СПОСОБ ПРОВЕРКИ ОДНОРОДНОСТИ СОСТАВА ТОПЛИВНОГО СТЕРЖНЯ

Abstract

Настоящее изобретение относится к способу (400) проверки однородности состава в отношении представляющего интерес делящегося изотопа химического элемента вдоль главной оси (X) тестируемого ядерного топливного стержня (15); способ содержит следующее: выполняют первое сканирование (401) тестируемого стержня (15) вдоль главной оси (X) с использованием первого детектора (20) гамма-излучения без предварительного возбуждения, при этом первое сканирование формирует первый гамма- профиль; выполняют второе сканирование (402) тестируемого топливного стержня (15) вдоль главной оси (X) с использованием второго детектора (25) гамма-излучения после возбуждения с помощью источника (30) нейтронов, при этом второе сканирование формирует второй гамма-профиль; получают третий гамма-профиль путем вычитания первого профиля из второго профиля; и оценивают (403) однородность состава в отношении представляющего интерес делящегося изотопа вдоль главной оси (X) тестируемого топливного стержня (15) по третьему гамма-профилю.

Description

[0001] TITLE: Method for controlling the uniformity of the composition of a nuclear fuel rod

[0002] The present invention relates to a method for controlling the uniformity of the composition of a fissile isotope of interest of a chemical element along a main axis of a nuclear fuel rod to be tested.

[0003] The present invention also relates to the corresponding control device.

[0004] The present invention finally relates to a standard nuclear fuel rod.

[0005] Nuclear reactors use the energy released by nuclear fission reactions that occur when fissile nuclear fuel is bombarded with neutrons.

[0006] Nuclear fuel generally contains uranium enriched in the isotope 235 at a content that depends on the subsequent use and the enrichment process. This content is, for example, between 3% and 5%. The composition of nuclear fuel can, however, vary depending on the technology used in the nuclear reactor and / or the sources of nuclear fuel used to supply this reactor.

[0007] The preparation of nuclear fuel most often includes a pelletizing step by sintering a nuclear fuel powder, comprising for example uranium oxides or mixed uranium and plutonium oxides.

[0008] Pelletizing is followed by a penciling step, during which the nuclear fuel pellets are placed in a cylindrical metal casing to form a nuclear fuel rod. A rod typically has a diameter of around 10 mm and a length of around 5 m.

[0009] The nuclear fuel rods are finally arranged in a network of appropriate meshes, to form a mechanically solid fuel assembly, which can be introduced into the nuclear reactor for combustion.

[0010] Proper control of the nuclear reaction in the nuclear reactor, particularly in terms of the energy output, temperature management within the nuclear reactor, and efficiency of use of the fissile material, results in high requirements on the quality of the pellets, rods and fuel assembly.

[0011] In particular, it is important that the fissile isotope composition is as uniform as possible along the fuel rods. Fuel rods therefore generally undergo a step of controlling their fissile isotope composition along their entire length, at the end of which they are either qualified for insertion into an assembly or scrapped.

[0012] Patent application US4822552 describes a method for determining the enrichment uniformity of a nuclear fuel rod. The method comprises moving the fuel rod along a linear path; repeatedly detecting natural, i.e., unexcited, gamma radiation of successive sections of the rod as the rod moves; and monitoring the average enrichment value of the fuel rod to detect deviations from a specified enrichment value, and / or detecting deviations in enrichment values ​​of successive sections of the rod that are greater than a predetermined percentage.

[0013] The method described by document US4822552 is described as particularly suitable for fuels comprising neutron absorbers, for which the methods of the prior art based on detection of gamma radiation induced by a neutron source are inapplicable.

[0014] However, in some cases, particularly if the fuel rod is prepared at least in part from recycled uranium, the inventors have found that the fissile isotope composition along the rod is not always correctly assessed using a passive method such as that described in document US4822552, so that the conclusions on the homogeneity of the enrichment may be wrong in a significant fraction of cases.

[0015] The aim of the invention is then to propose a determination method making it possible to control the uniformity of the composition in a fissile isotope of interest of a chemical element along a main axis of a nuclear fuel rod to be tested with a good level of confidence whatever the origin of the fuel present in the rod and / or whatever its components other than the fissile isotope, including in the case where the rod to be tested is prepared from recycled fissile isotope.

[0016] To this end, the invention relates to a method for controlling the uniformity of the composition of a fissile isotope of interest of a chemical element along a main axis of a nuclear fuel rod to be tested, the method comprising the following steps:

[0017] - first scan of the pencil to be tested along the main axis using a first gamma radiation detector without prior excitation of the pencil to be tested, the first scan producing a first gamma profile of the pencil to be tested; - second scan of the pencil to be tested along the main axis using a second gamma radiation detector after excitation of the pencil to be tested by means of a neutron source, the second scan producing a second gamma profile of the pencil to be tested;

[0018] - obtaining a third gamma profile of the pencil to be tested by subtracting the first profile from the second profile;

[0019] - evaluation of the uniformity of the fissile isotope composition of interest along the main axis of the test rod from the third gamma profile.

[0020] The first profile is obtained without prior excitation of the pencil. It is therefore representative of the spontaneous gamma emission along the pencil, due to all the spontaneous gamma emitters that are present in each successively scanned pencil segment.

[0021] The second profile is obtained with prior excitation of the rod by a neutron source. The absorption of neutrons by the fissile nuclei present in the rod leads to the fission of these nuclei and to the emission of gamma radiation by the excited nuclei produced, radiation not observed in the first profile. Simultaneously, the spontaneous gamma-emitting nuclei emit gamma radiation in the same way as for the first scan.

[0022] Obtaining the third profile by subtracting the first profile from the second profile therefore makes it possible to get rid of the signal component not representative of the fissile isotope of interest in the second profile and thus to draw correct conclusions on the uniformity of the composition of the fissile isotope of interest along the rod, even in the presence of spontaneous gamma-emitting radionuclides.

[0023] This is particularly advantageous when the rod is a uranium rod including recycled uranium.

[0024] The fissile isotope of interest is then uranium 235. The rod then also includes uranium 232, which is a radioactive decay product of plutonium 236 formed during combustion prior to the recycling of uranium, and which is a spontaneous gamma emitter.

[0025] According to other advantageous aspects of the invention, the method comprises one or more of the following characteristics, taken individually or in all technically possible combinations:

[0026] - obtaining the first profile comprises a step of correcting measurements of the first detector obtained during the first scan by applying a multiplicative correction factor characteristic of the assembly formed by the first detector and the second detector; - the method comprises, before the first scan and the second scan, a determination of the correction factor, this determination comprising:

[0027] * the supply of a standard nuclear fuel rod, comprising the fissile isotope of interest and at least one other isotope of a spontaneous gamma-emitting chemical element, and whose compositions in fissile isotope of interest and gamma-emitting isotope along a main axis of the standard rod are known;

[0028] * the production of a first reference gamma profile by a first preliminary scan of the standard pencil using the first detector without prior excitation of the standard pencil and of a second reference gamma profile by a second preliminary scan of the standard pencil using the second detector after prior excitation of the standard pencil by means of the neutron source;

[0029] * providing a value of the correction factor from the first reference gamma profile and the second reference gamma profile, as well as the compositions of the fissile isotope of interest and the gamma-emitting isotope along the main axis of the standard rod;

[0030] - the scan performed earliest among the first pre-scan and the second pre-scan and the scan performed earliest among the first scan and the second scan are separated by a duration less than a predetermined calibration period;

[0031] - the test rod contains recycled nuclear fuel;

[0032] - the fissile isotope of interest is uranium and the gamma-emitting isotope is uranium;

[0033] - for the second scan, the neutron source includes californium;

[0034] - the nuclear fuel is chosen from enriched non-recycled uranium, enriched recycled uranium and their mixtures; and

[0035] - the standard pencil consists of at least two pellets comprising uranium, the uranium content of at least one first pellet being known and equal to a nominal composition and the uranium content of at least one second pellet being known and different from the nominal composition;

[0036] The invention also relates to a device for controlling the uniformity of the composition of a fissile isotope of interest of a chemical element along a main axis of a nuclear fuel rod to be tested, the device comprising:

[0037] - a first gamma radiation detector, configured to produce a first gamma profile of the pencil to be tested without prior excitation of the pencil to be tested at the end of a first scan of the pencil to be tested along the main axis;

[0038] - a second gamma radiation detector comprising a neutron source and configured to produce a second gamma profile of the test rod at the end of a second scan of the test rod along the main axis after excitation of the test rod by means of the neutron source;

[0039] - a data processing device configured to determine a third gamma profile of the test rod by subtracting the first profile from the second profile and evaluating the uniformity of the fissile isotope composition of interest along the main axis of the test rod from the third gamma profile.

[0040] According to other advantageous aspects of the invention, the device comprises one or more of the following characteristics, taken individually or in all technically possible combinations:

[0041] - the device further comprises a standard nuclear fuel rod, comprising the fissile isotope of interest and at least one other isotope of a spontaneous gamma-emitting chemical element, and the compositions of which in fissile isotope of interest and in gamma-emitting isotope along a main axis of the standard rod are known, the data processing device being further configured to determine a value of a correction factor from a first reference gamma profile of the standard rod produced by the first detector without prior excitation of the standard rod and from a second reference gamma profile of the standard rod produced by the second detector after prior excitation of the standard rod by means of the neutron source, as well as the compositions of fissile isotope of interest and of gamma-emitting isotope along the main axis of the standard rod,the device being configured to multiply measurements obtained during the first scan by the correction factor for the production of the first profile; and,

[0042] - the first detector comprises means for moving the pencil to be tested along a first scanning axis and the second detector comprises means for moving the pencil to be tested along a second scanning axis different from the first scanning axis.

[0043] The invention also relates to a standard nuclear fuel rod comprising:

[0044] - at least one pellet comprising a fissile isotope of interest of a chemical element in a first known content and free of gamma-emitting radioisotope, and

[0045] - at least one pellet comprising the fissile isotope of interest of a chemical element in a second known content and at least one gamma-emitting radioisotope of interest in a third known content, the positions of each of the pellets along a main axis along which the standard rod extends being known. The invention will appear more clearly on reading the description which follows, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0046] [Fig 1] Figure 1 schematically represents a device for controlling the uniformity of the composition of a fissile isotope of interest of a chemical element along a main axis of a nuclear fuel rod to be tested according to the invention.

[0047] [Fig 2] Figure 2 is an example of the first profile (curve A2), second profile (curve B2) and third profile (curve C2) obtained for the nuclear fuel rod shown schematically in the lower part of Figure 2, the excitation being carried out to obtain the second profile with a neutron source of approximately 500 pg of 252 Cf. The counting rate N is plotted on the y-axis, the abscissa X represents the position along the tested rod.

[0048] [Fig 3] Figure 3 is an example of the second profile (curve B3) and third profile (curve C3) obtained for the pencil of Figure 2, the excitation being carried out to obtain the second profile with a neutron source of approximately 1000 pg of 252 Cf. The counting rate N is plotted on the y-axis, the abscissa X represents the position along the tested rod.

[0049] [Fig 4] Figure 4 represents in the form of a flowchart a particular embodiment of the method for controlling the uniformity of the composition of a fissile isotope of interest of a chemical element along a main axis of a nuclear fuel rod to be tested according to the invention.

[0050] [Fig 5] Figure 5 is an example of the first profile (curve A5), second profile (curve B5) obtained and third profile (curve C5) expected for the standard pencil shown schematically in the lower part of Figure 5, the excitation being carried out to obtain the second profile with a neutron source of approximately 500 pg of 252 Cf. The counting rate N is plotted on the y-axis, the abscissa X represents the position along the pencil to be tested.

[0051] The device 10 for controlling the uniformity of the composition in a fissile isotope of interest of a chemical element A along a main axis X of a nuclear fuel rod 15 to be tested, one embodiment of which is shown schematically in FIG. 1, comprises a first gamma radiation detector 20, a second gamma radiation detector 25 comprising a neutron source 30 and a data processing device 35 exchanging data with the first detector 20 and the second detector 25. The rod 15 comprises a metal casing, for example cylindrical, in which nuclear fuel pellets are placed.

[0052] The composition of pencil 15 may or may not be known a priori.

[0053] The fissile isotope of interest is advantageously uranium 235 ( 235 U). In this case, chemical element A is therefore uranium (92U).

[0054] Pencil 15 may comprise non-fissile isotopes of chemical element A, including gamma-emitting radioisotopes of that chemical element A.

[0055] In the case where the fissile isotope of interest is uranium 235 ( 235 U), the rod 15 advantageously comprises recycled uranium comprising uranium 232 ( 232 U).

[0056] Pencil 15 may also include other chemical elements than element A, in particular radioactive decay products of uranium 232.

[0057] The first detector 20 comprises means for moving the pencil 15 with a given speed of movement between an input 20A and an output 20B of the first detector 20, as well as means for detecting and counting the gamma photons emitted by a fraction of the pencil 15 which is located in a detection zone 20C inside the first detector 20.

[0058] The means for moving the pencil 15 are advantageously configured to move the pencil 15 continuously.

[0059] The means of detection and counting are for example a scintillation detector.

[0060] The first detector 20 is configured to provide a first gamma profile of the pencil 15.

[0061] The first profile comprises, for each fraction of rod 15 successively passed through the detection zone 20C, a counting rate N of gamma photons emitted spontaneously (i.e. without prior excitation) by the respective fraction of rod 15.

[0062] An example of the first profile can be observed on curve A2 of Figure 2, obtained for a model 40 rod comprising periodically alternating groups of non-recycled uranium 40A pellets enriched to 5.0% uranium 235 and groups of recycled uranium 40B pellets enriched to 4.0% uranium 235.

[0063] Another example of the first profile can be observed on curve A5 of figure 5, obtained for a standard pencil 50 comprising alternating groups of non-recycled uranium pellets 50A enriched to 4% in uranium 235 and groups of recycled uranium pellets 50B enriched to 4% in uranium 235.

[0064] In a particular embodiment, the counting rate N of the first profile corresponds to the number of gamma photons in a first energy range. The first energy range is advantageously chosen to comprise one or more gamma emission lines of a gamma-emitting radioisotope present or likely to be present in the rod 15.

[0065] For example, if the rod 15 comprises or is likely to comprise uranium 232, the first energy range is advantageously [0.2 MeV]. This first energy range comprises lines of several decay products of uranium 232, including thallium 208 ( 208 TI), lead 212 ( 212 Pb) and Bismuth 212 ( 212 Bi), which are the main contributors to the natural gamma spectrum of uranium-232.

[0066] Advantageously, each fraction of pencil 15 has a length of the order of a few millimeters.

[0067] In the example of figure 1, the second detector 25 is placed downstream of the first detector 20 in the direction of movement of the pencil 15.

[0068] The second detector 25 comprises means for moving the pencil 15 with a given movement speed between an input 25A and an output 25B of the second detector 25.

[0069] The means for moving the pencil 15 are advantageously configured to move the pencil 15 continuously.

[0070] The second detector 25 comprises a neutron source 30 near the input 25A, configured to excite the pencil 15 before the detection of gamma photons.

[0071] The neutron source is advantageously chosen to induce fission reactions of the fissile isotope of interest of chemical element A.

[0072] The neutron source is advantageously made up of 252 Cf. This arrangement is particularly advantageous in the case where the fissile isotope of interest is uranium 235.

[0073] Other neutron sources may be considered, including for example an Americium-Beryllium (Am-Be) source or a neutron generator.

[0074] The second detector 25 comprises, downstream of the source, means for detecting and counting the gamma photons emitted by a fraction of the pencil 15 which is located in a detection zone 25C inside the second detector 25.

[0075] The means of detection and counting are for example a scintillation detector.

[0076] The second detector 25 is configured to provide a second gamma profile of the pencil 15.

[0077] The second profile comprises, for each fraction of rod 15 successively passed through the detection zone 25C, a counting rate N of the gamma photons emitted by the respective fraction of rod 15. In the second detector 25, the emission is possibly both spontaneous and induced by excitation by means of the neutron source 30. No discrimination is made between the two emission modes in the second detector 25.

[0078] An example of a second profile can be observed on curve B2 of figure 2 and curve B3 of figure 3, obtained for a model 40 pencil, or on curve B5 of figure 5 for the standard 50 pencil.

[0079] In a particular embodiment, the counting rate N of the second profile corresponds to the number of gamma photons in a second energy range.

[0080] The second energy range is advantageously chosen to include one or more gamma emission lines of at least one fission product of the fissile isotope of interest present in the rod 15.

[0081] For example, if the rod 15 comprises uranium 235, the second energy range is advantageously [0.9 MeV], preferably [75 keV, 6 MeV],

[0082] This second energy range includes lines from several fission products of uranium 235, including krypton 93 ( 93 Kr), rubidium 93 (93 Rb) and yttrium 93 ( 93 Y), which are important contributors to the neutron activation-induced gamma spectrum of uranium-235, in addition to at least one line of a uranium-235 fission product, notably Strontium, iodine.

[0083] The first detector 20 and the second detector 25 are configured to exchange data with the data processing device 35.

[0084] In particular, the data processing device 35 is configured to receive the first (respectively second) profile of the first (respectively second) detector 20 (respectively 25).

[0085] The data processing device 35 is configured to generate a third profile by subtracting the first profile from the second profile.

[0086] An example of a third profile can be seen in curve C2 of Figure 2, obtained for the model 40 pencil from the first profile (curve A2) and the second profile (curve B2).

[0087] The data processing device 35 is configured to provide an assessment of the uniformity of the fissile isotope composition of interest along the rod to be tested from the third gamma profile. The method for controlling this uniformity is detailed below.

[0088] The method 400 for controlling the uniformity of the fissile isotope composition of interest along the rod to be tested from the third gamma profile is schematically represented in FIG. 4. The method 400 comprises a first scan 401 of the rod 15 using the first detector 20 without prior excitation of the rod 15 to be tested to obtain a first profile of the rod 15.

[0089] For this, the pencil 15 is moved at constant speed in the first detector 20 between its input 20A and its output 20B.

[0090] The first profile, comprising a counting rate N of the gamma photons emitted for each of the successive fractions of the pencil 15, is thus produced by the first detector 20.

[0091] The count rate N of the first profile is for example the number of gamma photons emitted in the first energy range.

[0092] The method 400 comprises a second scan 402 of the pencil 15 using the second detector 25.

[0093] For the second scan 402, the pencil 15 is moved at constant speed in the second detector 25 between its input 25A and its output 25B.

[0094] Excitation of rod 15 by means of neutron source 30 causes fission reactions of a small fraction of the nuclei of the fissile isotope of interest, so that fission products of the fissile isotope are formed.

[0095] At least a portion of these fission products are possibly gamma emitters, so that the gamma emission of a fraction of the rod 15 which passes after excitation into the detection zone 25C includes in superposition a possible emission induced by the excitation and a possible spontaneous emission, for cases where this fraction of the rod 15 would include gamma emitting radioisotopes before excitation.

[0096] For example, if rod 15 comprises uranium 235, spontaneous emission is observed. If the uranium 235 comes from recycling, rod 15 also comprises other radioisotopes which are sources of gamma emission induced by the excitation which will be observed in superposition with the spontaneous emission of uranium 235.

[0097] At the end of the second scan 402, the second profile of the pencil 15 is obtained.

[0098] The second profile includes a counting rate N of gamma photons emitted for each of the successive fractions of the pencil 15.

[0099] The count rate N of the second profile is for example the number of gamma photons emitted in the second energy range.

[0100] It will be noted that the order in which the first and second profiles are obtained has no impact on the method 400, and therefore that the second scan 402 can be carried out after the first scan 401 as shown in FIGS. 1 and 4, or before the first scan 401. Advantageously, if the first scan 401 is carried out after the second scan 402, the two scans 401 and 402 are separated by a predetermined duration, greater than or equal to a deactivation duration of a fraction of the rod 15 which was excited for the second scan 402.

[0101] In a particular embodiment, the first and second scans 401, 402 are carried out by means of a first detector 20 and a second detector 25 whose axes of movement of the pencil 15 for the scan are different.

[0102] The first profile and the second profile are transmitted respectively by the first detector 20 and the second detector 25 to the data processing device 35.

[0103] The data processing device 35 then subtracts the first profile from the second profile to produce the third gamma profile of the pencil 15.

[0104] The data processing device 35 can then provide a result of the control of the homogeneity of the composition of the fissile isotope of interest along the rod 15 to be tested from the third gamma profile.

[0105] For example, the processing device 35 can test whether the relative or absolute deviation between each of the count rates of the third profile for each fraction of the pencil 15 tested and the average value of these count rates is less than a threshold value.

[0106] If so, the result of the control is positive: the composition of the fissile isotope of interest is evaluated as homogeneous along the rod to be tested, to the precision corresponding to the chosen threshold value.

[0107] If not, the result of the control is negative: the composition of the fissile isotope of interest of at least one fraction deviates excessively from the average composition of the tested rod.

[0108] Advantageously, the result includes the location of each of the pencil fractions whose relative or absolute deviation from the mean is greater than the threshold value.

[0109] The method 400 according to the invention makes it possible to control the homogeneity of the composition of fissile isotope of interest along the rod 15 as well as or better than the methods of the prior art in all cases of rods 15 to be tested.

[0110] On the one hand, in the case where the rod 15 does not comprise a gamma-emitting radioisotope, after possible filtering of the ambient noise, the method provides a third profile identical to the second profile. The conclusions on the uniformity of the enrichment of the rod 15 will therefore be similar to those of the methods of the prior art implementing only an active scanning step.

[0111] On the other hand, in the case where the rod 15 comprises a gamma-emitting radioisotope, an example of which is shown in FIGS. 2 and 3, the second profile may comprise peaks corresponding to particularly high counting rates for the rod fractions in which the gamma-emitting radioisotope is located (fractions 40B for FIGS. 2 and 3). If the uniformity of the composition of the rod 15 in fissile isotope of interest is only controlled from the second profile, as in the methods of the prior art implementing only an active scanning step, the conclusions may be erroneous.

[0112] Thus, in the case of Figure 3, if we consider that the counting rate N of the zones 40A corresponds to an enrichment in accordance with the specifications, it is possible to conclude, on the basis of the second profile taken alone (profile B3) corresponding to the active method of the prior art, that the enrichment of the zones 40B is compliant and therefore that the rod 15 can be qualified for implementation in a fuel assembly. This conclusion of the method of the prior art is inadequate since the zones 40B are less rich in uranium 235 (enrichment 4.0%) than the zones 40A (enrichment at 5.0%). The amplitude of the gamma emission induced by the excitation is much greater in the case of Figure 3 than in Figure 2, the spontaneous gamma emission remaining independent of the excitation. Consequently, the spontaneous gamma emission is masked by the induced gamma emission and not observable on the profile B3.

[0113] It is then possible to conclude, on the basis of the second profile taken alone and corresponding to the active process of the prior art, that the enrichment is perfectly uniform since the profile B3 is flat. The active process of the prior art would therefore conclude that the rod 40 can be qualified for use in a fuel assembly. This conclusion is erroneous since the 40B zones are less rich in uranium 235 (4.0% enrichment) than the 40A zones (5.0% enrichment).

[0114] On the contrary, the third profile of figure 3 (profile C3) according to the invention is well representative of the enrichment of the rod 40, enrichment minima being observed at the level of the zones 40B less rich than the zones 40A of the rod 40.

[0115] Furthermore, if the neutron source is not chosen appropriately, prior art methods implementing only an active scanning step can lead to another type of error observable in Figure 2. The second profile in this Figure 2 was obtained on the same rod 40 as for Figure 3 but with excitation with a less massive californium 252 source (about 500 pg of 252 See for Figure 2 against approximately 1000 pg of 252 See Figure 3). The amplitude of the excitation-induced gamma emission is much less significant in the case of Figure 2 than in Figure 3, the spontaneous gamma emission remaining independent of the excitation. Consequently, the spontaneous gamma emission is disturbed by the induced gamma emission and a positive deviation is observed on the B2 profile while the 40B zones are less rich in uranium 235.

[0116] On the contrary, the third profile of figure 3 (profile C3) according to the invention is well representative of the enrichment of the rod 40, enrichment minima being observed at the level of the zones 40B less rich than the zones 40A of the rod 40.

[0117] Furthermore, if the uniformity of the composition of the rod 15 in fissile isotope of interest is only checked from the first profile (profile A2 in figure 2 and profile A5 in figure 5), as in the methods of the prior art implementing only a passive scanning step, the conclusions may also be erroneous.

[0118] Thus, in the case of Figure 5, if we consider that the counting rate of the 50A zones corresponds to an enrichment in accordance with the specifications, the passive method of the prior art would lead, on the basis of the first profile (profile A5) taken alone, to the conclusion that the enrichment of the 50B zones is excessive and therefore that the rod 15 cannot be qualified for implementation in a fuel assembly. This conclusion is inadequate since the 50B zones and the 50A zones have the same enrichment rate (enrichment at 4.0%).

[0119] On the contrary, the third profile according to the invention, which is in the case of figure 5 identical to the expected profile C5, is representative of the enrichment of the rod 40, an identical enrichment being observed at the level of the zones 50B and the zones 50A of the rod 40.

[0120] It is therefore understood that the methods of the prior art, whether based on passive scanning or active scanning, do not allow valid conclusions to be drawn on the homogeneity of the enrichment of a fuel rod in the fissile isotope of interest in many cases. On the contrary, the method according to the invention makes it possible to overcome these drawbacks by the judicious combination of the first profile and the second profile to obtain the third profile.

[0121] Advantageously, the method comprises a step 404 of correcting the measurements of the first detector.

[0122] For this step, a correction factor K is provided to a data processing device of the device 10, which may be included in the first detector 20 or alternatively be the data processing device 35.

[0123] The correction factor K is representative of the assembly formed by the first detector 20 and the second detector 25. This arrangement makes it possible to take into account the sensitivity of the first detector 20 and / or the second detector 25 as well as the environmental conditions if they are sensitive to them, in particular their operating temperature.

[0124] In addition, the technical characteristics of the first and second detectors, including the crystal efficiency of each of the detectors or the attenuation factor of the cladding of these detectors or the hardware configuration of the detector, may be relatively different so that the amplitudes of the signals measured to form the first profile and the second profile are not always directly comparable.

[0125] It is therefore advantageous to carry out the correction step 404 before subtracting the first profile and the second profile, to take into account the scale factor between the first detector 20 and the second detector 25.

[0126] In this case, the first profile therefore comprises a counting rate N of the gamma photons emitted in the first energy range for each of the successive fractions of the pencil 15 corresponding to the counting rate actually measured by the first detector 20 multiplied by the factor K.

[0127] The K factor can be obtained by simulation and / or by means of a standard.

[0128] Advantageously, the method 400 comprises a determination 405 of the correction factor K before the scan carried out earliest among the first scan 401 and the second scan 402.

[0129] For the determination step 405, a standard nuclear fuel rod 50 is provided, comprising the fissile isotope of interest and at least one other isotope of a spontaneous gamma-emitting chemical element present or likely to be present in a rod 15 to be tested subsequently.

[0130] More specifically, a standard pencil 50 comprises at least one pellet comprising the fissile isotope of interest in a first known content and devoid of gamma-emitting radioisotope and at least one pellet comprising the fissile isotope of interest in a second known content and at least one gamma-emitting radioisotope of interest in a third known content, and the positions of each of the pellets along the main axis of the standard pencil 50 are known.

[0131] The compositions of the fissile isotope of interest and the gamma-emitting isotope along the main axis of the standard rod of the 50 standard rod are for example obtained by a detailed gamma spectrometry analysis, including the analysis of each of the different lines of the passive gamma spectrum of the 50 standard rod.

[0132] An example of a standard rod is shown in the lower part of Figure 5. In this example, the standard rod comprises an alternation of zones 50A comprising pellets of non-recycled uranium enriched to 4.0% in uranium 235, which is in this case the fissile isotope of interest and zones 50B groups of pellets of recycled uranium enriched to 4.0% in uranium 235, comprising 30 parts per billion by weight of uranium 232, which is, as we have seen previously, a gamma emitter.

[0133] The number of 50B zones containing the gamma-emitting radioisotope, their lengths along the main axis of the 50 standard rod, and their two-by-two spacing can be chosen to gain precision.

[0134] Advantageously, the age of the pellets constituting the 50B zones comprising the gamma-emitting radioisotope is known.

[0135] This age is preferably homogeneous within a given 50 standard pencil.

[0136] Advantageously, if the gamma-emitting radioisotope is uranium 232, the age of the pellets constituting the 50B zones is of the order of 10 years. In this case, it has been observed that the gamma signal from such pellets is stable over a period of the order of several years, which makes it possible to use the same standard for successive iterations of the 400 method during this same period.

[0137] The determination step 405 comprises the production of a first reference gamma profile by a first preliminary scan 405a of the standard pencil 50 using the first detector 20 without prior excitation of the standard pencil 50, preferably under the conditions which will be used for the first scan 401 of the pencil 15 to be tested subsequently.

[0138] The determination step 405 also comprises the production of a second reference gamma profile by a second preliminary scan 405b of the standard pencil 50 using the second detector 25 after preliminary excitation of the standard pencil 50 by means of the neutron source, preferably under the conditions which will be used for the second scan 402 of the pencil 15 to be tested subsequently.

[0139] The order in which the first and second reference gamma profiles are obtained is irrelevant.

[0140] Advantageously, if the first preliminary scan 405a is carried out after the second scan 405b, the two preliminary scans 405a and 405b are separated by a predetermined duration, greater than or equal to a deactivation duration of a fraction of the standard pencil 50 which was excited for the second preliminary scan 405b.

[0141] The first reference profile and the second reference profile are transmitted to a data processing device, which is for example the data processing device 35, and which provides at the end of an analysis step 405c of these two reference profiles a value of the correction factor K.

[0142] Since the standard rod 50 is known, it is possible to generate a third expected reference profile for this standard rod 50. The third expected reference profile in the example of Figure 5 corresponds to curve C5: a flat signal is expected in this case since the standard rod 50 has been designed so that its fissile isotope content is constant over its entire length.

[0143] The data processing device 35 therefore determines the correction factor K to be applied to the values ​​measured by the first detector 20 to form a first reference profile such that the subtraction of the first reference profile from the second reference profile forms a profile identical to the expected third reference profile.

[0144] This arrangement makes it possible to determine the correction factor K by experiment, for each particular set of first detector 20 and second detector 25.

[0145] Advantageously, the determination step 405 is carried out iteratively, each time a duration equal to a predetermined calibration period has elapsed.

[0146] The calibration period is advantageously less than 24 hours, preferably less than 10 hours.

[0147] Advantageously, the determination step 405 is carried out iteratively, each time the hardware configuration of the first detector 20 and / or the second detector 25 is modified.

[0148] These provisions make it possible to take into account environmental and / or material variations, in particular temperature and exposure to radioactivity of the first detector 20 and the second detector and therefore the operating state of these detectors shortly before the scan carried out earliest among the first scan 401 and the second scan 402 for the control of a given rod 15 to be tested.

[0149] The accuracy of the 400 process is therefore further improved.

Claims

CLAIMS 1. Method (400) for controlling the uniformity of the composition of a fissile isotope of interest of a chemical element along a main axis (X) of a nuclear fuel rod (15) to be tested, the method comprising the following steps: - first scan (401) of the test pencil (15) along the main axis (X) using a first gamma radiation detector (20) without prior excitation of the test pencil (15), the first scan producing a first gamma profile of the test pencil (15); - second scan (402) of the test pencil (15) along the main axis (X) using a second gamma radiation detector (25) after excitation of the test pencil (15) by means of a neutron source (30), the second scan producing a second gamma profile of the test pencil (15); - obtaining a third gamma profile of the pencil to be tested (15) by subtracting the first profile from the second profile; and - evaluation (403) of the uniformity of the fissile isotope composition of interest along the main axis (X) of the test rod (15) from the third gamma profile.

2. Method according to claim 1, in which obtaining the first profile comprises a step of correcting (404) measurements of the first detector obtained during the first scan (401) by applying a correction multiplicative factor characteristic of the assembly formed by the first detector and the second detector.

3. Method according to claim 1 or claim 2, comprising, before the first scan (401) and the second scan (402), a determination (405) of the correction factor, this determination comprising: - providing a standard nuclear fuel rod (50), comprising the fissile isotope of interest and at least one other isotope of a spontaneous gamma-emitting chemical element, and whose compositions in fissile isotope of interest and in gamma-emitting isotope along a main axis (X) of the standard rod (50) are known; - producing a first reference gamma profile by a first pre-scan (405a) of the standard pencil (50) using the first detector (20) without prior excitation of the standard pencil (50) and a second reference gamma profile by a second pre-scan (405b) of the standard pencil (50) using the second detector (25) after prior excitation of the standard pencil (50) by means of the neutron source (30); - providing (405c) a value of the correction factor from the first reference gamma profile and the second reference gamma profile, as well as compositions in fissile isotope of interest and gamma-emitting isotope along the main axis (X) of the standard rod (50).

4. The method of claim 3, wherein the earliest of the first pre-scan (405a) and the second pre-scan (405b) and the earliest of the first scan (401) and the second scan (402) are separated by a time less than a predetermined calibration period.

5. Method according to any one of the preceding claims in which the test rod (15) comprises recycled nuclear fuel.

6. Method according to claim 3 in which the fissile isotope of interest is uranium 235 and the gamma emitting isotope is uranium 232.

7. A method according to any preceding claim wherein, for the second scan (402), the neutron source comprises californium 252.

8. Method according to any one of the preceding claims in which the nuclear fuel is chosen from enriched non-recycled uranium (UNE), enriched recycled uranium (URE) and their mixtures.

9. Method according to claim 8 and according to claim 3 in which the standard pencil consists of at least two pellets comprising uranium, the uranium 235 content of at least one first pellet being known and equal to a nominal composition and the uranium 235 content of at least one second pellet being known and different from the nominal composition.

10. Device (10) for controlling the uniformity of the composition of a fissile isotope of interest of a chemical element along a main axis (X) of a nuclear fuel rod (15) to be tested, the device (10) comprising: - a first gamma radiation detector (20), configured to produce a first gamma profile of the test pencil without prior excitation of the test pencil (15) at the end of a first scan (401) of the test pencil (15) along the main axis (X); - a second gamma radiation detector (25) comprising a neutron source (30) and configured to produce a second gamma profile of the test rod (15) at the end of a second scan (402) of the test rod (15) along the main axis (X) after excitation of the test rod (15) by means of the neutron source (30); - a data processing device (35) configured to determine a third gamma profile of the test rod (15) by subtracting the first profile from the second profile and evaluating the uniformity of the fissile isotope composition of interest along the main axis (X) of the test rod (15) from the third gamma profile.

11. Device (10) according to claim 10 further comprising a standard nuclear fuel rod (50), comprising the fissile isotope of interest and at least one other isotope of a spontaneous gamma-emitting chemical element, and whose compositions in fissile isotope of interest and in gamma-emitting isotope along a main axis of the standard rod (X) are known, the data processing device (35) being further configured to determine a value of a correction factor from a first reference gamma profile of the standard rod (50) produced by the first detector (20) without prior excitation of the standard rod (50) and from a second reference gamma profile of the standard rod (50) produced by the second detector (25) after prior excitation of the standard rod (50) by means of the neutron source (30), as well as the compositions in fissile isotope of interest and in gamma-emitting isotope gamma along the principal axis of the standard pencil,the device (10) being configured to multiply measurements obtained during the first scan (401) by the correction factor for the production of the first profile., 12. Device according to any one of claims 10 and 11, in which the first detector (20) comprises means for moving the pencil to be tested along a first scanning axis and the second detector (25) comprises means for moving the pencil to be tested along a second scanning axis different from the first scanning axis.

13. Standard nuclear fuel rod (50) comprising: - at least one pellet (50A) comprising a fissile isotope of interest of a chemical element in a first known content and free of gamma-emitting radioisotope, and - at least one pellet (50B) comprising the fissile isotope of interest of a chemical element in a second known content and at least one gamma-emitting radioisotope of interest in a third known content, the positions of each of the pellets along a main axis (X) along which the standard rod (50) extends being known.

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