Patent Number: 
Section: description

Reference should now be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components. As shown in the drawings, the in-pile creep test system 2 according to the preferred embodiment of the present invention comprises a creep tester 6, a detecting unit 10, a gas supply unit 18, and a control unit 20. The creep tester 6 is vertically installed in a water pool 4 of a research reactor. The detecting unit 10 is electrically connected to the creep tester 6, and detects the temperature of the heating unit 32 and creep strain of an irradiated specimen 64 during a creep test. The gas supply unit 18 is connected to the creep tester 6 through gas supply tubes 12, and controllably supplies helium gas from a helium gas reservoir tank 14 to the creep tester 6 or returns helium gas from the tester 6 to the tank 14 with the use of compressed air generated by an air compressor 16. The control unit 20 is electrically connected to both the detecting unit 10 and the gas supply unit 18 so as to control the operation of the creep test system 2 in response to results of a comparison of input data from the detecting unit 10 and the gas supply unit 18 with stored data. The water pool 4 is a unit of a conventional research reactor, and contains distilled water. The in-pile creep test for measuring and determining mechanical properties of irradiated nuclear materials is performed in the pool 4. Provided on the bottom of the water pool 4 is a base 22, with a plurality of tester installation slots 24 formed on the base 22 to vertically hold creep testers 6 on the base 22. The creep tester 6 comprises a fixing unit 26, a pressurizing unit 28, a movable unit 30, a heating unit 32, and a measuring unit 34, which are mounted to or set in a cylindrical capsule 36. The fixing unit 26 of the creep tester 6 is fixed to a tester installation slot 24 of the base 22 on the bottom of the water pool 4 so as to vertically hold the creep tester 6 on said base 22. The fixing unit 26 comprises a base plate 38 that is provided at an end of the capsule 36, with a fixing shaft 40 axially projecting from the center of the base plate 38. A stop plate 42 is movably fitted over the fixing shaft 40, with three stop rods 44 extending from the stop plate 42 toward the base plate 38 at three regularly spaced positions. Each of the stoop rods 44 is bent at a middle portion thereof. A coiled biasing member 46 is fitted over the fixing shaft 40 so as to bias the stop plate 42 in a predetermined direction. The pressurizing unit 28 is pressurized or depressurized by helium gas fed into a chamber 48 defined in the capsule 36 at a position adjacent to the fixing unit 26. The pressurizing unit 28 comprises a fixed first plate 50, a bellows tube 52, and a push plate 54. The first plate 50 is fixedly set in the capsule 36 at a position spaced apart from the base plate 38 at a predetermined interval. The bellows tube 52 is mounted to the first plate 50 at a position inside the chamber 48, while the push plate 54 is formed at the end of the bellows tube 52 so as to selectively push the movable unit 30. For contraction effect, a spring is installed on the push rod 56 so as to effectively pull the movable unit 3. A bushing 58 having a central hole 60 is set at the center of the first plate 50, while a push rod 56 of the movable unit 30 axially and movably passes through the central hole 60 of the bushing 58. In order to allow a compressive deformation of the specimen 64 during a compression creep test, a spring may be installed on the push rod 56 of the movable unit 30. The spring enhances a depressurizing effect of the pressurizing unit 28 as well as allowing an easy retraction of the movable unit 30. A push rod slot 62 is formed on the push plate 54 so as to receive the end of the push rod 56 of the movable unit 30, thus allowing the push plate 54 to accomplish precise pressure transmission to the push rod 56. The movable unit 30 is moved in accordance with an operation of the pressurizing unit 28, thus applying tensile or compressive force to the specimen, and comprises two holder units, that is, a first holder unit 66 and a second holder unit 70. The first holder unit 66 is set at a position adjacent to the first plate 50 of the pressurizing unit 28, and is divided on the basis of the central axis of the capsule 36 into two parts, that is, upper and lower holder plates spaced apart from each other by a predetermined gap. The second holder unit 70 is assembled with the first holder unit 66 while being partially overlapped with the first holder unit 66, and is movably assembled with a first guide stopper 68 which is set in the capsule 36 at a position spaced apart from the first plate 50 by a predetermined distance. In a detailed description, the first holder unit 66 comprises a guide slot 72 that is formed along the central axis of the movable unit 30, with the upper and lower holder plates 76 and 80 formed at upper and lower portions around the guide slot 72, respectively, such that they face each other. The upper holder plate 76 has a plurality of upper locking holes 74, while the lower holder plate 80 has a plurality of lower locking holes 78. The second holder unit 70 comprises the push rod 56, which has a predetermined length and movably passes through the central hole 60 of the first plate""s bushing 58. The second holding unit 70 also has a specimen holding rod 82 and a cylindrical holder pipe 90. The specimen holding rod 82 has a U-shape so as to stably hold the specimen 64, and is coupled to the end of the push rod 56. The cylindrical holder pipe 90 is connected to the end of the U-shaped specimen holding rod 82, and has a plurality of upper and lower locking holes 86 and 88 so as to hold the specimen 64 with the use of locking pins 84 inserted in the locking holes 86 and 88. A slide shaft 92 is connected to an end of the holder pipe 90, and movably passes through the central opening of the first guide stopper 68 welded in the capsule 36. The slide shaft 92 has a probe-seating slot 98 at the center of an end surface thereof, such that a probe 96 of a strain-measuring instrument 94, such as an LVDT, is precisely arranged along the central axis of the tester 6. An O-ring type bearing 100 made of a carbon plate is set in the central opening of the first guide stopper 68 so as to movably bear the slide shaft 92 in the guide stopper 68 while preventing frictional contact between the first guide stopper 68 and the slide shaft 92. The heating unit 32 is assembled with the movable unit 30 and is used for heating the specimen 64, and comprises a cylindrical heater housing 104 having a heater 108 made of silicon carbide. An axial slit 102 is formed at the sidewall of the heater housing 104 so as to allow the housing 104 to be fitted over the U-shaped specimen holding rod 82 from a side of the rod 82. Axially formed on the external surface of the heater housing 104 are two tube-seating grooves 106 which allow the gas supply tubes 12 to pass along the external surface of the housing 104. The heater 108 made of silicon carbide is set in the sidewall of the heater housing 104, and is electrically connected to the detecting unit 10. The measuring unit 34 senses a movement of the movable unit 30 to measure the creep strain of the tensioned or compressed specimen 64. The measuring unit 34 comprises the LVDT 94 and a fixing cap 114. The LVDT 94, which is a conventional strain-measuring instrument, axially passes through the center of a second guide stopper 110 such that the LVDT 94 is axially aligned with the slide shaft 92 of the movable unit 30 held by the first guide stopper 68. The fixing cap 114 is assembled with an end of the LVDT 94, and is attached to a third guide stopper 112 such that the LVDT 94 comes into contact with the slide shaft 92 at its probe 96. The capsule 36 is a cylindrical body consisting of first to fourth capsule parts 116, 118, 120 and 122. The first capsule part 116 supports the fixing unit 26, and houses the pressurizing unit 28 while defining the chamber 48. The pressurizing unit 28 is set in the first capsule part 116 by the first plate 50 welded to the internal surface of the first capsule part 116. The second capsule part 118 is assembled with an end of the first capsule part 116, and houses the movable unit 30. In such a case, the movable unit 30 is set in the second capsule part 118 through a process of welding the first guide stopper 68 to the inner surface of said second capsule part 118 for the convenience of assembling the capsule parts. The third capsule part 120 is assembled with an end of the second capsule part 118, and houses the measuring unit 34. In order to set the measuring unit 34 in the third capsule part 120 such that the measuring unit 34 is aligned with the central axis of the third capsule part 120, the second guide stopper 110 is welded to the inner surface of the third capsule part 120. The fourth capsule part 122 is assembled with an end of the third capsule part 120, and houses the fixing cap 114 of the LVDT 94. In order to set the fixing cap 114 in the fourth capsule part 122, the third guide stopper 112 is welded to the inner surface of the fourth capsule part 122. It is necessary to seal the fourth capsule part 122, through which the electric wires for both the heating unit 32 and the measuring unit 34 are led into the capsule 36 and the gas supply tubes 12 are led from the gas supply unit 18 into the capsule 36. In order to accomplish the above object, a sealing capsule part 126 having two porous sealing plates 124 is integrated with an end of the fourth capsule part 122. The detecting unit 10 is selected from conventional detecting units, and is electrically connected to the control unit 20, the silicon carbide heater 108 of the heating unit 32, and the LVDT 94, thus detecting temperature of the heating unit 32 and strain of the irradiated specimen 64. In such a case, the electric wires 128 connecting the detecting unit 10 to both the heater 108 and the LVDT 94 of the tester 6 extend in the capsule 36. The gas supply unit 18 is designed in a conventional manner, and is electrically connected to the control unit 20, thus being operated under the control of the control unit 20. The gas supply unit 18 is connected to both the air compressor 16 and the helium gas reservoir tank 14 through a gas pipe 130, and communicates with the second capsule part and the chamber 48 of the pressurizing unit 28 through the gas supply tubes 12. In the present invention, the gas supply unit 18 is provided with a vacuum pump (not shown) at a predetermined position. The gas supply tubes 12 comprise first and second tubes 132 and 134. During a gas supply mode for tensioning the irradiated specimen, the two tubes 132 and 134 are used to feed helium gas to the chamber 48 at the same time. However, during a gas return mode for compressing the irradiated specimen, one of the two gas supply tubes 132 and 134 is used for feeding compressed air to the chamber 48, and the other is used for returning helium gas from the chamber 48 to the reservoir tank 14. The first plate 50 is welded in the first capsule part 116, with the gas supply tubes 12 being connected to the plate 50. The first guide stopper 68 is welded in the second capsule part 118. As shown in FIG. 8, four first radial ribs 136 extend outwardly from the body of the first guide stopper 68 in four radial directions, and are welded to the internal surface of the second capsule part 118. Due to the radial ribs 136, a plurality of first spaces 136 are defined between the external surface of the first guide stopper 68 and the internal surface of the second capsule part 118, so the gas supply tubes 12 and the electric wires for electrically operated units, such as the heating unit 32, smoothly pass in the capsule 36 without being blocked by the first guide stopper 68. In an effort to prevent an undesired removal of the bearing 100 from the first guide stopper 68, a stop plate 140 is mounted to an end surface of the first guide stopper 68. The second guide stopper 110 is set in the third capsule part 120. As shown in FIG. 8, four second radial ribs 142 extend outwardly from the body of the second guide stopper 110 in four radial directions, and are welded to the internal surface of the third capsule part 120. Due to the radial ribs 142, a plurality of second spaces 144 are defined between the external surface of the second guide stopper 110 and the inner surface of the third capsule part 120, so the gas supply tubes 12 and the electric wires for the electrically operated units, such as the heating unit 32, smoothly pass in the capsule 36 without being blocked by the second guide stopper 110. The third guide stopper 112 is set in the fourth capsule part 122. In the same manner as that described for the first and second guide stoppers 68 and 110, four third radial ribs 146 extend outwardly from the body of the third guide stopper 112 in four radial directions, and are welded to the internal surface of the fourth capsule part 122. Due to the radial ribs 146, a plurality of third spaces 148 are defined between the third guide stopper 112 and the fourth capsule part 122, so the gas supply tubes 12 and the electric wires smoothly pass in the capsule 36 without being blocked by the third guide stopper 112. In the creep tester 6, the pressurizing unit 28, second capsule part 118, third capsule part 120 and the fourth capsule part 122 are isolated from each other while being independently sealed, so even though there is an unexpected breakage or damage to a part of the creep tester 6, the breakage or damage is not propagated to another part, but is limited to the originally broken or damaged part. Therefore, it is possible to reduce breakage or damage to the specimen during a procedure of disassembling the capsule into pieces after a materials irradiation test, as well as making it easy to disassemble the capsule. The specimen 64 installed in the tester 6 of the in-pile creep test system 2 is a specimen of a nuclear material, and has locking holes 150 at both ends so as to be held in the movable unit 30 by the locking pins 84. Of course, it is possible to selectively install any one of specimens 64 of a variety of materials, having different sizes and shapes, such as materials having cylindrical shapes, plate-type shapes or rod-type shapes, in the tester 6, as desired. The operation and effect of the in-pile creep test system of the present invention will be described herein below. In order to perform an in-pile creep test for measuring and determining mechanical properties of an irradiated material, the creep tester 6 of the special instrumented capsule type is vertically installed in the water pool 4 containing distilled water. That is, the creep tester 6 is vertically held on the base 22 in the water pool 4 with the use of the fixing unit 26. The control unit 20 controls the detecting unit 10 on the basis of stored data, thus allowing the heating unit 32 to generate heat. The control unit 20 also controls the gas supply unit 18 so as to controllably feed helium gas to the chamber 48 of the tester 6. Due to pressure of the helium gas in the chamber 48, the pressurizing unit 28 actuates the movable unit 30, thus causing expansion or shrinkage of the movable unit 30. Therefore, tensile load or compressive load or repeated low cyclic load is applied to the specimen 64 installed in the movable unit 30, thus applying tensile or compressive force to the specimen 64. In such a case, the measuring unit 34 installed in contact with the movable unit 30 measures creep strain of the specimen 64, and outputs a signal indicative of the creep strain to the control unit 20. The control unit 20 thus compares data signals from both the measuring unit 34 and the detecting unit 10 with stored date, and controls the gas supply unit 18 in response to the signal comparison results so as to controllably supply or return helium gas to or from the chamber 48. The operation of the in-pile creep test system 2 according to the present invention will be described in more detail herein below. In order to perform a tensile creep test for measuring and determining mechanical properties of an irradiated material using the system 2, a specimen 64 of the target material is axially installed on the U-shaped specimen holding rod 82, prior to assembling the creep tester 6. The U-shaped specimen holding rod 82 having the specimen 64 is, thereafter, set in both the first holder unit 66 and the holder pipe 90 of the movable unit 30 such that the middle portion of the U-shaped specimen holding rod 82 is exposed at a position between the first holder unit 66 and the holder pipe 90. Thereafter, the locking pins 84 are inserted into the two pairs of upper and lower holes 74 and 78, respectively, such that the locking pins 84 pass through the locking holes 150 of the specimen 64. After installing the specimen 64 in the movable unit 30, the heater housing 104 of the heating unit 32 is fitted over the exposed middle portion of the U-shaped specimen holding rod 82 from a side of the rod 82 through the axial slit 102 of the housing 104. Thereafter, the fixing unit 26 is mounted to the first capsule part 116. In addition, the pressurizing unit 28 and the movable unit 30 are housed in the first and second capsule parts 116 and 118, respectively, and the measuring unit 34 is housed in the third and fourth capsule parts 120 and 122. Thereafter, the sealing capsule part 126 having the two porous sealing plates 124 is integrated with the end of the fourth capsule part 122, thus completely assembling the parts of the creep tester 6 into a single structure. When the in-pile creep test system 2 starts a tensile creep test after the creep tester 6 is vertically installed in the water pool 4, the control unit 20 controls the gas supply unit 18 in accordance with stored data, thus feeding highly pressurized helium gas from the helium gas reservoir tank 14 to the chamber 48 of the tester 6 through the first and second gas supply tubes 132 and 134. The highly pressurized helium gas fed to the chamber 48 pushes the push plate 54, thus pushing the push rod 56 of the movable unit 30, so tensile force is imposed on the specimen 64 which is held in the first holder part 66 and the holder pipe 90 by the locking pins 84. In such a case, the control unit 20 also outputs a control signal to the detecting unit 10, and applies electric power to the silicon carbide heater 108 of the heating unit 32. The heater 108 thus generates heat. In addition, the gas supply unit 18 feeds highly pressurized helium gas from the helium gas reservoir tank 14 to the chamber 48 through the first and second gas supply tubes 132 and 134 in response to a control signal outputted from the control unit 20. In order to perform a compression creep test for measuring and determining mechanical properties of an irradiated material using the system 2, the bellows tube 52 of the pressurizing unit 28 is compressed to the maximum with the use of the gas supply unit 18 and the push rod 56 is welded to the push plate 54 during the procedure of installing a specimen 64 in the creep tester 6 while assembling the parts of the tester 6 into a single structure. The specimen 64 of a target material is axially installed on the U-shaped specimen holding rod 82, and the U-shaped specimen holding rod 82 having the specimen 64 is set in both the first holder unit 66 and the holder pipe 90 of the movable unit 30 such that the middle portion of the U-shaped specimen holding rod 82 is exposed at the position between the first holder unit 66 and the holder pipe 90. Thereafter, the locking pins 84 are inserted into the two pairs of upper and lower holes 74 and 78, respectively, such that the locking pins 84 pass through the locking holes 150 of the specimen 64. Thereafter, the heater housing 104 of the heating unit 32 is fitted over the exposed middle portion of the U-shaped 25 specimen holding rod 82 from a side of the rod 82 through the axial slit 102 of the housing 104. Thereafter, the fixing unit 26 is mounted to the first capsule part 116. The pressurizing unit 28 and the movable unit 30 are housed in the first and second capsule parts 116 and 118, respectively, and the measuring unit 34 is housed in the third and fourth capsule parts 120 and 122. Thereafter, the sealing capsule part 126 is integrated with the end of the fourth capsule part 122 through a welding process, thus completely assembling the parts of the creep tester 6 into a single structure. In such a case, a spring may be installed on the push rod 56 of the movable unit 30 in an effort to enhance the depressurizing effect of the pressurizing unit 28 as well as allowing an easy retraction of the movable unit 30, thus increasing operational efficiency of the creep tester 6 while compressing the specimen 64. When the in-pile creep test system 2 starts a compression creep test after the creep tester 6 is vertically installed in the water pool 4, the control unit 20 controls the gas supply unit 18 in accordance with stored data to feed compressed air to the chamber 48 through one of the two gas supply tubes 132 and 134, and return helium gas from the chamber 48 to the reservoir tank 14 through the other gas supply tube. Simultaneously the helium gas is supplied to the second capsule 118 from reservoir tank 14. Pressure in the chamber 48 is thus reduced and the pressure in the second capsule is thus increased. Due to the reduction in pressure in the chamber 48 inside the first capsule part 116 and the increase in the pressure in the second capsule, the push rod 56 of the movable unit 30 is pulled, so compressive force is imposed on the specimen 64 that is held in the first holder part 66 and the holder pipe 90 by the locking pins 84. In such a case, the control unit 20 also outputs a control signal to the detecting unit 10, and applies electric power to the silicon carbide heater 108 of the heating unit 32, so the heater 108 thus generates heat. In addition, the vacuum pump of the gas supply unit 18 is operated, in response to a control signal outputted from the control unit 20, to return the helium gas from the chamber 48 to the helium gas reservoir tank 14 through one of the first and second gas supply tubes 132 and 134. During a tensile, compressive or repeated loading or cyclic fatigue creep test, highly pressurized helium gas is fed from the tank 14 to the chamber 48 through the gas supply tubes 12, so the gas supply tubes 12 continuously maintain a low temperature. Since such low temperature gas supply tubes 12 pass along the tube-seating grooves 106 which are axially formed on the external surface of the heater housing 104, it is possible to prevent the heating unit 32 from being overheated. The detecting unit 10, which is electrically connected to the heating unit 32, detects the temperature of the heating unit 32, and outputs a data signal indicative of temperature of the heating unit 32 to the control unit 20. A temperature sensor may be mounted at a position on the specimen 64, and senses a temperature of the specimen 64. The temperature sensor outputs a signal indicative of the specimen""s temperature to the control unit 20, and the control unit 20 appropriately adjusts output power of the heating unit 32 in response to the signal from the temperature sensor. It is thus possible to obtain a desired experimental temperature for the specimen by repeatedly sensing the temperature of the specimen 64 and repeatedly adjusting the output power of the heating unit 32. During the creep test, the temperature of the specimen is reduced when the pressure of supplied helium gas is increased to enhance heat transfer efficiency, but the temperature is increased when the pressure of supplied helium gas is lowered to reduce heat transfer efficiency. In other words, during such a tensile creep test, highly pressurized helium gas is fed from the helium gas reservoir tank 14 to the chamber 48 through the gas supply tubes 12, and, 20 at the same time, the silicon carbide heater 108 of the heating unit 32 is turned on the detecting unit 10, thus continuously generating heat. Before or after a creep test, the detecting unit 10 and the gas supply unit 18 are initialized under the control of the control unit 20. When the specimen 64 is deformed to create tensile, compressive or cyclic strain, the slide shaft 92 of the movable unit 30 is moved in either direction under the guide of the first guide stopper 68, so the probe 96 of the LVDT 94 constituting the measuring unit 34 advances or retracts. When cutting both ends of the capsule 36, it is possible to remove the tested specimen 64 from the tester 6. In such a case, the specimen 64 is held in the first and second holder units 66 and 70 of the movable unit 30 with the use of the locking pins 84, so it is easy to remove the tested specimen 64 from the tester 6. Since the LVDT 94 is electrically connected to the detecting unit 10, the detecting unit 10 detects the strain of the specimen, and outputs a signal indicative of the specimen""s strain to the control unit 20, thus allowing the control unit 20 to measure and determine the creep testing results. It is possible to measure creep strain of any one of specimens having a variety of lengths and shapes, such as specimens having cylindrical shapes, plate-type shapes or rod-type shapes, with the use of the in-pile creep test system 2 of the present invention. Such a compatibility of the system 2 is simply and easily accomplished by appropriately adjusting the length of the slide shaft 92. In addition to creep tests for newly developed nuclear materials, the in-pile creep test system 2 of the present invention is used in creep tests for nuclear clad tubes. As shown in FIG. 9, which is a graph showing the stress-strain curve of an irradiated nuclear clad tube tested with the use of the in-pile creep test system 2, the system 2 of this invention is operated to precisely measure and determine mechanical properties of nuclear clad tubes. Therefore, it is noted that the in-pile creep test system 2 of the present invention is preferably and effectively used in creep tests for measuring and determining mechanical properties of irradiated materials having a variety of shapes and sizes during a procedure of developing new nuclear materials. As described above, the present invention provides an in-pile creep test system used for measuring and determining mechanical properties of nuclear materials irradiated in research reactors. The creep test system is advantageous in that it allows an effective and uniform irradiation to the area of the gauge length of a specimen during an in-pile creep test. The creep test system of the present invention is also fabricated with a minimum number of parts for applying tensile or compressive force to an irradiated specimen and a minimum number of instruments for measuring creep strain of the irradiated specimen in an effort to minimize the amount of nuclear wastes generated after a creep test. Therefore, the system is easily fabricated and reduces the amount of nuclear wastes. The creep test system is also designed such that it is easy to cut the capsule of a creep tester and disassemble a tested specimen from the capsule in a hot cell, while preventing damage or breakage of the specimen during a procedure of cutting the capsule and removing the tested specimen from the capsule. In the in-pile creep test system, the mechanical structure for applying load to the irradiated specimen during a creep test is designed such that it is not influenced by irradiation. The test system also maximizes the effect of irradiation to the specimen. Another advantage of the in-pile creep test system resides in that the test system is compatibly used in a tensile or compressive creep test for irradiated specimens having a variety of shapes and sizes. The test system of the present invention is used in low speed fatigue tests for irradiated specimens having various shapes and sizes. A further advantage of the test system resides in that it is possible to easily and appropriately control the temperature of heat transmitted to the irradiated specimen. Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.