JP3977979B2 - Fuel assembly - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to a fuel assembly loaded in a nuclear reactor core.
[0002]
[Prior art]
For example, the core of a boiling water reactor has a large number of fuel assemblies containing fuel bundles inside a rectangular tube channel box, and each fuel assembly has a nuclear fission such as uranium or plutonium. A large number of fuel rods filled with fuel pellets containing active substances and arranged in a square grid of n rows and n columns (n = 8, 9, 10, 11,...), And an upper tie plate that supports them vertically And a lower tie plate and a spacer or the like for maintaining a space between the fuel rods.
[0003]
This core is stopped after a predetermined period (= 1 cycle) of operation, and a part of the loaded fuel assembly is taken out and replaced with a new fuel assembly. The fuel loading of the new fuel assembly at the time of replacement is set so that the amount of fissile material necessary to keep the reactor critical for one cycle is loaded, but at the end of the operation period It is set in advance to be surplus. In other words, the reactor is in a state of exceeding criticality except at the end of operation. Therefore, the boiling water reactor core absorbs this extra neutron generated by the control rods inserted between the fuel assemblies and the combustible absorber added to the fuel of some fuel rods. Thus, the critical state is maintained throughout the operation period.
[0004]
In addition, as a combustible absorbent material, for example, a substance having a large thermal neutron absorption cross section such as gadolinia is used, but these are consumed by absorption of the thermal neutron, and the effect is reduced with combustion. Therefore, the combustible absorbent material is mainly used to suppress the excess reactivity (excess reactivity) at the early stage of combustion.
[0005]
Here, in recent years, from the viewpoints of extending the continuous operation period, effectively using fuel resources, and reducing the amount of spent fuel waste, the design of the fuel assembly has been increased (for example, target takeout average burnup) 40 GWd / t or more). In order to increase the burnup, it is desirable to increase the amount of fissile material loaded per fuel assembly (ie, increase the fuel enrichment).
[0006]
However, if the fuel enrichment is increased, the surplus reactivity increases, so the amount of combustible absorbent necessary to suppress the surplus reactivity also increases. Therefore, the fissile material in the fuel rod containing the combustible absorbent You will have to reduce the amount of loading. Combustible absorbents mixed with fuel rods tend to reduce the thermal conductivity and increase the fuel temperature (ie, the thermal margin for ensuring the integrity of the fuel rods decreases). In the fuel rod containing material, it is necessary to devise in advance to reduce the fuel enrichment and suppress the output. In this sense, the load of the fissile material of the fuel rod containing the combustible absorbent material must be reduced.
[0007]
As described above, even if the load of fissile material in the fuel rod containing the combustible absorbent is reduced, it is only necessary to increase the amount of fissile material loaded in the other fuel rods. From the standpoint of securing thermal margin, etc., there is an upper limit on the enrichment in the fuel assembly. For this reason, there is a limit to increasing the amount of fissile material loaded on other fuel rods. As a result, the average enrichment of the fuel assembly must be reduced, and the amount of fissile material loaded per fuel assembly is limited. Must be reduced.
[0008]
On the other hand, in recent years, from the viewpoint of nuclear fuel recycling in nuclear power plants, plutonium extracted from spent fuel by reprocessing is mixed with uranium, and as uranium / plutonium mixed oxide fuel (hereinafter referred to as MOX fuel as appropriate), It is proposed to use it in light water reactors (so-called pull thermal plan). MOX fuel is more than normal uranium fuel because its fissile material, plutonium 239 and plutonium 241 has a larger thermal neutron absorption cross section than uranium 235 and neutron absorption by plutonium 240 is larger than uranium 238. The ratio of thermal neutrons decreases and the neutron spectrum becomes hard.
[0009]
Here, usually usedGadoliniaCombustible absorbers such as these have a large absorption cross-section for thermal neutrons, so their combustion characteristics (ie absorption characteristics) are strongly dependent on the neutron spectrum, and the lower the neutron average energy (softer the neutron spectrum), the more the combustion As the neutron absorption effect increases, the neutron absorption effect becomes smaller as the neutron spectrum becomes harder. Therefore, the neutron absorption effect of the combustible absorbent is lower in the MOX fuel assembly equipped with the MOX fuel than the uranium fuel assembly equipped only with the uranium fuel, and more combustible in order to suppress the excess reactivity. This will reduce the amount of fissile material loaded per unit of fuel assembly.
From the above background, conventionally, for example, as described in JP-A-58-113785, JP-A-2-147890, and JP-A-10-10257, a combustible absorbent is used as a fuel rod. The method of making it contain in other fuel assembly structures is proposed.
[0010]
In the fuel assembly described in Japanese Patent Application Laid-Open No. 10-10257, a combustible absorbent material is provided on the outer peripheral side or the inner peripheral side of the channel box surrounding the square lattice array of fuel rods. Specifically, metals, alloys, intermetallic compounds, ceramics containing gadolinium, cadmium, samarium, boron, silver, indium, hafnium, etc. (or solid solution in a supersaturated state) are arranged, or channel boxes are arranged. The gadolinium or the like is added as an alloy element (or dispersed as a metal, an intermetallic compound, an oxide, a hydride, a nitride, or the like) to a constituent base material (zirconium alloy or the like).
[0011]
In the fuel assembly described in Japanese Patent Laid-Open No. 2-147890, in FIG. 6, a cross-shaped subchannel through which non-boiling water flows is provided in the channel box, and non-boiling is performed on the inner peripheral side of the wall surface of the subchannel. A film of flammable absorbent (zirconium boride) is coated so as to come into contact with water.
In FIG. 7, the fuel bundle is subdivided into a plurality of sub-bundles, sub-channels are arranged so as to surround each sub-bundle instead of the channel box, and non-boiling water is allowed to flow in each sub-channel. A coating of a flammable absorbent (zirconium boride) is coated on the inner peripheral side of the wall surface of the subchannel so as to be in contact with non-boiling water. At this time, by providing the combustible absorbent facing the non-boiling water, the initial excess reaction can be sufficiently suppressed even with boron having a smaller neutron absorption cross section than gadolinium.
[0012]
In the fuel assembly described in Japanese Patent Application Laid-Open No. 58-113785, a cladding tube of a moderator rod (water rod) disposed at the radial center of the fuel bundle for the purpose of flattening the fuel assembly radial output distribution Gadolinium, cadmium, samarium, eurobium, dysprosium, boron (boron), indium, hafnium between the outer and inner tubesEtc.Filled with flammable absorbent material.
[0013]
[Problems to be solved by the invention]
In general, boiling water reactors use light water (cooling water) as a coolant to remove heat generated by fission, but this cooling water also plays a role as a moderator for neutrons. (In detail, the hydrogen atom number density) has the property of decelerating neutrons more.
[0014]
Here, in the boiling water reactor, the flow path of the cooling water is divided by the channel box, and the cooling water flowing between the fuel rods in the channel box is free from bubbles generated by boiling due to heat generated from the fuel rods. It is a gas-liquid two-phase flow (hereinafter referred to as boiling water as appropriate), and the cooling water (gap water) flowing outside the channel box (in other words, between two adjacent fuel assemblies) is a liquid one-phase flow that does not contain bubbles. (Hereinafter referred to as non-boiling water as appropriate), and there is a difference in density of cooling water. For this reason, the fuel assembly of the boiling water reactor can be distributed such that the thermal neutron flux is lower in the central portion than in the outer peripheral portion near the channel box.
[0015]
Usually, boiling water reactor fuel is a fissionable material that has the property of being prone to fission mainly by low energy thermal neutrons, and fuel rods at high thermal neutron flux positions tend to produce high power, An output distribution is generated in the fuel assembly such that the fuel rod output is relatively high at the outer peripheral portion near the channel box having a large water density, and the fuel rod output is relatively low at the central portion. Therefore, in order to improve the distribution of thermal neutron flux as much as possible, normally, non-boiling water that does not contain bubbles is allowed to flow through the center of the fuel assembly in the radial direction.TheA moderator rod (so-called water rod) is often installed.
[0016]
On the other hand, as described above, the combustion of the combustible absorber strongly depends on the neutron spectrum, and the lower the neutron average energy (softer the neutron spectrum), the more the neutron absorption effect becomes, while the neutron absorption effect increases. The harder the spectrum, the smaller the neutron absorption effect. Therefore, in general, during normal operation where the water density becomes relatively small and the neutron spectrum becomes hard, the neutron absorption effect becomes small, the water density becomes relatively large and the neutron spectrum becomes small.SoftThe neutron absorption effect becomes large at the cold stop.
That is, when shifting from normal operation to cold shutdown, the amount of neutron absorption increases as the reactivity increases. The reactivity difference (neutron infinite multiplication factor difference) is moderated to some extent, and the hot / cold swing is reduced.
[0017]
However, there is a large difference in the degree of change in water density between the normal operation and the cold shutdown between the non-boiling water region outside the channel box and the boiling water region inside the channel box.
[0018]
In the non-boiling water area outside the channel box, the water density changes to some extent depending on the temperature as described above, but there is no change in phase such as the generation or disappearance of voids (bubbles) like the boiling water area inside the channel box. During normal operation of the neutron spectrum (relative to the spectrumhard) And cold shutdown (relative to spectrum)soft) Is smaller than the boiling water area inside the channel box.
[0019]
In the prior art described in the above-mentioned JP-A-10-10257, a flammable absorbent material is disposed near the non-boiling water region outside the channel box. The change in is smaller than the change in the center of the channel box. Therefore, the above-described hot / cold swing reducing action of the flammable absorbent, that is, the action of increasing the neutron absorption amount when shifting from the normal operation to the cold shutdown is reduced, so that the flammable absorbent is attached to the fuel rod. Hot cold swing tends to increase as compared with the case where it is contained.
In addition, if a flammable absorber is contained in the channel box, the channel box and the control rod face each other at a very close distance, so that the mutual neutron absorption effect cancels out, and there is a problem that the value of the control rod is reduced. .
[0020]
In the prior art described in Japanese Patent Laid-Open No. 2-147890, since a combustible absorbent is disposed in the non-boiling water region inside the subchannel, the normal neutron energy spectrum is the same as in Japanese Patent Laid-Open No. 10-10257. The change between the operation and the cold stop is small, and the hot / cold swing tends to increase.
[0021]
In the prior art described in Japanese Patent Laid-Open No. 58-113785, the structure is such that the cladding of the moderator rod is filled with a combustible absorbent, but the size of the moderator rod is usually not so large, and at most square. The size is such that seven fuel rods in a grid array are replaced. To make it larger, the space for fuel rods must be reduced. Because of these restrictions, the area where the moderator rod cladding tube is in contact with the surrounding cooling water usually has a certain upper limit, and as a result, the amount of neutron absorption is also limited. Difficult to do.
[0022]
The present invention has been made in view of the above-described problems of the prior art, and an object of the present invention is to provide a fuel assembly capable of reducing hot and cold swing while sufficiently suppressing excess reactivity. is there.
[0023]
[Means for Solving the Problems]
  (1) In order to achieve the above object, the present invention provides a neutron provided with a combustible absorber in a fuel assembly in which a plurality of fuel rods filled with a fissile material are arranged in a square grid of n rows and n columns. Absorbing means is provided, and the neutron absorbing means is a channel box at least partly arranged linearly in the row direction or the column direction in the gap between the lattice positions of the lattice-like array and surrounding the square lattice-like array Is placed in contact with the two-phase cooling water insideThe at least part of the neutron absorption means is at least one flat plate means arranged so that both surfaces thereof are in contact with the two-phase cooling water in the channel box.
[0024]
In the present invention, at least a part of the neutron absorbing means is in contact with the two-phase flow cooling water (boiling water) in the channel box. In the two-phase flow region in this channel box, when the reactor moves from normal operation to cold shutdown, a phase change occurs in addition to the temperature change, the generation and disappearance of voids (bubbles). Compared with the non-boiling water region of neutron spectrum during normal operation (relative spectrumhard) And cold shutdown (relative to spectrum)soft) And a large change. As a result, compared to the conventional structure in which a flammable absorber is disposed in the vicinity of the non-boiling water region, the neutron absorption amount of the flammable absorber can be significantly increased when shifting from normal operation to cold shutdown. Accordingly, the hot / cold swing can be greatly reduced.
[0025]
In general, the larger the area in which the combustible absorbent is in contact with the surrounding two-phase flow cooling water, the larger the neutron absorption amount. In the present invention, at least a part of the neutron absorbing means described above is arranged in the lattice arrangement. Combustible absorption in the cladding of the moderator rod by securing a sufficiently large area in contact with the cooling water of the two-phase flow in the channel box, which is arranged linearly in the gap between the grid positions in the row direction or the column direction. Unlike the conventional structure filled with the material, the reactivity suppression effect by the combustible absorbent material can be sufficiently secured.
[0027]
  Furthermore, at least a part of the neutron absorption means is at least one flat plate means arranged so that both surfaces thereof are in contact with the two-phase cooling water in the channel box.Therefore, it is possible to realize a configuration that can be manufactured at a low cost and can easily secure a sufficient contact area with the two-layer flow cooling water.
[0028]
  (2)the above(1)More preferably, a plurality of the flat plate means are provided, and the plurality of flat plate means includes row direction flat plate means arranged in the row direction of the lattice arrangement and the column of the lattice arrangement. Row direction plate means arranged in the direction.
[0029]
Thereby, a further sufficient contact area to the two-layer flow cooling water can be ensured.
[0030]
  (3)the above(2)Preferably, the row-direction flat plate means and the column-direction flat plate means are configured as a lattice-like plate member coupled in a substantially perpendicular state.
[0031]
  (4)In the above (1), preferably, n = 8 or 9, and the portion of the neutron absorption means on the center side in the radial direction of the fuel assembly is at least the second layer lattice from the outermost layer of the square lattice arrangement. It is arranged closer to the center side in the fuel assembly radial direction than the position.
[0032]
  In the two-phase flow region in the channel box, the fuel assemblyThe radially outer side, that is, the channel box sideDue to the influence of the non-boiling water region outside the channel box, the change of the neutron spectrum between normal operation and cold shutdown is reduced.Therefore, in the case of the fuel rod lattice array of 8 rows 8 columns or 9 rows 9 columns, if the fuel assembly radial direction center side from the vicinity of the lattice position of the second layer from the outermost layer of the square lattice array,The influence of the non-boiling water region outside the channel box is reduced, and the change of the neutron spectrum between normal operation and cold shutdown is increasedCan be estimated. In the present invention, by disposing at least the center side portion of the neutron absorbing means on the center side from the second layer,It can be surely placed at a position where the neutron spectrum changes greatly between normal operation and cold shutdown,Hot and cold swing can be reliably suppressed.
[0033]
  (5)In order to achieve the above object, the present invention also provides a fuel assembly in which a plurality of fuel rods filled with a fissile material are arranged in a square lattice, and includes a combustible absorbent, and the outer periphery 4 of the square lattice arrangement. It has four flat plate means respectively extended toward the center of the fuel assembly radial direction from the middle point of the side, the four flat plate means,Both sidesIt arrange | positions so that the two-phase flow cooling water in the channel box surrounding the said square grid | lattice arrangement | sequence may be touched.
[0034]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
[0035]
FIG. 2 is a longitudinal sectional view showing the overall structure of the boiling water nuclear reactor fuel assembly according to the present embodiment, and FIG. 3 is a cutaway perspective view showing the lower structure of the fuel assembly shown in FIG. FIG. 1 is a cross-sectional view taken along the line II in FIG.
[0036]
1 to 3, the fuel assembly 1 according to the present embodiment is loaded into the core of a boiling water reactor, arranged in a square grid of 8 rows and 8 columns, and a fissile fuel material therein. 60 fuel rods 2 filled with uranium-235, and fuel rods 2 for the purpose of increasing the amount of water in the central region in the radial direction (vertical direction or horizontal direction in FIG. 1) of a square lattice array Four water rods (moderator rods) 3 arranged in a region where one can be arranged, and a rectangular tube-type channel box 4 surrounding the fuel bundle formed by the fuel rods 2 and the water rods 3. And.
[0037]
The upper and lower portions of the fuel bundle are supported by an upper tie plate 5 and a lower tie plate 6, respectively, and spacers 7 for maintaining the distance between the fuel rod 2 and the water rod 3 at a plurality of axial positions of the fuel bundle. Is provided. Although not specifically shown or described in detail, the fuel rod 2 is composed of a plurality of types of fuel rods having different enrichment distributions, as is well known as this type of fuel assembly. Then, the axial output peaking is flattened by providing an appropriate axial concentration distribution for each type, or the radial output peaking is flattened by appropriately devising the arrangement of various fuel rods. . At this time, for the same purpose, a so-called short length fuel rod (also referred to as a partial length fuel rod) having an effective fuel length shorter than that of the normal fuel rod 2 may be appropriately disposed.
[0038]
The main part of the fuel assembly 1 of the present embodiment as described above is that the combustible absorbent containing member 8 including the combustible absorbent (not shown, details will be described later) is provided in the channel box 4. is there.
[0039]
The combustible absorbent-containing member 8 is disposed inside the channel box 4, that is, in a region where the gas-liquid two-phase flow flows (= boiling water region), and has a substantially cross shape (that is, horizontal) when viewed from the longitudinal direction of the channel box 4. The cross-sectional shape is a substantially cross shape).
[0040]
That is, the combustible absorbent-containing member 8 is composed of four flat plate members 8a, 8b, 8c, and 8d extending from the midpoint of the four outer sides of the square lattice array toward the center of the fuel assembly radial direction. Has been. These four flat plate members 8a to 8d are all arranged not in the lattice position of the lattice arrangement in which the fuel rods 2 are arranged, but in the gap between the lattice positions, as shown in FIG. The channel box 4 is arranged so as to be equally divided into four square areas 9A, 9B, 9C, 9D. With such an arrangement, both surfaces of the flat plate members 8 a to d are in contact with the two-phase flow cooling water in the channel box 4.
[0041]
Further, these four flat plate members 8a to 8d are arranged in a straight line in the row direction (for example, the horizontal direction in FIG. 1) of the lattice arrangement and the column direction (for example, FIG. The flat plate members 8b and 8d arranged in a straight line in the vertical direction in FIG. 1 are joined in a substantially perpendicular state to each other, thereby constituting a combustible absorbent containing member 8 which is a lattice plate member. . That is, the radial center side portion of each flat plate member 8a to d reaches the fuel assembly radial central axis at the joint portion of each flat plate member 8a to d. The end portions of the flat plate members 8a to 8d in the fuel assembly radial direction (that is, the channel box 4 side) are joined to the inner peripheral surface of the channel box 4 by welding or the like, so that the combustible absorbent containing member 8 Is fixed.
[0042]
The combustible absorbent-containing member 8 itself may be manufactured by a known method in Japanese Patent Laid-Open No. 58-113785, Japanese Patent Laid-Open No. 2-147890, Japanese Patent Laid-Open No. 10-10257, and the like.
[0043]
For example, the combustible absorbent material-containing member 8 is made up of one to two material plates (for example, a plate material of zirconium alloy, etc.), one of the two plates is provided with a dent, and the flammability is provided in the dent. After the absorbent material plate is disposed, the other material plate may be joined so that the combustible absorbent material plate is embedded.
Alternatively, a slit may be provided in the longitudinal direction of the plate at the center of the thickness of the combustible absorbent-containing member 8 so that the molten or melted combustible absorbent is poured into the slit.
In the above two cases, a position or a region where there is no flammable absorbent is generated in the flammable absorbent containing member 8, but for the purpose of securing a flow area of the two-phase flow between the fuel rods, If necessary, a hole is made in a position / region where there is no flammable absorbent so that boiling water freely flows between the four regions 9A to 9D in the four channel boxes 4 partitioned by the flammable absorbent containing member 8. It may be.
[0044]
In addition, as a combustible absorbent, gadolinium, cadmium, samarium, boron, silver, indium, hafnium, or the like (or a solid solution in a supersaturated state), an alloy, an intermetallic compound, a ceramic plate, or A material obtained by adding gadolinium or the like as an alloy element (or dispersed as a metal, an intermetallic compound, an oxide, a hydride, a nitride, or the like) to the base material (zirconium alloy or the like) constituting the channel box is considered. .
[0045]
In the above description, the flat plate members 8a and 8c constitute row-direction flat plate means arranged in the row direction of the lattice-like arrangement described in the claims, and the flat plate members 8b and 8d have the lattice-like arrangement. A row direction flat plate means arranged in the row direction is configured, and these four flat plate members 8a to 8d are respectively extended from the midpoints of the four outer sides of the square lattice array toward the center of the fuel assembly radial direction. Constitutes two flat plate means.
[0046]
And the combustible absorber containing member 8 which consists of these four flat plate members 8a-d comprises a lattice-shaped plate member, and also comprises the neutron absorption means provided with the combustible absorber.
[0047]
Next, functions and effects of the fuel assembly of the present embodiment configured as described above will be sequentially described below.
[0048]
(1) Hot / cold swing reduction action
According to the fuel assembly 1 of the present embodiment, the combustible absorbent containing member 8 as a neutron absorbing means is in contact with the two-phase flow cooling water (boiling water) in the channel box 4. In the two-phase flow region in the channel box 4, when the boiling water reactor shifts from the normal operation to the cold shutdown, a phase change such as generation and disappearance of voids (bubbles) occurs in addition to a change in temperature. Compared to the non-boiling water region (gap water region) outside the channel box 4, the normal operation of the neutron spectrum (relative to the spectrum)hard) And cold shutdown (relative to spectrum)soft) And a large change. Thereby, compared with the conventional structure (JP-A-2-147890 and JP-A-10-10257) in which a combustible absorbent is disposed in the vicinity of the non-boiling water region, the normal operation is shifted to the cold stop. Since the amount of neutron absorption of the combustible absorbent material can be greatly increased, hot / cold swing can be reduced accordingly.
[0049]
In addition, as an action to be obtained at this time, as described above, the larger the area where the combustible absorbent-containing member 8 is in contact with the surrounding two-phase flow cooling water, the greater the neutron absorption amount. The amount of increase in neutron absorption when shifting to time sometimes becomes larger as the area where the combustible absorbent-containing member 8 is in contact with the surrounding cooling water is larger. That is, according to the fuel assembly 1 of the present embodiment, the combustible absorbent-containing member 8 as the neutron absorbing means is connected to the channel box 4 via a wide contact area such as both surfaces of each flat plate member 8a, 8b, 8c, 8d. Because it is in contact with the two-phase flow cooling water (boiling water) in the inner area, the area in contact with the two-phase flow cooling water in the channel box cannot be made so large (JP-A-58-113785), Hot / cold swing can be slightly reduced. And since it is a flat plate, it can manufacture at low cost.
[0050]
Hereinafter, this operation will be specifically described with reference to a comparative example.
[0051]
FIG. 4 shows a fuel assembly 1 according to the embodiment of the present invention having a cross-shaped combustible absorbent-containing member 8 and a fuel assembly having the same structure (however, the combustible absorbent-containing member 8 is omitted). A first comparative example in which a flammable absorbent is contained in the cladding tube of the water rod 3 (substantially equivalent to JP-A-58-113785) and a second comparative example in which a flammable absorbent is contained in the channel box 4 ( The rate of change in the neutron infinite multiplication factor between the case where the void ratio is 40% during normal operation and the case where there is no control rod during cold shutdown (ie hot / cold swing) The analysis results are compared and shown.
[0052]
At this time, the content of gadolinium in each case was the same as the analysis condition. The water rod 3 was assumed to be a cylinder with a radius of 1.6 cm. In addition, the vertical axis represents hot / cold swing when a uranium fuel rod containing gadolinia is used in a fuel assembly having the same structure as the fuel assembly 1 of the present embodiment (however, the combustible absorbent-containing member 8 is omitted). The reference value is shown as a relative value obtained by subtracting the reference value from the hot / cold swing value in the above three cases (the present embodiment, the first comparative example, and the second comparative example).
[0053]
In FIG. 4, the hot / cold swing relative value in the fuel assembly 1 of the present embodiment is about 1/3 of the second comparative example in which the channel box 4 contains a combustible absorbent, and the water rod 3 is covered. Compared to the first comparative example in which a combustible absorbent is contained in the tube, it is about 3/5, and it can be seen that the hot and cold swings are suppressed to be smaller than those of the first and second comparative examples.
[0054]
As for the conventional structure of Japanese Patent Application Laid-Open No. 10-10257, the combustible absorbent material is not arranged on the outer peripheral side as much as the channel box, but is in contact with the surrounding cooling water in a larger area than the moderator rod. Since the surrounding cooling water is non-boiling water like the channel box, its characteristics are considered to be the same as those of the first comparative example or the second comparative example, or an intermediate characteristic between them. Therefore, the present embodiment can suppress the hot / cold swing to be smaller than this case.
[0055]
(2) Reactivity suppression action
As described above, according to the present embodiment, it is possible to secure a sufficiently large area in contact with the cooling water of the two-phase flow in the channel box 4 of the combustible absorbent-containing member 8. Therefore, unlike the conventional structure (Japanese Patent Laid-Open No. 58-113785) in which the cladding tube of the moderator rod is filled with the combustible absorbent, a sufficient reactivity suppression effect can be ensured by the combustible absorbent.
[0056]
This effect will be specifically described with reference to a comparative example.
[0057]
FIG. 5 shows a fuel assembly 1 according to the above embodiment of the present invention having a cross-shaped combustible absorbent containing member 8 and a fuel assembly having the same structure (provided that the combustible absorbent containing member is provided). In the first comparative example (substantially equivalent to JP-A-58-113785) in which the cladding tube of the water rod 3 contains a combustible absorbent and the channel box 4 contains a combustible absorbent. In the second comparative example (substantially equivalent to Japanese Patent Application Laid-Open No. 2-147890), the void ratio of boiling water is 0% when the control rod is inserted at the time of cold stop and when it is not inserted, and at the time of normal operation. , 40% and 70%, the comparison results of the analysis results of the neutron infinite multiplication factor in each case.
At this time, the analysis conditions were the same as in FIG. 4 above, assuming that the gadolinium content in each case was the same, and the water rod 3 was assumed to be cylindrical with a radius of 1.6 cm. The vertical axis shows the infinite neutron multiplication factor when a uranium fuel rod containing gadolinia is used in a fuel assembly having the same structure as that of the fuel assembly 1 of the present embodiment (however, the combustible absorbent-containing member 8 is omitted). The reference value is shown as a relative value obtained by subtracting the reference value from the neutron infinite multiplication factor in the above-described three cases (the present embodiment, the first comparative example, and the second comparative example).
[0058]
In Fig. 5, when the control rod is inserted at the time of cold stop and when it is not inserted, and when the void ratio of boiling water is 0%, 40% and 70% in normal operation, respectively, a total of 5 cases In any of the cases, the relative value of the neutron infinite multiplication factor of the first comparative example in which the flammable absorbent is contained in the cladding tube of the water rod 3 is the highest, and it is understood that the excessive reactivity suppression effect is insufficient. In contrast, the neutron infinite multiplication factor relative value in the fuel assembly 1 of the present embodiment is the first comparative example when the control rod is inserted at the time of cold stop and when the control rod is not inserted at the time of cold stop. Not only that, it is lower than the second comparative example in which the channel box 4 contains a combustible absorbent. In addition, when the void ratio of boiling water during normal operation is 0%, 40%, and 70%, respectively, it is slightly higher than the second comparative example, but is sufficiently small compared to the first comparative example. I understand.
[0059]
(3) Effects of this embodiment
(3-1) Main effects
As described above in (1) and (2), according to the fuel assembly 1 of the present embodiment, the conventional structure in which the combustible absorbent is disposed in the vicinity of the non-boiling water region or the cladding of the moderator rod is combustible. Unlike a conventional structure filled with a flammable absorbent, hot / cold swing can be reduced while sufficiently ensuring the reactivity suppression effect of the flammable absorbent.
[0060]
(3-2) Other effects
(a) More reliable reduction of hot / cold swing
In the two-phase flow region in the channel box,Radial circumferencePartIn other words, the channel box sideOutside the channel boxNon-boiling waterThe effect of neutron spectrum is large, and the change in neutron spectrum during normal operation and cold shutdown is small. According to the study by the inventors of the present application, in the case of the 8 × 8 or 9 × 9 fuel rod lattice arrangement, the fuel assembly radial direction center side from the vicinity of the lattice position of the second layer from the outermost layer of the square lattice arrangement If,The influence of the non-boiling water region outside the channel box is small, and the neutron spectrum is in a position where the change between normal operation and cold shutdown is large.It was found that it can be estimated.
[0061]
According to the present embodiment, as described above, the center side portion of the combustible absorbent-containing member 8 reaches the fuel assembly center axis, and is disposed closer to the center than the second layer lattice position. This ensures thatThe neutron spectrum can be placed at a position where there is a large change between normal operation and cold shutdown,Hot and cold swing can be reliably suppressed (see the result of FIG. 4 described above).
[0062]
(b) Prevention of control rod value decline
According to the fuel assembly 1 of the present embodiment, the channel box 4 does not contain a combustible absorbent. Therefore, it is possible to prevent the value of the control rod from being impaired as in the conventional structure in which the combustible absorbent material is contained in the channel box.
[0063]
(c) Strengthening of channel box strength
According to the fuel assembly 1 of the present embodiment, the combustible absorbent-containing member 8 supports the central portion of the side surface of the channel box 4, so that the strength of the channel box 4 can be reinforced and increased.
[0064]
In the above embodiment, all of the flat plate members 8a to 8d constituting the combustible absorbent-containing member 8 are linearly arranged in the row direction or the column direction in the gaps between the lattice positions of the lattice arrangement. Although arranged so as to be in contact with the two-phase cooling water in the channel box 4 surrounding the square lattice arrangement, the present invention is not limited to this. That is, it is partially arranged so as not to come into contact with the two-phase cooling water, is not arranged linearly in the row direction or the column direction, is curved, or is arranged at the lattice position of the lattice arrangement. May be. That is, at least a portion is arranged linearly in the row direction or the column direction in the gaps between the lattice positions of the lattice arrangement, and is in contact with the two-phase flow cooling water in the channel box 4 surrounding the square lattice arrangement. If it is placed in, it is enough.
[0065]
Moreover, although the above demonstrated the embodiment which actualized this invention by the structure shown in FIGS. 1-3, this invention is not limited to this, In the range which does not deviate from the meaning and technical scope. Various modifications are possible. Hereinafter, the modified examples will be sequentially described.
[0066]
(1) When used with conventional structure
That is, as shown in FIG. 6, some of the fuel rods 2 in the structure of FIG. 1 are fuel rods containing a combustible absorbent, or as shown in FIG. A combustible absorbent material 12 is filled between the inner tube 3Ai and the outer tube 3Ao (with a structure similar to that disclosed in Japanese Patent Laid-Open No. 58-113785), or in the structure shown in FIG. This is a case where the combustible absorbent 13 is disposed inside (the same structure as in JP-A-10-10257 is used in combination).
[0067]
In the case of FIG. 7, the four flat plate members 8a, 8b, 8c, and 8d of the combustible absorbent-containing member 8 are respectively joined to the outer peripheral portion of the large-diameter water rod 3A, and the position thereof is combustible absorption. This is the most radially central portion of the material-containing member 8. That is, this joining position is arranged closer to the center side in the fuel assembly radial direction than the lattice position of the third layer from the outermost layer in the square lattice arrangement.
[0068]
Even in these modified examples, the structure of the above-described embodiment is mostly adopted while the conventional structure is left, and therefore substantially the same effect as that of the above-described embodiment can be obtained.
Further, for example, in the structure of FIG. 6, when it is considered that the structure of the present invention is combined with the conventional structure, the number of the fuel rods 2A containing the combustible absorbent necessary for obtaining a sufficient surplus reactivity suppression effect, or It can also be considered that the combustible absorbent content per one fuel rod 2A can be reduced. Similarly, in the structure of FIG. 8, the amount of the combustible absorbent 13 that must be contained in the channel box 4 in order to obtain a sufficient surplus reactivity suppression effect can be reduced, and the decrease in the value of the control rod is kept low. You can also think that you can.
[0069]
(2) When using irregular shaped water rod
That is, as shown in FIG. 9, a single diamond-shaped water rod 3 </ b> B is used instead of the four circular rods 3 in FIG. 1. In this case, the four flat plate members 8a, 8b, 8c, 8d of the combustible absorbent containing member 8 are respectively joined to the four vertices of the rhomboid water rod 3B, and the positions thereof are combustible absorbent containing member 8 Is the most radially central portion. That is, this joining position is arranged slightly on the center side in the fuel assembly radial direction from the lattice position of the third layer from the outermost layer of the square lattice arrangement.
[0070]
Also by this modification, the same effect as the above-mentioned embodiment is acquired.
[0071]
Needless to say, the present invention can be applied not only to the rhombus-shaped water rod as described above but also to a quadrilateral cross-section or other shapes, and even when five or more water rods are used.
[0072]
(3) When using a well containing flammable absorbent material
That is, as shown in FIG. 10, in place of the cross-shaped lattice-shaped combustible absorbent-containing member 8 in FIGS. 1 to 3, a combustible absorbent-containing member 8 </ b> A that is a “well” -shaped lattice-shaped plate member is disposed. This is the case. In this case, the water rod 3 also corresponds to the fact that there is no central partition (a flat plate member passing through the midpoint of the four sides of the square lattice array) as shown in FIG. One large-diameter water rod 3C having a large deceleration effect is arranged in an area where four fuel rods 2 can be arranged.
[0073]
The combustible absorbent-containing member 8A is composed of four flat plate members 8Aa (in the column direction) linearly provided in a quadrangular square shape surrounding the outer peripheral side of each lattice position in the third layer from the outermost layer in a square lattice array. 8Ab (row direction), 8Ac (column direction), 8Ad (row direction), and eight flat plate members 8Ae (linearly extending from the four apexes of the substantially square shape toward the outside in the column direction and the row direction) Column direction), 8Af (row direction), 8Ag (row direction), 8Ah (column direction), 8Ai (column direction), 8Aj (row direction), 8Ak (row direction), 8Al (column direction) Yes.
[0074]
These flat plate members 8Aa to 8l are all arranged not in the lattice position of the lattice arrangement in which the fuel rods 2 are arranged, but in the gap between the lattice positions, and as shown in FIG. The inside is arranged so as to be divided into one large square area 10A, four small square areas 10B, 10C, 10D, and 10E, and four rectangular areas 10F, 10G, 10H, and 10I. With such an arrangement, both surfaces of each flat plate member 8Aa-l come into contact with the two-phase cooling water in the channel box 4.
At this time, the most radially central portion of the combustible absorbent-containing member 8A is constituted by the flat plate members 8Aa to d that form the four sides of the square, as described above. It is arranged closer to the center of the fuel assembly in the radial direction than the lattice position of the second layer from the outer layer. Further, the end of the eight flat plate members 8Ae to 8l in the fuel assembly radial direction (that is, the channel box 4 side) is joined to the inner peripheral surface of the channel box 4 by welding or the like, thereby containing a combustible absorbent. The member 8A is positioned and fixed.
[0075]
In the above description, the flat plate members 8Af, 8Ad, 8Ak and 8Ag, 8Ab, 8Aj constitute the row-direction flat plate means arranged in the row direction of the grid array described in the claims, and the flat plate member 8Ae. , 8Aa, 8Ah and 8Al, 8Ac, 8Ai constitute a row direction flat plate means arranged in the row direction of the grid-like arrangement, and these twelve flat plate members 8Aa to l constitute the flat plate means. And the combustible absorber containing member 8A consisting of these twelve flat plate members 8Aa to l constitutes a lattice plate member and also constitutes a neutron absorbing means provided with the combustible absorber.
[0076]
According to this modification, in addition to obtaining the same effect as the above embodiment, the area of the combustible absorbent material-containing member 8A in contact with the boiling water in the channel box 4 is increased, and the neutron absorption amount is increased. As compared with the above-described embodiment, the effect of suppressing the excess reactivity is further increased, and the hot / cold swing can be suppressed to be smaller.
[0077]
In addition, as shown in FIG. 11, the present invention may be applied to a 9 × 9 (or more) grid array. In FIG. 11, two water rods 3D are arranged in a region where seven fuel rods 2 can be arranged in the central region in the fuel assembly radial direction. Further, at this time, the most radially central portion of the combustible absorbent-containing member 8A is disposed closer to the fuel assembly radial center than the lattice position of the third layer from the outermost layer of the square lattice array.
[0078]
(4) When multiple flammable absorbent material containing cross-shaped grids are provided
That is, as shown in FIG. 12A, the combustible absorbent material-containing member 8B1, which is four cross-shaped lattice-shaped plate members smaller than the cross-shaped lattice-shaped combustible absorbent material-containing member 8 of FIGS. This is a case where 8B2, 8B3, and 8B4 are arranged. The water rod 3C is the same as that of the modified example (3).
[0079]
Each of the combustible absorbent-containing members 8B1, 8B2, 8B3, and 8B4 has four rows of fuel rods 2 on the upper left side, the lower left side, the lower right side, and the upper right side as viewed in FIG. It is arranged in the four-row arrangement square areas 11A, 11B, 11C, 11D. The four combustible absorbent-containing members 8B1 to B4 include four flat plate members 8Ba, 8Bb, 8Bc, and 8Bd that extend from the midpoint of the outer periphery of the 4 rows and 4 columns arrangement region toward the center of the region. It is composed of These four flat plate members 8Ba to 8d are all arranged not in the lattice position of the lattice arrangement in which the fuel rods 2 are arranged, but in the gap between the lattice positions, as shown in FIG. As described above, the corresponding four-row arrangement square regions 11A to 11D are arranged so as to be divided into approximately four equal parts. With such an arrangement, both surfaces of each flat plate member 8Ba to d are in contact with the two-phase flow cooling water in the channel box 4. These four flat plate members 8Ba to 8d are flat plate members 8Ba and 8Bc arranged linearly in the row direction of the grid array, and flat plate members 8Bb and 8Bd arranged linearly in the column direction of the grid array. Are connected in a state of being substantially perpendicular to each other, thereby constituting combustible absorbent-containing members 8B1 to B4 which are lattice-like plate members.
[0080]
At this time, in each combustible absorbent material-containing member 8B1 to B4, the most radially central portion is one of flat plate members 8Ba, 8Bb, 8Bc, 8Bd (in the combustible absorbent material-containing member 8B1, the flat plate member 8Bb or 8Bc, flat member 8Bc or 8Bd in combustible absorbent containing member 8B2, flat plate member 8Ba or 8Bd in combustible absorbent containing member 8B3, flat plate member 8Ba or 8Bb in combustible absorbent containing member 8B4, both square It is arranged closer to the center of the fuel assembly in the radial direction than the lattice position of the second layer from the outermost layer of the lattice arrangement.
[0081]
And the fuel assembly radial direction outer side end (that is, the channel box 4 side) end of each combustible absorbent-containing member 8B1 to B4 does not reach the inner peripheral surface of the channel box 4, but the inner peripheral surface of the channel box 4 Opposing via a predetermined gap. Correspondingly, each of the combustible absorbent-containing members 8B1 to B4 has upper and lower tie plates 5 and 6 or spacers 7 at the upper and lower ends (not shown) or at appropriate positions in the axial direction by welding or the like. By being joined to (see FIG. 2), the positioning is fixed.
[0082]
In the above, the flat plate members 8Ba, 8Bc of the combustible absorbent-containing members 8B1, 8B2, 8B3, 8B4 constitute the row-direction flat plate means arranged in the row direction of the grid array according to the claims. The plate members 8Bb and 8Bd constitute a row direction plate means arranged in the row direction of the grid-like arrangement, and 16 plate members 8Ba, 8Bb, which are a total of all the combustible absorbent-containing members 8B1 to 8B4, 8Bc and 8Bd constitute plate means. And the combustible absorber containing member 8B1-8B4 which consists of these 16 flat plate members 8Ba-d each comprises a grid | lattice-like plate member, and also comprises the neutron absorption means provided with the combustible absorber.
[0083]
In addition, as shown in FIG. 12B, a large number of flammable absorbent material-containing members 8C (12 in this example), which are smaller cross-shaped lattice-like plate members, may be arranged for the purpose of suppressing local output peaking. Good. Each combustible absorbent-containing member 8C is composed of flat plate members 8Ca and 8Cc that are linearly arranged in the row direction and flat plate members 8Cb and 8Cd that are linearly arranged in the column direction. This portion is arranged slightly on the center side in the fuel assembly radial direction from the lattice position of the second layer from the outermost layer of the square lattice arrangement.
[0084]
(5) When neutron absorption means is configured with non-plate material
That is, as shown in FIG. 13, each of the cross-shaped sides 8a ', 8b', 8c ', 8d' of the combustible absorbent-containing member 8 'is not a flat plate member but a thin tube containing the combustible absorbent 15 This is a case where 14 are arranged in a linear combination in the row direction or the column direction.
In this case, the combustible absorbent-containing member 8 ′ having four sides 8a ′, 8b ′, 8c ′, and 8d constitutes a neutron absorbing means provided with the combustible absorbent.
[0085]
The member containing the combustible absorbent is not limited to the above-described flat plate members 8a to 8d and the thin tube 14 and the like, and the member does not increase in volume and the member is boiled in the channel box 4. Any shape that can increase the area in contact with water is sufficient.
[0086]
In the above description, the present invention is applied to an uranium fuel assembly using uranium fuel as an example. However, the present invention is not limited to this, and the uranium / plutonium mixed oxide fuel in which plutonium is mixed with uranium ( The present invention can also be applied to a MOX fuel assembly using (MOX fuel). Also in this case, as in the above-described embodiment of the present invention, the effect of reducing hot / cold swing can be obtained while sufficiently ensuring the reactivity suppression effect by the combustible absorbent.
[0087]
【The invention's effect】
According to the present invention, at least a part of the neutron absorption means is in contact with the two-phase cooling water (boiling water) in the channel box, and this region has a neutron spectrum region as compared with the non-boiling water region outside the channel box. The change between normal operation and cold stop is large. Therefore, the amount of neutron absorption of the combustible absorbent when shifting from the normal operation to the cold shutdown can be greatly increased, so that the hot / cold swing can be greatly reduced.
Further, at least a part of the above-described neutron absorbing means is arranged linearly in the row direction or the column direction in the gap between the lattice positions of the lattice arrangement, and has a sufficient area in contact with the cooling water of the two-phase flow in the channel box. Since it can ensure large, the reactivity suppression effect by a combustible absorber can fully be ensured.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing the structure of a boiling water reactor fuel assembly according to an embodiment of the present invention.
FIG. 2 is a longitudinal sectional view showing the entire structure of the boiling water nuclear reactor fuel assembly shown in FIG.
FIG. 3 is a cutaway perspective view showing a lower structure of the fuel assembly shown in FIG. 2;
FIG. 4 shows a fuel assembly including the combustible absorbent-containing member shown in FIG. 1, a first comparative example in which a combustible absorbent is contained in a cladding tube of a water rod, and a combustible absorbent in a channel box. It is the figure which compared and showed the analysis result of the hot and cold swing in the 2nd comparative example made to contain.
FIG. 5 shows a fuel assembly including the combustible absorbent-containing member shown in FIG. 1, a first comparative example in which a combustible absorbent is contained in a cladding tube of a water rod, and a combustible absorbent in a channel box. It is the figure which compared and showed the analysis result of the value of the neutron infinite multiplication factor in the 2nd comparative example made to contain.
FIG. 6 is a cross-sectional view showing a modification in which some of the fuel rods are fuel rods containing a combustible absorbent.
FIG. 7 is a cross-sectional view showing a modified example in which a large diameter water rod is filled with a combustible absorbent.
FIG. 8 is a cross-sectional view showing a modified example in which a combustible absorbent material is disposed in a channel box.
FIG. 9 is a cross-sectional view showing a modification using a water rod having a diamond-shaped cross section.
FIG. 10 is a cross-sectional view showing a modification using a “well” -shaped combustible absorbent-containing member.
FIG. 11 is a cross-sectional view showing a modification applied to a 9-row 9-column grid array.
FIG. 12 is a cross-sectional view showing a modified example in which a plurality of cross-shaped lattice-shaped combustible absorbent-containing members are provided.
FIG. 13 is a cross-sectional view showing a modified example in which thin tubes containing a combustible absorbent material are linearly combined and arranged in a row direction or a column direction.
[Explanation of symbols]
1 Fuel assembly
2 Fuel rod
4 Channel box
8 Combustible absorber-containing member (grid-like plate member, neutron absorption means)
8 'flammable absorber-containing material (neutron absorption means)
8A Combustible absorbent material-containing member (lattice plate member, neutron absorbing means)
8Aa, c, e,
h, i, l Flat plate member (row direction flat plate means, flat plate means)
8 Ab, d, f,
g, j, k flat plate member (row direction flat plate means, flat plate means)
8B1-4 Combustible absorbent material-containing members (grid-like plate members, neutron absorption means)
8Ba, c Flat plate member (Row direction flat plate means, flat plate means)
8Bb, d flat plate member (row direction flat plate means, flat plate means)
8C Combustible absorbent material-containing member (lattice plate member, neutron absorbing means)
8Ca, c flat plate member (row direction flat plate means, flat plate means)
8Cb, d flat plate member (row direction flat plate means, flat plate means)
8a, c flat plate member (row direction flat plate means, flat plate means)
8b, d flat plate member (row direction flat plate means, flat plate means)

Claims (5)

In a fuel assembly in which a plurality of fuel rods filled with fissile material are arranged in a square lattice of n rows and n columns,
Provide neutron absorption means with combustible absorbers,
The neutron absorbing means is a two-phase flow in a channel box that is at least partially arranged linearly in the row direction or the column direction in the gaps between the lattice positions of the lattice array and surrounds the square lattice array. Placed in contact with the cooling water,
The fuel assembly according to claim 1, wherein the at least part of the neutron absorbing means is at least one flat plate means arranged so that both surfaces thereof are in contact with the two-phase cooling water in the channel box. 2. The fuel assembly according to claim 1 , wherein a plurality of the flat plate means are provided, and the plurality of flat plate means includes a row direction flat plate means arranged in the row direction of the lattice arrangement and the grid shape. A fuel assembly comprising: a row direction plate means arranged in the row direction of the array. 3. The fuel assembly according to claim 2 , wherein the row direction flat plate means and the column direction flat plate means are configured as a lattice plate member coupled in a substantially perpendicular state to each other. .   2. The fuel assembly according to claim 1, wherein n = 8 or 9 and a portion of the neutron absorbing means on the center side in the radial direction of the fuel assembly is at least a second layer lattice from the outermost layer of the square lattice array A fuel assembly, wherein the fuel assembly is disposed closer to the center in the radial direction of the fuel assembly than a position. In a fuel assembly in which a plurality of fuel rods filled with fissile material are arranged in a square lattice,
Comprising a burnable absorber, has the square lattice array of four flat section that extend respectively from the midpoint of the peripheral four sides toward the fuel assembly radial center of the four flat plate means, the A fuel assembly, characterized in that both surfaces are arranged so as to contact two-phase flow cooling water in a channel box surrounding the square grid array.

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