Patent Publication Number: US-2023134236-A1

Title: Reactor core

Description:
TECHNICAL FIELD 
     This present disclosure relates to a reactor core. 
     Priority is claimed on Japanese Patent Application No. 2020-079438, filed Apr. 28, 2020, the content of which is incorporated herein by reference. 
     BACKGROUND ART 
     As a type of a nuclear reactor, a type called a sodium-cooled fast reactor shown in Patent Document 1 below has been put into practical use. The reactor core of the sodium-cooled fast reactor is configured by a plurality of fuel assemblies loaded with fissile materials, and liquid metal sodium as a coolant for removing heat generated from the fuel assemblies. 
     In a case where a metal fuel is used for the fuel assembly, a cast metal fuel is generally used. The fuel assembly has a cylindrical wrapper tube (cladding tube), and a plurality of fuel pins accommodated inside the wrapper tube. The metal fuel as the fissile material and liquid metal sodium are enclosed in the inside of the fuel pin. 
     Here, as an index for evaluating the safety and stability of the reactor core, an index called a void reactivity (void coefficient of reactivity) is known. The void reactivity is a value that depends on a generation amount (void amount) of bubbles in the coolant in the reactor core. In a case where the temperature of the coolant in the reactor core is increased, the density of the coolant is decreased. As a result of the decrease in the density, the energy of neutrons is less likely to be absorbed by the coolant, and the neutrons are less likely to be decelerated. This causes a positive void reactivity, and may impair the stability of the reactor core. 
     In order to cancel such a positive void reactivity, for example, a method of reducing the reactor core height and providing a sodium plenum above the reactor core to promote neutron leakage to the outside of the reactor core can be considered. 
     CITATION LIST 
     Patent Document 
     [Patent Document 1] 
     Japanese Unexamined Patent Application, First Publication No. H5-323077 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, as described above, in the fuel assembly using a cast metal fuel in the related art, since the liquid metal sodium has to be loaded between the fuel and the cladding tube, a space (gas plenum) for releasing gas generated by nuclear fission needs to be arranged above the fuel. Therefore, it is not possible to secure a space for providing the sodium plenum in an upper portion. As a result, there are restrictions on improving the passive safety characteristics of the reactor core. 
     The present disclosure has been made to solve the above problems, and an object of the present disclosure is to provide a reactor core having higher stability. 
     Solution to Problem 
     In order to solve the above problems, a reactor core according to the present disclosure includes an inner core region that extends in a vertical direction, and has a plurality of first fuel pins accommodating an inner core fuel; an outer core region that extends in the vertical direction, is arranged to surround the inner core region from an outer peripheral side, and has a plurality of second fuel pins accommodating an outer core fuel; and a sodium plenum provided above the inner core region and the outer core region, in which a dimension of the outer core fuel in the vertical direction is larger than the dimension of the inner core fuel in the vertical direction, and the position of a center of the outer core fuel in the vertical direction is higher than the position of a center of the inner core fuel in the vertical direction. 
     Advantageous Effects of Invention 
     According to the present disclosure, it is possible to provide a reactor core having higher stability. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a vertical sectional view showing a configuration of a fast reactor according to an embodiment of the present disclosure. 
         FIG.  2    is a plan view showing a configuration of a reactor core according to the embodiment of the present disclosure. 
         FIG.  3    is a vertical sectional view showing a configuration of a fuel assembly according to the embodiment of the present disclosure. 
         FIG.  4    is a schematic sectional view showing a configuration of the reactor core according to the embodiment of the present disclosure. 
         FIG.  5    is a graph showing a distribution of neutron flux in a vertical direction of the reactor core according to the embodiment of the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a fast reactor  100  and a reactor core  1  according to an embodiment of the present disclosure will be described with reference to  FIGS.  1  to  5   . 
     (Configuration Example of Fast Reactor) 
     The fast reactor uses mixed oxide of uranium (U) and plutonium (Pu) as a fuel to fission Pu239, while allowing U238 to absorb generated excess fast neutrons and produce more plutonium than being burned. As shown in  FIG.  1   , the fast reactor  100  includes the reactor core  1 , a reactor vessel  2 , a guard vessel  3 , a coolant inlet pipe  4 , a coolant outlet pipe  5 , an upper core structure  7 , and a fixed plug  8 . 
     The reactor core  1  is a heat generation source containing a fissile material. The detailed configuration of the reactor core  1  will be described later. The reactor vessel  2  is a vessel that accommodates the reactor core  1 . The reactor vessel  2  has a cylindrical shape having a bottom surface. The reactor core  1  is fixed to a lower portion in the reactor vessel  2  via a core internal structure  12 . The opening at an upper portion of the reactor vessel  2  is covered by the fixed plug  8 . The fixed plug  8  is supported by the structure of a nuclear reactor building (reactor vessel pedestal  6 ). 
     The guard vessel  3  covers the reactor vessel  2  from the outside. That is, the reactor vessel  2  and the guard vessel  3  form a double wall structure. As a result, even in a case where the coolant leaks from the reactor vessel  2 , the coolant is held by the guard vessel  3 , and leakage to the outside is suppressed. 
     The coolant inlet pipe  4  guides liquid metal sodium as the coolant (primary coolant) guided from the outside, into the reactor vessel  2 . The end portion of the coolant inlet pipe  4  is positioned below the reactor core  1  in the reactor vessel  2 . As a result, the inside of the reactor vessel  2  is in a state of being filled with the coolant. The coolant outlet pipe  5  discharges the coolant in the reactor vessel  2  to the outside. The end portion of the coolant outlet pipe  5  is positioned above the reactor core  1  in the reactor vessel  2 . 
     The upper core structure  7  has a control rod drive mechanism  9 , a rotary plug  10 , and a rotary plug drive device  11 . The control rod drive mechanism  9  is a device for inserting and pulling out a control rod for controlling the progress of the fission reaction in the reactor core  1  described later. The control rod drive mechanism  9  moves the control rod up and down in the vertical direction. The rotary plug  10  is a device for positioning a device for exchanging the nuclear fuel (fuel assembly  30  described later) in the reactor core  1 . The rotary plug  10  is driven by the rotary plug drive device  11 . 
     (Configuration of Reactor Core) 
     Next, the configuration of the reactor core  1  will be described with reference to  FIG.  2   . As shown in  FIG.  2   , the reactor core  1  is an assembly of members each having a hexagonal sectional shape, and is arranged without gaps to form a hexagonal shape as a whole. The reactor core  1  has a neutron shield  21 , a radial blanket fuel  22 , a control rod  23 , a neutron source  24 , an outer core region  25 , and an inner core region  26 . 
     The neutron shield  21  is arranged on the outermost peripheral side of the reactor core  1 . A plurality of neutron shields  21  are arranged so as to form a hexagonal annular shape. A plurality of radial blanket fuels  22  are arranged inside the neutron shield  21 . The radial blanket fuels  22  are arranged in a hexagonal annular shape. The outer core region  25  is provided inside the radial blanket fuel  22 . The inner core region  26  is provided further inside the outer core region  25 . A plurality of control rods  23  can be inserted into a partial region in the inner core region  26 . The outer core region  25  and the inner core region  26  are formed by arranging a plurality of fuel assemblies  30  described later. 
     The outer core region  25  and the inner core region  26  generate heat by fission of the fissile material triggered by the neutrons generated from the neutron source  24 . The insertion amount of the control rod  23  is adjusted to control the progress of the fission reaction. In the radial blanket fuel  22 , the reaction progresses in a state where the fast fission reaction is reduced as compared with the outer core region  25  and the inner core region  26 . Further, in the radial blanket fuel  22 , the absorption amount of neutrons generated by the fission reaction is larger than that in the outer core region  25  and the inner core region  26 . The neutron shield  21  is provided to shield the neutrons and suppress leakage to the outside. 
     (Configuration of Fuel Assembly) 
     Subsequently, the configuration of the fuel assembly  30  will be described with reference to  FIG.  3   . The fuel assembly  30  has a wrapper tube  31 , an entrance nozzle  32 , a handling head  33 , a plurality of fuel pins  40  (first fuel pin  41 , second fuel pin  42 ), and an upper neutron shield  50 . 
     The wrapper tube  31  has a cylindrical shape centered on an axis line Ac extending in the vertical direction. Further, the wrapper tube  31  has a hexagonal sectional shape when viewed from the axis line Ac direction. The opening at a lower portion of the wrapper tube  31  is closed by the entrance nozzle  32 . In the inside of the entrance nozzle  32 , a flow path  32 F for guiding the coolant into the inside of the wrapper tube  31  is formed. An inlet H for the communication between the flow path  32 F and the outside is formed in the lower portion of the entrance nozzle  32 . The handling head  33  is attached to the opening at the upper portion of the wrapper tube  31 . The handling head  33  is a portion gripped by the device when the fuel assembly  30  is transported. 
     The plurality of fuel pins  40  are arranged inside the wrapper tube  31  and directly above the entrance nozzle  32 , at intervals in a direction orthogonal to the axis line Ac. Each fuel pin  40  has a cylindrical pin body  40 H extending in the vertical direction, an upper end plug  43  that closes the upper opening of the pin body  40 H, a lower end plug  48  that closes the lower opening of the pin body  40 H, fuel alloy particles  45  (core fuel) enclosed inside the pin body  40 H, and a lower blanket fuel  46 . The fuel alloy particles  45  are formed of fissile metal particles having two types of different outer diameters. The fuel alloy particles  45  are filled in a region upwardly biased in the pin body  40 H. 
     As will be described in detail later, the configuration of the fuel assembly  30  is different between the outer core region  25  and the inner core region  26 . Specifically, in the fuel pin  40  (second fuel pin  42 ) constituting the outer core region  25 , the dimensions of the fuel alloy particles  45  in the vertical direction are large as compared with the fuel pin  40  (first fuel pin  41 ) constituting the inner core region  26 . That is, the fuel alloy particles  45  (outer core fuel  45 B) of the second fuel pin  42  are set to have a larger dimension in the vertical direction than the fuel alloy particles  45  (inner core fuel  45 A) of the first fuel pin  41 . 
     The space between the fuel alloy particles  45  and the upper end plug  43  is an upper gas plenum  44  through which the gas generated from the fuel alloy particles  45  flows. A heat shield (not shown) is provided in the upper gas plenum  44 . The lower blanket fuel  46  formed of depleted uranium is filled below the fuel alloy particles  45 . It is also possible to adopt a configuration in which the lower blanket fuel  46  is not provided. Further, the second fuel pin  42  constituting the outer core region  25  does not include the lower blanket fuel  46 . 
     The space above the lower end plug  48  is a lower gas plenum  47  through which the gas generated from the fuel alloy particles  45  flows. The dimension of the lower gas plenum  47  in the vertical direction is larger than the dimension of the upper gas plenum  44 . The upper neutron shield  50  is provided to shield the leakage of neutrons upward. The upper neutron shield  50  is arranged above the fuel pin  40  with a gap. 
     The space around and above the fuel pin  40  configured as described above is a sodium plenum  49 . The liquid metal sodium as the coolant guided from the inlet H of the entrance nozzle  32  flows to the sodium plenum  49 . 
     Next, with reference to  FIG.  4   , the difference in the dimensions of the outer core region  25  and the inner core region  26  in the vertical direction will be described.  FIG.  4    shows a cross section including a central axis line X of the reactor core  1 . Further, in  FIG.  4   , the illustration of the neutron shield  21  in  FIG.  2    is omitted. As shown in  FIG.  4   , the dimension of the outer core fuel  45 B in the outer core region  25  in the vertical direction is larger than the dimension of the inner core fuel  45 A of the inner core region  26  in the vertical direction. Further, the upper end of the outer core fuel  45 B is at a higher position than the upper end of the inner core fuel  45 A. Further, as described above, the fuel pin  40  constituting the outer core region  25  is not provided with the lower blanket fuel  46 . As a result, the lower end of the inner core fuel  45 A is at a higher position than the lower end of the outer core fuel  45 B. 
     (Effects) 
     Here, as shown in  FIG.  5   , it is known that the peak position of the fast neutron flux (that is, the position with the largest number of fast neutrons passing per unit area and unit time) generated from the inner core fuel  45 A and the outer core fuel  45 B is the center position in the vertical direction of each core fuel. With the above configuration, the dimension of the outer core fuel  45 B in the vertical direction is larger than the dimension of the inner core fuel  45 A in the vertical direction. Further, the position of the center of the outer core fuel  45 B is higher than the position of the center of the inner core fuel  45 A. That is, the peak position of the fast neutron flux generated from the outer core fuel  45 B can be maintained above the peak position of the fast neutron flux generated from the inner core fuel  45 A. As a result, it is possible to secure a large amount of neutron leakage to the sodium plenum  49  positioned above the inner core fuel  45 A and the outer core fuel  45 B. As a result, even in a case where the temperature of the coolant is abnormally increased, a large negative reactivity can be generated. Therefore, the passive safety of the reactor core  1  is further improved, and the fast reactor  100  can be operated more safely. 
     Further, with the above configuration, the upper end of the outer core fuel  45 B is at a position higher than the upper end of the inner core fuel  45 A. That is, the peak position of the fast neutron flux generated from the outer core fuel  45 B can be maintained above the peak position of the fast neutron flux generated from the inner core fuel  45 A. As a result, the amount of neutron leakage to the sodium plenum  49  positioned above the inner core fuel  45 A and the outer core fuel  45 B can be further secured. 
     On the other hand, with the above configuration, the dimension of the outer core fuel  45 B is larger than the dimension of the inner core fuel  45 A in the downward direction as well as in the upward direction. As a result, the heat generation range of the outer core fuel  45 B is widened, and more output can be generated. That is, by keeping the height dimension of the inner core fuel  45 A small and by increasing the output sharing of the outer core fuel  45 B, the average output and peak output of the fuel can be suppressed without increasing the number of loads of the fuel assembly  30  and without increasing the radial dimension of the reactor core  1 . That is, it is possible to suppress the increase in the size of the reactor core system caused by flattening. 
     Further, with the above configuration, since the fuel alloy particles  45  are used as the fissile material, it is not necessary to enclose sodium inside the pin body  40 H. Therefore, a gas plenum (that is, a region filled with the inert gas), which is provided above the fuel alloy particles  45  in the related art, can be provided below the fuel alloy particles  45 . Further, dimensions of the first fuel pin  41  and the second fuel pin  42  in the vertical direction can be suppressed to be small by the amount that the region to be filled with sodium is eliminated and the amount that the gas pressure is suppressed to be low by installing the gas plenum below the fuel alloy particles  45 . As a result, the size of the reactor core  1  can be reduced. 
     The embodiments of the present disclosure have been described above. It is possible to make various changes and modifications to the above configuration as long as it does not deviate from the gist of the present disclosure. 
     [Additional Notes] 
     The reactor core  1  described in the embodiment is grasped as follows, for example. 
     (1) A reactor core  1  according to a first aspect includes an inner core region  26  that extends in a vertical direction, and has a plurality of first fuel pins  41  accommodating an inner core fuel  45 A; an outer core region  25  that extends in the vertical direction, is arranged to surround the inner core region  26  from an outer peripheral side, and has a plurality of second fuel pins  42  accommodating an outer core fuel  45 B; and a sodium plenum  49  provided above the inner core region  26  and the outer core region  25 , in which a dimension of the outer core fuel  45 B in the vertical direction is larger than a dimension of the inner core fuel  45 A in the vertical direction, and the position of a center of the outer core fuel  45 B in the vertical direction is higher than the position of a center of the inner core fuel  45 A in the vertical direction. 
     Here, it is known that the peak position (that is, the position with the largest number of fast neutrons per unit area and unit time) of the fast neutron flux generated from the inner core fuel  45 A and the outer core fuel  45 B is the center position in the vertical direction of each core fuel. With the above configuration, the dimension of the outer core fuel  45 B in the vertical direction is larger than the dimension of the inner core fuel  45 A in the vertical direction. Further, the position of the center of the outer core fuel  45 B is higher than the position of the center of the inner core fuel  45 A. That is, the peak position of the fast neutron flux generated from the outer core fuel  45 B can be maintained above the peak position of the fast neutron flux generated from the inner core fuel  45 A. As a result, it is possible to secure a large amount of neutron leakage to the sodium plenum  49  positioned above the inner core fuel  45 A and the outer core fuel  45 B. As a result, even in a case where the temperature of the coolant is abnormally increased, a negative reactivity is generated, and the void reactivity can be suppressed to a negative value. 
     (2) In the reactor core  1  according to a second aspect, an upper end of the outer core fuel  45 B is higher than an upper end of the inner core fuel  45 A. 
     With the above configuration, the upper end of the outer core fuel  45 B is at a position higher than the upper end of the inner core fuel  45 A. That is, the peak position of the fast neutron flux generated from the outer core fuel  45 B can be maintained above the peak position of the fast neutron flux generated from the inner core fuel  45 A. As a result, the amount of neutron leakage to the sodium plenum  49  positioned above the inner core fuel  45 A and the outer core fuel  45 B can be further secured. 
     (3) The reactor core  1  according to a third aspect further includes a lower blanket fuel  46  that is provided inside the first fuel pin  41  to be arranged below the inner core fuel  45 A. 
     With the above configuration, the lower blanket fuel  46  is provided below the inner core fuel  45 A. That is, the dimension of the inner core fuel  45 A in the vertical direction can be suppressed to be small by the amount of the lower blanket fuel  46 . As a result, a sufficient height difference from the outer core fuel  45 B can be secured. As a result, the peak position of the fast neutron flux generated from the outer core fuel  45 B can be sufficiently shifted. 
     (4) In the reactor core  1  according to a fourth aspect, the first fuel pin  41  and the second fuel pin  42  have a cylindrical pin body  40 H extending in the vertical direction, a plurality of fuel alloy particles  45  that are enclosed in the pin body  40 H and have two types of different outer diameters, and an inert gas filled between the fuel alloy particles  45 . 
     With the above configuration, since the fuel alloy particles  45  are used, it is not necessary to enclose sodium inside the pin body  40 H. Therefore, a gas plenum (that is, a region filled with the inert gas), which is provided above the fuel alloy particles  45  in the related art, can be provided below the fuel alloy particles  45 . Further, dimensions of the first fuel pin  41  and the second fuel pin  42  in the vertical direction can be suppressed to be small by the amount that the region to be filled with sodium is eliminated and the amount that the gas pressure is suppressed to be low by installing the gas plenum below the fuel alloy particles  45 . As a result, the size of the reactor core  1  can be reduced. 
     INDUSTRIAL APPLICABILITY 
     According to the present disclosure, it is possible to provide a reactor core having higher stability. 
     REFERENCE SIGNS LIST 
     
         
           100 : Fast reactor 
           1 : Reactor core 
           2 : Reactor vessel 
           3 : Guard vessel 
           4 : Coolant inlet pipe 
           5 : Coolant outlet pipe 
           6 : Reactor vessel pedestal 
           7 : Upper core structure 
           8 : Fixed plug 
           9 : Control rod drive mechanism 
           10 : Rotary plug 
           11 : Rotary plug drive device 
           12 : Core internal structure 
           21 : Neutron shield 
           22 : Radial blanket fuel 
           23 : Control rod 
           24 : Neutron source 
           25 : Outer core region 
           26 : Inner core region 
           30 : Fuel assembly 
           31 : Wrapper tube 
           32 : Entrance nozzle 
           32 F: Flow path 
           33 : Handling head 
           40 : Fuel pin 
           40 H: Pin body 
           41 : First fuel pin 
           42 : Second fuel pin 
           43 : Upper end plug 
           44 : Upper gas plenum 
           45 : Fuel alloy particle 
           45 A: Inner core fuel 
           45 B: Outer core fuel 
           46 : Lower blanket fuel 
           47 : Lower gas plenum 
           48 : Lower end plug 
           49 : Sodium plenum 
           50 : Upper neutron shield 
         Ac: Axis line 
         H: Inlet 
         X: Central axis line