Abstract:
Provided are a radioactive contamination monitoring device and a radioactive contamination monitoring method for enabling easy detection of radiation from an object to be monitored in a little surrounding space. The radioactive contamination monitoring device comprises a radiation detection unit, a photoelectric conversion unit for converting the light generated in the radiation detection unit to electricity, and a signal processing unit connected to the photoelectric conversion unit. The radiation detection unit includes a quadrangular prism-shaped light guide bar having a rectangular cross-section and a scintillator attached only to two adjacent side faces of the four side faces of the light guide bar.

Description:
TECHNICAL FIELD 
     The present invention relates to a radioactive contamination monitoring device and a monitoring method, and in particular, relates to a radioactive contamination monitoring device and a monitoring method of monitoring for radioactive contamination of an object to be monitored whose surrounding space is narrow. 
     BACKGROUND ART 
     As a main turbine in a nuclear power plant deteriorates due to secular changes, a turbine rotor or nozzle diaphragm is replaced by a new one to improve operation efficiency and enable longer utilization. During the replacement, radioactive substances adhering to an outer surface of the replaced old part such as a turbine rotor and nozzle diaphragm are decontaminated by blasting or the like and it is necessary to check the effect of decontamination by a radioactive contamination monitoring device so that the replaced part can be transported out of a radiation controlled area as domestic waste. On the other hand, surface metals removed by decontamination, sand containing radioactive substances, and parts from which radioactive substances cannot be removed due to a device structure or the like are packed in a drum and temporarily stored on a premise as low level radioactive waste before being transported to a low level radioactive waste burying center where the low level radioactive waste is buried in a concrete pit. 
     Because the space between nozzle wings to be monitored is very narrow, it has been very difficult to monitor for residual radiation of nozzle wings and the like after decontamination by blasting or the like using a commercial radioactive contamination monitoring device, prolonging a removal work period. For example, the space α between nozzle wings of a turbine rotor is about 3.9 mm to 24 mm and very narrow. Also, through-holes and non-through-holes formed on a horizontal joint surface of a turbine nozzle diaphragm have very small inside diameters. 
     Thus, to enable measurement of residual radiation of an object to be monitored whose surrounding space is narrow, a method of disassembling and pulling out a turbine rotor or cutting a nozzle diaphragm into three parts of outer rings, inner rings, and nozzle wings and then monitoring for residual radiation is commonly used. Because a nozzle wing is cast into an outer wing and inner wing, there has been no choice, but to mechanically cut or fuse for disassembly. 
     In one plant of 1100 MWh class, there are 8 stages×1 nozzle diaphragms into which the above nozzle wings are incorporated for a high-pressure turbine and 9 stages×3 nozzle diaphragms for a low-pressure turbine so that the number of nozzle plates amounts to about 10,000. A challenge is to reduce processes, save resources such as time, labor, and power, and also reduce installation costs of facilities by enabling storage of such nozzle plates as they are without cutting. 
     Fusing at high temperature is more efficient when compared with mechanical cutting, but radioactive substances may be melted and cured in a fusion zone, making work to grind/cut the fusion zone unavoidable to remove residual radioactive substances completely. Thus, it becomes necessary to reduce offcut (low level radioactive waste) in which radioactive substances are melted and generated by cutting, to be careful with fire, and to take safety measures including installation of a ventilator and a filter to remove fumes during cutting. Further, it is also necessary to secure a wide area within a limited radiation controlled area to make measurements with a radioactive contamination monitoring device. 
     Patent Literature 1 discloses a radioactive contamination monitoring device that enables easy monitoring for radioactive contamination of an inner surface of a tube. That is, a radiation detection unit configured by attaching scintillators or the like to a rod-like transparent guide line portion extended from a cylindrical photoelectric conversion unit and attaching a light shielding portion that allows radiation to pass through, but blocks light to an outer side thereof is disclosed. However, the radiation detection unit shown by Patent Literature 1 is intended for radiation measurement of an inner surface of a tube and has difficulty in monitoring a through-hole, a non-through-hole, or an object to be monitored whose surrounding space is narrow. 
     CITATION LIST 
     Patent Literature 
     PLT 1: Japanese Patent Application Laid-Open No. 2008-145427 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     To solve the above problems, an object of the present application is to provide a radioactive contamination monitoring device and a monitoring method capable of easily detecting radiation of a through-hole, a non-through-hole, or an object to be monitored whose surrounding space is narrow without disassembling, cutting, or fusing an object to be monitored. 
     Solution to Problem 
     A radioactive contamination monitoring device according to an embodiment of the present application includes a radiation detection unit, a photoelectric conversion unit that converts light generated in the radiation detection unit into electricity, and a signal processing unit connected to the photoelectric conversion unit. The radiation detection unit according to the present embodiment includes a quadrangular prism-shaped light guide bar having a rectangular cross-section and a scintillator mounted on only two adjacent side faces of four side faces of the light guide bar. 
     In the radioactive contamination monitoring device according to another embodiment of the present application, the light guide bar has a square cross-section, and the scintillator is further attached to a tip of the light guide bar. 
     In another embodiment of the present application, the scintillator is clad in a grid-like protective member. 
     In still another embodiment of the present application, the scintillator attached to the tip of the light guide bar having the square cross-section and the scintillator mounted on the two adjacent side faces are clad in a grid-like protective member. 
     In yet another embodiment of the present application, the grid-like protective member is formed of stainless steel. 
     In a radioactive contamination monitoring method according to an embodiment of the present application, the radiation detection unit in which the light guide bar has a square cross-section is inserted into the through-hole and half the surface of the inner surface of the through-hole is monitored for the radioactive contamination. According to this method, the radiation detection unit is inverted around an axis in a longitudinal direction thereof and the remaining surface of the inner surface of the through-hole is monitored for the radioactive contamination. 
     In a radioactive contamination monitoring method according to another embodiment of the present application, the radiation detection unit in which the light guide bar has a square cross-section and the scintillator is arranged at a bottom thereof is inserted into the non-through-hole and at least half the surface of the inner surface of the non-through-hole is monitored for the radioactive contamination. According to this method, the radiation detection unit is inverted around an axis in a longitudinal direction thereof and the remaining surface of the inner surface of the non-through-hole is monitored for the radioactive contamination. 
     In a radioactive contamination monitoring method according to still another embodiment of the present application, the radiation detection unit in which the light guide bar has a square cross-section is inserted into the narrow space surrounding the object to be monitored and one side of the object to be monitored is monitored for the radioactive contamination. According to this method, the radiation detection unit is inverted around an axis in a longitudinal direction thereof and the other side face of the object to be monitored is monitored for the radioactive contamination. 
     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 
     According to an embodiment of the present application, a radioactive contamination monitoring device and a monitoring method capable of easily detecting radiation of a through-hole, a non-through-hole, or an object to be monitored whose surrounding space is narrow without disassembling, cutting, or fusing an object to be monitored. 
     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 
         FIG. 1  is a schematic diagram of a radiation detection unit according to an embodiment of the present invention. 
         FIG. 2  is a schematic diagram of the radiation detection unit according to another embodiment of the present invention. 
         FIG. 3  is a schematic diagram of the radiation detection unit according to still another embodiment of the present invention. 
         FIG. 4  is a schematic diagram of a configuration of the radiation detection unit according to another embodiment of the present invention. 
         FIG. 5  is a schematic diagram of the structure of the radiation detection unit according to still another embodiment of the present invention. 
         FIG. 6  is a diagram illustrating a method of monitoring for radioactive contamination of an inner surface of a through-hole and a non-through-hole according to an embodiment of the present invention. 
         FIG. 7  is a diagram illustrating the method of monitoring for radioactive contamination of two surfaces of an object to be monitored whose surrounding space is narrow according to another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments according to the present invention will be described below with reference to drawings when appropriate. A detailed description of a common structure of a radioactive contamination monitoring device, for example, a radiation detection unit using scintillators, a photoelectric conversion unit that makes a photoelectric conversion of a detected signal by an electron multiplier tube, and a signal processing unit that is also described in Patent Literature 1 described above is omitted and instead, features of the present application will be described. 
       FIG. 1  shows a radiation detection unit according to an embodiment of the present invention. A light guide bar  11  of a radiation detection unit  10  shown in  FIG. 1  has a rod shape and is a regular quadrangular prism having a square cross-section. A scintillator  12  is arranged on two adjacent side faces of the light guide bar  11 . As described later, the radiation detection unit  10  is suitable for monitoring for radioactive contamination of an inner surface of a through-hole such as a tapped hole. 
       FIG. 2  shows the radiation detection unit according to another embodiment of the present invention. In a radiation detection unit  13  shown in  FIG. 2 , a scintillator  14  is also arranged at the tip of the light guide bar  11 , in addition to the two adjacent side faces where the scintillator  12  is arranged in the radiation detection unit  10  shown in  FIG. 1 . The radiation detection unit  13  is suitable for monitoring for radioactive contamination of an inner surface of a non-through-hole such as a tapped hole with a bottom. 
       FIG. 3  shows the radiation detection unit according to still another embodiment of the present invention. In a radiation detection unit  15  shown in  FIG. 3 , a light guide bar  16  has a flat plate shape and is a quadrangular prism having a rectangular cross-section. A scintillator  17  is arranged on two side faces containing a long side and short side adjacent at the bottom of the radiation detection unit  15 . In the shape shown in  FIG. 3 , the thickness of the scintillator  17  can be set to 2 mm and the thickness of the light guide bar  16  to 4 mm. As a result, the thickness of the radiation detection unit  15  becomes 6 mm so that the radiation detection unit  15  in a flat plate shape can be inserted deep into a narrow location. 
       FIG. 4  and  FIG. 5  show a configuration of the radiation detection unit in which the scintillator is covered with a grid-like protective member according to another embodiment of the present invention. The protective member is provided to prevent the scintillator of the radiation detection unit in a rod or flat plate shape from being contaminated or damaged by an impact or contact with other members. 
       FIG. 4  shows a protective member  19  formed in a cross shape to mechanically protect the scintillator  14  arranged at the tip of the rod-like radiation detection unit  13  shown in  FIG. 2  and a grid-like protective member  18  to mechanically protect the scintillator  12  arranged on the side face. In the protective member in a cross shape or grid shape, a portion of the cross or grid is made of a member of high strength and other portions are voids. As is easily understood by those skilled in the art, the cross shape or the grid shape is only an illustration and is not intended to limit the shape and other mesh shapes may also be applied. 
       FIG. 4  shows a photoelectric conversion unit  20  and a signal processing unit  21 . The photoelectric conversion unit  20  converts light generated when radiation enters the radiation detection unit  13  into an electric signal. The signal processing unit  21  is connected to the photoelectric conversion unit  20  and performs processing such as a wave height analysis of an electric signal output from the photoelectric conversion unit  20 . Though a figure in which the rod-like radiation detection unit  10  is clad in a grid-like protective member is not shown in  FIG. 1 , the configuration is almost the same as the configuration excluding the protective member  19  to protect the scintillator  14  arranged at the tip of the radiation detection unit  13  shown in  FIG. 4 . 
       FIG. 5  shows the configuration in which the outer circumference of the radiation detection unit  15  in a flat plate shape shown in  FIG. 3  is clad in a grid-like protective member  23 . A photoelectric conversion unit  20 ′ and a signal processing unit  21 ′ are also shown in  FIG. 5 . The functions of the photoelectric conversion unit  20 ′ and the signal processing unit  21 ′ are the same as those of the photoelectric conversion unit  20  and a signal processing unit  21  respectively. 
     The protective member  19  in a cross shape and the grid-like protective members  18  and  23  are formed of stainless steel and the opening ration thereof is set to 85%. This value of the opening ratio is a value that minimizes radiation shielding by stainless steel and also enables the maintenance of an impact resistance prevention function. 
     The radiation detection units shown in  FIGS. 1 to 5  have, as described above, a scintillator arranged on only two adjacent side faces of four side faces of a quadrangular prism. Thus, faces of an object to be monitored that can be monitored at a time are only faces opposite to the two adjacent side faces where the scintillator of the radiation detection unit and other faces cannot be monitored at the same time. If scintillators are arranged on all side faces of the quadrangular prism, radioactive contamination can be monitored for on all faces at the same time. However, as described above, dimensions allowing insertion into a tapped hole whose hole diameter is small or a narrow portion for monitoring of such locations are limited. Thus, as described above, the scintillator is arranged on only two adjacent side faces. 
     According to an embodiment of the present application, as is evident from the above description of embodiments, a radioactive contamination monitoring device capable of simplifying radiation detection work of a through-hole, a non-through-hole, or an object to be monitored whose surrounding space is narrow, simplifying the processing method, shortening processes, and reducing work costs and also easily detecting radiation of a through-hole, a non-through-hole, or an object to be monitored whose surrounding space is narrow can be provided. 
     Next, the method of monitoring for radioactive contamination using the above radioactive contamination monitoring device in an embodiment according to the present application will be described with reference to  FIG. 6 ,  FIG. 1 ,  FIG. 2 , and  FIG. 4  by taking a through tapped hole and a tapped hole that is a non-through-hole with a bottom as examples. 
     First, the method of monitoring for radioactive contamination of the inner surface of a through tapped hole  25  shown in  FIG. 6  will be described. In this case, the radiation detection unit  10  shown in  FIG. 1  is inserted into the through-hole  25  from, for example, the upper side in  FIG. 6 . It is assumed here that the scintillator  12  of the radiation detection unit  10  is arranged on the right side and the rear side of the radiation detection unit  10  in  FIG. 6 . In this case, the right side and the rear side in  FIG. 6  of the inner surface of the through-hole  25  are monitored for radioactive contamination. If the dimension of the scintillator  12  in the longitudinal direction is equal to the length of the through-hole  25  or more, monitoring of radioactive contamination is completed for half the inner surface of the through-hole  25 . Next, the radiation detection unit  10  is inverted around an axis in the longitudinal direction thereof. At this point, the scintillator  12  of the radiation detection unit  10  is positioned on the left side the front side of the radiation detection unit  10  in  FIG. 6 . In this state, the radiation detection unit  10  is inserted into the through-hole  25  again from the upper side in  FIG. 6 . In this manner, monitoring of radioactive contamination of the remaining inner surface of the through-hole  25 , which is not monitored, is completed. A case when monitoring is performed using the radiation detection unit  10  is described, but the radiation detection unit  13  shown in  FIG. 4  may be used for the monitoring. In such a case, however, the scintillator  14  arranged at the tip of the radiation detection unit and the protective member  19  are not necessarily needed. 
     If the length of a through-hole is long, as shown in  FIG. 6 , after the above monitoring from above is completed, the radiation detection unit is inserted again from below to perform monitoring in the same as way as the monitoring from above. 
     When monitoring of the inner surface of a non-through-hole  24  shown in  FIG. 6  is performed by using the radiation detection unit  13  having a scintillator also at the tip of the rod like radiation detection unit shown in  FIG. 2  and  FIG. 4 , monitoring can be performed by a method similar to the above monitoring method of the through-hole  25 . In this case, a scintillator is arranged at the tip of the radiation detection unit  13  and thus, not only an inner side face of the non-through-hole  24 , but also the bottom face of the non-through-hole  24  can be monitored for radioactive contamination. 
     Next, the method of monitoring for radioactive contamination when the space surrounding an object to be monitored is narrow by using the above radioactive contamination monitoring device in an embodiment according to the present application will be described with reference to  FIG. 7  by taking nozzle wings of a turbine rotor as an example. In this case, the radiation detection unit  15  in a flat plate shape shown in  FIG. 3  and  FIG. 5  is effectively used.  FIG. 7  shows a plurality of nozzle wings. In the description that follows, monitoring of radioactive contamination of a nozzle wing  52  will be described. Every nozzle wing extends in a direction perpendicular to the paper surface. First, as an example, the upper surface of the nozzle wing  52  in  FIG. 7  is monitored. The radiation detection unit  15  is inserted into a space between the nozzle wing  52  and a nozzle wing  51  from the upper left in  FIG. 7 . At this point, the radiation detection unit  15  is inserted into the space so that the scintillator  17  of the radiation detection unit  15  is opposite to the nozzle wing  52 . After the insertion, the upper surface of the nozzle wing  52  is monitored. Since, as described above, the nozzle wing  52  extends in the direction perpendicular to the paper surface, the upper surface of the nozzle wing  52  can be monitored without omission by moving the radiation detection unit  15  in the direction perpendicular to the paper surface. After the monitoring of the upper surface of the nozzle wing  52  is completed, the radiation detection unit  15  is pulled out of the space and inverted around the axis in the longitudinal direction thereof. At this point, the scintillator  17  of the radiation detection unit  15  is positioned on the upper side of the radiation detection unit  15  in  FIG. 7  so as to face the lower surface of the nozzle wing  52 . In this state, the radiation detection unit  15  is inserted into a space between the nozzle wing  52  and a nozzle wing  53  again from the left side in  FIG. 7  to monitor the lower surface of the nozzle wing  52  for radioactive contamination. In this manner, the monitoring of radioactive contamination of the upper and lower surfaces of the nozzle wing  52  is completed. 
     If monitoring of the right side of the nozzle wing  52  is not completed only from the left side in  FIG. 7 , the radiation detection unit  15  is inserted between wings from the right side in  FIG. 7  to perform monitoring in the same way as described above. 
     According to an embodiment of the present application, as is evident from the above description of embodiments, a radioactive contamination monitoring method capable of simplifying radiation detection work of a through-hole, a non-through-hole, or an object to be monitored whose surrounding space is narrow, simplifying the processing method, shortening processes, and reducing work costs and also easily detecting radiation of a through-hole, a non-through-hole, or an object to be monitored whose surrounding space is narrow can be provided. 
     The present invention is not limited to the above embodiments as they are 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 
     
         
         
           
               11 ,  16  Light guide bar 
               12 ,  14 ,  17  Scintillator 
               10 ,  13 ,  15  Radiation detection unit 
               18 ,  19 ,  23  Protective member 
               20 ,  20 ′ Photoelectric conversion unit 
               21 ,  21 ′ Signal processing unit 
               24  Non-through-hole 
               25  Through-hole 
               51 ,  52 ,  53  Nozzle wing