Method for checking component uniformity of nuclear fuel rod

Abstract

The invention relates to a method (400) for checking the homogeneity of components of a fission isotope of interest in chemical elements along a main axis (X) of a nuclear fuel rod (15) to be tested, the method comprising: performing a first scan (401) of the fuel rod (15) to be tested along the main axis (X) using a first gamma radiation counter (20) without a pre-excitation, wherein the first scan produces a first gamma distribution; performing a second scan (402) of the fuel rod (15) to be tested along the main axis (X) using a second gamma radiation counter (25) after excitation by the neutron source (30), where the second scan produces a second gamma distribution; obtaining a third gamma distribution by subtracting the first distribution from the second distribution; and evaluating (403) the homogeneity of the composition of the fission isotope of interest along the main axis (X) of the fuel rod (15) to be tested according to the third gamma distribution.

Description

Technical Field

[0001] The present invention relates to a method for examining the homogeneity of the composition of fission isotopes of interest of chemical elements along the main axis of a nuclear fuel rod to be tested.

[0002] The present invention also relates to a corresponding inspection device.

[0003] Finally, this invention relates to a reference nuclear fuel rod. Background Technology

[0004] Nuclear reactors use the energy released by nuclear fission reactions that occur when fissionable nuclear fuel is bombarded with neutrons.

[0005] Nuclear fuel typically contains uranium enriched to the isotope 235 in a quantity that depends on its subsequent use and enrichment methods. This quantity is, for example, between 3% and 5%. However, the composition of nuclear fuel can vary depending on the technology implemented in the nuclear reactor and / or the nuclear fuel source used to supply the reactor.

[0006] The preparation of nuclear fuel most typically involves a core pressing step by sintering nuclear fuel powder, which includes, for example, uranium oxide or a mixture of uranium and plutonium oxide.

[0007] Following core pressing is the rod manufacturing step, during which nuclear fuel pellets are placed inside a cylindrical metal cladding to form nuclear fuel rods. Rods typically have a diameter on the order of 10 mm and a length on the order of 5 m.

[0008] Nuclear fuel rods are eventually arranged in a grid of appropriate unit cells to form mechanically integrated fuel assemblies, which can be introduced into the nuclear reactor for its irradiation.

[0009] Proper control of nuclear reactions in nuclear reactors, particularly in terms of energy delivery, temperature management within the reactor, and the efficiency of fissile material utilization, leads to high requirements for the quality of pellets, rods, and fuel assemblies.

[0010] It is important to ensure that the composition of fission isotopes is as uniform as possible along the fuel rod.

[0011] Therefore, fuel rods typically undergo a step of examining the composition of their fission isotopes along their entire length, at the end of which they are eligible to be inserted into the assembly or scraped.

[0012] US Patent Application US4822552 describes a method for determining the enrichment homogeneity of nuclear fuel rods. The method includes: shifting the fuel rod along a linear path; repeatedly detecting natural gamma radiation, in other words, gamma radiation emitted without pre-excitation, from consecutive segments of the rod as it is gradually shifted; and examining the average enrichment value of the fuel rod to detect deviations relative to a specified enrichment value, and / or detecting deviations in the enrichment value of consecutive segments of the rod that are greater than a predetermined percentage.

[0013] The method described in document US4822552 is described as particularly suitable for fuels including neutron absorbers, where existing methods based on the detection of gamma radiation induced by neutron sources are not applicable.

[0014] However, it is noteworthy that in some cases, if the fuel rods are at least partially prepared from recycled uranium, the inventors have found that the fission isotopic composition along the rods is not always correctly assessed by passive methods (such as those described in document US4822552), making conclusions about enrichment homogeneity potentially erroneous in many cases. Summary of the Invention

[0015] Therefore, the object of the present invention is to provide a determination method that allows for the examination of the homogeneity of the composition of the fission isotopes of interest of the chemical elements along the main axis of the nuclear fuel rod to be tested with a good confidence level, regardless of the source of the fuel present in the rod and / or regardless of its composition other than the fission isotopes, including in the case where the rod to be tested is prepared from recycled fission isotopes.

[0016] Therefore, the present invention provides a method for examining the homogeneity of the composition of fissile isotopes of interest along the main axis of a nuclear fuel rod to be tested, the method comprising the following steps:

[0017] - Without pre-exciting the test bar, a first gamma radiation counter is used to perform a first scan of the test bar according to the main axis, and the first scan generates a first gamma distribution of the test bar;

[0018] -After the test rod is excited by a neutron source, a second gamma radiation counter is used to perform a second scan of the test rod according to the main axis, and the second scan generates a second gamma distribution of the test rod;

[0019] - The third gamma distribution of the test bar is obtained by subtracting the first distribution from the second distribution;

[0020] - Evaluate the homogeneity of the composition of the fission isotopes of interest along the main axis of the test rod based on the third gamma distribution.

[0021] The first distribution is obtained without prior excitation by the rod. Therefore, it represents spontaneous gamma emission along the rod, which is due to all spontaneous gamma emission present in each rod segment of the successive scan.

[0022] The second distribution is obtained by pre-exciting the rod using a neutron source. The absorption of neutrons by the fissile nuclei present in the rod leads to the fission of these nuclei and the emission of gamma radiation by the resulting excited nuclei, radiation not observed in the first distribution. Simultaneously, spontaneously emitting gamma-emitting nuclei emit gamma radiation in the same manner as in the first scan.

[0023] Therefore, obtaining the third distribution by subtracting the first distribution from the second distribution allows for the elimination of signal components that do not represent fission isotopes of interest in the second distribution, and thus allows for correct conclusions about the homogeneity of the composition of fission isotopes of interest along the rod, even in the presence of spontaneously gamma-emitting radionuclides.

[0024] This is particularly advantageous when the rod is a uranium rod containing recycled uranium.

[0025] The fissile isotope of interest is uranium-235. The rod also includes uranium-232, which is a product of the radioactive decay of plutonium-236 formed during previous irradiation prior to uranium recycling and is a spontaneous gamma emitter.

[0026] According to other advantageous aspects of the invention, the method includes one or more of the following features, individually or according to all technically possible combinations:

[0027] - Obtaining the first distribution includes the step of correcting the measurement of the first counter obtained during the first scan by applying the multiplication correction factor characteristic of the component formed by the first counter and the second counter;

[0028] - The method includes determining the correction factor before the first scan and the second scan, the determination including:

[0029] * A reference nuclear fuel rod is provided, the reference nuclear fuel rod containing the fission isotope of interest and at least one other isotope of a chemical element that serves as a spontaneous gamma emitter, and the composition of the fission isotope of interest and the gamma-emitting isotope of interest in the reference nuclear fuel rod is known along the main axis of the reference nuclear fuel rod;

[0030] * In the absence of pre-excitation of the reference nuclear fuel rod, a first reference gamma distribution is generated by using the first counter to perform a previous first scan of the reference rod, and a second reference gamma distribution is generated by using the second counter to perform a previous second scan of the reference rod after pre-excitation of the reference rod by the neutron source.

[0031] The value of the correction factor is provided based on the first reference gamma distribution and the second reference gamma distribution, and based on the composition of the fission isotope of interest and the gamma emission isotope along the main axis of the reference nuclear fuel rod;

[0032] - The earliest scan performed between the first and second prior scans is separated from the earliest scan performed between the first and second prior scans by a duration less than a predetermined calibration period;

[0033] - The rods to be tested contain recycled nuclear fuel;

[0034] - The fission isotope of interest is uranium-235, and the gamma-emitting isotope is uranium-232;

[0035] - For the second scan, the neutron source includes californium-252;

[0036] The nuclear fuel is selected from enriched non-recycled uranium, enriched recycled uranium, and mixtures thereof; and

[0037] - The reference rod consists of at least two cores containing uranium, wherein the uranium-235 content of at least one first core is known and equal to the nominal composition, and the uranium-235 content of at least one second core is known and different from the nominal composition.

[0038] The present invention also relates to an apparatus for examining the homogeneity of the composition of fissile isotopes of interest along the main axis of a nuclear fuel rod to be tested, the apparatus comprising:

[0039] - A first gamma radiation counter, configured to generate a first gamma distribution of the test bar at the end of a first scan of the test bar according to the main axis without pre-exciting the test bar;

[0040] - A second gamma radiation counter, comprising a neutron source and configured to generate a second gamma distribution of the test rod after the test rod is excited by the neutron source, at the end of a second scan of the test rod according to the main axis;

[0041] - A data processing device configured to determine a third gamma distribution of the test bar by subtracting the first distribution from the second distribution, and to evaluate the uniformity of the composition of the fission isotope of interest along the main axis of the test bar based on the third gamma distribution.

[0042] According to other advantageous aspects of the invention, the device includes one or more of the following features, individually or according to all technically possible combinations:

[0043] The apparatus further includes a reference nuclear fuel rod comprising the fission isotope of interest and at least one other isotope of a chemical element that serves as a spontaneous gamma emitter, and the composition of the fission isotope of interest and the gamma-emitting isotope along the principal axis of the reference rod is known.

[0044] The data processing apparatus is further configured to determine the value of a correction factor based on a first reference gamma distribution of the reference rod generated by the first counter without pre-excitation of the reference rod, and a second reference gamma distribution of the reference rod generated by the second counter after pre-excitation of the reference rod by the neutron source, and based on the composition of the fission isotopes of interest and the gamma emission isotopes along the principal axis of the reference rod.

[0045] The device is configured to multiply the measurements obtained during the first scan by the correction factor to produce the first distribution; and

[0046] - The first counter includes means for shifting the test bar according to a first scan axis, and the second counter includes means for shifting the test bar according to a second scan axis different from the first scan axis.

[0047] The present invention also relates to a reference nuclear fuel rod, comprising:

[0048] - At least one core element containing a fissile isotope of interest in a first known quantity of a chemical element and free of gamma-emitting radioactive isotopes, and

[0049] - At least one core containing a fission isotope of interest in a second known quantity of a chemical element and at least one gamma-emitting radioactive isotope of interest in a third known quantity.

[0050] The position of each core block along the main axis is known, and the reference rod extends along the main axis. Attached Figure Description

[0051] The invention will become more apparent from the following description, which is given by way of non-limiting example only and with reference to the accompanying drawings, wherein:

[0052] Figure 1 The apparatus according to the invention is schematically illustrated for examining the homogeneity of the composition of fissile isotopes of interest along the main axis of a nuclear fuel rod to be tested.

[0053] Figure 2 It is aimed at Figure 2 The lower part schematically illustrates examples of the first distribution (curve A2), second distribution (curve B2), and third distribution (curve C2) obtained from nuclear fuel rods, using approximately 500 μg of... 252 The Cf neutron source performs excitation to obtain the second distribution. The count rate N is plotted on the ordinate; the x-axis represents the position along the test bar.

[0054] Figure 3 It is aimed at Figure 2 Examples of the second distribution (curve B3) and third distribution (curve C3) obtained from the rod were obtained using approximately 1000 μg of [material / material]. 252 The Cf neutron source performs excitation to obtain the second distribution. The count rate N is plotted on the ordinate; the x-axis represents the position along the test bar.

[0055] Figure 4 A particular embodiment of the method according to the invention for checking the homogeneity of the composition of fissile isotopes of interest along the main axis of a nuclear fuel rod to be tested is illustrated in flowchart form.

[0056] Figure 5 Yes (in) Figure 5 The examples shown below (schematic representation) of the first distribution (curve A5), the obtained second distribution (curve B5), and the expected third distribution (curve C5) of the reference bar, using approximately 500 μg 252 A Cf neutron source is used to perform excitation to obtain a second distribution. The count rate N is plotted on the vertical axis; the horizontal axis X represents the position along the test rod. Detailed Implementation

[0057] An apparatus 10 for examining the homogeneity of the composition of the fissile isotopes of interest of chemical element A along the main axis X of the nuclear fuel rod 15 to be tested, one embodiment of which is in Figure 1 The diagram schematically shows a first gamma radiation counter 20, a second gamma radiation counter 25 including a neutron source 30, and a data processing device 35 that exchanges data with the first counter 20 and the second counter 25.

[0058] Rod 15 includes a metal cladding, such as a cylinder, in which nuclear fuel pellets are placed.

[0059] The components of rod 15 may be known a priori or unknown.

[0060] The fissile isotope of interest is preferably uranium-235 ( 235 In this case, chemical element A is therefore uranium (U). 92 U).

[0061] Rod 15 may include non-fission isotopes of chemical element A, particularly gamma-emitting radioactive isotopes of chemical element A.

[0062] The fissile isotope of interest is uranium-235 ( 235 In the case of U), rod 15 advantageously includes recycled uranium-232 containing uranium. 232 U).

[0063] Rod 15 may also include chemical elements other than element A, particularly radioactive decay products of uranium-232.

[0064] The first counter 20 includes means for shifting the rod 15 between the inlet 20A and the outlet 20B of the first counter 20 at a given displacement velocity, and means for detecting and counting gamma photons emitted by a portion of the rod 15 located in the detection region 20C inside the first counter 20.

[0065] The device for shifting rod 15 is advantageously configured to shift rod 15 continuously.

[0066] Detection and counting devices are, for example, flash counters.

[0067] The first counter 20 is configured to provide the first gamma distribution of the bar 15.

[0068] For each section of the rod 15 that continuously passes through the detection region 20C, the first distribution includes the count rate N of gamma photons spontaneously emitted (in other words, emitted without prior excitation) by the corresponding section of the rod 15.

[0069] exist Figure 2 An example of the first distribution can be seen on curve A2, which is obtained for model rod 40, which includes a group of periodically alternating groups of non-recycled uranium pellets 40A enriched to 5.0% in uranium-235 and a group of recycled uranium pellets 40B enriched to 4.0% in uranium-235.

[0070] exist Figure 5Another example of the first distribution can be seen on curve A5, which is obtained for reference rod 50, which consists of alternations of non-recycled uranium pellets 50A enriched with 4% uranium-235 and recycled uranium pellets 50B enriched with 4% uranium-235.

[0071] In one particular implementation, the count rate N of the first distribution corresponds to the number of gamma photons in a first energy range.

[0072] The first energy range is advantageously selected to include one or more gamma emission lines of gamma-emitting radioactive isotopes present or possibly present in rod 15.

[0073] For example, if rod 15 includes or may include uranium-232, then the first energy range is advantageously [0, 2 MeV]. This first energy range includes a line of several decay products of uranium-232, of which thallium-208 ( 208 Tl), Lead 212 ( 212 Pb) and bismuth-212 ( 212 Bi is a major contributor to the natural gamma spectrum of uranium-232.

[0074] Advantageously, each section of rod 15 has a length on the order of a few millimeters.

[0075] exist Figure 1 In the example, the second counter 25 is positioned downstream of the first counter 20 in the shifting direction of the rod 15.

[0076] The second counter 25 includes means for shifting the bar 15 between the inlet 25A and the outlet 25B of the second counter 25 at a given shifting speed.

[0077] The device for shifting rod 15 is advantageously configured to shift rod 15 continuously.

[0078] The second counter 25 includes a neutron source 30 near the inlet 25A, which is configured to excite the rod 15 before detecting gamma photons.

[0079] A neutron source is advantageously selected to induce a fission reaction of the fissionable isotope of interest in chemical element A.

[0080] Neutron source favorablely derived from 252 Cf composition. This arrangement is particularly advantageous when the fissile isotope of interest is uranium-235.

[0081] Other neutron sources could be considered, particularly, for example, americium-beryllium (Am-Be) sources or neutron generators.

[0082] The second counter 25 includes, downstream of the source, a device for detecting and counting gamma photons emitted by a portion of the detection region 25C located inside the second counter 25 of the rod 15.

[0083] Detection and counting devices are, for example, flash counters.

[0084] The second counter 25 is configured to provide a second gamma distribution for the bar 15.

[0085] For each section of rod 15 that continuously passes through detection region 25C, the second distribution includes the count rate N of gamma photons emitted by the corresponding section of rod 15.

[0086] In the second counter 25, the emission can be either spontaneous or caused by excitation through the neutron source 30. No distinction is made between the two emission modes in the second counter 25.

[0087] An example of the second distribution can be found in... Figure 2 curve B2 and Figure 3 As seen on curve B3, this is obtained for model rod 40, or even in Figure 5 The curve B5 was obtained with reference bar 50.

[0088] In one particular implementation, the count rate N of the second distribution corresponds to the number of gamma photons in the second energy range.

[0089] Advantageously, a second energy range is selected to include one or more gamma emission lines of at least one fission product of the fissionable isotope of interest present in rod 15.

[0090] For example, if rod 15 includes uranium-235, then the second energy range is advantageously [0, 9 MeV], preferably [75 keV, 6 MeV].

[0091] This second energy range includes lines of several fission products of uranium-235, including krypton-93 (… 93 Kr), Rubidium 93 ( 93 Rb) and yttrium-93 ( 93 Y) is a significant contributor to the gamma spectrum induced by neutron activation of uranium-235, and at least one line of uranium-235 fission products (especially strontium and iodine).

[0092] The first counter 20 and the second counter 25 are configured to exchange data with the data processing device 35.

[0093] It is worth noting that the data processing device 35 is configured to receive the first distribution and the second distribution from the first counter 20 and the second counter 25, respectively.

[0094] The data processing device 35 is configured to generate a third distribution by subtracting the first distribution from the second distribution.

[0095] It is possible Figure 2 An example of a third distribution is observed on curve C2, which is obtained for model bar 40 based on the first distribution (curve A2) and the second distribution (curve B2).

[0096] Data processing unit 35 is configured to provide an assessment of the homogeneity of the composition of the fission isotopes of interest along the bar to be tested from a third gamma distribution. The method for checking this homogeneity is described in detail below.

[0097] exist Figure 4 The diagram illustrates a method 400 for checking the homogeneity of the composition of the fission isotope of interest along a rod to be tested based on a third gamma distribution.

[0098] Method 400 includes performing a first scan 401 on the rod 15 using a first counter 20 without pre-exciting the rod 15 to be tested, in order to obtain a first distribution of the rod 15.

[0099] For this purpose, rod 15 is moved at a constant speed between its inlet 20A and its outlet 20B in the first counter 20.

[0100] Therefore, the first counter 20 generates a first distribution of the count rate N, which includes gamma photons emitted for each of the continuous portions of the rod 15.

[0101] The count rate N of the first distribution is, for example, the number of gamma photons emitted within the first energy range.

[0102] Method 400 includes performing a second scan 402 on bar 15 using a second counter 25.

[0103] For the second scan 402, bar 15 moves at a constant speed between its inlet 25A and its outlet 25B in the second counter 25.

[0104] The excitation of rod 15 by neutron source 30 induces a fission reaction in a small portion of the nucleus of the fissile isotope of interest, resulting in the formation of fission products of the fissile isotope.

[0105] At least a portion of these fission products may be gamma emitters, such that for a portion of rod 15 that includes a gamma-emitting radioisotope before excitation, the gamma emission of that portion of rod 15 passing through the detection zone 25C after excitation includes superimposed possible emission caused by excitation and possible spontaneous emission.

[0106] As an example, if rod 15 contains uranium-235, spontaneous emission is observed. If the uranium-235 is from recycling, rod 15 also contains other radioactive isotopes that are the source of gamma emission caused by excitation observed in superposition with the spontaneous emission of uranium-235.

[0107] At the end of the second scan 402, the second distribution of bar 15 is obtained.

[0108] The second distribution includes the count rate N of gamma photons emitted for each consecutive section of rod 15.

[0109] The counting rate N of the second distribution is, for example, the number of gamma photons emitted within the second energy range.

[0110] It should be noted that the order in which the first and second distributions are obtained has no effect on method 400, and therefore can be used as follows: Figure 1 and Figure 4 The second scan 402 is performed after or before the first scan 401 shown in the figure.

[0111] Advantageously, if the first scan 401 is performed after the second scan 402, the two scans 401 and 402 are separated for a predetermined duration, which is greater than or equal to the deactivation duration of a portion of the rod 15 excited for the second scan 402.

[0112] In one particular implementation, the first scan 401 and the second scan 402 are performed by a first counter 20 and a second counter 25, and the displacement axes of the bars used for scanning are different.

[0113] The first distribution and the second distribution are sent to the data processing device 35 by the first counter 20 and the second counter 25, respectively.

[0114] The data processing device 35 then subtracts the first distribution from the second distribution to produce the third gamma distribution of the bar 15.

[0115] The data processing device 35 can then provide results based on the third gamma distribution to examine the homogeneity of the composition of the fission isotopes of interest along the test rod 15.

[0116] For example, the processing device 35 can test whether the relative or absolute difference between each count rate of the third distribution of each part of the test bar 15 and the average of these count rates is less than a threshold.

[0117] If so, the result of the check is positive: the composition of the fission isotope of interest is assessed as being homogeneous along the bar being tested, with accuracy corresponding to the selected threshold.

[0118] If not, the result is negative: the composition of at least one fission isotope of interest deviates excessively from the average composition of the test bar.

[0119] Advantageously, the results include the location of each bar segment whose relative or absolute difference from the mean is greater than a threshold.

[0120] The method 400 according to the invention allows for the examination of the homogeneity of the composition of the fission isotopes of interest along the rod 15 in all cases, at least as in or better than prior art methods.

[0121] On the one hand, assuming that rod 15 does not contain gamma-emitting radioisotopes, this method provides a third distribution identical to the second distribution after filtering out potential environmental noise. Therefore, the conclusion regarding the uniformity of the concentration of rod 15 would be similar to that of prior art methods that perform only one active scanning step.

[0122] On the other hand, in cases where rod 15 includes a gamma-emitting radioactive isotope, examples are found in... Figure 2 and Figure 3 The text indicates that the second distribution may include the rod portion containing the gamma-emitting radioactive isotope. Figure 2 and Figure 3 The peak of the particularly high count rate corresponds to the portion 40B. If the homogeneity of the component of rod 15 in the fission isotopes of interest is examined only from the second distribution, as in the prior art method which performs only one active scanning step, the conclusion may be erroneous.

[0123] Therefore, in Figure 3 In the case where the count rate N of region 40A is considered to correspond to enrichment according to specifications, it can be concluded solely based on the second distribution (curve B3) corresponding to the prior art active method that the enrichment of region 40B is compliant, and therefore rod 15 is eligible for use in fuel assemblies. This conclusion from the prior art method is insufficient because region 40B has a lower concentration of uranium-235 (enriched 4.0%) than region 40A (enriched 5.0%). Figure 3 In this case, the amplitude of gamma emission caused by excitation is much greater than Figure 2 In the amplitude of the excitation, spontaneous gamma emission remains independent of the excitation. Therefore, spontaneous gamma emission is masked by induced gamma emission and is unobservable on distribution B3.

[0124] Then, based on a separate distribution corresponding to the prior art active method, it can be concluded that the enrichment is perfectly uniform because curve B3 is flat. Therefore, the prior art active method would conclude that rod 40 is qualified for use in fuel assemblies. This conclusion is incorrect because region 40B is less enriched in uranium-235 (4.0% enrichment) than in region 40A (5.0% enrichment).

[0125] Conversely, according to the present invention Figure 3 The third distribution (distribution C3) does indeed represent the concentration of rod 40, with the minimum concentration observed in region 40B, which has a lower concentration than region 40A of rod 40.

[0126] Furthermore, if the neutron source is not selected in an appropriate manner, existing techniques that only implement the active scanning step may lead to... Figure 2 Another type of error that can be observed in the process. Figure 2 The second distribution is in relation to Figure 3 Obtained on the same rod 40, but using a smaller mass of californium-252 source ( Figure 2 Approximately 500 μg 252 Cf and Figure 3 Approximately 1000 μg 252 Cf) excitation. In Figure 2 In this case, the amplitude of gamma emission caused by excitation is much smaller than Figure 3 In the amplitude, spontaneous gamma emission remains independent of excitation. Therefore, spontaneous gamma emission is disturbed by induced gamma emission, and a positive deviation is observed on distribution B2, while the enrichment of uranium-235 in region 40B is lower.

[0127] Conversely, according to the present invention Figure 3 The third distribution (distribution C3) does indeed represent the concentration of rod 40, with the minimum concentration observed in region 40B, which has a lower concentration than region 40A of rod 40.

[0128] Furthermore, if only from the first distribution ( Figure 2 Distribution A2 and Figure 5 The distribution of rod 15 in the fission isotopes of interest (A5) is examined to check the homogeneity of the composition of rod 15. However, in existing methods that only perform passive scanning steps, the conclusions may be erroneous.

[0129] Therefore, in Figure 5 In the case of assuming that the count rate of zone 50A corresponds to the concentration according to specifications, the passive approach of the prior art would lead to the conclusion that zone 50B is over-concentrated based solely on the first curve (curve A5), thus disqualifying rod 15 from use in fuel assemblies. This conclusion is insufficient because zone 50B and zone 50A exhibit the same concentration level (at 4.0% concentration).

[0130] On the contrary, Figure 5 In the case of the same distribution as expected C5, the third distribution according to the invention represents the concentration of rod 40, and the same concentration is observed in regions 50B and 50A of rod 40.

[0131] Therefore, it should be understood that existing methods, whether based on passive or active scanning, often fail to provide valid conclusions regarding the enrichment homogeneity of fuel rods in the fissile isotopes of interest. Conversely, the method according to the invention remedies these shortcomings by intelligently combining the first and second distributions to obtain a third distribution.

[0132] Advantageously, the method includes a calibration step 404 for the measurement of the first counter.

[0133] For this step, a correction factor K is provided to the data processing device of device 10, which may be included in the first counter 20 or may be the data processing device 35.

[0134] The correction factor K represents the component formed by the first counter 20 and the second counter 25.

[0135] This arrangement allows for consideration of the sensitivity of the first counter 20 and / or the second counter 25, as well as environmental conditions (if they are sensitive to them), particularly their operating temperature.

[0136] Furthermore, the technical characteristics of the first and second counters may differ, making the amplitudes of the signals measured to form the first and second distributions not always directly comparable. These technical characteristics stem from the crystal efficiency of each counter, the attenuation factor of the sheath of these counters, or the hardware configuration of the counters.

[0137] Therefore, it is advantageous to perform the correction step 404 before subtracting the first curve from the second curve to take into account the scaling factor between the first counter 20 and the second counter 25.

[0138] Therefore, in this case, for each consecutive portion of rod 15, the first distribution includes a count rate N of gamma photons emitted in the first energy range, which corresponds to the count rate actually measured by the first counter 20 multiplied by a factor K.

[0139] The factor K can be obtained through simulation and / or by reference.

[0140] Advantageously, method 400 includes determining a correction factor K 405 prior to the earliest scan performed in the first scan 401 and the second scan 402.

[0141] For step 405, a reference nuclear fuel rod 50 is provided, which includes at least one other isotope of the fission isotope of interest and a chemical element that is a spontaneous gamma emitter present or possibly present in the rod 15 to be subsequently tested.

[0142] More specifically, the reference rod 50 includes at least one core block comprising a first known amount of a fission isotope of interest and containing no gamma-emitting radioactive isotope. The reference rod 50 also includes at least one core block comprising a second known amount of a fission isotope of interest and a third known amount of at least one gamma-emitting radioactive isotope. The position of each core block along the main axis of the reference rod 50 is known.

[0143] The composition of fission isotopes of interest and gamma emission isotopes along the main axis of the reference rod 50 is obtained, for example, through detailed gamma spectral analysis, including analysis of each line in the different lines of the passive gamma spectrum of the reference rod 50.

[0144] Example of a reference bar in Figure 5 The lower part is indicated. In this example, the reference bar comprises alternating groups of regions 50A and 50B. Region 50A comprises non-recycled uranium pellets enriched to 4.0% in uranium-235, where uranium-235 is the fissile isotope of interest. Region 50B comprises recycled uranium pellets enriched to 4.0% in uranium-235, comprising 30 parts by weight of uranium-232, which, as mentioned above, is a gamma emitter.

[0145] The number of regions 50B that include gamma-emitting radioactive isotopes, their length relative to the main axis of reference rod 50, and their spacing (2×2) can be selected to obtain accuracy.

[0146] Advantageously, the age of the core block constituting region 50B containing the gamma-emitting radioactive isotope is known.

[0147] The age is preferably uniform within a given reference bar of 50.

[0148] Advantageously, if the gamma-emitting radioactive isotope is uranium-232, the age of the core constituting region 50B is approximately 10 years. In this case, it has been observed that the gamma signal of such a core is stable over a period of approximately several years, which allows for the use of the same reference bar for continuous iterations of method 400 during that same period.

[0149] Step 405 includes, preferably, generating a first reference gamma distribution using a first counter 20 through a previous first scan 405a of the reference bar 50 without pre-exciting the reference bar 50, under the condition of a first scan 401 of the bar 15 to be used for subsequent testing.

[0150] The determination step 405 further includes, preferably, generating a second reference gamma distribution by using a second counter 25 to perform a previous second scan 405b on the reference rod 50 after pre-excitation of the reference rod 50 by a neutron source, under the conditions of a second scan 402 of the rod 15 to be used for subsequent testing.

[0151] The order in which the first and second reference gamma distributions are obtained is irrelevant.

[0152] Advantageously, if the first scan 405a is performed after the second scan 405b, the two previous scans 405a and 405b are separated for a predetermined duration, which is greater than or equal to the deactivation duration of a portion of the reference bar 50 excited by the second scan 405b.

[0153] The first and second reference distributions are sent to a data processing device, such as data processing device 35, and the value of the correction factor K is provided at the end of the analysis step 405c of the two reference distributions.

[0154] Since the reference bar 50 is known, the expected third reference distribution can be generated for the reference bar 50.

[0155] Figure 5 The expected third reference distribution in the example corresponds to curve C5: a flat signal is expected in this case because reference bar 50 is designed such that its fission isotope content is constant over its entire length.

[0156] Therefore, the data processing device 35 determines a correction factor K to be applied to the value measured by the first counter 20 to form a first reference curve, such that subtracting the first reference curve from the second reference curve forms the same curve as the expected third reference curve.

[0157] Therefore, for each specific set of the first counter 20 and the second counter 25, this arrangement allows the correction factor K to be determined empirically.

[0158] Advantageously, the determination step 405 is performed iteratively each time a duration equal to the predetermined calibration period has elapsed.

[0159] The calibration cycle is advantageously less than 24 hours, preferably less than 10 hours.

[0160] Advantageously, the determination step 405 is performed iteratively whenever the hardware configuration of the first counter 20 and / or the second counter 25 is modified.

[0161] These arrangements allow for consideration of environmental and / or hardware changes, particularly the temperature and exposure to radiation of the first and second counters, and thus the operational status of these counters shortly before the earliest scans performed in the first scan 401 and the second scan 402, to examine the given rod 15 to be tested.

[0162] Therefore, the accuracy of method 400 is further improved.

Claims

1. A method (400) for examining the homogeneity of the composition of fissile isotopes of interest of a chemical element along the main axis (X) of a nuclear fuel rod (15) to be tested, the method comprising the following steps: - Without pre-excitation of the test rod (15), a first gamma radiation counter (20) is used to perform a first scan (401) on the test rod (15) according to the main axis (X), and the first scan (401) generates a first gamma distribution of the test rod (15); - After the test rod (15) is excited by the neutron source (30), the test rod (15) is scanned (402) according to the main axis (X) using the second gamma radiation counter (25), and the second scan produces a second gamma distribution of the test rod (15); - The third gamma distribution of the test bar (15) is obtained by subtracting the first distribution from the second distribution; and -Based on the third gamma distribution, evaluate (403) the homogeneity of the composition of the fission isotopes of interest along the main axis (X) of the test rod (15).

2. The method according to claim 1, wherein, Obtaining the first distribution includes step (404): correcting the measurement of the first counter obtained during the first scan (401) by applying the multiplication correction factor characteristics of the component formed by the first counter and the second counter.

3. The method according to claim 1 or claim 2, comprising: Before the first scan (401) and the second scan (402), the correction factor is determined (405), the determination including: - Provide a reference nuclear fuel rod (50) comprising the fission isotope of interest and at least one other isotope of a chemical element that is a spontaneous gamma emitter, and the composition of the fission isotope of interest and the gamma emission isotope of interest along the main axis (X) of the reference nuclear fuel rod (50) is known; - Without pre-excitation of the reference rod (50), a first reference gamma distribution is generated by using a first counter (20) through a previous first scan (405a) of the reference rod (50), and a second reference gamma distribution is generated by using a second counter (25) through a previous second scan (405b) of the reference rod (50) after pre-excitation of the reference rod (50) by the neutron source (30); - The value of the (405c) correction factor is provided based on the first reference gamma distribution and the second reference gamma distribution, and based on the composition of the fission isotopes and gamma emission isotopes of interest along the main axis (X) of the reference bar (50).

4. The method according to claim 3, wherein, The earliest of the previous first scan (405a) and the previous second scan (405b) is separated from the earliest of the first scan (401) and the second scan (402) by a duration less than a predetermined calibration period.

5. The method according to any one of the preceding claims, wherein, The test rod (15) contains recycled nuclear fuel.

6. The method according to claim 3, wherein, The fission isotope of interest is uranium-235, and the gamma-emitting isotope is uranium-232.

7. The method according to any one of the preceding claims, wherein, For the second scan (402), the neutron source includes californium-252.

8. The method according to any one of the preceding claims, wherein, The nuclear fuel is selected from UNE, URE and mixtures thereof, wherein UNE is enriched non-recycled uranium and URE is enriched recycled uranium.

9. The method according to claims 8 and 3, wherein, The reference rod consists of at least two cores containing uranium, wherein the uranium-235 content of at least the first core is known and equal to the nominal composition, and the uranium-235 content of at least the second core is known and different from the nominal composition.

10. An apparatus (10) for examining the homogeneity of the composition of fissile isotopes of interest of a chemical element along the main axis (X) of a nuclear fuel rod (15) to be tested, the apparatus (10) comprising: - A first gamma radiation counter (20) is configured to generate a first gamma distribution of the test rod (15) at the end of the first scan (401) of the test rod (15) according to the main axis (X) without pre-exciting the test rod (15); - A second gamma radiation counter (25) includes a neutron source (30) and is configured to generate a second gamma distribution of the test rod (15) at the end of a second scan (402) of the test rod (15) according to the main axis (X) after the test rod (15) is excited by the neutron source (30). - A data processing device (35) is configured to: determine a third gamma distribution (15) of the test bar by subtracting the first distribution from the second distribution, and evaluate the homogeneity of the composition of fission isotopes of interest along the main axis (X) of the test bar (15) based on the third gamma distribution.

11. The apparatus (10) according to claim 10, further comprising a reference nuclear fuel rod (50), the reference nuclear fuel rod (50) comprising the fission isotope of interest and at least one other isotope of a chemical element as a spontaneous gamma emitter, and the composition of the fission isotope of interest and the gamma emission isotope of interest along the principal axis of the reference rod (X) of the reference nuclear fuel rod (50) is known. The data processing device (35) is further configured to: determine the value of a correction factor based on a first reference gamma distribution of the reference rod (50) generated by the first counter (20) without pre-excitation of the reference rod (50), and a second reference gamma distribution of the reference rod (50) generated by the second counter (25) after pre-excitation of the reference rod (50) by the neutron source (30), and based on the composition of the fission isotopes of interest and the gamma emission isotopes along the main axis of the reference rod. The device (10) is configured to multiply the measurement obtained during the first scan (401) by the correction factor to produce the first distribution.

12. The apparatus according to any one of claims 10 and 11, wherein, The first counter (20) includes means for shifting the test bar according to a first scan axis, and the second counter (25) includes means for shifting the test bar along a second scan axis different from the first scan axis.

13. A reference nuclear fuel rod (50), comprising: - At least one core (50A) comprising a fission isotope of interest in a first known quantity of a chemical element and free of gamma-emitting radioactive isotopes, and - At least one core (50B) comprising a fission isotope of interest in a second known quantity of a chemical element and at least one gamma-emitting radioactive isotope of interest in a third known quantity. The position of each core block along the main axis (X) is known, and the reference bar (50) extends along this main axis.

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