Abstract:
Even when a radiation detector contacts a pipe arrangement or another member that is an object to be monitored, the damage of the detector is prevented without impairing the detection performance. An inside-tube-wall radioactive contamination monitor comprises: a rod-like light guide bar having a polygonal cross-section; a plurality of scintillators secured to the outer circumferential surface of the light guide bar; a net-like protective tube worn so as to cover the outer circumference of the scintillators with a space between the surfaces of the scintillators and the tube; and a guide portion attached to an end of the net-like protective tube, supporting an end of the light guide bar, and having a shape the diameter of which decreases as approaching the end. The monitor includes: a photoelectric conversion unit coupled to the base end of the net-like protective tube and incorporating a photoelectric conversion element; and a signal processing unit connected to the photoelectric conversion unit.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to monitoring of radioactive contamination, and in particular, relates to a monitor to monitor for radioactive contamination of an inside-wall surface of a small caliber tube, a monitoring device, and a monitoring method. 
       BACKGROUND ART 
       [0002]    A heat exchanger such as a condenser and feedwater heater for a main turbine of a nuclear power plant has many heat exchanger tubes, which are small caliber tubes, installed therein. These heat exchanger tubes are replaced and disposed of when the end of their life comes. However, according to a disposal method of shredding all applicable heat exchanger tubes and filling a drum with shredded heat exchanger tubes, a large number of drums will arise and the disposal cost needed to store and bury these drums will be vast. Thus, it is necessary to reduce the quantity of drums to be disposed of by decontaminating heat exchanger tubes. 
         [0003]    For this purpose, the reduction in quantity of drums to be disposed of has been achieved by the method described below. That is, after a heat exchanger tube being cut to fixed lengths, cut heat exchanger tubes are half-cut in the longitudinal direction and radioactive substances adhering to inner and outer surfaces are decontaminated by blasting or the like. Next, a decontamination result is checked by a survey meter and heat exchanger tubes whose radioactive contamination is not confirmed are not disposed of after being packed in a drum and only heat exchanger tubes whose radioactive contamination is confirmed are disposed of by being packed in a drum. 
         [0004]    However, according to the method of half-cutting heat exchanger tubes in the longitudinal direction and half-cut heat exchanger tubes are decontaminated and then making measurements by a survey meter, it is necessary to half-cut a vast amount of heat exchanger tubes, posing problems that the number of processes increases, time and labor are needed, and further energy such as power is needed and installation costs of facilities increase. 
         [0005]    Patent Document 1 discloses a radiation measuring device allowing radiation measurements of inside-tube-wall surface to be made easily without half-cutting a heat exchanger tube. That is, a radiation detector configured by mounting a scintillator unit on a rod-like transparent light guide unit extended from a photoelectric conversion unit and attaching a light-shielding portion that allows radiation to pass through, but blocks light to an outer surface thereof is disclosed. 
         [0006]    The radiation detector disclosed by Patent Literature 1 enables radiation measurements of inner-wall surface of a long tube so that monitoring of radioactive contamination of the inner-wall of a tube can be performed efficiently. However, when such a long radiation detector is inserted into a tube, a radioactive substance may adhere to the detector through contact between the tube and the detector, which makes it necessary to decontaminate the detector each time monitoring is performed or pay close attention so that the detector is not damaged by contact with the tube, other tools, or other members. Further, even if close attention is paid, the detector may be damaged by contact with the tube or other members. 
       Citation List 
     Patent Literature 
       [0007]    PLT 1: Japanese Patent Application Laid-Open No. 2008-145427 
       SUMMARY OF THE INVENTION 
       [0008]    1. Technical Problem 
         [0009]    An object of the present application is to provide an inside-tube-wall radioactive contamination monitor, a monitoring device, or a monitoring method capable of preventing damage of a detector without loss of monitoring performance even if the radiation detector and a tube to be monitored or another member come into contact. 
         [0010]    2. Solution To Problem 
         [0011]    An inside-tube-wall radioactive contamination monitor according to an embodiment of the present application includes a rod-like light guide bar whose section is polygonal, a plurality of scintillators fixed to an outer circumferential light guide bar, a net-like protective tube attached to cover an outer circumference of the plurality of scintillators with a space to a surface of these scintillators, and a guide portion in a shape in which a diameter decreases with a decreasing distance to a tip that is attached to a tip portion of the net-like protective tube and supports the tip portion of the light guide bar. An inside-tube-wall radioactive contamination monitor according to the present embodiment also includes a photoelectric conversion unit coupled to a base end of the net-like protective tube and containing a photoelectric conversion element and a signal processing unit connected to the photoelectric conversion unit. 
         [0012]    In the inside-tube-wall radioactive contamination monitor according to another embodiment of the present application, a light shielding film that allows radiation to pass through, but blocks light is arranged on the inner surface of the net-like protective tube. 
         [0013]    In another embodiment of the present application, the net-like protective tube is formed of stainless steel. 
         [0014]    In further embodiment of the present application, a plurality of radiation transmission holes in a parallel hexagonal shape in which one pair of opposite sides whose length is longer than other sides is formed reticulately in the net-like protective tube. The radiation transmission holes in the parallel hexagonal shape are arranged in such a way that sides whose length is longer than the other sides are parallel to an axis in a longitudinal direction of the net-like protective tube. 
         [0015]    In still another embodiment of the present application, the net-like protective tube has an opening ratio of 85%. 
         [0016]    In still another embodiment of the present application, the net-like protective tube is detachably mounted on a cabinet of the photoelectric conversion unit. 
         [0017]    In still another embodiment of the present application, the photoelectric conversion element is fixed by molding resin injected into the cabinet of the photoelectric conversion unit and cured. 
         [0018]    An inside-tube-wall radioactive contamination monitoring device according to an embodiment of the present application monitors for radiation of inner-wall surface of a tube by using an inside-tube-wall radioactive contamination monitor according to one of the above embodiments. The inside-tube-wall radioactive contamination monitoring device includes a workbench and the inside-tube-wall radioactive contamination monitor fixed onto the workbench. The monitoring device also includes a first roller group comprising a plurality of rollers arranged on the workbench along an axis of a radiation detection unit constituting the inside-tube-wall radioactive contamination monitor in such a way that rotation axes thereof are parallel to each other to transport the tube to be monitored and a second roller group comprising a plurality of rollers arranged on the workbench between the first roller group and the radiation detection unit along the axis of the radiation detection unit in such a way that the rotation axes thereof are parallel to each other to transport the tube to be monitored. Further, monitoring device includes at least a pair of longitudinal position regulation guide rollers provided on the workbench between the first roller group and the second roller group to regulate a position in a longitudinal direction of the transported tube to be monitored and a plurality of lateral position regulation guide rollers arranged in the first roller group or the second roller group to regulate the position in a lateral direction of the transported tube to be monitored. 
         [0019]    At least the one pair of longitudinal position regulation guide rollers and the plurality of lateral position regulation guide rollers in the inside-tube-wall radioactive contamination monitoring device according to another embodiment of the present application have an hourglass shape in which a center portion thereof is contracted. Moreover, the longitudinal position regulation guide rollers are installed upright on an installation member provided on the workbench and the installation member is provided with elastic members as if to sandwich the tube to be monitored while being transported between the pair of longitudinal position regulation guide rollers. 
         [0020]    In the inside-tube-wall radioactive contamination monitoring device according to another embodiment of the present application, a stopper to stop movement of the transported tube to be monitored is provided on the workbench between the photoelectric conversion unit and the second roller group. 
         [0021]    In the inside-tube-wall radioactive contamination monitoring device according to still another embodiment of the present application, when the radiation detection unit constituting the inside-tube-wall radioactive contamination monitor is calibrated, a radiation measurement calibration jig to place a radiation source above the radiation detection unit is installed on the workbench. The radiation measurement calibration jig has radiation transmission holes through which radiation from the radiation source passes formed in an upper surface thereof. 
         [0022]    An inside-tube-wall radioactive contamination monitoring method according to an embodiment of the present application monitors for radioactive contamination of an intratubal surface by using an inside-tube-wall radioactive contamination monitoring device. According to this monitoring method, the tube to be monitored is placed on the first roller group, the tube to be monitored is moved on the first roller group while being placed on the first roller group, a positional shift in the lateral direction of the tube to be monitored is regulated by the plurality of lateral position regulation guide rollers, the tube to be monitored is moved forward between at least the one pair of longitudinal position regulation guide rollers against the elastic members, and the positional shift in the longitudinal direction of the tube to be monitored is regulated. Moreover, the tube to be monitored is moved forward so that the radiation detection unit is inserted into the tube to be monitored while being placed on the second roller group and the inner surface of the tube to be monitored is monitored for radioactive contamination. 
         [0023]    In an inside-tube-wall radioactive contamination monitoring method according to another embodiment of the present application, the tube to be monitored whose monitoring of the radioactive contamination is completed is pulled out from the radiation detection unit and a position in an axial direction of the tube to be monitored is reversed. Then, the inner surface of the tube to be monitored whose position has been reversed is monitored for the radioactive contamination by the above method. 
         [0024]    As is understood by those skilled in the art, the present application can be carried out by other embodiments and details of some of them can be modified in various obvious modes without deviating from the scope of the present application. Therefore, drawings and descriptions should be considered actually intended as illustrations rather than limitations. 
       Advantageous Effects of Invention 
       [0025]    According to an embodiment of the present application, an inside-tube-wall radioactive contamination monitor, a monitoring device, and a monitoring method capable of preventing damage of a detector without loss of monitoring performance even if the radiation detector and a tube to be monitored or another member come into contact are provided. 
         [0026]    Other advantages, modes, and features of the present application will be clear to those skilled in the art from the following describing a preferred embodiment of the present application as an illustration of the most preferred embodiment to carry out the present application. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  is a diagram showing an appearance of an inside-tube-wall radioactive contamination monitor according to an embodiment of the present invention. 
           [0028]      FIG. 2  is a diagram schematically showing a section in the longitudinal direction of a radiation detection unit of an inside-tube-wall surface radiation detector according to an embodiment of the present invention. 
           [0029]      FIG. 3  is a diagram schematically showing a cross section of the radiation detection unit according to an embodiment of the present invention. 
           [0030]      FIG. 4  is an enlarged view of a tip portion of a net-like protective tube covering an outer circumferential surface of the radiation detection unit according to an embodiment of the present invention. 
           [0031]      FIG. 5  is a diagram illustrating an outline and a monitoring state of the inside-tube-wall radioactive contamination monitoring device according to an embodiment of the present invention. 
           [0032]      FIG. 6  is a diagram when the inside-tube-wall radioactive contamination monitoring device according to an embodiment of the present invention shown in  FIG. 5  is viewed from a side. 
           [0033]      FIG. 7  is a diagram exemplifying an embodiment of a longitudinal position regulation guide roller and an elastic member of the inside-tube-wall radioactive contamination monitoring device according to an embodiment of the present invention shown in  FIG. 5 . 
           [0034]      FIG. 8  is a diagram showing an embodiment of a radiation measurement calibration jig shown in  FIG. 6 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0035]    An embodiment according to the present invention will be described below with reference to drawings when appropriate.  FIG. 1  shows an appearance of an inside-tube-wall radioactive contamination monitor  12  using an inside-tube-wall surface radiation detector  11  according to an embodiment of the present invention. The inside-tube-wall radioactive contamination monitor  12  comprises the inside-tube-wall surface radiation detector  11  and a measuring device  13 . The inside-tube-wall surface radiation detector  11  comprises a radiation detection unit  14 , a photoelectric conversion unit  15  that converts an optical signal detected by the radiation detection unit  14  and originating from radiation into an electric signal, a signal processing unit  16  that processes a signal from the photoelectric conversion unit  15 , and a handle portion  17  mounted on the signal processing unit  16 , and the handle portion  17  and the measuring device  13  are connected via a cable  18 . 
         [0036]    The radiation detection unit  14 , whose details will be described later, has a net-like protective tube  19  attached to an outer circumference thereof and, for example, a bullet-like guide portion  20  is provided at the tip thereof. The shape of the guide portion  20  is not limited to a bullet shape and may be any shape like, for example, a cone shape whose diameter becomes smaller with a decreasing distance to the tip. The radiation detection unit  14  coaxially projects from the center of, for example, the cylindrical photoelectric conversion unit  15 . The signal processing unit  16  comprises a high-voltage circuit, a coincidence circuit, a logic circuit and the like and an error display unit  16   a,  a reset switch  16   b , a display unit  16   c  to indicate that contamination has been detected, a display unit  16   d  to indicate that no contamination has been detected, a measurement status display unit  16   e  and a measurement start switch  16   f  are provided on the upper surface thereof. The measurement status display unit  16   e  displays the status by color and, for example, green indicates that measurement is being made, yellow indicates that the device is under abnormal conditions, and red indicates high/low of background radiation. The measuring device  13  has a power switch  13   a,  a measurement start switch  13   b,  a reset switch  13   c,  an LCD display unit  13   d,  and a shoulder belt  13   e  to carry the device provided therein. 
         [0037]    Principal dimensions of the radiation detection unit  14  and the photoelectric conversion unit  15  according to the present invention are, for example, as follows: A length  20 L of the bullet-like guide portion  20  provided at the tip is 15 mm, an outside diameter  19 D of the net-like protective tube  19  is 11 mm and a length  19 L thereof is 420 mm, and a length  15 L of the photoelectric conversion unit  15  is 130 mm. However, these dimensions do not limit the present invention. 
         [0038]    By setting the outside diameter of the net-like protective tube  19  of the radiation detection unit  14  configured as described above to 11 mm, the detection unit  14  can be inserted into a small heat exchanger tube whose inside diameter is up to 13 mm. The longer the radiation detection unit  14 , the longer the heat exchanger tube that can be monitored, but attenuation of an optical signal detected and originating from radiation becomes more conspicuous with an increasing length of the radiation detection unit  14  and thus, the above lengths are set. As a result, a heat exchanger tube to be monitored whose length is up to 600 mm can be monitored. That is, by inserting the radiation detection unit  14  from one end of the heat exchanger tube to be monitored, the inner surface of the heat exchanger tube up to about 400 mm from the one end is measured. Next, by inserting the radiation detection unit  14  from the other end of the heat exchanger tube, the inner surface of the heat exchanger tube up to about 400 mm from the other end is measured. A portion of about 100 mm in the center of the heat exchanger tube is measured duplicately and thus, there is no portion that is not measured and the entire inner surface of the heat exchanger tube can be measured. 
         [0039]    By providing the net-like protective tube  19 , which will be described in detail later, when the radiation detection unit  14  is inserted into a heat exchanger tube whose inside diameter is small, a plastic scintillator and the like inside the radiation detection unit  14  can be protected from damage caused by a collision or contact with the heat exchanger tube or other members. 
         [0040]    For example, the bullet-like guide portion  20  is provided at the tip of the net-like protective tube  19 , as described above, and a base end of the net-like protective tube  19  has a male screw formed therein and is firmly fixed to, for example, a circular metallic mounting plate  10 . The mounting plate  10  is detachably screwed to the photoelectric conversion unit  15  to increase the stiffness of the thin and long radiation detection unit  14 . 
         [0041]      FIG. 2  is a diagram schematically showing a section in the longitudinal direction of the radiation detection unit  14  of the inside-tube-wall surface radiation detector  11  described above and  FIG. 3  is an A-A line sectional view of the radiation detection unit  14  shown in  FIG. 2 . An overview of the radiation detection unit  14 , the photoelectric conversion unit  15 , and the signal processing unit  16  will be provided below, but the basic configuration thereof is the same as that disclosed by Patent Document 1. 
         [0042]    A light shielding film  21  that allows radiation to pass through, but blocks light is arranged inside the net-like protective tube  19  to whose tip, for example, the bullet-like guide portion  20  is attached. A light guide bar  22  whose section is, for example, a hexagonal shape is arranged in the center of the radiation detection unit  14  and a scintillator  23  is mounted on each of six side surfaces around the light guide bar  22 . The light guide bar  22  is, for example, wavelength shift bar that shifts the wavelength of light generated after radiation enters the scintillator  23  to a longer wavelength. The scintillator  23  is mounted with a clearance to the net-like protective tube  19  so as to make damage caused by a collision or contact with a heat exchanger tube or other members less likely when the heat exchanger tube is monitored. The tip portion of the light guide bar  22  is supported by the guide portion  20 . 
         [0043]    The photoelectric conversion unit  15  contains a photoelectric conversion element  24  and the photoelectric conversion element  24  and one end of the light guide bar  22  are connected via a transparent acryl light guide  25 . The photoelectric conversion element  24  is firmly fixed by injecting molding resin  26  into a cabinet of the photoelectric conversion unit  15  and curing the molding resin  26 . The mounting plate  10  to which the net-like protective tube  19  is fixed is tightly screwed into an opening provided in a case of the photoelectric conversion unit  15 . By adopting such a configuration, the radiation detection unit  14  projecting long from the center of the photoelectric conversion unit  15  can be held without runout. The net-like protective tube  19  can easily be removed from the photoelectric conversion unit  15  for decontamination, replacement or the like. 
         [0044]      FIG. 4  is an enlarged view of a tip portion of the net-like protective tube  19  covering an outer circumference of the radiation detection unit  14  shown in  FIG. 2 . The guide portion  20  in a bullet shape made of plastics or metal is attached to the tip of the net-like protective tube  19  and the radiation detection unit  14  is guided into a heat exchanger tube to be monitored by the guide portion  20 . 
         [0045]    The net-like protective tube  19  is made of, for example, stainless steel and has, as is evident by referring to  FIG. 4 , a plurality of radiation transmission holes  27  in a long parallel hexagonal shape in which one pair of opposite sides are longer than other sides is formed reticulately. These radiation transmission holes are arranged on the entire surface like a honeycomb in such a way that sides whose length is longer than other sides are parallel to the axis in the longitudinal direction of the net-like protective tube  19 . Metal does not allow radiation to pass through and unless the total area of the plurality of radiation transmission holes  27  is large enough, the transmittance of radiation drops. However, if the opening area is too large, the function to mechanically protect the scintillator  23  of the net-like protective tube  19  will be lost. 
         [0046]    In a radiation detection unit having each of the above dimensions, it turned out that the optimum balance between maintenance of mechanical strength of the net-like protective tube  19  and the transmittance of radiation can be obtained by setting the opening ratio of the net-like protective tube  19  to 85%. It also turned out that by adopting the hexagonal shape described above as the shape of the radiation transmission hole  27 , the optimum relation between the above strength and the transmittance can be achieved. 
         [0047]    Next, a monitoring device and a monitoring method that monitor for radioactive contamination of an inner surface of a heat exchanger tube  28  to be monitored by using the radiation detection unit  14  according to an embodiment of the present invention will be described with reference to  FIGS. 5 to 8 .  FIG. 5  is a diagram showing an outline and a monitoring state of an inside-tube-wall radioactive contamination monitoring device according to an embodiment of the present invention and  FIG. 6  is a diagram when the monitoring device shown in  FIG. 5  is viewed from a side. In  FIG. 5 , at least a portion of the heat exchanger tube  28  where the radiation detection unit  14  is inserted into the heat exchanger tube  28  is illustrated transparently to enhance understanding. 
         [0048]    As shown in  FIGS. 5 and 6 , the radiation detection unit  14  is arranged on a workbench  29 . In this case, the radiation detection unit  14  is arranged in parallel with the workbench  29 . An edge of the net-like protective tube  19  on the side of the photoelectric conversion unit  15  is placed on a stopper  30  on the workbench  29 . The stopper  30  is fixed to the workbench  29  between the photoelectric conversion unit  15  and a second roller group  32  described later. The stopper  30  regulates the stop position when the heat exchanger tube  28  to be monitored moves from the right in  FIG. 5 . 
         [0049]    As shown in  FIG. 5 , a first roller group  31  and the second roller group  32  that are arranged along the axis of the radiation detection unit  14  and each of which comprises a plurality of columnar rollers to move the heat exchanger tube  28  are provided on the workbench  29 . Rotation axes of columnar rollers contained in each of the roller groups  31 ,  32  are arranged so as to be parallel to each other and perpendicular to the axis of the radiation detection unit  14 . The first roller group  31  is arranged on the axis of the radiation detection unit  14  and the second roller group  32  is arranged between the radiation detection unit  14  and the first roller group  31  and, like the first roller group  31 , on the axis of the radiation detection unit  14 . 
         [0050]    Lateral position regulation guide rollers  41 ,  42  in an hourglass shape in which the center portion is contracted are installed in two locations of an intermediate position of the first roller group  31  and a position closer to the radiation detection unit  14  respectively. As will be described later, the lateral position regulation guide rollers  41 ,  42  are provided so that when the heat exchanger tube  28  moves toward the radiation detection unit  14  after being placed on the first guide roller group  31 , the heat exchanger tube  28  moves straight without being oscillating in the lateral direction. While the two lateral position regulation guide rollers  41 ,  42  are shown in  FIG. 5 , still more lateral position regulation guide rollers may be provided, for example, one lateral position regulation guide roller for every several columnar rollers. 
         [0051]    A pair of longitudinal position regulation guide rollers  33 ,  34  is installed upright between the first roller group  31  and the second roller group  32  on the workbench  29 . Like the lateral position regulation guide rollers  41 ,  42 , the longitudinal position regulation guide rollers  33 ,  34  each have an hourglass shape in which the center portion is contracted and the position of the heat exchanger tube  28  in the longitudinal direction is regulated by the heat exchanger tube  28  being sandwiched between the guide rollers  33 ,  34 . In this manner, the heat exchanger tube  28  is aligned by the lateral position regulation guide rollers  41 ,  42  and the longitudinal position regulation guide rollers  33 ,  34  so that the heat exchanger tube  28  is in a coaxial relationship with the radiation detection unit  14 . 
         [0052]    As an example, as shown in  FIG. 7 , elastic members  43 ,  44  like springs that press the pair of longitudinal position regulation guide rollers  33 ,  34  in mutually opposite direction with an equal force are provided inside an installation member  35  that installs the pair of longitudinal position regulation guide rollers  33 ,  34  upright. When the heat exchanger tube  28  is inserted between the pair of longitudinal position regulation guide rollers  33 ,  34 , the heat exchanger tube  28  is inserted against a force by the elastic members caused by a pinching force of the elastic members  43 ,  44 . The elastic forces applied to the pair of longitudinal position regulation guide rollers  33 ,  34  are equal and thus, the heat exchanger tube  28  is always held in the same position on the same axis as the radiation detection unit  14 . 
         [0053]    According to the monitoring device described above, even if a radiation detector and a tube to be monitored or other members come into contact, damage of the detector can be prevented without loss of measurement performance. 
         [0054]    Next, the method of monitoring for radioactive contamination of the inner surface of the heat exchanger tube  28  using an inside-tube-wall radioactive contamination monitoring device configured as described above will be described. First, the heat exchanger tube  28  to be monitored is moved in an arrow direction, that is, in the direction of the radiation detection unit  14  after being placed on the first roller group  31  from the right side of the workbench  29  shown in  FIGS. 5 and 6 . 
         [0055]    On the first roller group  31 , for example, runout in the lateral direction of the heat exchanger tube  28  is regulated by the two lateral position regulation guide rollers  41 ,  42  and further, the coaxial/concentric physical relationship between the heat exchanger tube  28  and the radiation detection unit  14  is maintained by the heat exchanger tube  28  being passed through the longitudinal position regulation guide rollers  33 ,  34 . In this manner, the heat exchanger tube  28  is moved until the stopper  30  is reached so that the radiation detection unit  14  opposite to the workbench in parallel and mechanically protected by the net-like protective tube  19  is inserted into the heat exchanger tube  28 . 
         [0056]    As a result, if dimensions of the radiation detection unit  14  and the length of the heat exchanger tube to be monitored are as illustrated above with reference to  FIG. 1 , the radiation detection unit  14  is inserted into the heat exchanger tube  28  until at least half the length of the heat exchanger tube  28  is exceeded. Then, the heat exchanger tube  28  is pulled out and after being rotated by 180°, the process is similarly performed from the opposite side not yet monitored of the heat exchanger tube  28  to monitor the inner surface for radiation. Thus, radiation up to a position exceeding at least half the length of the heat exchanger tube  28  is monitored for in the first monitoring and in the subsequent monitoring, the remaining monitoring exceeding at least half the length is performed. Therefore, monitoring of the heat exchanger tube  28  can be completed without leaving any unmonitored location. In the case of dimensions illustrated above with reference to  FIG. 1 , about 100 mm in the center in the longitudinal direction of the heat exchanger tube  28  is measured duplicately so that no unmonitored location will arise. 
         [0057]    Further, work efficiency can be improved by arranging a plurality of similar monitoring device in positions indicated by alternate short and long dashed lines on the workbench  29  in  FIG. 5 . 
         [0058]    According to the monitoring method described above, contact between a radiation detector and a tube to be monitored is prevented and therefore, damage of the detector can be prevented without loss of measurement performance and also radiation of the inner surface of many tubes can be monitored for precisely and efficiently in a short time. 
         [0059]      FIG. 6  shows a radiation measurement calibration jig  36  installed on the workbench  29 . The radiation measurement calibration jig  36  is used for calibration of the radiation detection unit  14  to make monitoring reliable. In  FIG. 6 , the radiation measurement calibration jig  36  is shown together with the heat exchanger tube  28 , but this is for the convenience of display and the radiation measurement calibration jig  36  is to be used when the heat exchanger tube  28  is not measured.  FIG. 8  shows a diagram showing an embodiment of the radiation measurement calibration jig  36 . The radiation measurement calibration jig  36  comprises an upper surface  37   a  and a pair of side faces  37   b,    37   c  opposite to each other coupled to both ends of the upper surface  37   a  opposite to each other. The upper surface  37   a  of the radiation measurement calibration jig  36  is provided with a radiation transmission hole  38  that cuts through the upper surface and allows radiation to pass through and a reference radiation source  39  indicated by a dotted line is placed on the upper surface  37   a  to cover at least a portion of the radiation transmission hole  38 . The radiation detection unit  14  is calibrated by installing the radiation measurement calibration jig  36  as if to straddle the radiation detection unit  14 . Incidentally, for example, a level adjustment mechanism  40  as shown in  FIG. 8  can be provided at the bottom of the pair of side faces  37   b,    37   c  opposite to each other of the radiation measurement calibration jig  36 . Though a concrete configuration is not illustrated in  FIG. 8 , the level adjustment mechanism  40  can be configured by a hoisting and lowering mechanism using a screw. 
         [0060]    Further, work efficiency can be improved by arranging a plurality of similar monitoring devices in positions indicated by alternate short and long dashed lines on the workbench  29  in  FIG. 5 . 
         [0061]    The present invention is not limited to the above embodiment as it is and structural elements thereof may be modified for embodiment in the stage of working without deviating from the spirit thereof. Moreover, various inventions may be formed by suitably combining a plurality of structural elements disclosed in the above embodiment. For example, some structural elements may be removed from all structural elements shown in the embodiment. Further, structural elements across different embodiments may suitably be combined. 
       REFERENCE SIGNS LIST 
       [0062]      10  Mounting plate 
         [0063]      11  Inside-tube-wall surface radiation detector 
         [0064]      12  Inside-tube-wall radioactive contamination monitor 
         [0065]      13  Measuring device 
         [0066]      14  Radiation detection unit 
         [0067]      15  Photoelectric conversion unit 
         [0068]      16  Signal processing unit 
         [0069]      17  Handle portion 
         [0070]      18  Cable 
         [0071]      19  Net-like protective tube 
         [0072]      20  Guide portion 
         [0073]      21  Light shielding film 
         [0074]      22  Light guide bar 
         [0075]      23  Scintillator 
         [0076]      24  Photoelectric conversion element 
         [0077]      25  Transparent acryl light guide 
         [0078]      26  Molding resin 
         [0079]      27  Radiation transmission hole 
         [0080]      28  Heat exchanger tube 
         [0081]      29  Workbench 
         [0082]      30  Stopper 
         [0083]      31  First roller group 
         [0084]      32  Second roller group 
         [0085]      33 ,  34  Longitudinal position regulation guide roller 
         [0086]      35  Installation member 
         [0087]      36  Radiation measurement calibration jig 
         [0088]      37   a  Upper surface of the radiation measurement calibration jig 
         [0089]      37   b,    37   c  Side face of the radiation measurement calibration jig 
         [0090]      38  Radiation transmission hole 
         [0091]      39  Reference radiation source 
         [0092]      40  Level adjustment mechanism 
         [0093]      41 ,  42  Lateral position regulation guide roller 
         [0094]      43 ,  44  Elastic member