RU2316064C2 - Method and device for quality control of mox fuel rods - Google Patents

Abstract

FIELD: complex quality control of mixed-oxide fuel rods.

SUBSTANCE: proposed method for complex quality control of MOX fuel rod includes combination of at least two of following steps: measuring plutonium content in mentioned fuel rod pellets; checking for nonstandard pellets in mentioned rod; checking for internal structure and components in mentioned rod; measuring geometric characteristics of mentioned rod; measuring radioactive contaminants along mentioned rod surface and checking rod identifying data for compliance with plutonium enrichment. In the process fuel rod is moved along axis at the same time executing at least two steps along axially displaced fuel rod using radiometry, and/or radiography, and/or electrooptic identification for mentioned check.

EFFECT: minimized requirement for check-to-check buffer stores, reduced quality control region and capital outlays for mixed-oxide fuel production, reduced danger of personnel exposure to radioactive rays.

1 cl

Description

The present invention relates to a method for integrated quality control of MOX (mixed oxide) - fuel rods and a device for implementing this method.

Nuclear fuel rods are used in most power reactors (light-water nuclear reactors, heavy-water nuclear reactors, nuclear breeder reactors, etc.) as elements that isolate nuclear fuel from the medium. As such, they must meet high quality standards and undergo appropriate control before being released for installation in a fuel assembly (sometimes called a fuel element or a bundle of fuel elements), which is the final product used to transport this fuel, load it into the reactor, serve it in the reactor core, unload it from the reactor and, ultimately, remove it by intermediate storage for further processing or final storage defense.

Typically, the fuel rod 1 (FIG. 1) consists of a column of fuel pellets 2 made of a divisible material enclosed in a tube or casing 3, sealed at both open ends with an end cap, an upper end cap 4 and a lower end cap 5. Typically, the column The tablets are held in place by a spring 6 located in the gas collector 7 and designed to provide internal pressure to the fuel rod. This internal pressure arises as a result of the initial gas filling of the fuel rods, the presence of helium, which is formed mainly due to the alpha decay of plutonium and curium during irradiation, as well as gaseous fission products. In some designs of fuel rods, a gas collector is provided not only at the lower end of the rod, but also at the upper end. Sometimes, between the spring 8 of the gas collector and the adjacent fuel tablet 2, an insulating tablet 8 is installed.

Along with the tightness of the fuel rod associated with the welding of end caps, the most important quality indicators of any fuel rod are:

- the degree of enrichment of each tablet along the column, which determines the nominal power. Most often, it is checked by scanning natural or stimulated gamma radiation, or by X-ray fluorescence of the fuel rod;

- the absence of interstitial gaps that cause strong local power surges and can lead to damage to the shell. It can be checked by radiography of the fuel rod along its entire length;

- the length of the column of tablets, the length of the gas collector and the presence of springs and other structural elements (getters, insulating tablets from Al ₂ O ₃ , tablets from the reproduction zone from UO ₂ , ...). This is usually checked by radiography of the gas collector zones of each rod;

- the total length of the fuel rod, affecting the good operation of the fuel assembly. Usually it is checked by metrological methods;

- contamination of the outer surface of the fuel rod with radioactive material, affecting the internal exposure of personnel at subsequent stages of production and transportation of the rods, as well as the level of radioactivity of the entire primary circuit of the energy reactor and its cleaning system. As a rule, it is checked by measuring the alpha activity of the fuel rod or some of its sections. In fact, alpha radiation has very low penetrating power and, therefore, correctly displays the amount of nuclear fuel available on this surface;

- dependencies between the fuel rod identification code - sign and / or number - and the specific type and characteristics of the fuel. They are verified by comparing the fuel rod identification code with the results of the above quality checks.

The most widely used nuclear fuel is made from enriched uranium dioxide, in which the natural content of ²³⁵ U was increased; the waste from its production is depleted uranium with a lower content of ²³⁵ U than that of natural uranium. This enrichment is carried out on gaseous uranium fluoride and therefore allows to obtain enriched uranium of high homogeneity, which is typical of any process occurring in the gaseous phase. Reconversion of enriched uranium hexafluoride to uranium dioxide from which fuel rods are made does not affect the high homogeneity of the starting material and the resulting fuel. Therefore, the sole purpose of the aforementioned enrichment control of the tablets along the fuel rod is to verify that the “non-standard” tablets (not meeting the specifications for the manufacture of this batch) are not mixed with the standard ones.

Plutonium recovered as a result of reprocessing of spent nuclear fuel based on UO ₂ is a more active material than ²³⁵ U and, therefore, can be advantageously used for the production of nuclear fuel. For this, plutonium oxide is mixed with depleted or natural uranium dioxide in order to obtain a mixed oxide, usually called MOX fuel, which is functionally equivalent to enriched uranium fuel and can be loaded instead into the reactor core. The mechanical mixing of powders in order to obtain MOX fuel does not use homogeneity equal to the homogeneity of enriched uranium (acquired as a result of its processing in the gaseous phase). The control of enrichment along the axis of the fuel rod is intended to verify that the plutonium content of each tablet remains within a certain tolerance field and to detect any non-standard tablets in the column of fuel tablets.

In addition, unlike ²³⁵ U, which is a single fissile isotope, plutonium source material consists mainly of five isotopes of plutonium ( ²³⁸ Pu, ²³⁹ Pu, ²⁴⁰ Pu, ²⁴¹ Pu, ²⁴² Pu) and americium ( ²⁴¹ Am), which is a product of the decay of ²⁴¹ Pu. In this plutonium source material, only two isotopes are “fissile”, namely ²³⁹ Pu and ²⁴¹ Pu, while the other four are essentially neutron absorbers. Several batches of plutonium are used to make reloadable MOX fuel. Each of these parties is distinguished by its own isotopic composition and americium content. However, the resulting MOX fuel must contain plutonium with a homogeneous isotopic composition and americium content that remain within a certain tolerance range. This is achieved by taking the corresponding aliquots from different lots of plutonium feed material for mixing with uranium oxide. As a result of this, the quality control of MOX fuel rods should also include verification that the specified uniformity of the plutonium isotopic composition and americium content are indeed achieved in the final product. This presents additional difficulty in the selection of equipment for quality control.

The production of MOX fuel is also seen as the most efficient way to process excess weapons grade plutonium. Irradiation of MOX fuel derived from weapons-grade plutonium irreversibly causes the isotopic composition of this plutonium to degrade to such an extent that it becomes unsuitable for military use. Compared to reactor-grade plutonium, weapons-grade plutonium mainly consists of ²³⁹ Pu, includes a small amount of ²⁴⁰ Pu and a small amount of ²⁴¹ Pu. Thus, the heterogeneity of the isotopic composition and the content of americium is not a problem. However, the very low content of plutonium isotopes absorbing neutrons leads to a narrowing of the tolerance field specified for the plutonium content in each tablet. In addition, a small amount of americium, which is the strongest gamma emitter, lowers the sensitivity of detection of intercellular fluctuations in the plutonium content. So, ultimately, the issue of quality control is just as acute for MOX fuel using weapons grade plutonium as it is for MOX fuel reactor plutonium.

A higher neutron cross section and higher radiotoxicity of plutonium compared to ²³⁵ U leads to a decrease in the tolerance for surface contamination of MOX fuel rods compared to that of fuel rods based on enriched uranium. Therefore, additional attention should be paid to obtaining reliable and reproducible data on contamination with alpha-active substances through quality control.

Accounting for nuclear materials and safeguards enforcement are much stricter with respect to plutonium than enriched uranium. Therefore, the quality control of these fuel rods should provide more accurate data for the nuclear material accounting system and safeguards inspectors at the MOX fuel plant than at the uranium fuel plant.

A MOX fuel assembly is made of MOX fuel rods with 3-5 different Pu contents, while an equivalent uranium enriched fuel assembly usually contains fewer fuel rods with varying degrees of uranium enrichment. In addition, a surge in power due to the interstitial gap in the column of tablets, a non-standard tablet in the fuel rod, or a non-standard fuel rod in a certain position inside the fuel assembly is much stronger in MOX fuel than in enriched uranium fuel. Consequently, it is much more important for MOX fuel than for uranium fuel to ensure interstitial gaps in the column of tablets, non-standard tablets in the fuel rod, or the identification code of the rod does not match the type of fuel contained in the latter.

All plutonium and americium isotopes are gamma emitters, and such that gamma radiation from the MOX fuel rod is several orders of magnitude higher than that of the equivalent fuel rod with enriched uranium. Neutron activity, arising partly as a result of spontaneous fission and partly as a result of (α, n) reactions with oxide oxygen, is also many orders of magnitude higher than that of fuel rods with enriched uranium. Consequently, the number of manipulations with MOX fuel rods should be kept to a minimum, and their presence outside shielded storage locations should be as short as possible in order to minimize the dose burden on personnel.

Summarizing this problem, it can be said that the control of MOX fuel rods includes a greater number of quality indicators and requires more accurate verification than using uranium fuel rods, but the time devoted to quality control should be as short as possible in order to minimize the radiation dose to personnel .

Most existing control methods developed specifically for testing fuel rods containing mixed oxides (including not only uranium-plutonium but also uranium-gadolinium oxides here) relate to checking the uniformity of enrichment or a given axial distribution along the fuel rod. To implement this quality control method, a single device is usually used.

One example thereof is a method and apparatus for passive gamma-ray scanning of a nuclear fuel rod described in US Pat. No. 4,822,552. The fuel rod is supplied step by step along the linear path of movement, and its natural gamma radiation (proportional to enrichment) is calculated using a number of detectors located at an equal distance from each other, whose signals are summed to obtain the calculated enrichment of each length segment along this fuel rod. The stepwise advancement of the fuel rod through this device prevents its simultaneous use for monitoring other quality indicators, as mentioned in the statement of the problem described in the final part of the section "Background of the invention".

Another example is the system described in US Pat. No. 5,108,692 for non-destructive testing of fuel rods containing axially variable enriched uranium oxide tablets and / or to fuel rods containing uranium-gadolinium oxide tablets. The fuel rod is supplied at a uniform speed through the test channel in front of a sequentially located magnetometer sensitive to gadolinium content, a background gamma radiation detector, a densitometer consisting of a gamma radiation source and a scintillation detector, and an active scanner consisting of a neutron source and a gamma radiation detector . Based on the results of these calculations, the computer determines the gadolinium content and enrichment by adjusting the characteristics of the active scanner for background gamma radiation and for the density at each location along the fuel rod. Although this system is an example of non-destructive testing of fuel rods in relation to many quality indicators, it does not solve the specific problem of MOX fuel described above in the section "Background of the invention."

Another known method for determining the mass of fissile material in a test volume is by counting neutron coincidences. The time required to ensure good accuracy precludes the use of this method of estimating local fluctuations in the plutonium content for scanning fuel rods. In practice, this method is used only for the general control of nuclear fuel by safeguards inspectors.

X-ray fluorescence systems that calculate the x-rays emitted by Pu and U find industrial applications for directly measuring the degree of enrichment in MOX fuel pellets. An example of such systems is the system described in the report “Use of X-ray fluorescence for high precision measurement of uranium and plutonium in MOX” published by CGWilkins and H. Spottiswoode, in Proceedings of the ESARDA Symposium, Sevilla, May 4-6, 1999 This system can be used to implement the passive radiometric method in a fuel rod scanner, but its sensitivity is insufficient if this scanner should operate at the throughput required by industry. In addition, compared to gamma radiation, x-ray radiation has a weaker penetrating power in these materials. Therefore, this method can be used to measure mainly the peripheral parts of tablets, and it is practically insensitive to large inclusions of fissile material or inert inclusions or to other inhomogeneities lying in the center or closer to the center of the tablet.

All these known systems allow you to determine only one or several qualitative indicators that must be checked with MOX fuel rods manufactured industrially. In this regard, they do not solve the problem described above in the section "Background of the invention."

In choosing an approach to solving this problem, it was recognized that it should cover the largest possible number of non-destructive tests of quality indicators (hereinafter referred to as "quality checks"). This minimizes the number of buffer stores that need to be provided between quality checks. Due to this, the quality control zone is reduced, which in turn reduces the capital costs of the plant for the production of MOX fuel, transportation of fuel, and thereby the radiation exposure of workers.

As for the necessary quality checks, it turned out that some of them cannot be implemented within the framework of a single comprehensive quality control of moving fuel rods, namely:

- a helium leak test involving the placement of the test fuel rods in a vacuum chamber;

- X-ray quality control of the welds of the end caps, the low sensitivity of which requires that the fuel rod remains stationary for a long time under the source of x-rays;

- control of geometric characteristics, such as deflection of the fuel rod or ovality of the end cap, during which the fuel rod must be axially rolled on a flat marble sorting table or placed in a stand for measuring the geometry of the rod.

The present invention relates to a comprehensive method for non-destructive testing of all other quality indicators.

In order to get industrial use in modern medium and large plants for the production of MOX fuel, the duration of control should be 2-4 minutes per fuel rod, which corresponds to a control speed of 2-4 cm of the length of the fuel rod per second.

According to the invention, quality features are controlled by moving the MOX fuel rod along its axis and simultaneously taking measurements and checks using only radiometric and radiographic methods.

Other details and features of the invention will be apparent from the accompanying claims and the description of the method and device shown below in the form of non-limiting examples with reference to the accompanying drawings.

Figure 1 is a schematic axial section of a fuel rod that can be checked using the method and / or device according to the invention.

Figure 2 is a simplified schematic illustration of a device that can be used to apply the proposed control method.

Figure 3 is a simplified schematic representation of a γ-radiometric detector that can be used in the proposed device, and the fuel rod under test is oriented perpendicular to the plane of this drawing.

Figure 4 is a simplified schematic section in the direction IV-IV of figure 3.

5 is a simplified schematic representation of a γ-radiographic detector, which can be used in the proposed device, and the fuel rod under test is oriented perpendicular to the plane of this drawing.

6 is a simplified schematic representation of the same γ-radiographic detector as in FIG. 5, but the fuel rod is oriented parallel to the plane of this drawing.

7 is a simplified schematic representation of a detector of contamination with alpha-active substances, which can be used in the proposed device, and the fuel rod under test is oriented perpendicular to the plane of this drawing.

Fig.8 is a simplified schematic section in the direction VIII-VIII of Fig.7.

Fig.9 is a schematic representation of the blocks of the formation, collection and processing of signals that can be integrated into the device according to the invention.

In various figures, the same positions indicate the same or similar elements of the device.

To avoid unnecessary repetitions, this device and method are described together.

According to the invention, all or most of the quality indicators are monitored or verified by moving the MOX fuel rod 1 (FIG. 2) at a strictly constant speed in front of the control devices performing various types of control, from the moment before the fuel rod 1 passes in front of the first control device, until he passed before the last control device.

Types of control performed sequentially along the path of movement of the fuel rod 1 include:

- γ-radiometry (in device 12), based on scanning and processing of natural or stimulated gamma radiation emitted by fuel pellets 2 of rod 1. It provides signals related to important quality features of a column of fuel pellets: detection of non-standard tablets and content fluctuations Pu and isotopic composition (mainly ²³⁹ Pu, ²⁴¹ Pu, ²⁴¹ Am) along the column of fuel pellets. Integrating the results of measurements along the length of the column, it shows the total Pu content in fuel rod 1. Among other things, it provides supporting initial data for accounting for nuclear materials and for a safeguards inspection;

- γ-radiography (in device 13), based on the quantitative detection and processing of gamma radiation emitted by the gamma radiation source and penetrating through the fuel rod 1. It provides signals related to controlled quality indicators of the rod design: interstitial gaps, length a column of fuel pellets, the presence and location of insulating tablets 8 at the end or ends of this column, the presence of a gas collector spring 7 and a gas collector length;

- α-radiometry (in device 14), based on the calculation of alpha radiation from the surface of the fuel rod, in order to measure its local contamination with plutonium and, by summing, the total contamination with plutonium;

- electro-optical reading (in device 15) of the identification code or codes of the fuel rod;

- the formation and collection of signals, as well as the calibration and calculation of all received signals in order to adequately determine their location along the fuel rod 1, comparing these signals with the limits taken from the technical conditions on the fuel rod, or conversion to units used subsequently (% Pu, mm, Bq / cm ² , ...), and integrating the results regarding the features related to the entire fuel rod.

The sequence of arrangement and numbering of devices 12-15, as they are shown in figure 2, do not matter and can be changed.

A constant speed V of the fuel rod 1 is preferred for carrying out the invention. Alternatively, you can select the stepwise movement of the fuel rod 1 in front of the control devices 12-15. However, the stepwise movement cannot be adapted to detect non-standard tablets partially overlapping the collimating window of the control devices 12. In addition, when the required test speed is reached, the stepwise supply of the fuel rod 1 can cause it to shake, which may lead to a violation of the continuity of the column of tablets and / or damage to some tablets in their places contact with each other. Two pairs of rollers 16, 17 schematically depict a drive for supplying a rod to the proposed device.

γ-radiometry (more detailed examples of which are shown in FIGS. 3 and 4) used to detect non-standard tablets is preferably based on the measurement of gamma radiation emitted from radioactive decay of plutonium and americium isotopes, a simplified spectrometric method using one or more gamma -detectors 21 and various discriminators in energy. Non-standard or inappropriate tablets are those tablets that differ from the specified tablets in the fuel rod 1 by the degree of enrichment and / or isotopic composition. Gamma detectors 21 can be either scintillators or solid state semiconductors, for example, Si, Ge, CdTe, ....

NaI (TI) - crystal 21 is preferred for this embodiment of the invention because of its high detection sensitivity and suitability for operation at room temperature, as well as the possibility of producing large single crystals at low production costs. However, compared with two other semiconductor detectors, its ability to distinguish between gamma radiation with different energies is relatively small. As outlined below, this inconvenience can be compensated for by the choice of a large gamma-ray detection channel characteristic of a single Pu isotope.

Ge detectors have much better energy resolution, but suffer from the disadvantage that they must be used at very low temperatures, usually achieved by cooling them with liquid nitrogen. For these reasons, their use is limited to cases involving complex gamma-ray spectra or requiring detailed information on the isotopic composition of materials. In this regard, the scope of their use is practically limited to supervision of compliance with safeguards, laboratories and research centers. You might also consider using GeLi detectors.

CdTe and CdZnTe detectors do not require cooling and have the highest intrinsic detection efficiency of these three types of detectors. Their very high energy resolution speaks in their favor as a “candidate” for use instead of NaI detectors whenever this is due to the characteristics of the manufactured MOX fuel (for example, low content of ²⁴¹ Am and ²⁴¹ Pu). Consideration may also be given to using known CZT detectors.

The identification of custom tablets is preferably based on the measurement of isotopes that noticeably affect the spectrum of gamma radiation naturally emitted by this fuel (i.e. ²³⁹ Pu, ²⁴¹ Pu, ^total Pu and AM) using the following selection of radiation detection channels:

- ²⁴¹ Am strongly affects the low- and high-energy parts of the spectra (below 100 keV and above 500 keV);

- ²⁴¹ Pu (due to radioactive decay of ²³⁷ U) is the cause of most of the radiation in the range of 150-350 keV;

- ²³⁹ Pu is the cause of most of the radiation in the range from 350 to 500 keV;

- ^total Pu is the cause of the total gamma radiation in the range from 90 to 500 keV.

All these isotopes (U, Pu, Am) directly and indirectly contribute by emitting their x-ray and γ-radiation. This radiation is induced by the natural radioactivity of this fuel and, thus, reflects the overall degree of enrichment.

The gamma radiation detected by gamma detectors 21 is collimated by stationary collimators 23, the width of which is calculated taking into account the tablet length, plutonium content and isotopic composition, the number and size of the detectors 21 and the speed of the fuel rod V 1. NaI detectors (or CdTe detectors ) 21 can also be equipped with cadmium filters to reduce the calculation of low-energy gamma radiation. You can integrate as many gamma-detectors 21 as you need in order to ensure the necessary accuracy of quality control or reproducibility of its results.

A possible embodiment of the gamma detector includes a ring scintillator 21, a number of associated photomultipliers 20, a ring cadmium filter or screen 24 to attenuate the ²⁴¹ Am signal, collimators 23 and mechanical guides 25 for the fuel rod 1, as shown schematically in FIG. 3 and / or 4.

Given the typical gamma-ray spectra of various types of plutonium, it becomes clear that the statistical significance of the measurements associated with high-energy radiation detection channels is small due to the reduced count rate. At the other end, low-energy radiation is strongly absorbed by the fuel itself and the shell of the rod 3. This means that the radiation detection channel in the range of 150-350 keV is the most effective, and the measurements should be correlated with the content of ²⁴¹ Pu in tablets.

Measuring the gamma radiation emitted by ²⁴¹ Am and Pu with a narrow detector and detecting their low / high transient signals also allows obtaining information about the length and location of the column of fuel pellets.

Whenever the isotopic composition of plutonium processed into MOX fuel is unfavorable for its detection by passive γ-radiometry, described below, active γ-radiometry, which is specially discussed in this description, can be used. This is the case, for example, in the production of MOX fuel from weapons-grade plutonium, mainly consisting of ²³⁹ Pu, a small amount of ²⁴⁰ Pu and a small amount of ²⁴¹ Pu, but almost not including ²⁴¹ Am. Active γ-radiometry based on the activation of fuel by thermal neutrons from an external neutron source (for example, ²⁵² Cf or ²⁴¹ AmLi) and the measurement of gamma radiation resulting from neutron-induced fission reactions mainly ²³⁹ Pu, to a lesser extent ²⁴¹ Pu and to a small extent ²³⁵ U. Thus, it provides a direct measurement of the content of fissile material. This measurement can be performed by any of the above detectors.

γ-radiography (explained in more detail in FIGS. 5 and 6) is based, in a possible embodiment, on the measurement of at least one gamma detector 34 and at least one photomultiplier 35 of the penetrating radiation emitted by the gamma source - radiation 30 ²⁴¹ Am or ¹³⁷ Cs installed along the fuel rod 1 and narrowly collimated by the first collimator 32 near the source 30 and the second collimator 33 located in front of the scintillator 34.

The choice of gamma detectors 34 with a NaI crystal (TI) is preferred for this embodiment because of their counting efficiency, suitability for operation at room temperature, and low temperature sensitivity. The use of ²⁴¹ Am or ¹³⁷ Cs isotopes as sources of gamma radiation 30 is preferred because of their long half-life and their availability.

The necessary associated photomultiplier 35 and source screen 31 are shown in FIGS. 5 and 6.

The control of some quality parameters is based on the detection of low / high count transitions after appropriate processing, while other types of control are based on the count level. The location of the upper and lower ends of the fuel rod is measured by a low / high junction in one or more photocells. Low / high transitions of signals are converted to lengths using the constant speed of a moving rod.

It provides the required data on the length and location of all internal components of the fuel rod, such as the presence of a spring and insulating tablets, as well as the presence, location and size of any gap between the components and inside the column of fuel tablets. In addition, it provides data on the total length of the rod and the length of the gas collector. It facilitates the location of the relatively lower end plug of signals related to various qualitative parameters controlled by γ-radiometric methods. When necessary, data on the length of the rod, the location of components or clearances can be adjusted taking into account the temperature of the shell of the rod.

Some features are verified by sharing both gamma radiography and radiography. For example, the presence of pellets in the reproduction zone at the edge of a column of MOX fuel can be derived from measurements of the total length of a column of pellets using radiography, and the total length of a column of MOX fuel pellets using radiometry.

The aforementioned γ-radiographic method can be used to monitor certain parameters of the tablet manufacturing process, for example, tablet density.

α-Radiometry, which is explained in more detail in Figs. 7 and 8, is based, in a possible embodiment, on the detection of alpha particles by a ZnS (Ag) scintillation detector 41, coated with thin translucent sheets 42 and having a number of photomultipliers 40. The measured contamination is the sum of the residual fixed and tolerated pollution with alpha-active substances on the surface of the fuel rod 1. In view of the short distance of the propagation of α-radiation in air, the device 25 for the mechanical direction of the rod 1 should be chnym. You can also consider the possibility of using scintillators on CaF ₂ (Eu), Gd ₂ O ₂ S, Y ₂ O ₂ S, doped with Tb or Eu.

It should be understood that the present invention is in no way limited to the above-described embodiments, and its many modifications can be made without departing from the scope of the claims set forth below.

Electro-optical identification (device 15 in FIG. 2) consists in computer processing of image markings engraved on one of the end caps of the rod to indicate the degree of fuel enrichment, and / or in computer processing of images of a human or machine readable identification code provided on the surface of the rod, and / or in reading the identification bar code printed on the surface of the fuel rod.

More sophisticated image processing even allows for adequate lighting to detect substandard scratches on the surface of moving fuel rods.

Another possible example of the implementation of the γ-radiometric method can be based on complex γ-spectrometry, as is well known to specialists.

It should be noted that these methods, i.e. γ-radiometry, γ-radiography, α-radiometry, electro-optical identification of the rod, can be combined with other non-destructive testing methods to simultaneously perform comprehensive quality control of MOX fuel rods.

Non-limiting examples of the individual instruments and devices used to implement this integrated quality control method are provided below to explain this method.

A constant speed V of the fuel rods 1 in front of the control devices 12-15 can be achieved using a driving mechanism with cargo rollers 16, 17 driven by two stepper motors with precise electronic synchronization, one of them located on the loading side and the other on the output side side.

The rods 1 are loaded from the loading table and unloaded to the unloading table, with monolayers of the rods, thereby providing self-shielding of control devices.

In order to improve the count of natural γ-radiation in order to obtain appropriate accuracy, the ring geometry of the NaI detector or CdTe detector 21 is often preferred over asymmetric geometry with regard to the implementation of the present invention. For the same purpose, a number of photomultipliers 20 attached to a ring NaI detector or CdTe detector is also preferred, as well as a number of detectors 2 mounted in series. In this latter case, the distance between the detectors in series is set so that it is a multiple of the distance that results from multiplying the gating time by the speed of the rod; summation of the transmitted signals is performed after a time shift. In this case, serial detection devices including a scintillator, a collimator, photomultipliers can be designed to detect one specific Pu or Am isotope in order to optimize the associated characteristics (the latter are the result of optimizing the collimation width and the thickness of the cadmium filter depending on the gamma radiation of this isotope, scintillator reaction and rod velocity).

Similarly, to improve alpha particle counting to ensure its suitable accuracy, the ring geometry for the ZnS detector 41 is often preferred to asymmetric geometry with regard to the implementation of the present invention. For the same purpose, a number of photomultipliers 40 attached to an annular ZnS detector is also preferred.

As for γ-radiometric and γ-radiographic measurements, the collimators 23, 32 and 33 are usually made of tungsten or machined tungsten alloys.

As a non-limiting example, a system for generating, collecting and processing radiation signals (Fig. 9) transmitted by photomultipliers 20 or 40 includes a summing 50, preamplifier 51 and amplifier 52 blocks, an optional pulse magnitude analyzer 53, a single-channel analyzer 55, a spectrum stabilizer 54, a counter 56, and a device for interfacing 57 with the computer 58. All of these devices are widely used in radiometry. Figure 9 shows a possible combination of these blocks.

The spectrum stabilizer 54 is tuned to a specific energy peak of either MOX fuel or a gamma radiation source built into the scintillator in order to correct various drifts of the measuring devices. For this purpose, mainly NaI crystals doped with ¹³⁷ Cs or CdZnTe crystals can also be used.

Periodically, this system can be calibrated by passing any standard and / or reference fuel rod through this device, containing all the characteristic features and quality features to be detected. Alternatively, this calibration can also be done by computer simulating gamma radiation and transmitting active gamma radiation.

This simulation can be performed on any microcomputer that has parallel and serial interfaces for signal collection and corresponding peripheral devices for outputting reports and archiving data.

The above γ-radiometric and γ-radiographic methods can also be used to control MOX fuel rods for fast breeder reactors containing a reproduction zone UO ₂ , as well as MOX fuel rods for light-water nuclear reactors containing plutonium with different enrichment inside the same same shell.

Although the above invention relates to quality control of MOX fuel rods, it is clear that it can also be used to control fuel rods made from any plutonium-containing fuel pellets, such as

- plutonium-containing fuel with burnable or non-burnable sinks, for example B, Gd, Er, Hf;

- mixed oxide fuel (MOX) made from thorium and plutonium;

- plutonium nitrides and carbides;

- inert matrix plutonium-containing fuel for burning plutonium.

The feasibility of implementing the present invention was demonstrated on MOX fuel rods using the following hardware:

- a combination of two mechanisms with a stepper motor, moving the rod 1 at a speed of 3 mm / s, constant at 1/1000;

- γ-radiometric measuring system, including 3 ring NaI (TI) detectors, each equipped with a 15 mm wide collimator, 1 mm thick cadmium filter, two photomultipliers and three radiation detection channels;

- a γ-radiographic detection device consisting of a ²⁴¹ Am source with an activity of 300 mCi and a thin asymmetric NaI (TI) detector equipped with a collimator 2 mm wide and one photomultiplier;

- a device for detecting α particles, consisting of a ring scintillation ZnS (Ag) crystal and equipped with one photomultiplier;

- a unit for generating, collecting and pre-processing a signal optimized for channels with a length of 10 mm; and

- a microcomputer with a Pentium processor from Intel, running under the Windows NT operating system and having a program in language C;

and allowed to detect non-standard tablets, inconsistencies in the structure of the rod, i.e. such as those related to clearances, springs, gas collector, and contamination of the surface of the rod with the following characteristics reported for a confidence level corresponding to one standard deviation:

* reproducibility, measured by repeatedly checking the calibrated rod:

fuel rod length 0.2 mm pill bar length 0.4 mm gas pan length 0.2 mm clearance length (clearances> 0.5 mm) 30 rel.% ^total Pu per channel 0.8 rel.% ²³⁹ Pu per channel 0.8 rel.% ²⁴¹ Pu per channel 0.7 rel.% ²⁴¹ Am per channel 0.8 rel.% alpha active pollution substances (for general activity> 75 Bq  on the whole core): 20 rel.%

* detection limits:

non-standard tablet: any tablet that has one or more deviations on

7% relative to neighboring measurements for ²³⁹ Pu, ²⁴¹ Pu and / or ^total Pu, and 15% for ²⁴¹ Am

alpha pollution: 15 Bq (whole core) gap: 0.5 mm

* throughput: one 4 m long fuel rod every 2 minutes.

Claims (14)

1. A method for implementing comprehensive quality control of an MOX fuel rod, comprising a combination of at least two of the following steps: measuring the plutonium content in the tablets contained in said rod; checking for the presence of non-standard tablets in the indicated rod; checking the internal structure and components in the specified rod; measuring the geometric characteristics of the specified rod; measuring radioactive contamination along the surface of the specified rod and checking for compliance of the identifying data of the rod with plutonium enrichment, characterized in that the fuel rod is moved along the axis; at least two stages are carried out simultaneously along the fuel rod moving along the axis; use radiometry and / or radiography, and / or electro-optical identification for the above control. 2. The method according to claim 1, characterized in that when carrying out the indicated measurements and checks, the fuel rod is continuously moved at a constant speed. 3. The method according to claim 1, characterized in that the plutonium content is simultaneously measured and the presence of non-standard tablets is checked by scanning the natural gamma radiation emitted by the plutonium and americium contained in the tablets using at least one NaI and / or CdTe- detectors. 4. The method according to claim 1, characterized in that the plutonium content is simultaneously measured and the presence of non-standard tablets is checked by scanning the natural gamma radiation emitted by the plutonium contained in the tablets after activation by thermal neutrons from a ²⁵² Cf or ²⁴¹ AmLi source, the measurement is performed by at least one NaI and / or CdTe detectors. 5. The method according to one of claims 3 or 4, characterized in that the aforementioned gamma radiation is detected using several detectors, the signals of which are recognized using several radiation detection channels; shift the transmitted signals in time and summarize these signals shifted in time. 6. The method according to one of claims 1 or 2, characterized in that the internal structure is checked by exposing the rod to an external gamma radiation source of ²⁴¹ Am or ¹³⁷ Cs and scanning the penetrating gamma radiation with one gamma detector. 7. The method according to one of claims 1 or 2, characterized in that the external contamination of the fuel rod with alpha-active substances is measured with a single ring detector without contact with it. 8. The method according to one of claims 1 or 2, characterized in that they verify that the identity of the rod matches the content of plutonium by electro-optical reading or image processing of the identification code or codes. 9. The method according to claim 1, characterized in that the fuel rod is moved using two independent but precisely synchronized drive mechanisms, one of which is mounted on the loading side of the device for performing the indicated at least two stages, and the other on its output side so that the fuel rod is continuously advanced through said device from one end plug of the fuel rod to the other, both of which are located at a distance shorter than the length of the rod nya. 10. A device for comprehensive quality control of an MOX fuel rod, comprising means for moving the fuel rod along its axis at a constant speed, including at least two independent but precisely synchronized drive mechanisms, one of which is located on the loading side of the rod control device, and the other on the output side of the specified device; a number of measuring and control systems installed in one line along the path of movement of the specified fuel rod and working simultaneously; and said systems are selected from the group consisting of radiometry, radiography, and electro-optical identification systems, and further comprises at least one of NaI and / or CdTe detectors configured to measure plutonium content and detect non-standard tablets by scanning natural gamma radiation emitted by plutonium and americium contained in tablets. 11. The device according to claim 10, characterized in that for measuring plutonium content and detecting non-standard tablets, the device contains a ²⁵² Cf or ²⁴¹ AmLi source of thermal neutrons for plutonium activation and at least one NaI and / or CdTe detector for scanning gamma radiation emitted by activated plutonium contained in tablets. 12. The device according to claim 10, characterized in that the device is equipped with at least one annular detector configured to measure plutonium content and detect non-standard tablets. 13. The device according to claim 10, characterized in that the device is equipped with several discriminators in energy and means for summing time-shifted γ-samples. 14. The device according to claim 10, characterized in that for measuring radioactive contamination of the shell of the rod, the device includes at least one ring alpha particle detector for scanning alpha radiation emitted by the outer surface of the fuel rod.

Images