Calorimeter for measuring the residual linear power density of at least one portion or one piece of a spent or new nuclear fuel rod

Abstract

The invention essentially consists of a calorimeter, the core of which is delimited by a block of material for absorbing radioactive and thermal radiation surrounded by a housing made of thermal insulation material which thermally insulates the core from the ambient environment, the block and the housing each consisting of two parts which can be moved apart from each other in order to easily and quickly insert a spent fuel rod. The calorimeter is instrumented with temperature measurement sensors which are distributed along it and which are inserted into the block and / or into the housing in order to measure the residual linear power density of the inserted rod.

Description

[0001] 1

[0002] Description Title: Calorimeter for measuring the linear residual power of at least one portion or piece of a spent or new nuclear fuel rod. Technical field 5 The present invention relates to the field of fuel rod instrumentation for nuclear reactors. The invention essentially aims to provide a device that can measure, in a simple and reliable manner, the burnup rate or the residual power of a spent nuclear fuel rod. 10 By "nuclear reactors", throughout the application, is understood the usual meaning of the term to date, namely power plants for producing energy from nuclear fission reactions using fuel elements in which fissions occur which release the heat power, the latter being extracted from the elements by heat exchange with a heat transfer fluid which ensures their cooling. 15 By "nuclear fuel rod",throughout the application, the official meaning defined for example in the dictionary of Nuclear Sciences and Technology is understood to be a narrow tube of small diameter, closed at both ends, constituting the core of a nuclear reactor and containing fissile material. Thus, a "nuclear fuel rod" is a nuclear fuel rod but the terminology is used20 for fast neutron reactors. The invention is applicable to any nuclear fuel rod or rod. By "spent fuel rod" is meant a fuel assembly irradiated to such a degree that it cannot be used subsequently in a reactor without having undergone appropriate treatment. In other words, by spent fuel rod is meant a rod in which there is no longer sufficient fissile material to maintain nuclear reactions within a reactor. The invention relates to all fuel rods,in particular of the ceramic type made of uranium or uranium and plutonium oxide (U, Pu)O2 or MOX (acronym for "mixed oxide"), which can be dedicated to all types of nuclear reactors for electrogenic and / or calogenic or experimental purposes, such as Boiling Water Reactors (BWR), Pressurized Water Reactors (PWR) and all advanced 3rd and 4th generation reactors.Previous technique In the sites of nuclear reactors and irradiated fuel processing plants, there are counting wells which make it possible to estimate the combustion rates of the entire irradiated fuel assembly. The combustion rate or "burn-up" characterizes both the degree of irradiation and the energy supplied by the fuel when it leaves the reactor. It is expressed as an average for an object (average of the refill,average in the fuel assembly). Patent FR2988837B1 thus proposed a device for measuring the residual power of a spent fuel assembly before its unloading from the vessel of a liquid metal-cooled reactor, such as an RNR-Na or SFR reactor (English acronym for "Sodium Fast Reactor"). To date, there is no simple measurement in the nuclear industry of the burnup rate or residual power for a spent fuel rod in its intact state, i.e. whose casing has not been physically modified, in particular by cutting. After passing through a reactor, a fuel has been the site of nuclear reactions which have resulted in a particular composition of more or less radioactive isotopes and dissipating an energy specific to each. These energies are linked to the different radioactive radiations (alpha, beta, gamma, X,neutron) and they are deposited in the surrounding matter at a greater or lesser distance. The sum of these energies dissipated over a period of time constitutes a residual power which can vary throughout the rod which is specific to the isotopic composition of the fuel and it evolves over time. When this power is associated with a given length, we then speak of the linear residual power of a fuel rod. This linear power value contributes to the validation of neutron calculation codes which simulate the passage in the reactor and calculate a composition of a fuel rod according to the fission yields. However, this linear power measurement also serves as basic data in the design and operation of nuclear reactors but also throughout the downstream part of the nuclear cycle (storage, transport,reprocessing and storage). Research and tests on residual power measurement have been carried out since the 1950s. It appears that measuring residual power is a good way to trace and qualify the decay of isotopes in irradiated fuel and it is the method using elementary fission curves that was initially used. Some experimental measurements, in the laboratory, on fuel rods have been carried out. We can cite the experiment called "MERCI" which consisted of placing a portion of irradiated fuel rod in a calorimeter called "CALMOS". This experiment allowed a single measurement of the residual power of this portion of rod: [1], [2]. The burnup rate was 3.5 GWd / t and the measurement could take place for cooling times between 40 minutes and 40 days. Another experiment was carried out in the Swedish laboratory for deep storage studies,on an assembly placed in a calorimeter with water and where the temperature was recorded throughout its rise: [3], [4]. The combustion rate was between 30 and 50 GWj / t and the measurement could take place for cooling times of a few years to several decades. Another experiment called "PRESTO" is planned: [5]. This experiment will consist of irradiating a portion of fuel rod in the Jules Horowitz experimental reactor (RJH) and, at the end of the irradiation,to insert this portion into a tungsten cylinder to then measure the temperature rise. Processing this temperature rise using an inverse numerical method will allow the residual power to be determined. The power measurement is similar to the reaction heat measurements which are determined essentially in calorimeters using differential scanning calorimetry (DSC), which is a thermal analysis technique. This measurement consists of recording the differences in heat exchange between a sample to be analyzed and a reference. DSC devices as well as thermobalances operating in DSC are extremely fragile devices which are not intended to operate in shielded cells and are therefore unsuitable for spent fuel rods. A so-called Langavant calorimeter is also known, which consists of a semi-adiabatic vessel,closed by an insulating cap and placed in a rigid envelope, and which is used to measure the hydration heat of mortars: [6]. A mortar box is filled with a grout, instrumented with temperature sensors and the whole thing is placed in the vase. Since the setting of mortars is a slow process, the measurement can be carried out over several days. Therefore, the thermal leakage of this calorimeter is determined beforehand. This calorimeter also requires a relatively tight closure. Such a device is not really suitable for nuclear fuel rods. Thus, there is a need to propose a device that is both simple and reliable for measuring the residual power of nuclear fuel rods. The aim of the invention is to meet this need at least in part. Disclosure of the inventionTo do this, the invention relates, in one of its aspects,a device for measuring the linear residual power of at least one portion or piece of a spent nuclear fuel rod of a spent or new fuel rod, forming a calorimeter extending around a central axis and comprising:- a block with two block portions made of thermal and radioactive radiation absorption material, each in the form of a cylindrical crown portion closed at each of its longitudinal ends by a cylindrical portion pierced in its center by a through hole, the two block portions being able to be brought closer to each other from an open position in which they are spaced apart from each other to allow the insertion and housing of a spent fuel rod,to a closed position in which they are folded back on each other, delimiting a radioactive absorption chamber of cylindrical shape adapted to house at least part of the spent fuel rod with its other parts which extend on either side of the chamber, from the through holes forming a cylindrical passage;- a casing comprising two shells of thermal insulating material, each in the form of a portion of a cylindrical crown closed at each of its longitudinal ends by a cylindrical portion pierced in its center by a through hole,the two shells being able to be brought together from an open position in which they are spaced apart from each other to allow the insertion and housing of the radioactive absorption chamber to a closed position in which they are folded back on each other, delimiting a housing adapted to house the radioactive chamber with the other parts of the fuel rod which extend on either side of the housing from the cylindrical passage of the block passing through the through holes also forming a cylindrical passage; - temperature measuring sensors, mounted in a cross-section through one and / or the other of the portions of the absorption block and through one and / or the other of the shells and adapted to measure the temperature along at least the part of the spent fuel rod in the closed positions of the block and the housing. Preferably,the thermal and radioactive radiation absorption material of the block is lead (Pb) or a lead alloy. The linear power is linked to the radiation of the nuclear fuel, some of which easily passes through any low-density material. The choice of lead or a lead alloy for the block is advantageous because it allows the absorption of radiation, thus causing a rise in temperature in the material. Usually used in the nuclear field to protect against radiation because it absorbs, lead has two important advantages: high density and abundance, which make this material inexpensive. Lead alloys are readily available on the market and have the advantage of being easy to work with and whose shaping poses no problem during ingot casting, machining, and drilling, which is perfectly suited to the design of the block according to the invention. In addition,we can know precisely the heat capacity of this material to calculate the power. For example, a block of 96% by mass of lead and 4% by mass of antimony has a heat capacity which is a linear combination between these two materials, i.e.: Cp = 0.96x0.129 + 0.04x0.210 = 0.132 Jg-1.K-1. The thickness of lead can be about 5 cm, which allows to absorb the majority of the radiations which thus deposit their specific energies there. Thus, in addition to the function of absorbing radioactive radiation, lead presents a very good compromise between compactness, machinability and low cost. The thickness chosen for the block can be a compromise which takes into account the handling capacity of standard telemanipulators, in a shielded cell, for example of the order of 7 kg. To close perfectly around the pencil, the block is preferably machined leaving a recess corresponding to the internal geometry of the pencil. In order to lighten the upper block portion,one or more centering pieces can be used to reposition the top portion. The bottom block portion is preferably drilled in different places to allow the insertion of temperature sensors. According to an advantageous variant, the block further comprises an adapter made of thermal and radioactive radiation absorption material and in the form of a cylindrical crown portion with an internal diameter suitable for housing a spent fuel rod and an external diameter suitable for housing in one of the two block portions. This adapter makes it possible to adapt to any existing nuclear fuel rod geometry. According to an advantageous variant, the two block portions made of thermal and radioactive radiation absorption material, such as lead (Pb) or a lead alloy,have male / female machining which promotes self-centering and a perfect fit of the two portions. This variant contributes greatly to the speed of measurement implementation and therefore to the calculation of the linear residual power. In other words, this improves the accuracy of the measurements. According to an advantageous embodiment, the calorimeter comprises an outer protective casing with two casing portions each housing, with complementary shapes, one of the two shells made of thermally insulating material of the housing. Preferably, this outer protective casing is made of stainless steel. Stainless steel is a very good compromise between, on the one hand, the protection, support and adjustment between the block and the housing sought and, on the other hand, the precise guidance of the temperature sensors that it allows. In addition, this material is easily cleaned. According to an advantageous variant,the calorimeter comprises at least one hinge around which the two shells of the housing are articulated together, so as to bring them closer or further apart, from the open position to the closed position or vice versa. Advantageously, the calorimeter comprises at least one means for locking the shells in the open position. This facilitates the insertion of a pencil into the calorimeter, which is advantageous in particular in the case of remote handling in a shielded cell. Preferably, the thermal insulating material of the housing is polystyrene. More preferably, the thermal insulating material of the housing is in direct contact with the thermal and radioactive radiation absorption material in the closed position. With this dimensional adjustment, no air layer is present between the block and the housing in their closed position, which is optimal for the interpretation of the measurements. In other words,this adjustment guarantees the absence of heat transfer by gas convection between the block and the housing in their closed position. Advantageously, the temperature measuring sensors are resistance probes and / or thermocouples. Advantageously again, the sensors are guided and blocked by cable glands positioned outside and / or inside the housing. According to another advantageous embodiment, the calorimeter comprises a self-supporting support frame, adapted to support the block, the housing, the temperature sensors and the spent fuel rod, and where appropriate the external protective casing. This support frame which can advantageously be in the form of cradles or feet to support the entire calorimeter and keep it perfectly still during the loading of a fuel rod or during the measurement. Thus, the support frame can be placed directly on the ground,in particular of a shielded cell without having to provide a suitable support within the cell. The invention also relates to a shielded cell housing at least one calorimeter as described above, the electrical wires connected to the temperature sensors being connected to connectors mounted across the wall of the shielded cell for the transmission of signals from the sensors to an analog / digital converter outside. Thus, the processing of the measurements is done remotely, protected by the shielded cell. The invention also relates to the use of a calorimeter as described above or of the shielded cell as described above, for measuring the linear residual power of at least a portion or piece of a spent or new fuel rod whose fissile material is chosen from uranium (IV) oxide (UO2), mixed oxide (U,Pu)O2 or a mixed mixture based on uranium oxide and reprocessed plutonium oxides (MOx) or any other radioactive or thermal source based on fission products, actinides or activation products. Thus, the invention essentially consists of a calorimeter whose core is delimited by a block of material for absorbing radioactive and thermal radiation surrounded by a casing of thermal insulation material which makes it possible to thermally isolate the core from the ambient environment, the block and the casing each being in two parts which can move away from each other for the easy and rapid introduction of a spent fuel rod. The calorimeter is instrumented by temperature measurement sensors distributed along and which are inserted into the block and / or into the casing for a measurement of the residual linear power of the inserted rod. A calorimeter according to the invention has numerous advantages among which one can, citer :- a simple and reliable measurement of the linear residual power of a spent fuel rod, integrated, without prior sampling or cutting of the measurement,- a measurement that can be carried out directly in a shielded chain during storage of a rod after its passage in a nuclear reactor,- the possibility of a non-destructive examination on the same spent rod, which can be carried out in the same place at different times or periods,- a robust device without interaction with the radioactivity of the ambient environment,- the possibility of increasing the precision by statistical processing of several measurement values,- the interchangeability of the internal components, in particular the adapter, allows adaptation to any geometry of nuclear fuel rod,- an easy and rapid change of the measurement sensors in the event of failure,- an implementation of the calorimeter in an existing shielded chain.,- a possibility of remote manipulation of the calorimeter and, beforehand, of making it enter very simply into a shielded cell through a standard airlock by the waste bin transfer device. Other advantages and characteristics of the invention will emerge more clearly on reading the detailed description of examples of implementation of the invention given for illustrative and non-limiting purposes with reference to the following figures. Brief description of the drawings [Fig 1] Figure 1 is a schematic view in longitudinal section of a nuclear fuel rod according to the state of the art, as implemented in a PWR type nuclear reactor. [Fig 2A], [Fig 2B] Figures 2A and 2B are photographic reproductions of a calorimeter according to the invention, respectively in the open position of its casing and closed around the block of radioactive radiation absorption material.[Fig 3A], [Fig 3B] Figures 3A and 3B are schematic cross-sectional and longitudinal views of a calorimeter according to the invention, in the open position of both its casing and the block of radioactive radiation absorption material in which a nuclear fuel rod is housed.[Fig 4] Figure 4 is a schematic side view of a calorimeter according to the invention, with the different sides where temperature sensors are inserted.[Fig 4A], [Fig 4B], [Fig 4C], [Fig 4D], [Fig 4E] Figures 4A to 4E are cross-sectional views of the calorimeter of Figure 4, respectively along AA, BB, CC, DD, EE.[Fig 5] Figure 5 illustrates in the form of curves the temperature readings measured experimentally by the different temperature sensors according to Figures 4A to 4E.Detailed description It is specified that throughout the application, the terms "vertical", "lower", "upper", "lower", "higher", "below" and "above" are to be understood by reference to a calorimeter as it is in the horizontal configuration of use, that is to say housing a fuel rod horizontally, and which opens along a horizontal median plane. In Figure 1, a conventional nuclear fuel rod 1 is shown in its configuration of use in a PWR nuclear reactor, that is to say in a vertical position with the pellets 6 towards the lower part as specified below. The rod 1 consists of a cladding 2 conventionally made of Zircaloy-4 (Zr4) closed at each of its ends by a respectively upper 3 and lower 4 plug which is welded thereon.This sealed rod is filled with helium, typically at 25 bars when cold for usual fuels, to partially counterbalance the effect of the external pressure of 150 bars of the heat transfer fluid. The interior of the cladding is essentially divided into two compartments, one of which 5 in the upper part, between the top of the fissile column and the upper plug 3, constitutes a gas expansion chamber and the other houses the fissile column formed by the stack of nuclear fuel pellets 6 which each extend in the longitudinal direction XX' of the rod 1. The expansion chamber is a free volume intended to receive the Fission Products in gaseous form, usually called Fission Gas (FG). In the stack shown, each pellet 6 has substantially the same length or height H.A helical compression spring 7, generally made of Inconel®, is housed in the expansion chamber 5 with its lower end resting against the upper face of the pellet 6, the highest in the stack of pellets, and its other end resting against the upper plug 3. In addition to maintaining the stack of pellets 6 along the longitudinal axis XX' and absorbing the longitudinal swelling of the pellets 6 over time, the other function of this spring 7 is to prevent buckling of the cladding section in its ovalization mode. In other words, it must prevent extreme ovalization of the cladding section. The primary function of a fuel rod is to produce, then transmit the heat produced by the fission reactions within the fuel. Once fuel rod 1 is used up, when it leaves the reactor it releases a residual power, which, associated with a determined length of the stack of pellets 6, is a linear power.Figures 2A, 2B, 3A, 3B show an example of a device forming a calorimeter 10 according to the invention which makes it possible to quickly and reliably measure such a linear power, without prior sampling or cutting of the rod. For example, this calorimeter 10 may have an external diameter Ø equal to 22 cm and an overall length L equal to 27 cm. Such a geometry of the calorimeter 10 makes it compatible with its transport in a container in a sealed double-door airlock of usual size of 270 mm in diameter.The calorimeter 10 extending around a central axis (X1) in an axisymmetric shape essentially comprises from the inside to the outside a block 11 made of thermal and radioactive radiation absorption material, a casing 12 made of thermal insulating material for housing the block 11 and a protective casing 13 in which the casing is housed and fixed, as well as temperature sensors C1 to C10 inserted and mounted in a crossing manner through the absorption block and / or the casing and adapted to measure the temperature along at least the part of the spent fuel rod in the closed positions of the block and the casing. The block 11 comprises two block portions 110, 111 each in the form of a cylindrical crown portion closed at each of its longitudinal ends by a cylindrical portion pierced in its center by a through hole 112.The two block portions 110, 111 can be brought together from an open position in which they are spaced apart from each other to allow the insertion and housing of a spent fuel rod 1, to a closed position in which they are folded back on each other, delimiting a radioactive absorption chamber of cylindrical shape adapted to house at least a part of the spent fuel rod with its other parts which extend on either side of the chamber, from the through holes 112 forming a circular passage. An adapter 113 also a thermal and radioactive radiation absorption material can be housed in the lower portion 111 of the block. This adapter has a semi-cylindrical crown shape whose internal diameter is adapted to house a spent fuel rod 1. The calorimeter can thus be adapted to any existing rod geometry.For a piece of pencil, this adapter can be closed at these axial ends over a length approximately equivalent to the radius of the block 111. It is specified that with or without such an adapter, the dimensioning is carried out so as not to leave a layer of air around the pencil 1. The thermal absorption material and radioactive radiation of the block 11 is preferably lead (Pb) or a lead alloy. For example: - the material of the block 11 is an alloy of 96% by mass of lead and 4% by mass of antimony; - in its closed position, the block 11 is a cylindrical crown 5.32 cm in diameter and 15 cm long and 5 cm thick; - to lighten the upper portion 110 which is the one to be handled for the insertion of a pencil, it is produced, in particular machined on a smaller portion than the lower portion 111.The upper portion 110 can thus be machined at an angle of approximately 110° and therefore 250° for the lower portion 111; - the mass of the lead block 11 is of the order of 13 kg, of which approximately 8 kg for the lower portion 111 and approximately 5 kg for the upper portion 110, which allows handling by a standard remote manipulator; - the lower portion 11 can be pierced at 6 different locations to allow the passage of as many temperature measurement sensors. The housing 12 comprises two shells 120, 121 made of thermal insulating material, each in the form of a semi-cylindrical crown closed at each of its longitudinal ends by a cylindrical portion pierced in its center by a through hole 122.The two shells 120, 121 can be brought together from an open position in which they are spaced apart from each other to allow the insertion and housing of the radioactive absorption chamber to a closed position in which they are folded back on each other to delimit a housing 123 adapted to house the radioactive chamber delimited by the block 11 with the other parts of the fuel rod which extend on either side of the housing from the cylindrical passage of the block passing through the through holes 122. The lower portion of block 111 is adjusted in height in the calorimeter in order to perfectly align the through holes 122 with those 112 of the block. The lower shell 121 is positioned in a cradle. When the calorimeter is closed, the two horizontal surfaces of the shells 120, 12 are in contact over their entire surface. They are drilled to allow the passage of temperature sensors.The thermal insulating material of the housing 12 is preferably polystyrene. In the closed position, the thermal insulating material of the housing 12 is directly in contact with the material of the block 11 for absorbing heat and radioactive radiation. For example: - the polystyrene has a thickness of 6 cm around any point of the block 11, in the closed position; - the mass of each polystyrene shell is approximately 170 g. The outer protective casing 13 has two portions 130, 131 each housing, with complementary shapes, a shell 120, 121 made of thermal insulating material of the housing 12. The two portions 130, 131 are pierced to allow the passage of the sensors. Preferably, this outer protective casing 13 is made of stainless steel.For example: - the wall thickness of the stainless steel casing 13 is 3 mm, which allows welding; - the closing half-discs on the ends of the casing 13 are welded and adjusted in the closed position of the calorimeter. A hinge 132 may be fixed in particular by welding to allow articulation between them of the shells 120, 121 of the housing 12 and the envelope portions 130, 131 which are integral therewith, so as to bring them closer or move them apart, from the open position to the closed position or vice versa. In addition, a means 133 for locking the shells in the open position may be fixed and facilitate the handling of the upper portion 110 of the block 11 and the pencil 1 before or after the measurement. The temperature measurement sensors C1 to C10 are mounted in a crossing through one and / or the other of the portions 110, 111 of the absorption block and through one and / or the other of the shells 120, 121 of the housing and the envelope portions 130, 131.These sensors C1 to C10 are adapted to measure the temperature along at least the portion of the spent fuel rod in the closed positions of the block and the housing. These sensors C1 to C10 may be paired platinum resistance probes and / or thermocouples. Preferably, they are guided and blocked by cable glands positioned outside and / or inside the housing. As shown in Figures 4 to 4E, these temperature sensors are inserted into the block 11 and / or into the material of the housing 12. The instrumentation of the sensors C1 to C10 is arranged at the bottom of the calorimeter 10, in order to leave the entire upper part of it free to move around the hinge 132.For example: - sensors C1 to C10 are provided in the temperature range 20 °C to 100 °C; - sensors C1 to C10 have a diameter of 3 mm, which makes it possible to minimize the impacts on the block 11; - the sheath of the sensor is thermally insulating, for example made of polytetrafluoroethylene (PTFE or Teflon) or alumina. The location of sensors C1 to C10 is judiciously chosen for the subsequent digital exploitation of the acquisitions of temperature changes over time. Thus, it is preferable to choose positions both on the internal and external edges of the block 11 and the housing 12 but also in the center, representative of the largest quantity of material. Certain positions in the block 11 and in the housing 12 are advantageously opposite each other to best characterize the heat transfer resistances at the interfaces.In the example illustrated in Figure 4, the number of sensors C1 to C10 of the calorimeter is limited to 10 so as not to significantly disturb the measurements at the heart of the device. It goes without saying that a larger or smaller number can be chosen depending on the configurations. For example, for operation of a calorimeter 10 in shielded cells, the number is smaller. For the positioning of the sensors, drillings respectively in the casing 13, the housing 12 and the block 11 are made on a radius in the plane perpendicular to the axis of the cylinder. For example, the drilling diameter is of the order of 3.5 mm. For example, the choice and installation of the sensors C1 to C10 illustrated in Figures 4 to 4E are indicated in [Table 1] below. It is specified that a Pt100 probe is a platinum resistor, of 100 ohms at 0 °C which varies according to the temperature. Class 1 / 10B Pt100s have an accuracy of ± 0.03 °C.

[0003] [Table 1]The calorimeter 10 may comprise a self-supporting frame 14 consisting of four feet or preferably two cradles. The frame 14 may also advantageously comprise force-recovery bars 141 secured to a base 142 provided with ground-supporting feet 143. The block 111 is supported by rods 140 made of non-conductive material with high mechanical strength fixed to the base 131 of the casing at several points. More precisely, at each of four locations of the block portion 111, preferably arranged in a square or rectangle, a thread for receiving a rod 140 has been made. Each of these rods 140 connects the casing 131 to the block 111 by a rigid thread via screwing in the lower part into a nut secured to the lower portion of the casing 131 to the female thread of the upper part of the block 111. Beforehand, the lower shell 121 has been drilled and put in place.For example: - the 140 rods are made of nylon; - the 140 rods have a diameter of 6 mm.The installation of a spent fuel rod 1 in a calorimeter 10 which has just been described consists of a series of the following steps: i / opening of the calorimeter 10 by separating the upper shell 120 and the upper portion 130 of the envelope from the lower shell 121 and the lower portion 131 which remain fixed and integral with the frame 14; ii / removal of the upper portion 110 of the block 11; iii / insertion and housing of the rod 1 in the holes 112, 122 provided for this purpose; iv / replacement of the upper portion 110 of the block 11 on its lower portion 111 to obtain the closed position in which at least a fissile part of the rod 1 is housed in the radioactive and thermal radiation absorption chamber; v / closing of the calorimeter 10, by bringing the upper shell 120 and the upper portion 130 closer together of the envelope of the lower shell 121 and of the lower portion 131.Since the accuracy of the calorimeter measurement depends on the time elapsed between the installation of the rod and the closing of the calorimeter, steps iii / , iv / and v / are carried out quickly. Typically, these steps iii / to v / can be carried out with a remote manipulator in less than 1 minute. The operation of a calorimeter 10 is as follows. Initially, block 11 absorbs the radioactive radiation as close as possible to the rod by its maximum quantity of dense material. The material of block 11 is also chosen to be thermally conductive in order to be able to accurately measure the temperature rise in this material in a second step. The energy balance can be calculated simply by the formula: Q = M * Cp * ∆T with Q: energy exchanged in Joules M: mass of the material in kilograms ∆T: temperature variation in °C of said material from T initial to T final Cp: heat capacity Cp of the material in J / kg.The precise knowledge of all the masses of all the components of the calorimeter as well as their heat capacity but also the very precise implementation of the measurement chain with the judicious and controlled installation of the sensors for the transmission of the measurement signals, in particular in a shielded cell, makes it possible to evaluate the thermal power released by a pencil 1. The precision of the measurement depends on the processing of the data and two approaches have been validated by the inventors. The first approach is by thermal balance by making the hypothesis of a system at equilibrium corrected or not by a thermal leak makes it possible to determine a linear power with an uncertainty of approximately 15 mW.The second approach consists of a more complex method of numerical processing by inverse method, using a direct numerical code solving the thermal of the experiment: the resolution of spatio-temporal differential equations makes it possible to resolve the transient regimes and to determine a linear power with an uncertainty of approximately 3%. The measurable power in absolute terms depends on the stability of the environment but, under usual conditions, the power measurement range can vary by at least 3 orders of magnitude (from 10 mW to 10 W). The inventors carried out an experimental test, in the laboratory, to validate the performance of a calorimeter 10 according to the invention. The test was carried out with the materials, characteristics and dimensions of the different components of the calorimeter, given as an example above.The calorimeter 10 is initially in thermal equilibrium with the ambient medium which contributes to a temperature of the internal components on the basis of which the relative measurements are made. The temperature readings as a function of the different thermocouples are shown in Figure 5. It is specified that Tamb designates the temperature in the environment (thermostat) at a distance of approximately one meter from the calorimeter 10. In the test that was implemented, a temperature recording began at the chronology 0:00 in Figure 5. After 32 minutes of no-load operation, that is to say without any external element in the radioactive absorption chamber, an 18.67 g stainless steel bar previously heated to 100°C was introduced into the calorimeter 10. With these data, the energy balance can be calculated between the stainless steel of the bar which loses its energy in the lead of block 11 which goes from 25.36 °C to 26.04 °C.For this assessment, the inventors assumed that this last highest temperature measured by the C3 sensor is also the final temperature of the stainless steel bar. The numerical application leads to a given energy transfer of 690 Joules. It is specified here that Q data = Minox x Cpinox x (TinitInox – TmaxC3), equal to 18.67 x 0.500 x (100 - 26.04) = 690 J. To calculate the energy received by the lead of block 11, the inventors considered that each part of lead which locally does not increase at the same temperature and the integration by part allows to calculate an energy received equal to 635 J. The energy balance between stainless steel and lead is correct to within 9%. The inventors estimated a bias on the performance of the test: in fact, the stainless steel bar was initially taken out of boiling water, dried and then placed in the calorimeter 10 while it lost energy during the installation.By balancing the received and given energies, we calculate that the temperature of the stainless steel was rather 94 °C, instead of the theoretical 100 °C. The exchanged power, that is to say the energy exchanged over time during the exchange, can be calculated because the duration of the phenomenon is approximately 8 minutes. We obtain an average power value of approximately 1.2 W which even rises to 2.7 W during the first 3 minutes of the exchange. We can also approach the precision on the power measurement when the average ambient temperature varies little, which happens during 1 hour between the chronological markers 40:00 and 1:40:00 for which the ambient temperature value is approximately 25.4 °C. The energy of calorimeter 10 then drops from 642 J to 610 J, which allows a thermal loss flux of approximately 9 mW to be calculated for a temperature gradient of approximately 0.6 °C. The accuracy of the power measurements can then be confused with the thermal loss flux.In conclusion of this test, the calorimeter 10 sized with the previous figures and which has been tested in the laboratory works well with performances on its precision of approximately 15 mW and with an uncertainty of approximately 10% if simple numerical tools of integration by part are used. By using inverse numerical methods, the inventors believe that this precision could be increased and the uncertainty reduced to around 3%. The invention is not limited to the examples which have just been described; it is possible in particular to combine characteristics of the illustrated examples within non-illustrated variants. Other variants and improvements can be envisaged without departing from the scope of the invention. List of cited references[1]: JC Jaboulay, S. Bourganel, “Analysis of MERCI decay heat measurement for PWRUO2 fuel rod”, Nuclear Technology 177(2012) 73-82.[2]: H. Carcreff, L. Salmon, and C.Courtaux, “First In-Core Measurement Results Obtainedwith the Innovative Mobile Calorimeter CALMOS inside the OSIRIS Material Testing Reactor”, IEEE Transactions on Nuclear Science, Vol.61, N°.4, Aug 2014[3]: F. Sturek, L. Agrenius, Svensk Karnbranslehantering "Measurements of heat decay inspent nuclear fuel at the Swedish interim storage facility, Clab”, AB, RepportR-05-62, December 2006. [4]: ​​P. Jansson, M. Bengtsson, Ul. Bäckström, K. Svensson, M. Lycksell, A. Sjöland, “Data from calorimetric decay heat measurements of five used PWR 17x17 nuclear fuel assemblies”, Data in brief 28 (2020) 104917. [5]: F. Muratori, « Étude de faisabilité et preconception d'une expérience de measure de la puissance résiduelle de un combustible nucléaire irradiaté aux temps très courts sur le réacteur RJH », PhD thesis from Aix-Marseille University, defended on October 26th, 2020. https: / / theses.hal.science / tel-03045906 / file / 2020_11_26_manuscrit_FINAL.pdf [6]: https: / / www.calorimetre-de-langavant.com / .

Claims

Claims1. Device (10) for measuring the linear residual power of at least one portion or piece of a spent or new fuel rod, forming a calorimeter extending around a central axis (X1) and comprising:- a block (11) with two block portions (110, 111) made of thermal and radioactive radiation absorption material, each in the form of a portion of a cylindrical crown closed at each of its longitudinal ends by a cylindrical portion pierced in its center by a through hole (112), the two block portions being able to be brought closer to each other from an open position in which they are spaced apart from each other to allow the insertion and housing of a spent fuel rod,to a closed position in which they are folded back on each other, delimiting a radioactive absorption chamber of cylindrical shape adapted to house at least a part of the spent fuel rod with its other parts which extend on either side of the chamber, from the through holes forming a cylindrical passage; - a housing (12) with two shells (120, 121) made of thermal insulating material, each in the form of a portion of a cylindrical crown closed at each of its longitudinal ends by a cylindrical portion pierced in its center by a through hole (122),the two shells being able to be brought together from an open position in which they are spaced apart from each other to allow the insertion and housing of the radioactive absorption chamber to a closed position in which they are folded back on each other, delimiting a housing adapted to house the radioactive chamber with the other parts of the fuel rod which extend on either side of the housing from the cylindrical passage of the block passing through the through holes also forming a cylindrical passage; - temperature measuring sensors (C1 to C10), mounted in a cross-section through one and / or the other of the absorption block portions and through one and / or the other of the shells and adapted to measure the temperature along at least the part of the spent fuel rod in the closed positions of the block and the housing.

2. Calorimeter according to claim 1,the heat and radioactive radiation absorption material of the block being lead (Pb) or a lead alloy., 3. Calorimeter according to claim 1 or 2, the block further comprising an adapter (113) made of thermal and radioactive radiation absorbing material and in the form of a cylindrical crown portion with an internal diameter suitable for housing a spent fuel rod and an external diameter suitable for housing in one of the two block portions.

4. Calorimeter according to one of the preceding claims, comprising an outer protective casing (13) with two casing portions (130, 131) each housing, with complementary shapes, one of the two shells made of thermal insulating material of the housing.

5. Calorimeter according to one of the preceding claims, comprising at least one hinge (132) around which the two shells of the housing are articulated together, so as to bring them together or separate them, from the open position to the closed position or vice versa.6.Calorimeter according to one of the preceding claims, comprising at least one locking means (133) in the open position of the shells.

7. Calorimeter according to one of the preceding claims, the thermal insulating material of the housing being polystyrene.

8. Calorimeter according to one of the preceding claims, the thermal insulating material of the housing being in direct contact with the thermal and radioactive radiation absorption material in the closed position.

9. Calorimeter according to one of the preceding claims, the temperature measuring sensors being resistance probes and / or thermocouples.

10. Calorimeter according to one of the preceding claims, the sensors being guided and locked by cable glands positioned outside and / or inside the housing.11.Calorimeter according to one of the preceding claims, comprising a self-supporting support frame (14), adapted to support the block, the housing, the temperature sensors and the spent fuel rod, and where appropriate the external protective casing.

12. Shielded cell housing at least one calorimeter according to one of the preceding claims, the electrical wires connected to the temperature sensors being connected to connectors mounted across the wall of the shielded cell for the transmission of signals from the sensors to an analog / digital converter outside.13.Use of a calorimeter according to one of claims 1 to 11 or of the shielded cell according to claim 12, for measuring the linear residual power of at least a portion or piece of a spent or new fuel rod whose fissile material is chosen from uranium (IV) oxide (UO2), mixed oxide (U, Pu)O2 or a mixed mixture based on uranium oxide and reprocessed plutonium oxides (MOx) or any other radioactive or thermal source based on fission products, actinides or activation products.

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