Patent Publication Number: US-2017349496-A1

Title: Production method of nuclear reactor structure

Description:
TECHNICAL FIELD 
     The present invention relates to a production method of a nuclear reactor structure for nuclear reactors. 
     BACKGROUND ART 
     In view of the facts that since graphite that is utilized as a core material of a nuclear reactor structure has a high neutron absorption cross section and large capability for neutron moderation, it has a high neutron moderation ratio and high heat resistance and is easy to provide large materials, the graphite is utilized as a neutron moderator or a reflector of nuclear reactors. In particular, the graphite is an important material as materials of neutron moderators, reflectors, and so on for gas-cooled reactors, such as a magnox reactor, an advanced graphite reactor (AGR reactor), a high temperature gas-cooled reactor, etc. 
     Patent Document 1 discloses a graphite structure having enhanced strength without impairing nuclear and thermal performances, by covering a surface of a nuclear reactor structure configured by solid graphite, such as a neutron moderator, a reflector, etc., by a heat-resistant ceramic, such as silicon carbide SiC, etc., or the like. 
     CITATION LIST 
     Patent Document 
     Patent Document 1: JP-U-S61-206897 
     SUMMARY OF INVENTION 
     Technical Problem 
     In order to stably operate a nuclear reactor, various movable mechanisms, such as a control rod, etc., are provided in the inside thereof. In addition, for the purpose of exchange or maintenance of a nuclear fuel, or the like, there is a case where members, nuclear fuels, and the like of the inside of the nuclear reactor are carried in or carried out. Then, as for the nuclear reactor, for example, in a high temperature gas-cooled reactor using helium as a coolant, there are exemplified a block type high temperature gas-cooled reactor and a pebble bed type high temperature gas-cooled reactor according to a difference of the fuel shape. 
     The block type high temperature gas-cooled reactor is, for example, configured by a hexagonal graphite block (fuel column) having fuel rods accommodated in the inside thereof, a hexagonal graphite block (movable reflector) not having fuel rods accommodated in the inside thereof, and a permanent reflector surrounding the outsides of the foregoing graphite blocks. In addition, the graphite structure of Patent Document 1 is a technology concerning the block type high temperature gas-cooled reactor. 
     On the other hand, in the pebble bed type high temperature gas-cooled reactor, a fuel ball (pebble) formed by mixing covered fuel particles and graphite particles and molding the mixture in a spherical shape is used, and a plurality of such fuel balls are piled up at random within a space formed of a graphite block to form a reactor core. A diameter of the fuel ball is about 6 cm. The pebble bed type high temperature gas-cooled reactor is characterized in that the fuel balls, a nuclear reaction of which has been reduced, are taken out from the lower part during the operation, and at the same time, new fuel balls are supplied from the upper part, thereby continuously exchanging the fuel balls. For this reason, according to the pebble bed type high temperature gas-cooled reactor, the matter that the operation is stopped to exchange the fuel as in the block type high temperature gas-cooled reactor is not needed, so that an operation period of the nuclear reactor can be made long. 
     However, in the block type high temperature gas-cooled reactor, there is a case where friction occurs in the heat-resistance ceramic following the motion of the control rod or movable reflector, and furthermore, on the occasion of exchanging the graphite block, an impact is applied to the heat-resistant ceramic. In addition, in the pebble bed type high temperature gas-cooled reactor, the fuel ball with a high density moves while rolling on the surface of the graphite block, and therefore, high strength is required. 
     In view of the foregoing problems, an object of the present invention is to provide a production method of a nuclear reactor structure having high durability. 
     Solution to Problem 
     In order to solve, the above-described problem, a production method of a nuclear reactor structure of the present invention includes, 
     (1) a core material processing step of processing a core material including graphite into a quadrangular prism, a bottom surface of which is an approximately isosceles trapezoid, a step of obtaining a base material by covering the core material with an aggregate including a ceramic fiber, and a CVD step of putting the base material into a CVD reactor and forming a SiC matrix in gaps of the aggregate, thereby forming a ceramic/ceramic composite material on a surface of the core material. 
     (2) The step of obtaining the base material includes a step of impregnating a resin after covering the core material with the aggregate. 
     (3) The step of obtaining the base material includes a step of heating after the step of impregnating the resin. 
     (4) The step of obtaining the base material includes a step of simultaneously covering the core material with the aggregate and a resin. 
     (5) The step of obtaining the base material includes a step of heating after the step of simultaneously covering the core material with the aggregate and the resin. 
     (6) The resin is a resin containing an organosilicon-based resin or a silicide-based ceramic particle. 
     (7) The aggregate is a wound body of the ceramic fiber winding the core material. 
     (8) The aggregate is a cloth including the ceramic fiber covering the core material. 
     (9) The aggregate is a woven fabric including the ceramic fiber covering the core material. 
     (10) The ceramic fiber is a SiC fiber. 
     Advantageous Effects of Invention 
     According to the production method of the nuclear reactor structure of the present invention, a nuclear reactor structure capable of enhancing durability, preventing cracking, etc. from occurring, and preventing exposure of graphite as a core material from occurring can be provided. That is, since a ceramic/ceramic composite material having high durability is formed on a surface of a core material including graphite, the graphite is hardly exposed and difficultly consumed. In addition, according to the production method of a nuclear reactor structure of the present invention, since the core material occupying the majority is graphite, and the ceramic/ceramic composite material covers the surface of the core material, a production method of a nuclear reactor structure that scarcely affects the capability for neutron moderation of the graphite and has excellent durability can be provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view showing an example of a pebble bed type nuclear reactor using a nuclear reactor structure according to the present invention. 
         FIG. 2  is a schematic view showing an example of a pebble accommodating space constituted of a nuclear reactor structure according to the present invention, in which (a) is a longitudinal cross section, and (b) is a lateral cross section. 
         FIG. 3  is a block diagram showing an example of a production process of a nuclear reactor structure according to the present invention, in which (A) is a core processing step; (B) is a step of obtaining a base material; (C) is a CVD step; and (B 1 ) to (B 5 ) are five step patterns of the step (B) of obtaining a base material. 
         FIG. 4  shows the steps (A) to (C) of  FIG. 3 , in which (a 1 ) to (a 3 ) are each a conceptual perspective view; (b 1 ) to (b 3 ) are each a conceptual cross-sectional view; (a 1 ) and (b 1 ) are each concerned with the core process step (A); (a 2 ) and (b 2 ) are each concerned with the step (B) of obtaining a base material; and (a 3 ) and (b 3 ) are each concerned with the CVD step (C). 
         FIG. 5  is a conceptual view showing a specific example of the step (B) of  FIG. 3 , in which (a) is concerned with spray coating; (b) is concerned with sheet sticking; (c) is concerned with a nuclear reactor structure by (a) and (b); (d) is concerned with winding; and (e) is concerned with a nuclear reactor structure by (d). 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of the production method of a nuclear reactor structure according to the present invention are described in detail on the basis of  FIGS. 1 to 5 . 
       FIG. 1  is a schematic view showing an example of a pebble bed type nuclear reactor (high temperature gas-cooled reactor). In a pebble bed type nuclear reactor  1 , a reactor core  3  is accommodated within a nuclear reactor vessel  2 , and a plurality of pebbles  4  that are a fuel ball are loaded within the reactor core  3 . In the reactor core  3 , a pebble accommodating space  20  configured by, for example, graphite blocks that are a plurality of nuclear reactor structures  10  in the upper part, the lower part, and the surrounding is formed. In addition, the reactor core  3  is configured by piling up the plurality of nuclear reactor structures  10 , thereby minimizing the leakage amount of neutrons to the outsides as far as possible. In addition, the lower part of the nuclear reactor vessel  2  is connected to a piping  5  for coolant, and the upper part thereof is coupled with a power conversion apparatus  6  having a power generator in the upper part, a gas turbine or a compressor in the middle, and a cooler in the lower part, respectively. 
       FIG. 2  is a schematic view showing an example of the pebble accommodating space  20  constituted of the nuclear reactor structures  10  of the present embodiment, in which (a) is a longitudinal cross section, and (b) is a lateral cross section. 
     The pebbles  4  to be loaded within the pebble accommodating space  20  have a spherical shape having a diameter of about 6 cm and for example, have a structure in which a fuel region configured by a large number of covered fuel particles containing uranium oxide as a nuclear fuel substance and a graphite matrix involving the covered fuel particles is surrounded by a graphite shell. Then, the pebble  4  is completed in such a manner that in order to contain this covered fuel particle in a graphite material working as a neutron moderator, the covered fuel particle is mixed with a graphite powder, filled within a spherical molding die, and then subjected to primary pressing to produce a primary ball (core); this primary ball is subjected to secondary pressing together with a graphite powder to form a spherical particle with shell; and in order to process this into a true spherical shape, the resulting spherical particle is subjected to surface grinding, followed by pre-calcination and calcination. 
     The nuclear reactor structure  10  configuring the pebble accommodating space  20  includes a core material  11  including graphite and a ceramic/ceramic composite material  12  covering the surface of the core material  11 . In detail, the core material  11  is covered by an aggregate  13  including a ceramic fiber as described later to form a base material; the base material is put into a CVD furnace; and a SiC matrix is formed in gaps of the aggregate  13 , thereby forming the ceramic/ceramic composite material  12  on the surface of the core material  11 . In addition, in the present embodiment, the nuclear reactor structure  10  is formed in a quadrangular prism, a bottom surface of which is an approximately isosceles trapezoid, thereby forming the pebble accommodating space  20  in a columnar shape. According to this structure, the graphite is able to efficiently moderate the neutrons generated from the nuclear fuel substance to convert into heat energy. 
     The pebble  4  is formed by solidifying particles prepared by covering the nuclear fuel with pyrolytic carbon, SiC, or the like, and therefore, the pebble  4  is hard and has high ability of wearing the nuclear reactor structure  10 . In consequence, by covering the nuclear reactor structure  10  with a substance having the same material quality as SiC that is the hardest among those contained in the pebble  4 , even when a pressure is applied from the pebble  4 , it becomes possible to render the nuclear reactor structure  10  to be hardly broken. In addition, the ceramic/ceramic composite material  12  including SiC is less in neutron absorption, and therefore, it scarcely affects a chain reaction of nuclear fission. 
     A production process of the nuclear reactor structure  10  is explained by reference to  FIGS. 3 to 5 . 
     A basic production process includes three steps of (A) a core material processing step, (B) a step of obtaining a base material, and (C) a CVD step. 
     In the core material processing step (A), the core material  11  including graphite is processed into a quadrangular prism, a bottom surface of which is an approximately isosceles trapezoid (see  FIG. 3  and  FIGS. 4 ( a   1 ) and  4 ( b   1 )). In  FIGS. 4 ( a   1 ) to  4 ( a   3 ), the nuclear reactor structure is described sideways, and actually, it is used in such a manner that the Z-Z′ direction is a vertical direction. For this reason, the core material  11  is formed in a shape of quadrangular prism, whose cross section in the X-Y plane is formed in an approximately isosceles trapezoid and whose bottom surface is also formed in an approximately isosceles trapezoid. 
     In the step (B) of obtaining a base material, the core material  11  is covered with an aggregate  13  including a ceramic fiber to obtain the base material (see  FIG. 3  and  FIGS. 4 ( a   2 ) and  4 ( b   2 )). 
     In the CVD step (C), the base material is put into a CVD furnace, and a SiC matrix is formed in gaps of the aggregate  13 , thereby forming the ceramic/ceramic composite material  12  on the surface of the core material  11  (see  FIG. 3  and  FIGS. 4 ( a   3 ) and  4 ( b   3 )). 
     The gaps of the aggregate  13  are gaps formed among the ceramic fibers configuring the aggregate  13 . In general, as for a fibrous substance, the space can be completely filled with a fibrous substance under an extremely restricted condition. The restricted condition is, for example, a condition under which the following state is formed.
         In a cross section orthogonal to the fibrous substance, no gap exists, and the fibrous substance is arranged linearly and in the same direction. For example, a fibrous substance of triangular, quadrangular, or pentagonal prism is disposed.   A tabular substance is laminated, and a fibrous substance is filled in the tabular substance without gaps. The term “tabular” means, for example, a state where a fibrous substance of quadrangular prism is arranged laterally to constitute a tabular substance, or a state where a fibrous substance of quadrangular prism is wound up in a plane.       

     For this reason, in the case where the ceramic fiber is a woven fabric, a nonwoven fabric, or a papermaking sheet-like body, or in the case where the cross section of the ceramic fiber is circular, gaps are inevitably formed. The gap includes, in the addition to the case where the ceramic fibers are far from each other, a cavity of the surface formed by the adjacent ceramic fibers to each other. 
     In the present embodiment, the core material  11  is covered with the ceramic fiber-containing aggregate  13 , and also a CVD step of forming the SiC matrix is included. Accordingly, the nuclear reactor structure  10  capable of more enhancing the durability, preventing cracking, etc. from occurring, and preventing exposure of graphite of the core material  11  from occurring is provided. For this reason, since in the nuclear reaction structure, the core material occupying the majority is graphite, and the ceramic/ceramic composite material covers the surface of the core material, a production method of a nuclear reactor structure that scarcely affects the capability for neutron moderation of the graphite and has excellent durability can be provided. 
     In the above-described matrix formation, the ceramic matrix is filled in the surrounding of the ceramic fiber that is the aggregate  13 . In the CVD process, the core material  11  is put into the CVD furnace, and a raw material gas is introduced in a heated state thereinto. When the raw material gas is diffused within the CVD furnace and brought into contact with the heated aggregate  13 , pyrolysis occurs, whereby the ceramic matrix corresponding to the raw material gas is formed on the surface of the ceramic fiber constituting the aggregate  13 . 
     In the case wherein the objective ceramic matrix is SiC, a mixed gas of a hydrocarbon gas and a silane-based gas, an organosilane-based gas including carbon and silicon, and the like can be utilized. As such a raw material gas, a gas in which hydrogen is substituted with a halogen can also be utilized. As the silane-based gas, chlorosilane, dichlorosilane, trichlorosilane, and tetrachlorosilane can be utilized; and in the case of the organosilane-based gas, methyltrichlorosilane, methyldichlorosilane, methylchlorosilane, dimethyldichlorosilane, trimethyldichlorosilane, and the like can be utilized. In addition, these raw material gases may be properly mixed and used, and furthermore, hydrogen, argon, or the like can also be used as a carrier gas. Hydrogen which is used as the carrier gas is able to participate in adjustment of the equilibrium. 
     Next, the process of obtaining the base material is described in detail by reference to  FIG. 3 . In the present embodiment, in the step (B) of obtaining the base material, five step patterns are existent. In  FIG. 3 , the five step patterns are expressed by from (B 1 ) to (B 5 ). (B 1 ) is a step of covering the core material  11  with the aggregate  13  including a ceramic fiber. In (B 2 ), a step of impregnating a resin is added after the above-described step (B 1 ). In (B 3 ), a step of further heating is added after the above-described step (B 2 ). (B 4 ) is a step of simultaneously covering the core material  11  with the aggregate  13  and the resin. In (B 5 ), a step of heating is added after the above-described step (B 4 ). 
     In the step (B 1 ) of covering the core material  11  with the aggregate  13  including a ceramic fiber, aggregates including ceramic fibers in various forms can be utilized. Examples thereof include a sheet-like fiber, a single fiber, a strand resulting from bundling single fibers, a chopped fiber resulting from cutting a ceramic fiber, a milled fiber resulting from milling a ceramic fiber, and the like. Examples of the sheet-like fiber include a woven fabric and a nonwoven fabric. Furthermore, examples of the nonwoven fabric include a papermaking sheet resulting from papermaking a chopped fiber or milled fiber, a felt sheet resulting from laminating a chopped fiber or milled fiber, and the like. As for the aggregate covering the core material, though such materials may be used alone, they can also be used in combination. For example, a strand-like ceramic fiber may be provided outside the sheet-like ceramic fiber. The strand-like ceramic fiber tightens the sheet-like ceramic fiber, whereby the base material and the aggregates can be brought into intimate contact with each other. Furthermore, the base material and the ceramic/ceramic composite material obtained therefrom can be brought into intimate contact with each other. 
     Next, the step (B 1 ) of covering the core material  11  with the aggregate  13  including a ceramic fiber is specifically explained. For example, the ceramic fiber is applied onto the surface of the core material  11  utilizing method of blowing the ceramic fiber, such as a milled fiber, etc., together with a solvent onto the surface of the core material  11  by means of spraying or the like (see  FIG. 5( a ) ), or coating it together with a solvent using a coater or the like; a method of sticking a sheet-like ceramic fiber onto the surface of the ceramic fiber  11  (see  FIG. 5( b ) ); or the like. Then, the ceramic/ceramic composite material  12  can be formed on the surface of the core material  11  by the CVD step (C) (see  FIG. 5( c ) ). 
     In addition, it is also possible to wind a ceramic fiber in a single fiber or strand-like state, etc. around the entire surface of the core material  11 , thereby forming the aggregate  13  in a wound body  13   a  (see  FIG. 5( d ) ), and the ceramic/ceramic composite material  12  can be formed on the surface of the core material  11  by the CVD step (C) (see  FIG. 5( e ) ). 
     The winding method is not particularly limited. For example, hoop winding of winding the ceramic fiber in a ring while rotating the core material  11 ; helical winding of helically winding the ceramic fiber while keeping gaps between the ceramic fibers; and the like can be utilized, and a combination thereof can also be used. In addition, in the case where the ceramic fiber is made of a combination of hoop winding and helical winding, a ceramic fiber-reinforced ceramic composite material with high strength, in which a large number of points of contact of the ceramic fibers crossing each other are present at an interface thereof, can be obtained. 
     In the embodiment of  FIG. 2 , an example in which the entire surface of the core material  11  is covered with the aggregate  13  to form the ceramic/ceramic composite material  12  by the CVD method is shown. The ceramic/ceramic composite material  12  may be formed on only an inner wall facing the pebble accommodating space  20 . It is possible to properly select the forming surface of the ceramic/ceramic composite material  12  in conformity with the use condition. 
     In (B 2 ), a step of impregnating a resin is added after the above-described step (B 1 ). In order to increase the adhesion of the ceramic fiber to the core material  11  to obtain the aggregate  13  with higher strength, the resin to be impregnated contains, for example, an organosilicon-based resin or a silicide-based ceramic particle. In the case of using an organosilicon-based resin, the resin per se is converted into a ceramic under heating. Examples of the organosilicon-based resin include a polycarbosilane and the like. The polycarbosilane is converted into SiC under heating. 
     In the case of using a silicide-based ceramic particle, the ceramic particle is able to form the SiC matrix in the gaps of the aggregate. The silicide-based ceramic particle is not particularly limited, and examples thereof include SiC, SiO 2 , and the like. A resin which is used together with the silicide-based ceramic resin is not particularly limited, and for example, a phenol resin, polyvinyl alcohol, polyethylene glycol, and the like can be utilized. Such a resin functions as a binder. In addition, in the case of SiO 2  as the silicide-based ceramic particle, SiO 2  can be bonded to Si to become a raw material of the SiC matrix. 
     The method of heating the resin is not particularly limited, on the occasion of undergoing the CVD step of (C), the resin can be treated at the same time of heating before introducing the raw material gas; but, a heating step may also be separately added (see  FIG. 3 (B 3 )). 
     In the step of impregnating a resin, which is adopted in the production method of  FIG. 3 (B 2 ) or  3 (B 3 ), a solution containing the resin or the molten resin may be blown by means of spraying or the like, dipping, painting with a brush, or the like. In addition, it is possible to melt a powder or film-like solid resin, thereby impregnating the molten resin. 
     In (B 3 ), a step of heating is added after the above-described step (B 2 ). In this step of obtaining a base material, by adding the step of heating, before the CVD step, the ceramic fiber and the impregnated resin can be firmly bonded to each other, the aggregate does not rise in the CVD step, and the base material and the ceramic/ceramic composite material can be brought into intimate contact with each other. In addition, a decomposed gas is scarcely generated in the inside of the CVD furnace, and the inside of the CVD furnace can be made to be hardly contaminated. Thus, the purity of the SiC matrix to be formed within the CVD furnace can be increased, and the performance as a nuclear reactor structure, such as capability for neutron moderation, etc., can be increased. 
     In addition, in the step (B) of obtaining a base material, after the step of (B 4 ) of simultaneously covering the core material  11  with the aggregate  13  and the resin, the ceramic/ceramic composite material  12  can also be formed by the CVD step (C). The simultaneous covering with the aggregate  13  and the resin can be realized by containing the resin in the aggregate  13  from the beginning. Though a method of containing the resin in the aggregate  13  is not particularly limited, for example, a method of dipping the aggregate in the resin or a resin solution, a method of dispersing a powdered or fibrous resin in the aggregate, and the like can be applied. In order to increase the adhesion of the ceramic fiber to the core material  11  to obtain the aggregate  13  with higher strength, for example, the resin can contain an organosilicon-based resin or a silicide-based ceramic particle. In addition, a step of heating can be added after the step of (B 4 ) as in the step of (B 5 ). 
     Then, the aggregate  13  including a ceramic fiber, which covers the core material  11 , may be a cloth or a woven fabric including a ceramic fiber. 
     Though the ceramic fiber is not particularly limited so long as it has heat resistance and strength and has a low neutron absorption cross section, for example, ZrC, SiC, or a carbon fiber can be utilized. In particular, the ceramic fiber is desirably a SiC fiber. Since the SiC fiber is excellent in corrosion resistance and oxidation resistance and has high strength, by using SiC, even in the case where the ceramic matrix is damaged in a high-temperature corrosive atmosphere, the ceramic fiber stops development of cracking, whereby it can be safely used. In addition, since the SiC fiber is less in neutron absorption, it scarcely affects a chain reaction of nuclear fission. 
     The invention is not restricted to the above-described embodiment, and suitable modifications, improvements, and the like can be made. Moreover, the materials, shapes, dimensions, numerical values, forms, numbers, installation places, and the like of the components are arbitrarily set as far as the invention can be attained, and not particularly restricted. 
     It is to be noted that the present application is based on a Japanese patent application filed on Dec. 22, 2014 (Japanese Patent Application No. 2014-258777), the entireties of which are incorporated by reference. 
     INDUSTRIAL APPLICABILITY 
     The production method of a nuclear reactor structure according to the present invention is applicable to an application of a nuclear reactor utilizing a pebble. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 : Pebble bed type nuclear reactor 
               2 : Nuclear reactor vessel 
               3 : Reactor core 
               4 : Pebble 
               10 : Nuclear reactor structure 
               11 : Core material 
               12 : Ceramic/ceramic composite material 
               13 : Aggregate 
               20 : Pebble accommodating space