JP4944455B2 - Natural circulation boiling water reactor - Google Patents

Description

  The present invention relates to a boiling water reactor.

  In a forced circulation boiling water reactor (hereinafter referred to as “BWR”) that has been commercially operated so far, a fuel assembly having a square cross section is placed in the X-axis direction and the Y-axis direction in a cylindrical core shroud. The core is made up of forests. A control rod having a substantially cross-shaped cross section is disposed so as to be inserted between four fuel assemblies surrounding the control rod, and the unit of the four fuel assemblies surrounding the control rod is defined as a control rod cell. It is called.

  In the core, LPRM (Local Power Range Monitor) detector assemblies are discretely arranged so as to surround the 2 × 2 array of control rod cells at the corner positions of the 2 × 2 array. The neutron flux distribution in the reactor is measured. The in-core neutron flux distribution measured by the LPRM detector assembly is calculated from the power distribution in the reactor core to calculate the fuel assembly thermal limit [MCPR (Minimum Critical Power Ratio) and MLHGR (Maximum It is used for core performance calculation by a three-dimensional nuclear thermal hydraulic calculation code that calculates margins for the operation limit value of Linear Heat Generation Rate (maximum linear power density).

  The arrangement of this LPRM detector assembly is such that the fuel assembly in the core is centered at an angle of 45 ° with respect to the X and Y axes passing through the center in the cross section of the core and the X or Y axis. Considering 1/8 symmetry axis of symmetry, 1/4 core mirror symmetry, 1/4 core 90 ° rotational symmetry, 1/2 core mirror symmetry, 1/2 core 180 ° rotational symmetry, 1/8 core mirror symmetry It is something that uses the arrangement with the loading pattern such as. In other words, in the arrangement of the LPRM detector assembly, the neutron flux at the location where the LPRM detector assembly is not disposed at the corner of the control rod cell is the LPRM detector assembly in a symmetrical position on the cross section of the core. It is supposed to replace with the measured neutron flux.

In recent years, a natural circulation type BWR has been proposed, and in the natural circulation type BWR, a chimney that does not exist in the forced circulation type BWR is provided on the core in order to secure a driving force for natural circulation (see Patent Document 1).
Japanese Patent Publication No. 7-27051 (FIGS. 1 and 2)

By the way, the natural circulation type BWR described in Patent Document 1 has a structure in which the inside of the chimney is divided by a partition plate in the vertical direction and has a plurality of lattice channels.
In this case, the coolant flow is as follows. First, the coolant is distributed to each fuel assembly when it flows into the core from the lower plenum, and the fuel assembly is cooled to become a gas-liquid two-phase flow of the coolant. Further, the coolant exiting the individual fuel assemblies is joined by a plurality of fuel assemblies corresponding to one lattice channel at the inlet of the chimney lattice channel. Thereafter, the entire gas-liquid two-phase flow of each grid channel merges in the upper plenum. Therefore, the coolant flow rate distributed to the individual fuel assemblies is the difference between the specific gravity of the coolant, which is mainly determined by the pressure loss of the fuel assembly pressure loss plus the lattice flow pressure loss, and the void fraction in the lattice flow channel. Determined by the driving force. In other words, the flow rate of the coolant flowing through each lattice channel depends on the amount of vapor generated in the fuel assembly corresponding to the lattice channel.

  This grid channel needs to be divided so that the gas-liquid two-phase flow exiting the core has a uniform void rate in the cross section, but from the viewpoint of reducing the frictional pressure loss when passing through the chimney, It is preferable to use a grid channel having a larger channel cross-sectional area than the control rod cell unit. In addition, from the viewpoint of exchanging fuel assemblies and the like through the lattice channel while chimney is installed inside the reactor pressure vessel during the periodic inspection of the reactor, the flow channel partition wall of the lattice channel is: It is preferable that the upper lattice plate of the core is constructed so as not to be blocked in such a manner as to cross the square lattice holes opened in units of the control rod cells.

However, the central control rod cell is arranged so that the control rod is located at the center of the cross section of the core, and depending on the configuration of the channel partition walls forming the chimney lattice channel, The arrangement of the lattice channels is asymmetric with respect to the X axis and Y axis of the cross section of the core.
In that case, even if the fuel assembly loading pattern is configured to be one of the above-mentioned various symmetry in the cross section of the core, the symmetry of the fuel assembly loading pattern and the lattice flow arrangement of the chimney cross section If the symmetry of is deviated, the assumption of the use of symmetry in the core arrangement of the LPRM detector assembly is broken, and an error occurs in the calculation of the power distribution in the core of the fuel assembly.

  An object of the present invention is to provide a natural circulation boiling water reactor capable of more accurately calculating the output of the fuel assembly in the core in the core performance calculation. .

In order to solve the above-mentioned problem, the invention according to claim 1 is a natural circulation type comprising a core loaded with a plurality of fuel assemblies, a neutron detector installed in the core, and a chimney installed on the core. In a boiling water reactor, the upper grid plate that determines the horizontal position of the fuel assembly in the core constitutes one control rod cell by arranging a control rod at the center position of the 2 × 2 array of fuel assemblies, A square lattice hole corresponding to the control rod cell is arranged in the center of the cross section of the core, and further, a number of lattice holes are arranged in the X-axis direction and the Y-axis direction of the orthogonal XY coordinates. The arrangement based on the fuel assembly enrichment / flammable poison design type and the residence time in the furnace, at least with respect to two 1/8 symmetry axes that form an angle of 45 ° to both the X and Y axes Te is capable of loading configuration in a mirror symmetrical, chimney, the flow path partition wall The partitioned grid channel is configured to include a 2 × 2 array of grid holes in the upper grid plate and the cross-sectional arrangement of the grid channel is symmetrical with one of the two 1/8 symmetry axes The neutron detector is arranged in at least one of the symmetrical positions corresponding to the mirror symmetry of the one-eighth symmetry axis of the cross-sectional arrangement of the lattice flow path in the cross section of the core. It is arranged.
The invention according to claim 2 is a natural circulation boiling water reactor comprising a core loaded with a plurality of fuel assemblies, a neutron detector installed in the core, and a chimney installed on the core. The upper grid plate that determines the horizontal position of the fuel assembly in the core constitutes one control rod cell by arranging a control rod at the center position of the 2 × 2 array of fuel assemblies, and corresponds to the control rod cell. A square lattice hole is arranged in the center of the cross section of the core, and a number of lattice holes are arranged in the X-axis direction and the Y-axis direction of the right-angle XY coordinates. The layout based on the design type of the flammable poison and the duration of stay in the reactor is divided into four by dividing the cross section of the core into four by the X axis and Y axis passing through the center of the cross section of the core at least in the right-angle XY coordinates. Can be loaded 90 ° rotationally symmetrically at the center of the core cross section with respect to the core The chimney includes a lattice flow defined by the flow partition, and the lattice flow corresponding to the center of the cross section of the core includes only one lattice hole at the center of the cross section of the core. The grid flow path is configured to include a 2 × 2 array of grid holes in the upper grid plate, and is configured to be 90 ° rotationally symmetric with respect to the quarter core. The neutron detector is arranged at least at one of the symmetrical positions corresponding to the symmetry of 90 ° rotational symmetry with respect to the quarter core of the cross-sectional arrangement of the lattice channel.

  With this configuration, the symmetry in which the fuel assembly loading pattern in the cross section of the core can be used, the symmetry used in the arrangement of the neutron detectors, and the symmetry of the arrangement of the lattice channels in the chimney cross section , Matches.

  According to the present invention, the power distribution of the fuel assembly in the core can be measured more accurately with a small number of neutron detectors. As a result, the symmetry in the arrangement of the cross section of the core of the neutron detector can be used, and the power distribution in the core can be calculated with high accuracy by using a small number of neutron detectors.

<< First Embodiment >>
Next, the natural circulation boiling water reactor according to the first embodiment of the present invention will be described in detail with reference to FIGS.

(Outline of the reactor)
In general, there are two ways to drive coolant (light water) in boiling water reactors, one is forced circulation using a recirculation pump, and the other is natural without using a recirculation pump. It is a method by circulation. This embodiment is the latter method based on natural circulation.
As shown in FIG. 1, the natural circulation method uses voids generated in a reactor core 7 accommodated in a reactor pressure vessel (hereinafter referred to as a pressure vessel) 6, that is, vapor (gas phase) and a liquid phase of a saturation temperature. The driving force required for natural circulation is obtained by the specific gravity difference between the coolant having a low density mixed with the coolant and the coolant in the liquid phase mixed with the feed water supplied from the feed water pipe 16b.

  As shown in FIG. 1, a natural circulation boiling water reactor (hereinafter referred to as “reactor”) 1 of the present embodiment is provided in a cylindrical pressure vessel 6 and a core shroud 8 is provided in a concentric cylindrical shape. It has been. The core shroud 8 forms an annular space in the gap between the outer peripheral surface thereof and the inner peripheral surface of the pressure vessel 6, and this is referred to as a downcomer 9. Further, the core 7 in which a large number of fuel assemblies 21 are arranged is housed inside the core shroud 8.

Above the downcomer 9, the coolant supplied from the feed water inlet nozzle 17 into the pressure vessel 6 after being heated by the feed water heater 5 from the condenser 3 via the feed water pump 4 is evenly placed in the pressure vessel 6. A water supply sparger (not shown) is provided in an annular shape.
The core shroud 8 is supported by a shroud leg 8a. The coolant descending the downcomer 9 is introduced into the core lower plenum (hereinafter referred to as the lower plenum) 10 below the core 7 from the flow path between the shroud legs 8a.

A core support plate 22 is provided in the lower part of the core 7 and an upper lattice plate 23 is provided in the upper part, and the lateral arrangement of the fuel assemblies 21 and the control rods 24 is determined.
The core support plate 22 is provided with circular through holes (not shown) at predetermined intervals. A control rod guide tube 25 is inserted into the through holes, and the lower portion of the control rod guide tube 25 penetrates the bottom of the pressure vessel 6. Thus, it is combined with an upper portion of a control rod drive mechanism housing (hereinafter referred to as CRD housing) 26a that houses a control rod drive mechanism (hereinafter referred to as CRD) 26 that moves the control rod 24 in the vertical direction.
The fuel assembly 21 is placed on a fuel support bracket (not shown) attached to the upper end of the control rod guide tube 25, and the load is applied to the bottom of the pressure vessel 6 via the control rod guide tube 25 and the CRD housing 26a. Reportedly.

The fuel support fitting (not shown) has a coolant inlet on a side surface, and an orifice (not shown) is provided there to regulate the coolant flow rate. An opening is provided in the side surface of the control rod guide tube 25 corresponding to the coolant inlet of the fuel support bracket, and the coolant guided to the lower plenum 10 is guided into the fuel assembly 21 through the fuel support bracket.
Each fuel assembly 21 is surrounded by a rectangular tube channel box (not shown) to form individual flow paths in the axial direction. The channel box reaches the upper surface of the upper lattice plate 23. On the outside of the channel box of the rectangular tube, there is formed a flow path having a gap between the adjacent fuel assembly 21 and the flow rate of a predetermined ratio of coolant.
The control rod 24 has an effective portion containing a neutron absorbing material (not shown), and the effective portion is inserted between the four fuel assemblies 21 with the outer surface of the channel box as a guide.

  Further, an LPRM (Local Power Range Monitor) detector assembly (hereinafter simply referred to as LPRM) 33 that includes a plurality of neutron detectors and measures the neutron flux in the output region is disposed in the core 7. Has been. The LPRM 33 is housed in an in-core nuclear instrument housing 33a whose lower part passes through a through hole provided in the bottom of the pressure vessel 6, and a signal cable extends from the lower end of the in-core nuclear instrument housing 33a.

Provided on the core 7 is a chimney 11A (represented by the chimney 11 in FIG. 1) that guides the gas-liquid two-phase flow coolant flowing out of the core upward and increases the natural circulation driving force. The chimney 11A has, for example, a cylindrical chimney cylinder 11d concentric with the pressure vessel 6, and has a lattice flow path 11a in which the inside is partitioned by a partition plate. Hereinafter, the partition plate constituting the lattice channel 11a is referred to as a channel partition 11b.
In addition, the upper plenum 11c is provided in the upper part of the chimney 11A so that the coolant flowing upward through the individual lattice channels 11a merges in the upper part of the chimney 11A.
Note that the upper lattice plate 23 and the lower end of the chimney 11A have a combined structure in which the coolant that descends the downcomer 9 and the coolant that exits the core 7 are not mixed.

The upper end of the chimney 11A is closed by the shroud head 12a. The shroud head 12a is provided with a predetermined number of coolant passage holes, and the holes are provided in the steam / water separator 12 for separating the saturated steam and the saturated water from the gas / liquid two-phase flow through the stand pipe 12b. linked. A steam dryer 13 is provided above the steam / water separator 12 to remove moisture contained in the saturated steam exiting the steam / water separator 12. The steam that passes through the steam dome 14, the steam outlet nozzle 15, and the main steam pipe 16 a is sent to the turbine 2.
The shroud head 12a, the stand pipe 12b, and the steam / water separator 12 are integrally assembled, and are configured to be integrally removable from the upper end of the chimney 11A when changing fuel.

Thus, in the nuclear reactor 1 schematically described, the coolant supplied from the feed water inlet nozzle 17 is mixed with the saturated water separated by the steam separator 12, and the coolant indicated by the arrow A in FIG. Moves down the downcomer 9, flows into the core shroud 8 from a flow path formed by a gap (not shown) of the shroud leg 8 a, and is heated by the core 7. By the heating from the core 7, the coolant A becomes a saturated gas-liquid two-phase flow indicated by an arrow B, and this gas-liquid two-phase flow passes through the lattice channel 11a, the upper plenum 11c, and the stand pipe 12b to separate the gas and water. The vessel 12 separates the gas phase saturated steam indicated by the arrow C and the liquid phase saturated water indicated by the arrow D. The saturated steam C passes through the steam dryer 13, is led from the steam outlet nozzle 15 to the turbine 2 through the main steam pipe 16 a, and is used for power generation.
On the other hand, the saturated water D is mixed with the coolant in the pressure vessel 6, further mixed with the coolant supplied from the feed water inlet nozzle 17, and descends the downcomer 9 again to circulate in the pressure vessel 6.

(Core structure-overview)
Next, referring to FIGS. 2 and 3 (refer to FIG. 1 as appropriate), a detailed configuration of the cross section of the core 7 (hereinafter referred to as the core plane) will be described.
The fuel assembly 21 is provided on the upper lattice plate 23 at predetermined intervals in the X-axis 51 direction and the Y-axis 52 direction, as shown in FIG. Corresponding to the square lattice holes 32 indicated by the solid solid lines, four fuel assemblies 21 indicated by the thin solid squares are arranged in groups, and the substantially cylindrical core 7 is formed. Constitute. A control rod 24 having a cross-shaped cross section is provided between the four fuel assemblies 21 so as to pass through the fuel support fitting (not shown) from the lower control rod guide tube 25 (FIG. 1). reference). A unit of four bodies of the fuel assembly 21 is hereinafter referred to as a control rod cell 31. In FIG. 2, only the central control rod cell 31 (C) that is typically arranged at the center of the core plane and the control rod cells 31 at other positions are shown, and only the control rod cell 31 is representatively shown in the control rod 24. Is shown. The control rod cell 31 of the L part in (a) is expanded and shown in (b).
The core 7 is configured to include 1132 fuel assemblies 21 and 269 control rods 24.

(Core structure-Symmetry of the core plane)
The core 7 has an axis of symmetry of the X axis 51 and the Y axis 52 passing through the center in the core plane, and 1/8 symmetrical at an angle of 45 ° with respect to the X axis 51 or the Y axis 52 through the center. It has symmetry axes (hereinafter referred to as １ ／ symmetry axes) 53 and 54.
Usually, each area of the core plane divided by the X axis 51 and the Y axis 52 is referred to as “1/4 core”, and “1/4 core” is further divided by 2 by 1/8 symmetry axes 53, 54. “1/8 core”.
The core 7 is a fuel assembly having a 1/4 core mirror symmetry, a 1/4 core 90 ° rotational symmetry, a 1/2 core mirror symmetry, and a 1/2 core 180 ° rotational symmetry with respect to the X axis 51 and the Y axis 52. The loading pattern can be configured. In addition, the core 7 can have a loading pattern of a fuel assembly having a 1/8 core mirror symmetry, a 1/2 core mirror symmetry, and a 1/2 core 180 ° rotational symmetry with respect to the 1/8 symmetry axes 53 and 54.

(Core structure-LPRM arrangement)
Next, the arrangement of the LPRM 33 on the core plane (hereinafter referred to as LPRM arrangement) will be described with reference to FIG.
As shown in FIG. 3A, in the core 7, LPRMs 33 are basically arranged at two corners adjacent to each control rod cell 31 except for the control rod cells 31 on the outer periphery of the core plane. ing. This is because 12 LPRMs 33 are arranged at the diagonal corner positions of the control rod cell 31 on the 1/8 symmetry axis 53 except for the outermost periphery of the core plane, as indicated by the mark ●, on the 1/8 symmetry axis 53. The LPRM 33 is arranged at the corners of the control rod cell 31 alternately along the X-axis 51 direction and the Y-axis 52 direction except for the outer peripheral portion of the core plane. The control rod cell 31 of L1 part in (a) is expanded and shown in (b). LPRM33 in this Embodiment is a total of 134 bodies.

  With such an LPRM arrangement, when the fold is centered on the 1/8 symmetry axis 53, the LPRM 33 is arranged at all corners except for the outer peripheral portion of the core plane of the control rod cell 31 of the core 7. By utilizing the symmetry of the 1/8 symmetry axis 53, it is reduced to 52.3% with respect to the position of each corner (256 places in total) of the control rod cell 31 excluding the outermost peripheral position of the core plane. Will be.

(Configuration of chimney lattice flow path)
Next, a detailed arrangement configuration of the lattice flow path 11a in the cross section of the chimney 11A installed on the core 7 will be described with reference to FIGS.
In the nuclear reactor 1 of the present embodiment, as shown in FIG. 4A which is a plan view taken along the line HH in FIG. 1, the lattice flow path 11a has a flow path partition shown by a solid line in the figure. There is a lattice flow path 11a indicated by a right oblique line portion surrounded by 11b, and a lattice flow path 11a indicated by a left oblique line portion surrounded by a flow path partition wall 11b and a chimney cylinder 11d. Such a cross-sectional arrangement of the lattice channel 11a is such that the central lattice channel 11a (C) including the central control rod cell 31 (C) located at the center of the core plane is connected to the central control rod cell 31 (C). The grid channels 11a are arranged so as to be located in the lower left corner, and in principle, each grid channel 11a includes a 2 × 2 array of control rod cells 31 in the central region of the core plane. Therefore, as shown to (a), arrangement | positioning of the lattice flow path 11a is mirror-symmetrical arrangement | positioning with respect to the 1/8 symmetry axis 53 of a core plane.
In addition, the flow path partition wall 11b of the lattice flow path 11a has a shape that does not cross the lattice hole 32 indicated by the middle thick solid square in the upper lattice plate 23. The control rod cell 31 of the L2 part in (a) is enlarged and shown in (b).

(Effects of the relationship between Chimney's grid channel arrangement and LPRM arrangement)
FIG. 5A shows the LPRM arrangement [see FIG. 3A] and the arrangement of the chimney 11A lattice channels 11a [see FIG. 4A] in an overlapping manner. The control rod cell 31 of the L3 part in (a) is enlarged and shown in (b). In the figure, the same components as those described in FIGS. 3 and 4 are denoted by the same reference numerals, and description thereof is omitted.
As shown to (a), the arrangement | positioning of the grating | lattice flow path 11a is mirror-symmetrical with respect to the 1/8 symmetry axis 53, and when LPRM33 folds around the 1/8 symmetry axis 53 as an axis | shaft, The LPRM 33 is arranged at each corner except for the outer peripheral portion of the core plane.
The loading pattern of the fuel assembly 21 in the core 7, that is, the concentration of the fuel assembly 21, the arrangement of the design type of the flammable poison, the distribution of the fuel assembly average burnup, etc. are mirror-symmetric with respect to the 1/8 symmetry axis 53. When arranged so that the void distribution in the cross section of the chimney 11A is mirror-symmetric with respect to the 1/8 symmetry axis 53 due to the arrangement configuration of the lattice channels 11a in the chimney 11A. Therefore, coupled with the loading pattern of the fuel assembly 21 of the core 7 being mirror-symmetrical with respect to the 1/8 symmetry axis 53, the void of the gas-liquid two-phase flow into the lattice channel 11a of the cross-section of the chimney 11A. The distribution is also mirror-symmetric with respect to the 1/8 symmetry axis 53, and even if the void distribution in the cross section of the chimney 11A affects the flow distribution to the fuel assemblies 21, the power distribution in the core radial direction (the individual fuel assemblies 21 as a whole) The power distribution to the fuel assemblies 21 in the core plane is also mirror symmetric with respect to the 1/8 symmetry axis 53.

  As described above, according to the present embodiment, the LPRM 33 is disposed only at one of the mirror symmetry positions with respect to the 1/8 symmetry axis 53 of the core plane among all the corner positions of the control rod cell 31, and on the other side. Even if the LPRM 33 is not arranged, the neutron flux at the location where the LPRM 33 is not arranged at the corner of the control rod cell 31 can be replaced with the neutron flux measured by the LPRM 33 at a symmetrical position on the core plane. Even with a small number of LPRMs 33, an accurate in-core power distribution calculation equivalent to the conventional forced circulation type BWR can be performed.

(First Modification of LPRM Arrangement in First Embodiment)
Next, a first modification of the LPRM arrangement in the first embodiment will be described with reference to FIGS. 6 and 7.
Normally, the nuclear reactor 1 designs a loading pattern to the core of the fuel assemblies 21 so that the radial power distribution of the core 7 becomes flat. In addition, the pattern of the insertion position of the control rod 24 for controlling the surplus reactivity during operation and adjusting the power distribution is also designed so that the radial power distribution of the core 7 becomes flat. The void ratio at the core outlet of the assemblies 21, that is, at the upper end outlet of the channel box, does not vary greatly between the fuel assemblies 21 except for the fuel assemblies arranged on the outer peripheral portion of the core plane.
Therefore, the void ratio is substantially uniform between the lattice flow paths 11a in the central region of the remaining core plane excluding the lattice flow paths 11a (see FIG. 7) including the fuel assemblies 21 on the outer periphery of the core plane. It can be said that the asymmetry of the arrangement of the lattice flow paths 11a in the 11A cross section has little influence on the flow distribution and output between the fuel assemblies 21 at the symmetrical positions of the loading pattern on the core plane.

Therefore, in the LPRM arrangement shown in FIG. 6A, all LPRMs 33 on the 1/8 symmetry axis 53 are deleted from the LPRM arrangement shown in FIG. 3, and the LPRM 33 on the 1/8 symmetry axis 53 in FIG. Starting from the position, every other LPRM 33 arranged in the corners of the control rod cell 31 along the X-axis 51 direction and the Y-axis 52 direction is deleted. In this case, 64 LPRMs 33 are arranged on the core plane, and the LPRM 33 is reduced to 25% with respect to the positions of the respective corners of the control rod cell 31 excluding the outermost peripheral portion of the core plane (256 places in total). Will be placed.
FIG. 7A shows the LPRM arrangement of this modification and the arrangement of the lattice channels 11a of the chimney 11A in an overlapping manner. The arrangement of the lattice channels 11a is mirror symmetric with respect to the 1/8 symmetry axis 53, and when the LPRM 33 is folded back on the 1/8 symmetry axis 53, the core plane outer peripheral portion at the diagonal position of the control rod cell 31 on the core plane is obtained. It will be placed in all corners except. The control rod cells 31 of L4 and L5 in FIGS. 6A and 7A are shown in FIGS. 6B and 7B.

  In this way, when the core radial power distribution is flat, the asymmetry of the arrangement of the lattice channels 11a in the cross section of the chimney 11A shown in FIG. 4 with respect to the core plane has little influence on the core radial power distribution. Even in this modified example of the LPRM arrangement, in the power distribution calculation in the core, the accuracy equivalent to that of the conventional forced circulation type BWR can be ensured with the LPRM 33 which is smaller than the LPRM arrangement of the first embodiment.

(Second Modification of LPRM Arrangement in First Embodiment)
Next, a second modification of the LPRM arrangement in the first embodiment will be described with reference to FIG. In the LPRM arrangement according to the first embodiment shown in FIG. 3, the present modification is the core plane peripheral region (from the outermost periphery of the core plane to the fourth control rod cell 31 in the control rod cell 31 unit). Area) is an LPRM arrangement in which LPRMs 33 are arranged only at corners that are diagonal to the control rod cell 31 even when folded around the 1/8 symmetry axis 53, and LPRMs 33 are subtracted in the peripheral area around the core plane. In this case, as shown in FIG. 8A, the LPRM 33 is disposed on the 106 core plane. The control rod cell 31 of L6 part of (a) is shown in (b).

In this LPRM arrangement, the output of the fuel assembly 21 tends to be high in the central region of the core, and the margin for the operation limit values of the thermal limit values (MCPR, MLHGR) tends to be small. This is a modification of the LPRM arrangement from the viewpoint of taking as much as possible and improving the accuracy of the power distribution calculation in the core central region as compared with the first modification.
In this modification, the number of corners of the control rod cell 31 excluding the outermost peripheral portion of the core plane is reduced to 41.4% with respect to the positions of the corners (256 places in total).

(Third Modification of LPRM Arrangement in First Embodiment)
Next, a third modification of the LPRM arrangement in the first embodiment will be described with reference to FIG. This modification is different from the LPRM arrangement in the first embodiment shown in FIG. 3 in the core plane central region (in terms of the control rod cell 31 unit, the control rod on the center side from the fourth layer from the outermost periphery of the core plane. The region of the cell 31) is obtained by subtracting the LPRM 33 in the central region of the core plane so that the LPRM 33 is arranged only at the corner at a diagonal position with respect to the control rod cell 31 even if the 1/8 symmetry axis 53 is folded back. It is. In the case of the LPRM arrangement shown in FIG. 9A, the LPRM 33 is arranged in the 92 core plane. The control rod cell 31 of L7 part of (a) is shown in (b).

This is contrary to the second modified example of the LPRM arrangement. In the central region of the core, the output of the fuel assembly 21 tends to be high, and the margin for the operation limit value of the thermal limit values (MCPR, MLHGR) tends to be small. From the trend, it is normal to design the core radial power distribution flat, and conversely, in the peripheral region outside it, the radial power tends to be lower than the central region due to the effect of neutron leakage from the core, In many cases, a large difference in output between the fuel assemblies 21 is used and combined, and the difference in the exit void ratio of the fuel assemblies 21 is also large. Therefore, as many neutron flux measurement signals as possible from the LPRM 33 are obtained. This is a modification of the LPRM arrangement from the viewpoint of improving the accuracy of the fuel assembly output calculation in the core peripheral region as compared with the first modification.
In this modified example, the number of corners of the control rod cell 31 excluding the outermost peripheral portion of the core plane is reduced to 35.9% with respect to the positions of the corners (256 places in total).

<< Second Embodiment >>
Next, a second embodiment in which the arrangement of the lattice channels 11a in the first embodiment is modified will be described with reference to FIG. The same components as those in the first embodiment are denoted by the same reference numerals and description thereof is omitted.
In the present embodiment, as shown in FIG. 10A, the arrangement of the cross section of the lattice flow path 11a of the chimney 11B is a central lattice flow including the central control rod cell 31 (C) located at the center of the core plane. The path 11a (C) includes a central control rod cell 31 (C) so as to be located in the lower right corner, and each lattice channel 11a is basically a 2 × 2 array of control rod cells 31 in the central region of the core plane. Is an arrangement including Therefore, as shown to (a), arrangement | positioning of the lattice flow path 11a is mirror-symmetric arrangement | positioning with respect to the 1/8 symmetry axis 54 of a core plane. The example shown in (a) of FIG. 10 uses the mirror symmetry of the chimney 11B cross-section relative to the 1/8 symmetry axis 54 and the mirror symmetry of the loading pattern on the core plane, except for the outermost peripheral portion. This is an LPRM arrangement corresponding to the LPRM arrangement at all the corners of the diagonal position of the control rod cell 31. In other words, the control rod cells 31 except for the outermost peripheral portion of the core plane are arranged by being reduced to 25% with respect to the positions of the respective corners (256 places in total). The control rod cell 31 of L8 part of (a) is shown in (b).
The arrangement of the lattice channels 11a in the cross section of the chimney 11B is simply 90 ° in the counterclockwise direction of the arrangement of the lattice channels 11a in the cross section of the chimney 11A in FIG. 4 in the first embodiment. For the arrangement of the lattice channels 11a, the LPRM arrangement shown in FIG. 3 of the first embodiment, the third of the first embodiment, A modification of the fourth LPRM arrangement may be combined with an LPRM arrangement rotated 90 ° counterclockwise around the core plane center. The operation and effect are the same as those of the first embodiment and the modification of the first embodiment.

<< Third Embodiment >>
Next, a nuclear reactor 1 according to a third embodiment of the present invention will be described with reference to FIG. The configuration of the longitudinal section of the nuclear reactor 1 of the present embodiment, the arrangement of the fuel assemblies 21 and the control rods 24 on the core plane are the same as those of the first embodiment (see FIGS. 1 and 2). The same components as those in the first embodiment are denoted by the same reference numerals and description thereof is omitted.
In the present embodiment, as shown in FIG. 11A, the arrangement of the cross section of the lattice channel 11a of the chimney 11C is different from that of the first embodiment. A central lattice channel 11a (C) including a central control rod cell 31 (C) positioned at the center of the core plane is positioned at the center of the 3 × 3 array of control rod cells 31. In addition, each lattice channel 11a is basically arranged in the central region of the core plane so as to include the 3 × 3 array of control rod cells 31. Therefore, as shown in (a), the arrangement of the lattice flow paths 11a is symmetric with respect to the X axis 51 and the Y axis 52 with respect to the X axis 51 and the Y axis 52, and 1/4 axis 90 ° rotational symmetry. , 1/2 core mirror symmetry, 1/2 core 180 ° rotationally symmetrical arrangement configuration. Further, the 1/8 symmetry axis 53 and 54 are arranged in a 1/8 core mirror symmetry, a 1/2 core mirror symmetry, and a 1/2 core 180 ° rotational symmetry.
The LPRM arrangement combined with this is based on the symmetry with respect to the X axis 51, the Y axis 52, and the 1/8 symmetry axes 53 and 54 in the core plane, and all the corners of the control rod cell 31 except for the outermost peripheral portion. This is an LPRM arrangement equivalent to the LPRM arrangement. In other words, the control rod cells 31 except for the outermost peripheral portion of the core plane are arranged by being reduced to 25% with respect to the positions of the respective corners (256 places in total). The control rod cell 31 of L9 part in (a) is shown in (b).

  In the present embodiment, since the grid flow passage 11a has the same symmetry as the symmetry of the core plane, there is no restriction on the loading pattern of the core fuel assembly 21 like the chimney 11A in the first embodiment. Even if the void fraction in the lattice flow path 11a affects the flow distribution and output of the fuel assembly 21 corresponding to the lattice flow path 11a, the symmetry is not broken, so the number of LPRMs 33 is smaller. The core performance can be evaluated with the same accuracy as the forced circulation type BWR.

<< Fourth Embodiment >>
Next, a nuclear reactor 1 according to a fourth embodiment of the present invention will be described with reference to FIG. The configuration of the longitudinal section of the nuclear reactor 1 of the present embodiment, the arrangement of the fuel assemblies 21 and the control rods 24 on the core plane are the same as those of the first embodiment (see FIGS. 1 and 2). The same components as those in the first embodiment are denoted by the same reference numerals and description thereof is omitted.
In the present embodiment, as shown in FIG. 12A, the arrangement of the cross section of the lattice flow path 11a of the chimney 11D is different from that of the first embodiment. The central lattice channel 11a (C) including the central control rod cell 31 (C) located at the center of the core plane includes only one central control rod cell 31 (C). The other lattice channels 11a are arranged to include the control rod cells 31 in a 2 × 2 arrangement in principle in the central region of the core plane. At this time, the lattice flow path 11a indicated by the left hatched portion of the lattice flow path 11a substantially corresponding to the quarter core of the first quadrant includes the control rod cell 31 on the X axis 51 so as to include the control rod cell 31 in the second quadrant. The lattice flow path 11a indicated by the left oblique line in the lattice flow path 11a substantially corresponding to the 1/4 core substantially corresponds to the 1/4 core in the third quadrant so as to include the control rod cell 31 on the Y axis 52. Among the lattice flow paths 11a corresponding to the quarter core of the fourth quadrant so that the lattice flow paths 11a indicated by the left oblique lines in the lattice flow paths 11a include the control rod cells 31 on the X axis 51. The lattice channel 11a indicated by the left hatched portion is configured to include the control rod cell 31 on the Y axis 52. The control rod cell 31 of L10 part in (a) is shown in (b).

Therefore, as shown in (a), the arrangement of the lattice channels 11a is a 1/4 core 90 ° rotationally symmetric and 1/2 core 180 ° rotationally symmetric arrangement with respect to the X axis 51 and the Y axis 52. . Further, the ½ core is 180 ° rotationally symmetric with respect to the １ ／ symmetry axes 53 and 54.
The LPRM arrangement combined with this is based on the symmetry with respect to the X axis 51, the Y axis 52, and the 1/8 symmetry axes 53 and 54 in the core plane, and all the corners of the control rod cell 31 except for the outermost peripheral portion. This is an LPRM arrangement equivalent to the LPRM arrangement. In other words, the control rod cells 31 except for the outermost peripheral portion of the core plane are arranged by being reduced to 25% with respect to the positions of the respective corners (256 places in total).

1 is a longitudinal sectional view of a natural circulation boiling water reactor according to a first embodiment of the present invention. It is a core top view of the GG arrow of FIG. It is a core top view which shows LPRM arrangement | positioning in 1st Embodiment. It is a cross-sectional arrangement drawing of the chimney lattice channel in the first embodiment. It is a figure explaining the relationship between the cross-sectional arrangement | positioning of the chimney lattice flow path of 1st Embodiment, and LPRM arrangement | positioning. It is a core top view which shows the 1st modification of LPRM arrangement | positioning in 1st Embodiment. It is a figure explaining the relationship between the cross-sectional arrangement | positioning of the chimney lattice flow path of 1st Embodiment, and the LPRM arrangement | positioning of a 1st modification. It is a figure explaining the relationship between the cross-sectional arrangement | positioning of the chimney lattice flow path of 1st Embodiment, and the LPRM arrangement | positioning of a 2nd modification. It is a figure explaining the relationship between the cross-sectional arrangement | positioning of the chimney lattice flow path of 1st Embodiment, and the LPRM arrangement | positioning of a 3rd modification. It is a figure explaining the relationship between the cross-sectional arrangement | positioning of the chimney lattice flow path of 2nd Embodiment, and LPRM arrangement | positioning. It is a figure explaining the relationship between the cross-sectional arrangement | positioning of the chimney lattice flow path of 3rd Embodiment, and LPRM arrangement | positioning. It is a figure explaining the relationship between the cross-sectional arrangement | positioning of the chimney lattice flow path of 4th Embodiment, and LPRM arrangement | positioning.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Natural circulation boiling water reactor 7 Core 11, 11A, 11B, 11C, 11D Chimney 11a Lattice flow path 11a (C) Central lattice flow path 11b Flow path partition wall 21 Fuel assembly 23 Upper lattice plate 24 Control rod 31 Control Rod cell 31 (C) Central control rod cell 32 Lattice hole 33 LPRM detector assembly (neutron detector)
51 X axis (Axis of symmetry)
52 Y axis (Axis of symmetry)
53, 54 1/8 symmetry axis

Claims (2)

In a natural circulation boiling water reactor comprising a core loaded with a plurality of fuel assemblies, a neutron detector installed in the core, and a chimney installed on the core,
The upper lattice plate for determining the horizontal position of the fuel assembly in the core constitutes one control rod cell by arranging a control rod at the center position of the fuel assembly in a 2 × 2 array. Is arranged in the center of the cross section of the core, and further, a number of the lattice holes are arranged in the X-axis direction and the Y-axis direction of a right-angle XY coordinate,
The core has an arrangement based on the concentration of the fuel assembly, the design type of the flammable poison, and the staying period in the reactor , at least the X axis and the Y axis passing through the center of the cross section of the core in the right-angle XY coordinates. It can be loaded mirror-symmetrically with respect to two 1/8 symmetry axes that form an angle of 45 ° with respect to both
The chimney is configured so that a lattice channel partitioned by a channel partition includes a 2 × 2 array of lattice holes of the upper lattice plate, and the cross-sectional arrangement of the lattice channels is the two ones. / 8 is configured to have one of the symmetry axes as a symmetry axis,
In cross-section of said core, of the symmetrical position corresponding to the mirror symmetry of the one-eighth the axis of symmetry of the cross-section arrangement of the lattice channel, and characterized in that a neutron detector in at least one location Natural circulation boiling water reactor. In a natural circulation boiling water reactor comprising a core loaded with a plurality of fuel assemblies, a neutron detector installed in the core, and a chimney installed on the core,
The upper lattice plate for determining the horizontal position of the fuel assembly in the core constitutes one control rod cell by arranging a control rod at the center position of the fuel assembly in a 2 × 2 array. Is arranged in the center of the cross section of the core, and further, a number of the lattice holes are arranged in the X-axis direction and the Y-axis direction of a right-angle XY coordinate,
The core has an arrangement based on the concentration of the fuel assembly, the design type of the flammable poison, and the period of stay in the reactor, at least the X axis and the Y axis passing through the center of the cross section of the core in the orthogonal XY coordinate. With respect to a quarter core obtained by dividing the cross section of the core into four parts, the center of the cross section of the core can be loaded with 90 ° rotational symmetry,
The chimney, a lattice flow passages divided by the flow path partition walls, the grid channel corresponding to the center of the cross section of the core, only only contains one said grid hole of the cross-section center of the core, other the grid passages are configured such that the configuration be Rutotomoni, arrangement of the 90 ° rotational symmetry with respect to the front SL 1/4 core to contain 2 × 2 array of grating holes in the top guide ,
In the cross section of the core, a neutron detector is disposed at at least one of the symmetrical positions corresponding to the 90 ° rotational symmetry with respect to the 1/4 core of the cross section of the lattice channel. A natural circulation boiling water reactor characterized by that .

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