides and not their activity, secondly the impossibility of identifying the actinides present finally, the impossibility of lowering the sensitivity limit sufficiently to determine the category in which the waste must be stored.
• the present invention aims to improve the systems of the prior art by providing a device capable of effectively and simply detect the prompt and delayed neutrons. It is known that the ratio between the fast neutrons and the delayed neutrons varies greatly with the type of actinide considered.
• the present invention allows the measurement of fast neutrons in addition to the measurement of delayed neutrons, allows to determine this report is therefore to determine the type of actinides present in the package.
• the present invention also aims an active method having a high sensitivity of detection, and able to focus this sensitivity on a specific area inside the package. It also aims to accurately measure any amount of actinides present in a large object.
• At least one of the above objectives is achieved with a novel method for probing an object containing potentially radio-transmitter elements.
• This object is irradiated by means of a particle beam whose energy is sufficient to cause photofission of the radio-transmitter elements of the object.
• the neutrons produced by photofission are measured by means of at least one neutron detector.
• the particles that probe the object by photofission are electrons directly irradiating the object, and the conversion of the electrons into photons capable of generating the photofission is performed directly by the object to be probed.
• the electrons are used directly to strike the object to be probed, and no heavy metal target, such as tungsten, is added to the interior of this object to convert the electrons in photons able to generate the photofission of the potentially radiating elements.
• the object to be probed generally consists of a peripheral container, and a content that occupies the central part.
• the container is generally of absorbent concrete, thick, and the content comprises nuclear material.
• the invention is particularly advantageous since it gives rise to brake radiation mainly at the concrete container.
• the constituents of the concrete have a low atomic number, on average around 20 (the heaviest component, the barium of atomic number 56 being in a very small amount), so that the photofission reaction does not create substantially direct neutrons to blind the detectors.
• the measurement is favored by arranging around the object to be probed or in its container a material having a sufficiently high atomic number so that the Bremsstrahlung reactions are sufficient in number for the photofission, and sufficiently low to limit the production of neutrons.
• this range can be located, without limitation, between 10 and 50.
• the electron-photon conversion takes place on a high atomic number material, such as tungsten, in a few mm.
• the invention makes it possible to make full use of the electron directivity to explore the contents of the object by choosing a number material container.
• sufficient part is meant an amount sufficient so that the resulting photofission reactions can be measured with a signal-to-noise ratio compatible with the needs.
• the above features combine to locate the photofission reactions as close as possible to the nuclear materials of the package, and, if desired, to distribute them relatively homogeneously over the entire volume of the nuclear material to be probed.
• the process according to the invention has many advantages.
• a first advantage is that the electrons can be manipulated more easily than photons: we can vary the size of the electron beam so as to vary the accuracy of the irradiation, we can focus the electron beam on a part of the nuclear material, we can transport the electrons, deflect them, carry out a well targeted or delimited scan, etc ... We can thus make sure that the electron beam sweeps all or part of the nuclear material, or even the totality of the object to be probed, which may be a cask for example. The electron beam is then moving relative to the object. In the same way, the object can be moved in translation and / or rotation relative to the electron beam so that the electron beam radiates all or part of the contents of the package.
• the electron beam that irradiates the package is pulsed, and the at least one neutron detector detects the delayed neutrons, generated by the photofission, after the resulting pulsed flash of the electron beam.
• Said at least one neutron detector can also detect the prompt neutrons emitted during the electron beam pulse.
• the nuclear material can be irradiated by transporting the electron beam via a vacuum line.
• particles emitted by the scanned parts are simultaneously detected so as to locate the nuclear material within the object.
• the second advantage is that the electrons have a penetrating power in the material. As a result, they can get as close as possible to the actinides present in the package, and thus increase the chances of a nuclear reaction. This results in a considerable improvement in energy efficiency and greater measurement sensitivity.
• the third advantage is that the direct use of electrons makes it possible to minimize the noise induced by gammas during the first impact (Bremsstrahlung gamma flash) which blinds the neutron detectors.
• the electron beam irradiates the nuclear material in the form of at least one pulse, it is possible to detect and measure fast neutrons emitted during the pulse. It is also possible to detect and measure the delayed neutrons emitted after the pulse.
• the present invention thus allows both the measurement of the fast neutron flux and the measurement of the delayed neutron flux.
• the second measurement makes it possible to determine the mass of actinides as taught in the documents FR 2 764383 (equivalent US6452992) or US4497768, but with a much better resolution (absence of noise, better distribution of the photon-probe).
• the ratio of these two measurements makes it possible to further determine the isotopic composition of actinides present in the material (elements radio transmitters). Indeed, as already seen, the various actinides have a fast neutron report on different delayed neutrons. The experimental measurement of this ratio will therefore be a characteristic of the isotope considered.
• the total amount of actinides present in the radio - transmitter elements from the fast neutrons and the measured delayed neutrons can also be determined.
• a device for probing an object containing radio-transmitter elements comprises means for irradiating said object by means of a particle beam whose energy is sufficient to cause a photofission of the radio-transmitter elements of the object, and at least one neutron detector for measuring the neutrons produced by photofission.
• said particles are electrons directly irradiating the nuclear material, the conversion of the electrons into photons able to generate the photofission being carried out directly by the object to be probed.
• the electron beam is transported via a vacuum line whose exit window is brought into contact with the object.
• the means for detecting fast neutrons are arranged behind the object when the electron beam arrives from the front of this object.
• FIG. 1 is a simplified schematic view of a device characterizing nuclear waste according to the prior art
• FIG. 2 is a graph illustrating the photon spectrum emitted by Bremsstrahlung for a 50 MeV energy electron on a 5 mm thick tungsten target, according to the prior art
• FIG. 3 is a simplified schematic view of a nuclear material characterization device according to the present invention.
• FIG 1 there is a large object to be probed 6 containing a very small amount of nuclear material 7, an actinide.
• This material 7 is located at a precise point within the object 6.
• This beam 13 is transported by a vacuum line 9 which is a vacuum tube connecting the output of the accelerator 8 to the object to be probed 6.
• a scanning device 10 is installed.
• magnetic deflection elements techniques well known in the field of accelerators
• An exit window is also provided at the end of the vacuum line 9. This window may come into contact, for example with the scanning device 10, with the object 6 so as to avoid the passage of the electron beam through the air.
• the present invention makes it possible to measure the prompt neutrons emitted during the electron pulse, while continuing to measure the delayed neutrons coming from the object to be probed, thus from a few ms to 100 ms, or even more after the pulse.
• the devices are identical to those described in documents FR 2 764383 (equivalent US6452992) or US4497768.
• detectors 11 of delayed neutrons are arranged around the object 6.
• the use of electrons as probe beam makes it possible to overcome the background noise of the "gamma flash". Indeed, the electrons striking only a small part of the surface of the object, will end (as well as all the various particles produced by the successive reactions in the object) either by being backscattered (towards the back), or to be absorbed into the object itself. It is thus possible to measure the prompt neutrons emitted during pulsing with fast neutron detectors 12. It is shown that the optimal position of these detectors from the point of view of the signal-to-noise ratio is to place them behind the object to be probed.
• the detected fast neutron signal (much more intense than that of the delayed neutrons mentioned above) is used to deduce the amount of nuclear material present in the package.
• the delayed neutron signal is also measured between two pulses. Moreover, it is the conjunction of two simultaneous information (delayed signal and delayed signal) that allows, by performing their ratio, to go back to the isotope of the actinide tested.
• a movement of the object (vertical and / or in rotation around the vertical axis) makes it possible to precisely locate the place where the material 7 is in the object 6. The precision is determined by the size of the beam of 13.
• the size of the electron beam can be as small as desired (in the limit of the emittance of the beam), simply placing a proper focus (not shown) in the vacuum transport line 9.
• Another feature of the present invention is the pulse width of the probe beam. While with photons or neutrons, the pulses used are generally very intense (up to 200 mA) and very short (generally less than 4 ⁇ s), it is shown that in the present case, long pulses for example 4 ms, which is a thousand times higher, could as well be used with intensities that can be much lower, some microamperes are enough.

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