Abstract:
An instrumented capsule for material irradiation tests in research reactors. The instrumented capsule performs an optimum material irradiation test under a testing environment similar to the operational environment of a real reactor. The capsule minimizes the influence of flow-induced vibration caused by forced-circulation-type coolant flow in a research reactor, and overcomes the problems experienced in the conventional breakable parts of instrumented capsules which may be broken during the process of loading/unloading the capsules in vertical irradiation holes of reactor pools. The instrumented capsule includes a capsule main body installed in the vertical irradiation hole. The capsule main body consists of a shell and several instruments, such as thermocouples, dosimeters, a vacuum control pipe, and heaters housed in the shell. The capsule main body also includes heat media, specimens set in the heat media, insulators interposed between adjacent heat media, upper and lower end plugs to seal the ends of the shell, an upper guide spring unit to vertically place the capsule main body in the irradiation hole, and a reinforced lower fixing unit assembled with the lower end plug. The instrumented capsule also includes a connecting means for connecting the capsule main body to a capsule control system installed outside the reactor pool.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an instrumented capsule for the material irradiation tests, which is designed to monitor the irradiation history of specimens and simultaneously to control the environment during material irradiation tests. 
     2. Description of the Prior Art 
     There are several essential prerequisites to be solved for developing new types of nuclear reactors, such as next generation reactors. For example, it is necessary to qualify fuel and structural material performance that is compatible with the features of advanced reactors in the design stage. The important essential prerequisites for the development of nuclear reactors are the close examination into several phenomena related to irradiation in reactors, and the development of advanced materials with the resistance of irradiation aging. 
     In recent years, next generation light water reactors (NGLR), advanced pressurized water reactors (APWR) and liquid metal reactors (LMR) have been actively studied and developed, and, therefore, advanced structural materials and fuels which are compatible with the features of such reactors are under active study and development. When designing such reactors, it is necessary to evaluate and determine neutron resistance of conventional structural materials or advanced structural materials that may be used in the reactors. 
     Degradation in structural material performance caused by a reduction in a variety of mechanical properties of materials, due to irradiation of fast neutrons to the materials in a real nuclear reactor, is the most serious factor, resulting in a reduction in both integrity and life span of a nuclear power plant. 
     Therefore, material irradiation testing in research reactors for qualification of neutron resistance of structural materials is recognized as a very important test for developing advanced structural materials or for newly planning the essential components of reactors. 
     Such material irradiation test in research reactor has been typically performed with the use of various material testing facilities. Such a material testing facility includes an in-pile test section, a so-called “capsule”. The capsule is the most important unit of the material testing facility, which houses specimens of a variety of target materials and is installed in an irradiation hole of the research reactor. The conventional capsules used in the material irradiation tests are classified into two types: instrumented capsules and non-instrumented capsules. The instrumented capsule has a connection channel through which control wires pass to connect the instruments of the capsule to a capsule control system installed outside the reactor pool, so it is possible to remotely control the test environments of the capsule, such as the inner temperature and atmosphere of the capsule, during a material irradiation test. On the contrary, the non-instrumented capsule does not have such a connection channel, so it is impossible to control the test environment of the capsule during a material irradiation test. 
     In other words, the non-instrumented capsule is an in-pile test unit lacking any means for remotely controlling the inner temperature and atmosphere of the capsule, so the irradiation temperature and atmosphere for target specimens housed in the capsule cannot be controlled. Therefore, the non-instrumented capsule, during a material irradiation test, does not provide a testing environment similar to the operational environments of real reactors. However, the instrumented capsule, related to the present invention, is an in-pile test section provided with an improvement in design of such a non-instrumented capsule. The construction of such instrumented capsules may be variously designed in accordance with irradiation test purposes, and may be equipped with various instruments, such as a thermocouple, a sub-heater, a pressure sensor, a strain gauge, and a dosimeter, in accordance with irradiation test purposes. 
     Uses of the instrumented capsules are wide, such that the capsules are preferably used in the qualification of nuclear fuel materials performance. However, the instrumented capsule related to the present invention is limitedly used in the qualification of performance of a variety of materials of reactor&#39;s essential elements, other than fuel. 
     The main body  10  of an instrumented capsule  1  comprises heat media  13  collaterally acting as specimen holders at portions  14 , specimens  2 , dosimeters  29 , and thermocouples  25 , which are housed in a stainless steel shell  11  as shown in FIGS. 4 a ,  5   a  and  5   b . The shell  11  of the capsule is a cylindrical body of about 1 m in length and 60 mm in outer diameter. The instrumented capsule also has a vacuum control pipe and heaters. The vacuum control pipe is used for controlling the pressure of helium gas in the capsule main body to control the degree of vacuum in said capsule main body, while the heaters are used for heating the specimens  2  in order to control the temperature of the specimens  2  during a material irradiation test. In the pool of a research reactor, a protection tube extends from the top end of the shell of the capsule installed in an irradiation hole, while a guide tube extends from the protection tube to a junction box. The protection tube and the guide tube, both air- and water-tight, guide the vacuum control pipe and the control wires while isolating them from coolant. The junction box connects the vacuum control pipe and the control wires to the capsule control system. Due to this unique construction of the instrumented capsule, it is possible to easily accomplish target irradiation temperature of specimens housed in the capsule, so an optimum material irradiation test under a testing environment similar to the operational environment of a real reactor may be accomplished. 
     The junction box has a role of connecting the capsule main body, installed in the irradiation hole of the reactor pool, and the capsule control system, installed at the upper portion of the research reactor, and connects the vacuum control pipe and a variety of control wires, such as a heater control wire and a thermocouple control wire, to the capsule control system. In such a case, the vacuum control pipe and the control wires extend from the interior of the shell of the capsule main body to the junction box guided by a protection tube and a guide tube. The junction box is an essential instrument necessarily used for detecting and controlling the specimen temperature during a material irradiation test. However, non-instrumented capsules do not have such a junction box. In the prior art, a junction box  110  of FIG. 14 has been used as the junction means. However, the conventional junction unit  110  has a complex construction with several problems whenever connecting the vacuum control pipe to the capsule control system within a limited space. The complex construction of the junction box  110  also causes difficulty in operation and fabrication of the instrumented capsules. In addition, the junction box  110  is quite heavy, thus sometimes overloading the flexible guide tube during a process of moving, loading or unloading the capsule main body in a research reactor. In such a case, the guide tube may be excessively bent at a radius of curvature larger than an allowable radius of curvature, thus causing severe problems. 
     Furthermore, The desired structural integrity of instrumented capsules and related systems for in-pile material irradiation tests must be accomplished. In an effort to secure such structural integrity of the instrumented capsules and related systems, it is necessary to perform a seismic analysis and structural analysis of the instrumented capsules and related systems in terms of dead loads, operational basic earthquake (OBE) and safe shutdown earthquake (SSE) in accordance with regulations of AMSE B&amp;PV Code, Section III, Div. 1, Part NF. Particularly, since the irradiation hole of a reactor pool, in which the capsule main body is loaded, is located at a forced convectional area, the essential instruments of the capsule must be designed in consideration of several important design factors. 
     In the case of a typical research reactor in which coolant flows upward, the capsule for material irradiation tests is loaded into a vertical irradiation hole inside a reactor pool. However, due to forced-circulation-type coolant flow in such a research reactor, the capsule may be vibrated in the irradiation hole, so structural integrity of the capsule and related systems must be maintained. Therefore, a variety of capsule fixing devices and capsule loading/unloading methods compatible with the features of research reactors have been developed and used. The capsule fixing devices are used for fixing the capsules in the reactor pools during material irradiation tests, and the capsule loading/unloading methods are for loading and unloading of the capsule main bodies in the irradiation holes inside the reactor pools before and after the material irradiation tests. 
     In order to fix a capsule main body in an irradiation hole of a reactor pool before a material irradiation test, a grapple head  84 , provided at the uppermost end of the capsule main body, as shown in FIG. 3, is rotated. When the grapple head  84  is rotated during the process of fixing the capsule main body, a torsion force is applied to the capsule main body. The capsule main body is thus stressed by the torsion force, and, in such a case, the guide pins  38   a  provided at the lower portion of the capsule main body may become overstressed leading to breakage. Therefore, the guide pins  38   a  must be designed such that they effectively endure such torsion force. 
     The inventors of the present invention thus have developed an instrumented capsule which houses specimens of various target materials and is equipped with a variety of instruments for controlling the temperatures of the specimens during a material irradiation test, and maintains desired structural integrity when the capsule is loaded into an irradiation hole of a research reactor pool, and which more effectively performs the material irradiation test in the research reactor. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an instrumented capsule for material irradiation tests in research reactors which is designed such that it houses specimens of target materials in its shell and easily and precisely controls the irradiation condition and the irradiation temperature, thus performing a material irradiation test under an optimum test environment, such as an irradiation temperature similar to the operational temperature of a real reactor. 
     Another object of the present invention is to provide an instrumented capsule for material irradiation tests in research reactors which has a guide spring means in addition to a conventional capsule fixing device, thus being stably held in an irradiation hole of a reactor pool while being prevented from excessive vibration caused by flow-induced vibration, and which does not interfere with adjacent structures during material irradiation tests, but safely performs the irradiation tests. 
     A further object of the present invention is to provide an instrumented capsule for material irradiation tests in research reactors which has a simple and safe junction box for simply and safely connecting a vacuum control pipe and a variety of control wires, such as a heater control wire and a thermocouple control wire, to a capsule control system installed outside a research reactor pool, and also which has a fixing unit capable of supporting the capsule main body in an irradiation hole of the reactor pool while maintaining the desired structural integrity of the capsule in the reactor pool where coolant flows upward, thus being compatible with the features of capsule loading/unloading methods. 
     In order to accomplish the above objects, the present invention provides an instrumented capsule for material irradiation tests in research reactors, including a capsule main body installed in a vertical irradiation hole of a research reactor pool, the capsule main body consisting of a shell opened at upper and lower ends thereof, a plurality of heat media set in the shell, a plurality of specimens set at a center and peripheral areas of each of the heat media, upper and lower reflectors installed on an upper end of an uppermost heat medium and under a lower end of a lowermost heat medium, respectively, a plurality of insulators interposed between adjacent heat media and positioned above and under the upper and lower reflectors, respectively, a spacer set in the shell at a position above an uppermost insulator, a spring seat installed above the spacer, a specimen compressing spring to bias the spring seat, thus compressing the specimens, a temperature control means for controlling a temperature inside the shell, the temperature control means consisting of a vacuum control pipe and a heater, a detecting means consisting of both a thermocouple used for detecting a temperature of the specimens and a dosimeter used for detecting a quantity of neutron radiation, upper and lower end plugs mounted to the upper and lower ends of the shell so as to seal the ends of the shell, and a lower fixing unit assembled with the lower end plug, and a connecting means for connecting the capsule main body to a capsule control system installed outside the reactor pool. 
     In the instrumented capsule, the shell of the capsule main body is a cylindrical body of about 0.6 m in diameter and 1 m in length. In order to stably and safely perform the material irradiation tests, an upper guide spring unit is fitted over the upper end of the shell so as to vertically place the capsule main body at the center of the vertical irradiation hole inside and minimize the influence of flow-induced vibration caused by forced-circulation-type coolant flow in the research reactor. The upper guide spring unit consists of upper and lower fixing rings, and a plurality of wire springs connected between the upper and lower fixing rings at regular intervals. 
     The temperature control means includes the vacuum control pipe and the heater. The vacuum control pipe is connected to the upper end of the capsule main body and controls the degree of vacuum in the capsule main body, thus controlling the quantity of transferred heat. The heater heats the specimens so as to control the temperature of the specimens. The control of the degree of vacuum and heater&#39;s operation is performed in response to a signal indicative of specimens&#39; temperature detected by the thermocouples. 
     The connecting means includes a rigid protection tube connected to an upper end of the capsule main body so as to air- and water-tightly guide the vacuum control pipe and the control wires extending from the thermocouple and the heater inside the capsule main body while protecting the vacuum control pipe and the control wires, a flexible guide tube connected to the protection tube so as to guide the vacuum control pipe and the control wires, and a junction box connected to the guide tube pipe so as to connect the vacuum control pipe and the control wires to the capsule control system installed outside the reactor pool, thus acting as a medium which transmits signals to the capsule control system. 
     The lower fixing unit includes a lower end cap mounted to the lower end plug, a rod tip connected to a center of the lower end cap and vertically extending downward, with a plurality of locking blades formed on a lower portion of the rod tip and locked to a fixing slot formed on a receptacle provided in the irradiation hole, a stopper movably fitted over the rod tip, and a stopper spring fitted over the rod tip at a position between the stopper and the lower end cap, thus normally biasing the stopper downward in a vertical direction. 
     The stopper of the lower fixing unit includes a plurality of holes formed in the stopper so as to allow a coolant flowing from the bottom of the irradiation hole to smoothly flow upward through the stopper without being disturbed by the stopper, and a plurality of guide pins projected on a circumferential surface of the stopper in radial directions such that the guide pins come into contact with the inner surface of the irradiation hole when the capsule main body is installed in the irradiation hole. 
     The stopper also includes an annular ring that connects the outside ends of the guide pins so as to support the guide pins. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic view showing an instrumented capsule according to a preferred embodiment of the present invention, which is installed in an irradiation hole of a research reactor and performs a material irradiation test; 
     FIG. 2 is a sectional view of a main body of the instrumented capsule according to the present invention; 
     FIG. 3 is a sectional view of a tube structure used for guiding a vacuum control pipe and several control wires from the capsule main body to a capsule control system installed outside the reactor pool while protecting the pipe and wires; 
     FIGS. 4 a  and  4   b  are sectional views of parts of the capsule main body, in which: 
     FIG. 4 a  is a sectional view of an upper part of the capsule main body; and 
     FIG. 4 b  is a sectional view of a lower part of the capsule main body; 
     FIGS. 5 a  to  5   d  are views of a heat medium housed in the shell of the capsule main body of the present invention, in which: 
     FIG. 5 a  is a front view of the heat medium; 
     FIG. 5 b  is a plan view of the heat medium, with several specimens axially set at the center and peripheral areas of the heat medium; 
     FIG. 5 c  is a longitudinal sectioned view of the heat medium taken along the line A-A′ of FIG. 5 b ; and 
     FIG. 5 d  is a development view of the heat medium; 
     FIGS. 6 a  and  6   b  are views of a reflector housed in the shell of the capsule main body according to the present invention, in which: 
     FIG. 6 a  is a sectional view of the reflector; and 
     FIG. 6 b  is a plan view of the reflector; 
     FIGS. 7 a  and  7   b  are views of an insulator housed in the shell of the capsule main body according to the present invention, in which: 
     FIG. 7 a  is a sectional view of the insulator; and 
     FIG. 7 b  is a plan view of the insulator; 
     FIG. 8 is a sectional view of an upper end plug included in the capsule main body according to the present invention; 
     FIGS. 9 a  to  9   c  are views of a lower fixing unit of the capsule main body according to the present invention, in which: 
     FIG. 9 a  is an exploded sectional view of the lower fixing unit of the present invention; 
     FIG. 9 b  is a perspective view of a guide pin assembly used in the lower fixing unit according to the present invention, with three guide pins being held by a holding ring at their ends; and 
     FIG. 9 c  is a perspective view of a conventional guide pin assembly, with three guide pins being left free at their ends; 
     FIG. 10 is a plan view of a stopper included in the lower fixing unit of the present invention; 
     FIG. 11 is a perspective view of a receptacle provided in the irradiation hole of the research reactor; 
     FIG. 12 shows the construction of a guide spring included in the capsule main body of the present invention, in a sectional view and a perspective view; 
     FIGS. 13 a  and  13   b  are views of a junction box used for connecting the heater control wire, thermocouple control wire, and vacuum control pipe of the capsule to the capsule control system according to the present invention, in which: 
     FIG. 13 a  is a front view of the junction box; and 
     FIG. 13 b  is a rear view of the junction box; and 
     FIG. 14 is a view of a conventional junction unit used for connecting the heater control wire, thermocouple control wire, and vacuum control pipe to a capsule control system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 FIG. 1, the instrumented capsule  1  according to a preferred embodiment of the present invention comprises a capsule main body  10  that is installed in an irradiation hole  103  of the research reactor pool  100 . The instrumented capsule  1  also has a rigid protection tube  60 , a flexible guide tube  70  and a junction box  80 , which guide and connect a vacuum control pipe and several control wires extending from the interior of the main body  10  to a capsule control system  90  installed outside the reactor pool  100 . 
     In a detailed description of the instrumented capsule  1  of the present invention, the main body  10  of the instrumented capsule comprises a shell  11  that defines the appearance of the main body  10  as shown in FIG.  2 . Housed in the shell  11  are heat media  13  used for transmitting heat from heaters to specimens. The heat media  13  collaterally hold the specimens  2  of target materials at the center and peripheral areas thereof. A plurality of insulators  23  are interposed between adjacent heat media  13  and positioned above and under the upper and lower reflectors  19 , respectively. The Thermocouples  25  are set in each of the heat media  13 , and are used for sensing the temperature of the specimens  2 . A dosimeter  29  is installed in each of the heat media  13  so as to measure the quantity of neutron radiation. The capsule main body  10  is loaded into the irradiation hole  103  of the research reactor. 
     The shell  11  of the main body  10  is a hollow cylindrical body, and the heat media  13  are sequentially set into the shell  11  along an axial direction of the shell  11 . The specimens  2 , made of a variety of target materials and having various shapes, are longitudinally set into the heat media  13 , so the specimens  2  create a multi-staged specimen arrangement. The specimens  2  are fabricated in the form of rods with the same length and circular or rectangular cross-sections, and are installed in the shell  11  while being axially set at the center and peripheral areas of the heat media  13 . 
     The number of the heat media  13  may be changed in accordance with test purposes and test environments. The heat media  13  collaterally act as specimen holders, and each have a plurality of specimen seating holes at the center and peripheral areas thereof as shown in FIG. 5 b . The specimen seating holes of the heat media  13  have circular or rectangular cross-sections, and receive the specimens  2  therein. The circumferential surfaces of the heat media  13  come into close contact with the inner surface of the shell  11 , and two adjacent media  13  are connected to each other by a plurality of connecting pins  15 . 
     The thermocouples  25  are set into the circumferential portion of each heat medium  13  so as to detect the temperatures of the specimens  2  in the heat medium  13 . The dosimeter  29  is installed in each heat medium  13  so as to measure the total neutron fluence. 
     A coiled heater  27  is installed around the circumferential surface of each heat medium  13  so as to generate heat. The heat from the heaters  27  is transferred to the specimens  2  through the heat media  13 , thus heating the specimens  13  to the target temperature. As shown in FIGS. 4 a  and  4   b , each of the heaters  27  is a sheath-heater, and is wrapped along a spiral groove  28  formed around the circumferential surface of an associated heat medium  13 . 
     An upper reflector  19  is installed on the upper end of the uppermost heat medium  13 , and a lower reflector  19  is installed under the lower end of the lowermost heat medium  13 . The two reflectors  19  prevent upward and downward heat transfer from the uppermost and lowermost heaters  27  in axial directions of the shell  11 . In order to fabricate each of the two reflectors  19 , a plurality of circular discs  18  are layered and fastened together into a single body by using a fastening pin  20 . The reflectors  19  thus have a multi-layered structure as shown in FIG. 6 a.    
     In an effort to minimize axial heat transfer between adjacent heat media  13  having specimens  2 , an insulator  23  is interposed between the adjacent heat media  13  as shown in FIGS. 7 a  and  7   b . In such a case, the insulators  23  are each fabricated in the form of a circular disc having the same diameter of the heat media  13 , and are locked to adjacent media  13  by the connecting pins  15 . 
     A lower end plug  31  is mounted to the open lower end of the shell  11 , while an upper end plug  33  is mounted to the open upper end of the shell  11 . The upper and lower end plugs  33  and  31  thus seal the upper and lower ends of the shell  11 . An upper guide spring unit  53  is fitted over the upper end of the shell  11 , and comes into elastic and frictional contact with the inner surface of the irradiation hole  103  when the capsule main body  10  is installed in the irradiation hole  103 . The upper guide spring unit  53  vertically places the shell  11  of the main body  10  at the center of the vertical irradiation hole  103 . As shown in FIG. 12, the upper spring unit  53  has upper and lower fixing rings  52 , at which the spring unit  53  is fitted over the shell  11 . A plurality of wire springs  51  are connected between the upper and lower fixing rings  52  at regular positions, and are bent outward at their middle portions to project outward in radial directions. The wire springs  51  are thus brought, at their bent portions, into elastic and frictional contact with the inner surface of the irradiation hole  103  when the main body  10  is loaded in the irradiation hole  103 . In the preferred embodiment of FIG. 12, the upper guide spring unit  53  has six wire springs  51  such that the capsule equipped with the spring unit  53  are loaded in an irradiation hole having a hexagonal cross-section. Of course, the number of the wire springs  51  may be changed in accordance with the cross-section of the irradiation hole in which the capsule main body  10  is installed. 
     A specimen compressing spring  43  is installed under the upper end plug  33 , and compresses the specimens  2 . In order to seat the specimen compressing spring  43 , a spring seat  45  is installed in the shell  11  at a position above the uppermost insulator  23 . Two spacers  47  and  49  are sequentially set in the shell  11  at a position between the spring seat  45  and the uppermost insulator  23 , thus spacing the spring seat  45  from the uppermost insulator  23  at a desired interval. 
     As shown in FIG. 8, the upper end plug  33  has a central pipe hole  34  and six peripheral pipe holes  34 . A vacuum control pipe  55 , used for controlling the pressure of helium gas in the capsule main body  10  to control the degree of vacuum in said main body  10 , passes through the central pipe hole  34  of the upper end plug  33 , while six wiring pipes  57 , which house the control wires extending from the thermocouples  25  and the heaters  27 , pass through the six peripheral pipe holes  34 . In such a case, the vacuum control pipe  55  and the six wiring pipes  57  are firmly held in the pipe holes  34  of the upper end plug  33  while accomplishing a sealing effect at the junctions of the pipes  55  and  57  and the pipe holes  34 , and are guided to the junction box  80  by the protection tube  60  and the guide tube  70  while being protected by said tubes  60  and  70 . The junction box  80  is installed outside the reactor pool  100 , and connects the pipes  55  and  57  to the capsule control system  90 . 
     As described above, the vacuum control pipe  55  and the wiring pipes  57  extending from the capsule main body  10  are guided to the junction box  80  via the protection tube  60  and the guide tube  70 . Both the protection tube  60  and the guide tube  70  shield the vacuum control pipe  55  and the wiring pipes  57  from coolant in the reactor pool  100 , and accomplish the air-tightness of the pipes  55  and  57 . The junction box  80  is installed outside the reactor pool  100 , and connects the pipes  55  and  57  to the capsule control system  90 . 
     As shown in FIG. 13 a , the junction box  80  has a guide tube connector  92  on its front surface, and the connector  92  connects the guide tube  70  to the junction box  80 . In the junction box  80 , the vacuum control pipe  55  and the control wires, such as wires extending from the heaters  25  and the thermocouples  27 , are separated from each other. In order to separately connect the vacuum control pipe  55  and the control wires to the associated parts of the capsule control system  90 , the rear surface of the junction box  80  is provided with several connectors, that is, a thermocouple control wire connector  93 , a heater control wire connector  94 , a vacuum control pipe connector  95 , and a pressurizing tube connector  96 . 
     A grab hook  83  and a grapple head  84  are provided at the uppermost end of the protection tube  60  connected to the upper end of the capsule main body  10  as shown in FIG.  3 . The grab hook  83  and the grapple head  84  are used in the process of moving, loading or unloading the capsule main body  10 . In a detailed description, the capsule main body  10  is movable in the research reactor, with the grab hook  83  caught by an overhead crane (not shown) positioned above the reactor pool  100 . The grapple head  84  is used for rotating the capsule main body  10  so as to fix or remove the main body  10  to or from a receptacle  105  provided at the bottom of the irradiation hole  103 . That is, the capsule main body  10  is loaded or unloaded in or from the irradiation hole  103 . 
     A lower fixing unit  35 , which is used for fixing the shell  11  of the capsule main body  10  to the receptacle  105  of the irradiation hole  103 , is mounted to the shell  11  at a position under the lower end plug  31 . As shown in FIG. 9 a , the lower fixing unit  35  comprises a lower end cap  41 , a rod tip  36 , a stopper  38 , and a stopper spring  40 . The lower end cap  41  is mounted to the lower end plug  31 , while the rod tip  36  is connected to the center of the lower end cap  41  and vertically extends downward. The stopper  38  is movably fitted over the rod tip  36 , while the stopper spring  40  is fitted over the rod tip  36  at a position between the stopper  38  and the lower end cap  41 , thus normally biasing the stopper  38  downward in a vertical direction. 
     The rod tip  36  is a slim shaft, with two locking blades  37  formed on the lower portion of the rod tip  36  at diametrically opposite positions as shown in FIG. 9 a . The rod tip  36  with the two locking blades  37  passes through the fixing slot  106  of the receptacle  105  provided at the bottom of the irradiation hole  103 . The fixing slot  106  has two blade spaces allowing the two locking blades to pass through the fixing slot  106 , and two locking recesses  106   a  are formed on the lower surface of the receptacle  105  such that the locking recesses  106   a  cross the locking slot  106  having the two blade spaces. The stopper  38  is provided with a plurality of holes  39  which allow the coolant flowing from the bottom of the irradiation hole  103  to smoothly flow upward through the stopper  38  without being disturbed by the stopper  38 . The guide pins  38   a  are provided at the circumferential surface of the stopper  38  such that the guide pins  38   a  are bent outward in radial directions. The guide pins  38   a  thus come into contact with the inner surface of the irradiation hole  103  when the capsule main body  10  is installed in the irradiation hole  103 . The upper ends of the guide pins  38   a  are connected to an annular ring  38   b , thus being supported by the ring  38   b , as best seen in FIG. 9 b . That is, the lower fixing unit  35  of the capsule main body  10  according to the present invention reinforces the guide pins  38   a  by the annular ring  38   b , so the lower fixing unit  35  effectively resists the torsion force applied thereto and effectively endures the stress caused by the torsion force even when the grapple head is rotated during the process of fixing the capsule main body  10  in the irradiation hole  103 , different from a conventional lower fixing unit lacking such an annular ring, as shown in FIG. 9 c.    
     The process of assembling and installing the instrumented capsule  1  of the present invention and a material irradiation test performed with the capsule  1  will be described herein below. 
     In order to fabricate the main body  10  of the instrumented capsule  1  for a material irradiation test, the heat media  13  with the specimens  2 , lower fixing unit  35 , lower end plug  31 , insulators  23 , reflectors  19 , thermocouples  25 , dosimeters  29 , heaters  27 , spacers  47  and  49 , specimen compressing spring  43 , upper end plug  33 , and the guide spring unit  53  are set in or mounted to the shell  11 , thus assembling the capsule main body  10 . 
     Thereafter, at the upper end plug  33  of the capsule main body  10 , the vacuum control pipe  55  and the wiring pipes  57  for the control wires extending from the thermocouples  25  and the heaters  27  are inserted into the protection tube  60  so as to be air- and water-tightly guided to the junction box  80  through the protection tube  60  and the guide tube  70 . The outside end of the guide tube  70  is connected to the guide tube connector  92  which is provided on the front surface of the junction box  80  installed at the upper portion of the reactor pool  100 . In addition, at the rear surface of the junction box  80 , the vacuum control pipe  55  and the control wires are separately connected to the associated parts of the capsule control system  90  through the several connectors provided at the rear surface of the junction box  80 . The instrumented capsule  1  for the material irradiation test is thus completely installed in a research reactor. 
     In other words, the protection tube  60  is connected at its inside end to the upper end plug  33 , and at its outside end to the guide tube  70 , thus guiding the vacuum control pipe  55  and the control wires to the guide tube  70 . The outside end of the guide tube  70  is connected to the guide tube connector  72  provided at the front surface of the junction box  80 , and so the vacuum control pipe  55  and the control wires are connected to the junction box  80 . The vacuum control pipe  55  and the control wires are, thereafter, connected to the capsule control system  90  through the connectors provided at the rear surface of the junction box  80 . 
     Thereafter, the grab hook  83  of the capsule main body  10  is coupled to the overhead crane (not shown) positioned above the reactor pool  100 , and primarily places the capsule main body  10  in the irradiation hole  103 . Thereafter, the grapple head  84  is rotated to fix the capsule main body  10  in the irradiation hole  103  of the reactor pool  100 . 
     During the process of installing the capsule main body  10  in the irradiation hole  103  of the reactor pool  100 , the lower fixing unit  35  provided at the lower end of the shell  11  is fixed to the receptacle  105  which is placed on the bottom of the irradiation hole  103  as shown in FIG.  11 . During the process of fixing the lower fixing unit  35  to the receptacle  105 , the receptacle  105  primarily catches the stopper  38  of the fixing unit  35 . In such a case, only the rod tip  36  passes through the fixing slot  106  of the receptacle  105 , while the stopper spring  40  is compressed by an external force. After the rod tip  36  completely passes through the slot  106  of the receptacle  105 , the capsule main body  10  is rotated at an angle of 90° by the grapple head  84  such that the two locking blades  37  of the rod tip  36  are positioned under the two locking recesses  106   a  of the receptacle  105 . Thereafter, the external force is removed from the capsule main body  10 , and so the stopper  38  is biased upward by both the liquid pressure of the coolant flowing upward from the position under the receptacle  105  and the restoring force of the stopper spring  38 . The two locking blades  37  of the rod tip  36  are seated into the two locking recesses  106   a  of the receptacle  105 . The installation of the capsule main body  10  in the irradiation hole  103  is accomplished. 
     After the capsule main body  10  is completely loaded into the irradiation hole  103  as described above, the protection tube  60 , placed in the coolant inside the reactor pool  100 , is supported by a clamp robot arm  108 . The instrumented capsule  1  is completely installed in the reactor pool  100 . 
     Thereafter, a desired material irradiation test using the capsule  1  is performed. During the material irradiation test, the specimens  2  housed in the shell  11  of the capsule main body  10  are irradiated. In such a case, the temperature inside the shell  11  is controlled by the thermocouples  25  and the heaters  27  wound around the spiral grooves  28  of the heat media  13 , in addition to the helium gas atmosphere inside the shell  11 . 
     That is, the thermocouples  28  installed on the heat media  13  detect the temperatures of the specimens  2 , and output temperature signals to the capsule control system  90 . Upon receiving the temperature signals from the thermocouples  28 , the capsule control system  90  controls the pressure of the helium gas flowing to the vacuum control pipe  55 , thus controlling the heat transfer rate inside the shell  11  and controlling the output power of the heaters  27 . For example, when the temperature of the specimens  2  is lower than a predetermined reference point or a predetermined target point, for example, 290° C.±10° C., the degree of vacuum in the shell  11  is increased to reduce the quantity of heat transferred from the interior of the shell  11  to the coolant flowing around the shell  11  and the sheath-heaters  27  are operated to generate heat. In such a case, heat dissipated from the heaters  27  is uniformly transferred to the surfaces of the specimens  2  through the heat media  13  surrounding the specimens  2 , thus increasing the temperature of the specimens  2  to a desired point. The dosimeters  29 , installed around the specimens  2  in the heat media  13 , detect and measure the quantity of neutron, radiation of the irradiated specimens  2 . During such a material irradiation test in a research reactor, the capsule main body may interfere with adjacent structures due to its vibration caused by flow-induced vibration. Therefore, it is necessary to install the capsule in the research reactor in accordance with regulations defined by law. That is, the upper guide spring unit  53  is fitted over the upper end of the shell  11  as shown in FIGS. 2,  4   a  and  12 , and comes into elastic and frictional contact with the inner surface of the irradiation hole  103  when the capsule main body  10  is installed in the irradiation hole  103 . The upper guide spring unit  53  thus vertically places the shell  11  of the main body  10  at the center of the vertical irradiation hole  103  inside, and prevents the shell  11  from being unexpectedly eccentrically placed in the irradiation hole  103 . The desired structural integrity of the capsule main body  10  is thus maintained. The lower fixing unit  35  provided at the lower end of the capsule main body  10  firmly fixes the capsule main body  10  in the vertical irradiation hole  103 . 
     When loading the instrumented capsule  1  of the present invention in an irradiation hole  103  of a reactor pool  100 , the grab hook  83  provided at the upper end of the capsule main body  10  is coupled to an overhead crane, and is moved to a desired position above the reactor pool  100  by the crane. Thereafter, the grapple head  84  of the capsule main body  10  is connected to an appropriate tool (not shown), and is rotated by the tool so as to fix the capsule main body  10  to the receptacle  105  provided at the bottom of the irradiation hole  103 . 
     In the instrumented capsule  1  of the present invention, the guide pins  38   a  are reinforced by the annular ring  38   b  which supports the upper ends of the guide pins  38   a  as shown in FIGS. 9 b  and  10 . The present invention thus allows the guide pins  38   a , which have been recognized as the most easily breakable parts in the case of conventional instrumented capsules, to have a stable structure capable of effectively resisting both a tensile load applied to the guide pins  38   a  in an axial direction of the capsule main body  10  and a bending load applied to the guide pins  38   a  in a transverse direction of the capsule main body  10 . 
     As described above, the present invention provides an instrumented capsule for material irradiation tests in research reactors. In the instrumented capsule of the present invention, specimens are housed in the shell of a capsule main body such that the specimens create a multi-staged specimen arrangement. The temperature of the specimens during a material irradiation test is detected by thermocouples, and is controlled by heaters, spirally wound around the external surfaces of the heat media, in accordance with the detected results. In addition, the temperature of the specimens during the material irradiation test is also indirectly controlled by controlling the heat transfer rate inside the shell. In such a case, the heat transfer rate inside the shell is controlled by controlling pressure of the helium gas atmosphere in the shell. Therefore, it is easy to control the temperature of the specimens housed in the shell of the capsule main body, so the capsule of the present invention performs an optimum material irradiation test. 
     In the capsule of the present invention, the vacuum control pipe and several control wires extending from the heaters and thermocouples are connected to the capsule control system through a junction box. The junction box of the present invention has a small size and light weight, different from conventional junction units, so it is easy and convenient for workers to handle the junction box. During a material irradiation test in a research reactor, the capsule main body may interfere with adjacent structures due to its vibration caused by flow-induced vibration. In order to prevent such interference of the capsule main body with adjacent structure, an upper guide spring unit is fitted over the upper end of the shell such that the guide spring unit comes into elastic and frictional contact with the inner surface of the irradiation hole when the capsule main body is loaded into the irradiation hole. The upper guide spring unit thus vertically places the capsule main body at the center of the vertical irradiation hole inside, and prevents the capsule main body from being unexpectedly and eccentrically placed in the irradiation hole. Desired structural integrity of the capsule main body is thus maintained. 
     In the instrumented capsule, the guide pins, provided at the lower end of the capsule main body, are reinforced by an annular ring, thus having a stable structure, different from conventional guide pins which have been recognized as the most easily breakable parts of instrumented capsules. The guide pins thus more effectively endure a tensile load and a bending load during the process of loading/unloading the capsule main body. 
     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.