A DEVICE FOR THE MEASUREMENT DURING OPERATION OF A DIMENSION OF AN ELEMENT OR A STACK OF ELEMENTS AND/OR ITS CLADDING

Abstract: A device for the measurement during operation of a dimension of an element (4) or a stack of elements (4), provided with a first casing (l) on the bottom of which the element or stack of elements can be placed, a first measuring rod (ll) one end (ll") of which is movably connected to the top of the first casing; a second casing (2) surrounding the first casing and rigidly connected thereto; and a measuring unit (3) connected to the second casing, said measuring unit being provided with a body (l5) having a central opening through which one end (l9′) of a second measuring rod (l9) is passed into the second casing, the displaceable member (2l") of a displacement transducer (2l) being connected to the other end (20) of the second measuring rod, and means being provided for longitudinally moving the first and the second measuring rod.

Description: This invention relates to a device for the measurement under operating conditions of a dimension of an element or a stack of elements and/or its cladding. This invention particularly relates to the measurement under operating conditions of the length of a stack of fuel tablets and/or the cladding of a fuel rod formed by such a stack. Fuel rods have long since been used in fast breeder reactors, but without any provision for monitoring the dimensions, in particular the length, of the stack of fuel tablets and/or the cladding of the fuel rod during operation. So far such data have been obtainable only after the irradiation of the fuel rods has been terminated and after the irradiation capsule has been dismantled, or by means of neutron radiography at intervals between the irradiation periods. However, the data obtained in these two cases relate to the cold fuel rod. There are many problems which stand in the way of obtaining dimensional data from hot fuel rods under irradiation. These results principally from the fact that the rods are usually of completely sealed construction and that they are enclosed in a double-walled casing with a liquid metal filling. In addition, it is undesirable to situate a transducer for obtaining dimensional data close to the points of interest because of the adverse effects of nuclear radiation and the elevated temperatures on the accuracy of the transducer. These conditions further render it impossible to replace the transducer, if required. On the other hand, the long-distance mechanical transmission of dimensional data to a transducer is undesir­ able because errors may then be introduced owing to thermal expansion of the components of such a mechanical system. It is an object of the invention to provide a device with which the above problems can be overcome and with which information on the dimensions of the fuel rod in a capsule which is substantially equal to a conven­ tional irradiation capsule with a fuel rod in a double casing can be transferred in a sufficiently accurate manner. It is a further object of the invention to provide a device with which three different measurements can be selectively carried out at the fuel rod, namely, the measurement of the length of the stack of fuel tablets; of the length of the cladding of the fuel rod, and of the difference between the length of the stack of fuel tablets and that of the cladding. The last mentioned measure­ ment may be chosen for reasons of improved accuracy, or the results obtained by the two first mentioned measurements can be verified therewith. By means of the measurements it is possible, among other things, to study the effect of the fuel tablets on the cladding when the diameter of the fuel tablets increases. In fact, it is possible that the diameter of a fuel tablet in the stack increases to such a degree that this tablet gets jammed against the cladding. When the length of the stack of fuel tablets increases further, the cladding will be forced to follow this change of length, which may lead to rupture of the cladding, the more so as this embrittles under the influence of irradiation. The invention provides a device for the measure­ ment during operation of a dimension of an element or a stack of elements, comprising a first casing on the bottom of which the element or stack of elements is placed, a first measuring rod one end of which is movably connected to the top of the first casing; a second casing surround­ ing the first casing and rigidly connected thereto, and a measuring unit connected to the second casing, said measuring unit being provided with a body having a central opening through which one end of a second measuring rod is passed into the second casing, the displaceable member of a displacement transducer being connected to the other end of the second measuring rod, and means being provided for longitudinally moving the first and the second measuring rod. With such a device it is possible to determine the length of an element or a stack of elements, particularly fuel tablets, arranged within the first casing. Preferably, the first and second casings are gas-tight and the other end of the second measuring rod is surrounded by a third casing in likewise gas-tight condition, the second and third casings each communicating with a line for supplying a pressurized fluid, preferably a gas. The measuring rods may then be displaced by controlling the pressure of the fluid in the second and third casings. The invention further provides a device of the above-described type in which a stop is formed in the first casing, said first casing communicating with a line for supplying a pressurized fluid, the first measuring rod being enlarged at the other end, which enlarged portion can cooperate with the bottom of the stop. Such a device also enables measurement of the length of the first casing, particularly of the cladding disposed around a stack of fuel tablets in said first casing, in the latter case the stop being connected to the cladding. Although the invention will hereinafter be discussed for use in carrying out measurements at fuel tablets in nuclear reactors, the invention is applicable wherever a dimension, particularly the length, of an element or stack of elements and/or a casing or cladding disposed around such elements is to be accurately determined under dangerous conditions. In case of application in nuclear reactors the pressurized fluid will be a gaseous medium, but for other applications the use of other fluids, such as liquids, is possible, and it is further possible to displace the first and the second measuring rod mechanically, e.g., by means of a motor or electromagnet. The invention will be elucidated herein below by means of an example of embodiment shown in the figure. The figure shows a modified capsule for a fuel rod, comprising a first casing l and a second casing 2. The first casing comprises an inner covering and an outer covering. A measuring unit 3 is attached to the second casing. A stack of fuel tablets 4 is placed within the first casing; by way of example six of such tablets 5 are shown in the figure, but in practice this number is very large. The stack 4 is surrounded by a cladding 6 forming the first covering. A cylinder 7 from molybdenum is disposed around the cladding 6, liquid metal 8 being arranged within the interspace between the cladding 6 and the cylinder 7 and between the cylinder 7 and the wall of the first casing l. The wall of the first casing forms the second covering for the fuel tablets. The cylinder 7 from molybdenum and the liquid metal serve for transporting the heat developed in the fuel tablets. The first casing l and the second casing 2 may have a common bottom which then forms a first plane of reference 9 in respect of which the measurement of the length of the stack 4 and the cladding 6 is effected. However, the first casing l may also be attached to a collar formed in the second casing, which collar then forms the plane of reference. Arranged within the cladding 6, at some distance above the fuel tablets 5, is a collar-­ shaped stop l0 connected to the cladding and provided with a central passageway for a measuring rod ll. The measuring rod ll has an enlarged lower part ll′ which may abut on the stop l0 when the measuring rod ll is moved upwards. The measuring rod ll is preferably made of titanium because this is a material having a low coefficient of expansion so that the change in length of the measuring rod continues to be small and does not adversely affect the measurements. The measuring rod ll is passed by means of two bellows l2 and l3, respectively, to the exterior of the inner casing. The first bellows l2 consists essentially of two series-connected bellows l2′ and l2", said bellows l2′, on the one hand, being attached to the cladding 6 and, on the other hand, to an enlarged upper part ll" of the measuring rod ll, and the bellows l2", on the one hand, likewise to the cladding and, on the other hand to the measuring rod ll. The bellows l2" may then be directed to the bottom of this measuring rod or like bellows l2′ be connected to the enlarged end ll" thereof. The second bellows is, on the one hand, attached to the wall of the first casing l and, on the other hand, likewise attached to the top ll" of the measuring rod ll. There are provided two bellows l2′ and l2" to ensure a superior and reliable sealing between the interior of the first covering of the first casing and the interior of the second covering thereof. In fact, it turns out in practice that the bellows sealing is most critical and is susceptible to failure. The top surface of the enlarged part ll" of the measuring rod ll forms a stop for carrying out measurements by means of the measuring unit 3. A second plane of reference is formed in the second casing 2 as a collar l4 extending into the interior of the casing. The bottom of the measuring unit 3 is mounted in a central opening of this collar l4. The measuring unit 3 comprises a body l5 which is preferably made of titanium and is passed through a central opening in the upper surface 2′ of the second casing 2 in gas-tight condition by means of a sealing ring l7. A third casing l6 is formed around the top of the body l5 in such a manner that there is a gas-tight sealing between the interior of the casing l6 and the surrounding space. This sealing consists of a collar l5′ formed at the body l5, with a sealing ring l8 provided in a circumferential groove thereof. A second measuring rod l9 is passed through a central opening in the body l5, said measuring rod having an enlarged lower part l9′, the top of which is connected to a piston 20. This measuring rod is preferably also made of titanium. The piston 20 is provided with a circumfer­ ential groove having contained therein a sealing ring which sealingly abuts on the inner wall of a cylindrical space in the body l5 of the measuring unit, in such a manner that the piston can move up and down through this cylindrical part. The movable part 2l" of a known per se displacement transducer 2l is connected to the top of the piston. The movable part 2l" is surrounded by the fixed part 2l′ of the transducer, said fixed part being connected to the body l5 of the measuring unit. Finally, there are provided three gas supply lines 22, 23 and 24, respectively. The first gas supply line 22 communicates with the interior of the casing l6 of the measuring unit 3; the second gas supply line 23 with the interior of the second casing 2 and the third gas supply line 24 with the interior of the first casing l. All of the three gas supply lines are connected to a gas source or gas sources not shown, with a possibility of separately controlling the pressure of the gas supplied to each line. The device according to the figure enables measurement, in the manner described herein below, of first the length of the stack 4 of fuel tablets 5; second the length of the cladding 6 and third the difference between the length of the stack 4 and that of the cladding 6. The measurement of the length of the stack 4 consisting of fuel tablets 5 is effected by making, via gas line 23, the gas pressure in the interior of the second casing so much higher than the gas pressure in the first casing that the measuring rod ll with the enlarger lower part ll′ thereof contacts the top of the uppermost fuel tablet 5 in the stack 4. When in a position of rest, the lower part l9′ of the second measuring rod l9 abuts on the bottom of the second plane of reference l4, but when a gas pressure is supplied via line 22 to the interior of the casing l6 of the measuring unit, the piston 20 will move downwards under the influence of the gas pressure until the enlarged lower part l9′ of the second measuring rod abuts on the enlarged upper part ll" of the first measuring rod. Information on the length of the stack of fuel tablets can be derived from the difference of the output signal of the transducer 2l in a position of rest and in the position in which the bottom of the second measuring rod abuts on the top of the first measuring rod. The length of the cladding 6 is in fact measured in the same way as described above with respect to the length of the stack of fuel tablets, with the exception of the fact that the gas pressure applied via line 24 to the first casing is higher than that prevailing in the second casing so that the measuring rod ll will move upward until the enlarged lower part 11′ thereof contacts the collar-shaped stop l0 of the cladding. When subsequently the lower part l9′ of the second measuring rod is moved from the rest position in which it abuts on the stop l4, by increasing the gas pressure in the third casing l6, until the part l9′ abuts on the upper part of the measuring rod ll, information on the length of the cladding 6 can be derived from the signals of the transducer 2l in the rest position and in the position in which the measuring rods make contact. Finally, it is possible to determine the difference between the length of the cladding and the length of the stack of fuel tablets by applying gas pressures to respec­ tively the first casing, the second casing and the measuring unit in such a manner that the lower part l9′ of the second measuring rod permanently abuts on the upper part ll" of the first measuring rod, while this measuring rod moves over its measuring range from the stop l0 to the top of the uppermost fuel tablet 5. Because during operation the part of the second casing between the first stop 9 and the second stop l4 is in permanent contact with cooling water from the reactor which, under operating conditions, has an average temperature of about 30°, the temperature gradient along the casing from the first stop to the second stop will be substantially constant, with the exception of small fluctuations occurring during the operating cycle of the reactor so that the position of the second plane of reference in regard to the first plane of reference can be determined with only a small margin of error. When the measurements are carried out, all information with respect to the length of the cladding and of the stack of fuel tablets is referred to the second plane of reference. When the resulting measuring values are analysed, these may be adjusted so as to determine the length in regard to the first plane of reference, that is to say, the bottom of the first casing. The output signals of the transducer 2l are supplied to a remote reading unit in a manner known per se. In the device according to the invention the measuring unit can be easily detached from the second casing or mounted in it again. This is particularly advanta­ geous in an irradiation capsule, especially when this is for single use, because the measuring unit can then be used again and again. With conventional irradiation capsules having a closed construction a repair is usually not possible after the irradiation has started, owing to the radioactivity induced in the material of the capsule. In the device according to the invention, however, the measuring unit can be replaced after the irradiation has started. Preferably, at least the lower part of the measuring unit which is most exposed to irradiation is made of titanium. A characteristic feature of this material is that the radioactivity induced therein soon becomes ineffective, and further that the material has a low coeffi­ cient of thermal expansion.

Claims: 1. A device for the measurement during operation of a dimension of an element or a stack of elements, charac­ terized by a first casing on the bottom of which the element or stack of elements can be placed, a first measuring rod one end of which is movably connected to the top of the first casing; a second casing surrounding the first casing and rigidly connected thereto; and a measuring unit connected to the second casing, said measuring unit being provided with a body having a central opening through which one end of a second measuring rod is passed into the second casing, the displaceable member of a displace­ ment transducer being connected to the other end of the second measuring rod, and means being provided for longitu­ dinally moving the first and the second measuring rod. 2. A device as claimed in claim l, characterized in that the first and the second casing are fluid-tight, and that around the body of the measuring unit a gas-tight third casing is formed, in which casing a piston attached to the other end of the second measuring rod can be moved up and down, the displaceable member of the displacement transducer being connected to the piston, and that a line for supplying a pressurized fluid is connected to the second and the third casing. 3. A device as claimed in claim l or 2, characterized in that the second measuring rod is substantially in line with the first measuring rod, and that the end of the second measuring rod extending into the second casing is enlarged and can cooperate with the bottom of a collar formed in the second casing. 4. A device as claimed in claim 2, characterized in that a stop is formed in the first casing and a line for supplying a pressurized fluid is connected to the first casing, and that the other end of the first measuring rod is enlarged and can cooperate with the bottom of the stop. 5. A device as claimed in claim 2, 3 or 4, character­ ized in that the pressurized fluid is a gas. 6. A device as claimed in claim 4, characterized in that a cladding is provided around the element or elements, and that the stop is formed at the cladding above the elements. 7. A device as claimed in claim 4 or 6, characterized in that one end of a first bellows is connected to the wall of the first casing, the other end of which bellows is connected to one end of the first measuring rod, and that one end of the second double bellows is always connected to the cladding, the other ends of which bellows are connected to the first measuring rod. 8. A device as claimed in at least one of the preceding claims, characterized in that the part of the measuring unit extending into the second casing and the second measuring rod are made of titanium. 9. A device as claimed in at least one of the preceding claims, characterized in that the first measuring rod is made of titanium.

Inventors: HALE, RONALD GEOFFREY, MOSS, RAYMOND LLOYD, ZEISSER, PETER RUDOLF
Applicants: EUROPEAN ATOMIC ENERGY COMMUNITY (EURATOM)
Application year: 1987
Main classification: G21C17
IPC codes: G21C17