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

Abstract

Calorimeter for measuring the linear residual power of at least a portion or piece of a spent or new nuclear fuel rod. The invention essentially consists of a calorimeter whose core is delimited by a block of radioactive and thermal radiation-absorbing material surrounded by a housing of thermal insulation material that thermally isolates the core from the ambient environment. The block and the housing are each in two parts that can be separated for the easy and rapid insertion of a spent fuel rod. The calorimeter is instrumented with temperature sensors distributed along its length, which are inserted into the block and / or the housing to measure the linear residual power of the inserted fuel rod. Figure for the abstract: Fig. 2A

Description

Title of the invention: Calorimeter for measuring the linear residual power of at least a portion or piece of a spent or new nuclear fuel rod. technical field

[0001] The present invention relates to the field of fuel rod instrumentation for nuclear reactors.

[0002] The invention essentially aims to provide a device that can measure, in a simple and reliable way, the combustion rate or residual power of a spent nuclear fuel rod.

[0003] For the purposes of this application, "nuclear reactors" means the current common meaning of the term, namely power plants that produce energy from nuclear fission reactions using fuel elements in which fissions occur that release heat power, the latter being extracted from the elements by heat exchange with a heat transfer fluid that ensures their cooling.

[0004] In the application as a whole, the term "nuclear fuel rod" is understood to mean the official definition, for example, found in the Dictionary of Nuclear Science and Technology, namely, 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 needle" is a nuclear fuel rod, but the terminology is used for fast neutron reactors. The invention is applicable to any nuclear fuel rod or needle.

[0005] The term "spent fuel rod" means a fuel assembly that has been irradiated to such a degree that it cannot be used subsequently in a reactor without having undergone appropriate treatment. In other words, a spent fuel rod means a rod in which there is no longer sufficient fissile material to sustain nuclear reactions within a reactor.

[0006] The invention relates to all fuel rods, in particular of the ceramic type made of uranium oxide or uranium and plutonium (U,Pu)O2 or MOX (acronym for "mixed oxide"), which can be dedicated to all types of nuclear reactors for power generation and / or heat generation or experimental purposes, such as Boiling Water Reactors (BWRs), Pressurized Water Reactors (PWRs) and all advanced 3rd and 4th generation reactors. Previous technique

[0007] At the sites of nuclear reactors and fuel processing plants For irradiated fuel assemblies, there are counting wells that allow us to estimate the combustion rates of the entire irradiated fuel assembly.

[0008] The burnup rate or "bum-up" characterizes both the degree of irradiation and the energy supplied by the fuel upon exiting the reactor. It is expressed as an average for an object (average of the refueling, average in the fuel assembly).

[0009] 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 a Na-NR or SFR reactor (English acronym for "Sodium Fast Reactor").

[0010] 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.

[0011] After passing through a reactor, a fuel has been the site of nuclear reactions resulting in a particular composition in isotopes that are more or less radioactive and dissipate a specific energy for each one.

[0012] These energies are linked to the different radioactive radiations (alpha, beta, gamma, X-rays, neutron) and are deposited in the surrounding matter at varying distances. The sum of these energies dissipated over a period of time constitutes a residual power that can vary along the length of the fuel rod, is specific to the isotopic composition of the fuel, and evolves over time.

[0013] When this power is associated with a specific length, it is referred to as the linear residual power of a fuel rod. This linear power value contributes to the validation of neutron calculation codes that simulate reactor operation and calculate the composition of a fuel rod based on fission efficiencies.

[0014] Now, this linear power measurement also serves as a basic data point in the design and operation of nuclear reactors but also in the entire downstream of the nuclear cycle (storage, transport, reprocessing and disposal).

[0015] Research and tests of residual power measurement have been taking place since the 1950s. It appears that the measurement of residual power is a good way to trace and characterize the decay of isotopes in irradiated fuel and it is the method using elemental fission curves that was initially employed.

[0016] Some experimental measurements were carried out in the laboratory on fuel rods.

[0017] One example is the experiment called "MERCI" which consisted of placing a portion of irradiated fuel rod in a calorimeter called "CALMOS". This The experiment allowed a unique measurement of the residual power of this portion of the fuel rod: [1], [2]. The combustion rate was 3.5 GWj / t and the measurement was able to take place for cooling times between 40 minutes and 40 days.

[0018] Another experiment was carried out in the Swedish laboratory for deep geological disposal studies, on an assembly placed in a calorimeter with water and where the temperature was recorded throughout its ascent: [3], [4]. The combustion rate was between 30 and 50 GWd / t and the measurement could take place for cooling times of a few years to several decades.

[0019] Another experiment called "PRESTO" is planned: [5]. This experiment will consist of irradiating a portion of a fuel rod in the Jules Horowitz experimental reactor (RJH) and, after irradiation, inserting 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.

[0020] Power measurement is similar to heat of reaction measurements, which are primarily determined in calorimeters employing Differential Scanning Calorimetry (DSC), a thermal analysis technique. This measurement involves recording the differences in heat exchange between a sample to be analyzed and a reference. DSC instruments, as well as thermobalances operating in DSC mode, are extremely fragile and are not designed for use in shielded cells, making them unsuitable for use with spent fuel rods.

[0021] A calorimeter known as the Langavant calorimeter is also known. It consists of a semi-adiabatic vessel, closed by an insulating stopper and placed in a rigid casing, and is used to measure the heat of hydration of mortars: [6]. A mortar box is filled with grout, instrumented with temperature sensors, and the vessel is placed inside. Since the setting of mortars is a slow process, the measurement can be carried out over several days. Therefore, the heat leakage of this calorimeter is determined beforehand. This calorimeter also requires a relatively airtight seal. Such a device is not really suitable for nuclear fuel rods.

[0022] Thus, there is a need to propose a device that is both simple and reliable for measuring the residual power of nuclear fuel rods.

[0023] The aim of the invention is to meet at least part of this need. Description of the invention

[0024] To this end, the invention relates, in one of its aspects, to a device for measuring the linear residual power of at least a portion or piece of a spent or new nuclear fuel rod, forming a calorimeter extending around a central axis and comprising:

[0025] - a block with two block portions made of thermal absorption material and radioactive radiation, each in the form of a portion of a closed cylindrical ring at each of its longitudinal ends by a cylindrical portion pierced in its center by a through hole, the two portions of block being able to be brought together from an open position in which they are separated from each other to allow the insertion and housing of a spent fuel rod, to a closed position in which they are folded down on each other delimiting a cylindrical radioactive absorption chamber adapted to house at least part of the spent fuel rod with its other parts extending on either side of the chamber, from the through holes forming a cylindrical passage;

[0026] - a housing comprising two shells made of thermally insulating material, each in form of a portion of a closed cylindrical crown 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 separated from each other to allow the insertion and housing of the radioactive absorption chamber to a closed position in which they are folded down on each other delimiting a suitable housing to accommodate the radioactive chamber with the other parts of the fuel rod which extend on either side of the casing from the cylindrical passage of the block through the through holes also forming a cylindrical passage;

[0027] - temperature measurement sensors, mounted in a through-hole configuration through one and / or the other 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 casing.

[0028] Preferably, the heat-absorbing and radioactive radiation-resistant material of the block is lead (Pb) or a lead alloy. The linear power is related to the radiation from the nuclear fuel, some of which readily penetrates 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 generating a temperature rise in the material. Commonly used in the nuclear field for radiation protection because it absorbs radiation, lead offers two important advantages: both 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 presents no problems during ingot casting, machining, and drilling, which is perfectly suited for the design of the block according to the invention. Furthermore, one... The specific heat capacity of this material can be precisely determined to calculate the power. For example, a block composed of 96% lead by mass and 4% antimony by mass has a specific heat capacity that is a linear combination of these two materials: Cp = 0.96 x 0.129 + 0.04 x 0.210 = 0.132 Jg·K. The lead layer can be approximately 5 cm thick, allowing it to absorb most of the radiation, which then deposits its specific energies. Thus, in addition to its ability to absorb radioactive radiation, lead offers a very good compromise between compactness, machinability, and low cost.

[0029] The thickness chosen for the block can be a compromise that takes into account the handling capacity of standard telemanipulators, in a shielded cell, for example, on the order of 7 kg. To close perfectly around the pencil, the block is preferably machined with a recess corresponding to the internal geometry of the pencil. To lighten the upper portion of the block, one or more centering pieces can be used to reposition it. The lower portion of the block is preferably the one drilled in different places to allow the insertion of temperature sensors.

[0030] According to an advantageous embodiment, the block further comprises an adapter made of a heat-absorbing and radioactive radiation-resistant material and in the form of a cylindrical ring portion with an inner diameter suitable for housing a spent fuel rod and an outer diameter suitable for fitting into one of the two block portions. This adapter allows it to accommodate any existing nuclear fuel rod geometry.

[0031] According to an advantageous embodiment, the calorimeter comprises an outer protective envelope with two envelope portions, each housing, with complementary shapes, one of the two shells made of thermal insulating material of the casing.

[0032] Preferably, this outer protective casing is made of stainless steel. Stainless steel offers an excellent compromise between the desired protection, support, and fit between the block and the housing, and the precise guidance it provides for the temperature sensors. Furthermore, this material is easy to clean.

[0033] According to an advantageous embodiment, the calorimeter includes at least one hinge around which the two shells of the casing are articulated to each other, so as to bring them closer together or further apart, from the open position to the closed position or vice versa.

[0034] Advantageously, the calorimeter includes at least one means for locking the shells in the open position. This facilitates the insertion of a pencil into the calorimeter, which is particularly advantageous in the case of remote manipulation in a shielded cell.

[0035] Preferably, the thermal insulation material of the housing is polystyrene.

[0036] Preferably, the thermal insulation material of the housing is directly made of Contact with the thermal and radioactive radiation absorption material occurs in the closed position. With this dimensional adjustment, no air gap exists between the block and the casing when closed, which is optimal for measurement interpretation. In other words, this adjustment guarantees the absence of heat transfer by gas convection between the block and the casing when closed.

[0037] Advantageously, the temperature measurement sensors are resistance probes and / or thermocouples.

[0038] Advantageously, the sensors are guided and blocked by cable glands positioned outside and / or inside the housing.

[0039] According to another advantageous embodiment, the calorimeter comprises a self-supporting frame adapted to support the block, the housing, the temperature sensors, and the spent fuel rod, and optionally the outer protective enclosure. This frame may 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 measurement. Thus, the frame can be placed directly on the floor, particularly in a shielded cell, without having to provide a suitable support within the cell.

[0040] 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 through the wall of the shielded cell for the transmission of signals from the sensors to an external analog-to-digital converter. Thus, the processing of the measurements is carried out remotely, protected by the shielded cell.

[0041] The invention also relates to the use of a calorimeter as described above or of the shielded cell as described above, for the measurement of 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.

[0042] Thus, the invention essentially consists of a calorimeter whose core is delimited by a block of radioactive and thermal radiation absorption material surrounded by a case of thermal insulation material which makes it possible to thermally isolate the core from the ambient environment, the block and the case each being in two parts which can be separated from each other for the easy and rapid introduction of a spent fuel rod.

[0043] The calorimeter is instrumented by temperature measurement sensors distributed across the long and which fit into the block and / or the casing for a measurement of the linear residual power of the inserted pencil.

[0044] A calorimeter according to the invention has many advantages, including: - a simple and reliable measurement of the linear residual power of a spent fuel rod, intact, without prior sampling or cutting of the measurement, - a measurement that can be carried out directly on a shielded chain during the storage of a fuel rod after it has been processed in a nuclear reactor, - the possibility of a non-destructive examination of the same worn pencil, which can be carried out in the same place at different times or periods, - a robust device with no interaction with ambient radioactivity, - the possibility of increasing accuracy through statistical processing of multiple measurement values, - The interchangeability of internal components, particularly the adapter, allows it to adapt to any nuclear fuel rod geometry. - Easy and quick replacement of the measuring sensors in case of failure, - Implementation of the calorimeter in an existing shielded chain. - the possibility of remotely manipulating the calorimeter and, beforehand, to do so very simply enter the armored cell through a standard airlock via the waste bin transfer device.

[0045] Other advantages and features of the invention will become clearer from the detailed description of examples of implementation of the invention given by way of illustration and not limitation with reference to the following figures. Brief description of the drawings

[0046] [Fig-1] [Fig.1] is a schematic longitudinal cross-sectional view of a pencil nuclear fuel according to the state of the art, as implemented in a PWR type nuclear reactor.

[0047] [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.

[0048] [Fig. 3A], [Fig. 3B] Figures 3A and 3B are schematic cross-sectional views transverse and longitudinal 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.

[0049] [Fig.4] [Fig.4] is a schematic side view of a calorimeter according to the invention, with the different sides where temperature sensors are inserted.

[0050] [Fig.4A], [Fig.4B], [Fig.4C], [Fig.4D], [Fig.4E] Figures 4A to 4E are views in cross-section of the calorimeter of [Fig.4], respectively along AA, BB, CC, DD, EE.

[0051] [Fig. 5] [Fig. 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

[0052] It is specified that in the whole of the application, the terms "vertical", "lower", "upper", "bottom", "top", "below" and "above" are to be understood by reference to a calorimeter as it is in horizontal configuration of use, that is to say housing a fuel rod horizontally, and which opens along a horizontal median plane.

[0053] In [Fig.1], a conventional nuclear fuel rod 1 is shown in its configuration for use in a PWR nuclear reactor, i.e. in a vertical position with the pellets 6 towards the lower part as specified below.

[0054] The fuel rod 1 consists of a sheath 2 conventionally made of Zircaloy-4 (Zr4) closed at each end by a cap 3 at the top and 4 at the bottom, respectively, which is welded to it. This sealed fuel rod is filled with helium, typically at 25 bar cold for common fuels, to partially counteract the effect of the external pressure of 150 bar from the heat transfer fluid.

[0055] 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 cap 3, constitutes a gas expansion chamber and the other houses the fissile column formed by the stacking of nuclear fuel pellets 6 which each extend along the longitudinal direction XX' of the rod 1.

[0056] The expansion chamber is a free volume intended to receive the Fission Products in gaseous form, usually called Fission Gas (FG).

[0057] In the stack shown, each pellet 6 has substantially the same length or height H.

[0058] A helical compression spring 7, generally made of Inconel®, is housed in the expansion chamber 5 with its lower end bearing against the upper face of the pad 6, the highest in the stack of pads, and its other end bearing against the upper plug 3.

[0059] In addition to maintaining the stacking of the 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 sheath cross-section in its oval shape. In other words, it must prevent extreme ovalization of the cross-section of the sheath.

[0060] The primary function of a fuel rod is to produce, and then transmit, the heat produced by the fission reactions within the fuel.

[0061] Once the fuel rod 1 is used up, at its exit from the reactor it releases a residual power, which associated with a determined length of the stack of pellets 6, is a linear power.

[0062] Figures 2A, 2B, 3A, 3B show an example of a device forming a calorimeter 10 according to the invention which allows to measure quickly and reliably such linear power, without sampling or cutting of the pencil beforehand.

[0063] By way of example, this calorimeter 10 may have an outside diameter 0 of 22 cm and an overall length L of 27 cm. Such a geometry of the calorimeter 10 makes it compatible with its transport in a container in a double-door airtight airlock of standard dimensions of 270 mm in diameter.

[0064] The calorimeter 10 extending around a central axis (XI) in an axisymmetric shape essentially comprises from the inside out a block 11 of thermal and radioactive radiation absorption material, a housing 12 of thermal insulation material to house the block 11 and a protective envelope 13 in which the housing is housed and fixed, as well as temperature sensors Cl to CIO inserted and mounted through the absorption block and / or the housing 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.

[0065] Block 11 comprises two block portions 110, 111 each in the form of a portion of a closed cylindrical crown at each of its longitudinal ends by a cylindrical portion pierced in its center by a through hole 112.

[0066] The two block portions 110, 111 can be brought together from an open position in which they are separated from each other to allow the insertion and housing of a spent fuel rod 1, to a closed position in which they are folded down on each other, delimiting a cylindrical radioactive absorption chamber adapted to house at least part of the spent fuel rod with its other parts extending on either side of the chamber, from the opening holes 112 forming a circular passage.

[0067] An adapter 113, also made of a thermal and radioactive radiation-absorbing material, can be housed in the lower portion 111 of the block. This adapter has a semi-cylindrical ring shape with an inner diameter adapted to accommodate a spent fuel rod 1. The calorimeter can thus be adapted to any existing fuel rod geometry. For a piece of fuel rod, this adapter can be closed at its axial ends for a length approximately equivalent to the radius of the block 111.

[0068] It is specified that with or without such an adapter, the dimensioning is carried out so as not to leave an air layer around the pencil 1.

[0069] The thermal and radioactive radiation absorption material of block 11 is preferably lead (Pb) or a lead alloy.

[0070] For example:

[0071] - the material of block 11 is an alloy of 96% by mass lead and 4% by mass of antimony;

[0072] - in its closed position, block 11 is a cylindrical ring of 5.32 cm diameter and 15 cm long and 5 cm thick;

[0073] - to lighten the upper portion 110 which is the one to be manipulated for the insertion of a pencil, it is made, in particular machined on a smaller portion than the lower portion 111. The upper portion 110 can thus be machined at an angle of about 110° and therefore of 250° for the lower portion 111;

[0074] - the mass of the lead block 11 is approximately 13 kg, of which about 8 kg is for the lower portion 111 and approximately 5 kg for the upper portion 110, which allows handling by a standard telemanipulator;

[0075] - the lower portion 11 can be pierced in 6 different places to allow the passing through as many temperature measurement sensors.

[0076] The housing 12 comprises two shells 120, 121 made of thermal insulating material, each in the form of a hemisylindrical crown closed at each of its longitudinal ends by a cylindrical portion pierced in its center by a through hole 122.

[0077] The two shells 120, 121 can be brought together from an open position in which they are separated from each other to allow the insertion and housing of the radioactive absorption chamber to a closed position in which they are folded down on each other delimiting 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 casing from the cylindrical passage of the block through the opening holes 122.

[0078] The lower portion of block 111 is adjusted in height in the calorimeter in order to perfectly align the open holes 122 with those 112 of the block.

[0079] The lower shell 121 is positioned in a cradle configuration. When the calorimeter is closed, the two horizontal surfaces of the shells 120, 12 are in contact over their entire surface. They are perforated to allow the passage of the temperature sensors.

[0080] The thermal insulation material of the housing 12 is preferably polystyrene

[0081] In the closed position, the thermal insulating material of the housing 12 is in direct contact with the material of the thermal absorption and radioactive radiation block 11.

[0082] By way of example: - the polystyrene has a thickness of 6 cm around every point of block 11, in the closed position; - the mass of each polystyrene shell is approximately 170 g.

[0083] The outer protective casing 13 has two portions 130, 131, each housing, with complementary shapes, a shell 120, 121 made of thermal insulating material for the casing 12. The two portions 130, 131 are pierced to allow the passage of the sensors.

[0084] Preferably, this outer protective cover 13 is made of stainless steel.

[0085] For example: - the wall thickness of the stainless steel casing 13 is 3 mm, which allows for welding; - the closing half-discs on the ends of the envelope 13 are welded and adjusted in the closed position of the calorimeter.

[0086] A hinge 132 can be fixed in particular by welding to allow articulation between the shells 120, 121 of the housing 12 and the envelope portions 130, 131 which are attached to it, so as to bring them closer together or further apart, from the open position to the closed position or vice versa.

[0087] In addition, a locking means 133 for locking the shells in the open position can be fixed and facilitate the handling of the upper portion 110 of the block 11 and the pencil 1 before or after the measurement.

[0088] The Cl to CIO temperature measurement sensors are mounted 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 casing and the envelope portions 130, 131. These Cl to CIO sensors are 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.

[0089] These Cl to CIO sensors can be matched platinum resistance probes and / or thermocouples.

[0090] Preferably, they are guided and secured 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 housing material 12.

[0091] The instrumentation of the Cl to CIO sensors is arranged at the bottom of the calorimeter 10, so as to leave its entire upper part free to move around the hinge 132.

[0092] By way of example: - Cl to CIO sensors are designed for the temperature range of 20 °C to 100 °C; - The Cl to CIO sensors have a diameter of 3mm, which allows for minimize the impacts on block 11; - the sensor sheath is thermally insulating, for example made of polytetrafluoroethylene (PTFE or Teflon) or alumina.

[0093] The placement of the Cl sensors at CIO is judiciously chosen for the subsequent digital exploitation of the acquisitions of temperature evolutions over time.

[0094] Thus, it is preferable to choose positions both on the inner and outer edges of block 11 and housing 12, but also at the center, which represents the largest quantity of material. Certain positions in block 11 and in housing 12 are advantageously opposite each other to best characterize the thermal transfer resistances at the interfaces.

[0095] In the example illustrated in [Fig. 4], the number of Cl to CIO sensors in the calorimeter is limited to 10 so as not to significantly interfere with the measurements at the core of the device. It goes without saying that a larger or smaller number can be chosen depending on the configuration. For example, for operation of a 10-cell calorimeter with shielded cells, the number is less important.

[0096] For positioning the sensors, holes are drilled in the casing 13, the housing 12, and the block 11, respectively, on a radius in the plane perpendicular to the axis of the cylinder. For example, the diameter of the hole is approximately 3.5 mm.

[0097] By way of example, the selection and placement of the Cl to CIO sensors illustrated in Figures 4 to 4E are shown in [Table 1] below. It should be noted that a Pt100 probe is a platinum resistor, 100 ohms at 0 °C, whose resistance varies with temperature. Class 1 / 10B Pt100s have an accuracy of ±0.03 °C.

[0098] [Tables 1] Sensor Type of sensor Sensor position Distance from left end of calorimeter 10 Sensor insertion length block 11 Sensor insertion length in housing 12 C1 PtlOO 7.2 cm 34 mm - C2 PtlOO 7.2 cm 7 mm - C3 PtlOO 11 cm 34 mm C4 PtlOO 11 cm 7 mm C7 PtlOO 16 cm 10 mm C8 PtlOO 16 cm 50 mm C5 PtlOO 11.2 cm 55 mm C6 PtlOO 11.2 cm 15 mm C9 PtlOO 24 cm 105 mm C10 PtlOO 26 cm 95 mm

[0099] The calorimeter 10 may include a self-supporting frame 14 consisting of four feet or preferably two cradles.

[0100] The frame 14 may also advantageously include load-bearing bars 141 attached to a base 142 with ground support feet 143.

[0101] The support for block 111 is achieved by rods 140 made of a non-conductive material with high mechanical strength, fixed to the base 131 of the casing at several points. More specifically, at each of four locations on the portion of block 111, preferably arranged in a square or rectangle, a threaded hole has been formed to receive a rod 140.

[0102] Each of these rods 140 connects the casing 131 to the block 111 by means of a rigid thread via a screw in the lower part into a nut attached to the lower portion of the casing 131 to the female thread in the upper part of the block 111. Previously, the lower shell 121 was drilled and put in place.

[0103] By way of example: - the 140 rods are made of nylon; - the 140 rods have a diameter of 6 mm.

[0104] The placement of a spent fuel rod 1 in a calorimeter 10, which has just been described, consists of a sequence of the following steps:

[0105] 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 attached to the building 14;

[0106] ii / removal of the upper portion 110 of block 11;

[0107] iii / insertion and housing of the pencil 1 in the holes 112, 122 provided for this purpose;

[0108] iv / repositioning the upper portion 110 of the block 11 onto its lower portion 111 to obtain the closed position in which at least a fissile part of the pencil 1 is housed in the radioactive and thermal radiation absorption chamber;

[0109] v / closing of the calorimeter 10, by bringing the upper shell 120 and the upper portion 130 of the envelope closer to the lower shell 121 and the lower portion 131.

[0110] Since the accuracy of the calorimeter measurement depends on the time elapsed between the installation of the pencil and the closure of the calorimeter, care is taken to perform steps iii, iv, and v quickly. Typically, these steps iii to v can be performed with a telemanipulator in less than 1 minute.

[0111] The operation of a calorimeter 10 is as follows.

[0112] Initially, block 11 absorbs radioactive radiation as close as possible to the pencil due to its maximum amount of dense material. The material of block 11 is also chosen to be thermally conductive so that the temperature rise in this material can be precisely measured subsequently.

[0113] The energy balance can be calculated simply using the formula:

[0114] Q=M*Cp*AT with

[0115] Q: energy exchanged in Joules

[0116] M: mass of the material in kilograms

[0117] AT: temperature variation in °C of said material from Tinitiaie to Tfinaie

[0118] Cp: specific heat capacity Cp of the material in J / kg.

[0119] The precise knowledge of all the masses of all the constituents of the calorimeter as well as their heat capacity but also the very precise implementation of the measurement chain with the judicious and controlled placement 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.

[0120] The accuracy of the measurement depends on the data processing and two approaches have been validated by the inventors.

[0121] The first approach is by thermal balance, assuming a system in equilibrium, corrected or not by a thermal leak, allows us to determine a linear power with an uncertainty of approximately 15 mW.

[0122] The second approach consists of a more complex numerical processing method 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 solve the transient regimes and to determine a linear power with an uncertainty of about 3%.

[0123] The absolute measurable power depends on the stability of the environment but, under normal conditions, the power measurement range can vary by at least 3 orders of magnitude (from 10 mW to 10 W).

[0124] The inventors carried out an experimental test in the laboratory to validate the performance of a calorimeter 10 according to the invention.

[0125] The test was carried out with the materials, characteristics and dimensions of the different components of the calorimeter, given as an example above.

[0126] The calorimeter 10 is initially in thermal equilibrium with the ambient environment which contributes to a temperature of the internal components on the basis of which the relative measurements are carried out.

[0127] The temperature readings from the different thermocouples are shown in [Fig. 5]. It should be noted that Tamb denotes the temperature in the environment (thermostat) at a distance of approximately one meter from the calorimeter 10.

[0128] In the test which was implemented, a temperature recording began at timecode 0:00 in [Fig.5].

[0129] After 32 minutes of operation without an external element in the radioactive absorption chamber, a 18.67 g stainless steel rod previously heated to 100°C was introduced into the calorimeter 10.

[0130] 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.

[0131] For this assessment, the inventors assumed that this last highest temperature measured by sensor C3 is also the final temperature of the stainless steel bar.

[0132] The numerical application leads to a given energy transfer of 690 Joules. It is specified here that Qdonnée = Minox x Cpinox x (Tinitlnox - TmaxC3), or equal to 18.67 x 0.500 x (100 - 26.04) = 690 J.

[0133] To calculate the energy received by the lead in block 11, the inventors considered that each part of lead does not locally increase at the same temperature and integration by part allows to calculate an energy received equal to 635 J.

[0134] The energy balance between stainless steel and lead is correct to within 9%.

[0135] The inventors considered a bias in the execution of the test: indeed, 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 placement.

[0136] By balancing the energies received and given, it is calculated that the temperature of the stainless steel was rather 94 °C, instead of the theoretical 100 °C.

[0137] The power exchanged, 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. An average power value of approximately 1.2 W is obtained, which even rises to 2.7 W during the first 3 minutes of the exchange.

[0138] We can also approach the accuracy of the power measurement when the average ambient temperature varies little, which happens during 1 hour between the time markers 40:00 and 1:40:00 for which the ambient temperature value is approximately 25.4 °C.

[0139] The energy of the calorimeter 10 then decreases from 642 J to 610 J, which allows the calculation of a heat loss flux of approximately 9 mW for a temperature gradient of approximately 0.6 °C. The accuracy of the power measurements can then be considered equivalent to the heat loss flux.

[0140] In conclusion of this test, the calorimeter 10 dimensioned with the previous figures and which was tested in the laboratory works well with performance on its accuracy of about 15 mW and with an uncertainty of about 10% if simple numerical tools of integration by part are used.

[0141] By using inverse numerical methods, the inventors believe that this accuracy could be increased and the uncertainty decreased to around 3%.

[0142] The invention is not limited to the examples just described; in particular, features of the illustrated examples can be combined in unillustrated variants.

[0143] Other variants and improvements may be envisaged without departing from the scope of the invention. List of cited references

[0144] [1]: JC Jaboulay, S. Bourganel, “Analysis of MERCI decay heat measurement for PWR UO2fuel rod”, Nuclear Technology 177(2012) 73-82.

[0145] [2]: H. Carcreff, L. Salmon, and C. Courtaux, “First In-Core Measurement Results Obtained with the Innovative Mobile Calorimeter CALMOS inside the OSIRIS Material Testing Reactor”, IEEE Transactions on Nuclear Science, Vol.61, N°.4, Aug 2014

[0146] [3]: F. Sturek, L. Agrenius, Svensk Karnbranslehantering "Measurements of decay heat in spent nuclear fuel at the Swedish internal storage facility, Clab AB, RepportR-05-62, December 2006.

[0147] [4]: ​​P. Jansson, M. Bengtsson, Ul. Bâckstrôm, K. Svensson, M. Lycksell, A. Sjôland, “Data from calorimetric decay heat measurements of five used PWR 17x17 nuclear fuel assemblies”, Data in brief 28 (2020) 104917.

[0148] [5]: F. Muratori, « Etude de faisabilité et preconception d'une expérience de mesure de la puissance résiduelle de un combustible nucléaire irradié aux temps très courts sur le réacteur RJH », PhD thesis from Aix-Marseille University, defended on October 26th, 2020.

[0149] https: / / theses.hal.science / tel-03045906 / file / 2020_l l_26_manuscrit_FINAL.pdf

[0150] [6]: https: / / www.calorimetre-de-langavant.com /

Claims

Demands

1. Device for measuring (10) the linear residual power of at least a portion or piece of a spent or new fuel rod, forming a calorimeter extending around a central axis (XI) and comprising: - a block (11) with two portions (110, 111) of block made of thermal and radioactive radiation absorbing material, each in the form of a portion of a cylindrical ring closed at each of its longitudinal ends by a cylindrical portion pierced in its center by a through hole (112), the two portions of block being able to be brought together from an open position in which they are separated from each other to allow the insertion and housing of a spent fuel rod, to a closed position in which they are folded down on each other delimiting a cylindrical radioactive absorption chamber adapted to house at least a part of the spent fuel rod with its other parts extending 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 ring 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 separated from each other to allow the insertion and housing of the radioactive absorption chamber to a closed position in which they are folded down on each other delimiting a housing adapted to accommodate 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 through the through holes also forming a cylindrical passage; - temperature measurement sensors (Cl to CIO), mounted in a through-hole configuration through one and / or the other portion of the absorption block and through one and / or the other of the shells and adapted to measure the temperature along at least the portion of the spent fuel rod in the closed positions of the block and the casing.

2. Calorimeter according to claim 1, the thermal 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 portion of a cylindrical ring of inner diameter adapted to house a spent fuel rod and of outer diameter to fit into one of the two portions of the block.

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

5. Calorimeter according to any one of the preceding claims, comprising at least one hinge (132) around which the two shells of the casing are articulated to each other, so as to bring them closer together or further apart, from the open position to the closed position or vice versa.

6. Calorimeter according to any one of the preceding claims, comprising at least one means for locking (133) the shells in the open position.

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

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

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

10. Calorimeter according to any one of the preceding claims, the sensors being guided and blocked by cable glands positioned outside and / or inside the housing.

11. Calorimeter according to any one of the preceding claims, comprising a self-supporting support frame (14), adapted to support the block, the casing, the temperature sensors and the spent fuel rod, and where applicable the outer protective casing.

12. Shielded cell housing at least one calorimeter according to any one of the preceding claims, the electrical wires connected to the temperature sensors being connected to connectors mounted through 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 any 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 selected 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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