JP2002055188A - Fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the breeding ratio, to simultaneously set the void reactivity to negative, and to effectively accumulate voids. SOLUTION: This fuel assembly has a plurality of fuel rods for filling a channel box with fissile materials and a neutron leakage pipe disposed therein. The both top/bottom both ends of the neutron leakage pipe are closed by end plugs 4 and 6, the upper end plug 6 is provided with a plurality of tapered cooling material channel holes 5 with throttled upper ends, and a cooling material inlet 7 is provided near the lower end pug 6.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel assembly for a water-cooled nuclear reactor, and more particularly to a fuel assembly for making void reactivity negative.

[0002]

2. Description of the Related Art In general, the core of a water-cooled nuclear reactor is constituted by arranging a large number of fuel assemblies in which a number of fuel rods loaded with a fissile material (fuel) are bound, and heat generated by the fissile material is generated. Water is used as coolant for removal.

[0003] Since water has a large neutron moderating ability of hydrogen atoms contained therein, in a conventional water-cooled reactor having a large proportion of water, high-energy neutrons generated by nuclear fission are greatly slowed down, and thermal neutrons having low energy are generated. Most of the neutrons.

When neutrons having low energy are absorbed by a fissile material, the rate of a capture reaction in which nuclear fission takes place in the nucleus without fission, instead of a fission reaction in which about three neutrons are generated. That is, the number of neutrons generated per neutron absorption is reduced in fission by low energy neutrons.

On the other hand, in the case of high-energy neutrons, since the rate of the capture reaction is small, the average number of neutrons generated per absorption can be set to two or more even if the effect of the capture is included. Used for maintenance, the other one is U-
It is possible to efficiently generate fissile material by absorbing it into a parent material such as 238. If the ratio of the generation and extinction of fissile material is 1 or more, fuel has been proliferated, and breeder reactors have been developed in various countries from the viewpoint of securing resource energy.

However, in the conventional water-cooled reactor, the ratio of water to fuel is large, and neutrons have low energy. Therefore, neutrons cannot be propagated, and the ratio of generation and extinction of fissile material (referred to as the growth ratio). If this ratio is 1 or less, the ratio is referred to as the conversion ratio, but here all are simply referred to as the proliferation ratio.), The value was 1 or less (about 0.5). Therefore, in a breeder reactor, only about 1% of uranium resources that can be converted into 100% heat energy can be used in principle.

However, if the energy spectrum is high, there is a possibility that the reactivity (void reactivity) due to boiling of the coolant becomes positive as in a conventional large fast reactor. In a water-cooled reactor, it is important to set the void reactivity to a negative value from the viewpoint of core stability and safety.

For this reason, conventionally, a structure has been proposed in which a neutron leak tube having a hollow inside is provided in a fuel assembly to reduce void reactivity. That is, the fuel assembly has a structure in which a large number of fuel rods are arranged inside the channel box, and a neutron leakage tube is provided at the center of the large number of fuel rods. In this type of fuel assembly, a large number of fuel rods are arranged and arranged in a cylindrical channel box having a regular hexagonal cross section, a neutron leak tube is arranged along a central axis, and cooling water is supplied from below to above. It is configured to flow to
Since the void fraction of the coolant flowing inside the neutron leak tube increases as the power of the reactor core increases, neutron leakage increases. Due to this effect, the void reactivity is reduced.

[0009]

However, in a conventional water-cooled reactor, a neutron leak tube is provided in a channel box to increase the breeding ratio, and at the same time, a fuel assembly that makes the void reactivity negative is water. There is a risk that hydrogen will be generated by the activation of the molecules of the molecule and the reaction between the cladding tube and water, and this hydrogen will be confined inside the neutron leak tube and, in the worst case, will explode.

Further, in the conventional neutron leak tube, a large amount of vapor accumulated in the neutron leak tube escapes rapidly or unevenly due to an acute angle at the lower end, so that the void reactivity rapidly changes with time, and Has a negative impact on tube integrity. At the time of the transient change, the thermal integrity of the core is deteriorated, so it is necessary to further reduce the void reactivity, reduce the core output, and further improve the thermal margin of the core.

In order to effectively leak neutrons, it is necessary to increase the area ratio of the neutron leak pipe to the entire cross-sectional area including the coolant flow path and the neutron leak pipe. As in the prior art, in a configuration in which the coolant flow path is an annular portion and the neutron leakage pipe is provided in the center, if the cross-sectional area ratio of the neutron leakage pipe is increased, the inner wall of the coolant flow path in the annular section (the outer wall of the neutron leakage pipe) )
There is a possibility that the gap between the inner wall and the outer wall becomes narrower, the diameter of the flow path becomes narrower, the flow resistance increases, the flow rate flowing through the coolant flow path decreases, or the flow velocity increases, which causes a problem of hydraulic vibration.

Further, as the core characteristics of the dense bundle, the output increases, the void fraction increases, and the void reactivity (the rate of increase in the reactivity with respect to the increase in the void rate) tends to become positive. Therefore, as the void fraction of the core increases,
The neutron leakage rate in the neutron leak tube needs to be increased.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has been developed for a water-cooled nuclear reactor having an increased breeding ratio and at the same time improved performance in a water-cooled nuclear reactor having a negative void reactivity. The purpose is to provide a fuel assembly.

Further, according to the present invention, it is possible to easily remove air from the inside of the neutron leak tube without impairing the function of the neutron leak tube in the operating state, and to effectively store voids inside the neutron leak tube when the output is increased. To provide an aggregate. Another object of the present invention is to provide a fuel assembly in which the void reactivity does not change rapidly with time and does not adversely affect the soundness of the fuel cladding tube.

[0015]

According to a first aspect of the present invention, there is provided a channel box, a plurality of fuel rods disposed in the channel box and loaded with fissile material, and a plurality of fuel rods interposed between the plurality of fuel rods. And a neutron leak pipe, wherein the neutron leak pipe is closed at its upper and lower ends by an upper end plug and a lower end plug, and a plurality of tapered coolants whose upper ends are narrowed by the upper end plug. It has an outflow hole, and has a coolant inflow hole at or near the lower end plug. According to the present invention, when the void ratio in the neutron leak tube rises, it is possible to make it difficult for the void to escape from the outlet hole at the upper end.

The invention corresponding to claim 2 is characterized in that a step-shaped groove is provided on the inner surface of the tapered coolant outflow hole. According to the present invention, the flow resistance is increased by roughening the tapered surface of the outflow hole and providing a step-like groove, and the void from the outflow hole at the upper end portion is increased when the void ratio in the neutron leak tube increases. It is hard to fall out.

According to a third aspect of the present invention, there is provided a fuel assembly including a plurality of fuel rods loaded with fissile material and a neutron leak tube, wherein a coolant is provided from a lower portion to an upper portion inside the neutron leak tube. It is characterized by having a closed thin tube bundle having a flow channel and a plurality of thin tubes closed at both ends.

The invention corresponding to claim 4 is characterized in that a thin-walled honeycomb sealing structure is provided inside the neutron leak tube. The invention corresponding to claim 5 is a fuel assembly comprising a plurality of fuel rods loaded with fissile material and a neutron leak tube, wherein the neutron leak tube has a lower end opened and an upper portion closed with an upper lid, Coolant flows upward from the lower end opening inside, and a tube formed of a shape memory alloy is joined to the upper lid, and a heating element by neutron absorption or gamma ray absorption is provided below the neutron leak tube. It is characterized by.

According to a sixth aspect of the present invention, a metal cylinder is attached to the upper lid in place of the tube formed of the shape memory alloy, and a metal having a larger coefficient of thermal expansion than the metal of the metal cylinder in the metal cylinder. And a plurality of plates provided above and below the metal cylinder. The invention corresponding to claim 7 is characterized in that the upper lid is made of a nickel plate.

The invention corresponding to claim 8 is characterized in that the corner of the lower end opening of the neutron leak tube is chamfered into a spherical shape. The invention corresponding to claim 9 is characterized in that a plurality of holes are provided near the lower end opening of the neutron leak tube.

According to a tenth aspect of the present invention, the upper lid is formed to have a larger diameter than the outer diameter of the neutron leak pipe, and a flow path outside the neutron leak pipe is narrowed near the large diameter upper lid. It is characterized by. An invention corresponding to claim 11 is characterized in that the large-diameter upper lid is provided with a plurality of vertical holes.

According to a twelfth aspect of the present invention, there is provided a fuel assembly including a plurality of fuel rods loaded with fissile material and a neutron leak tube, wherein the neutron leak tube has a vertical coolant passage formed therein. An inner tube is provided, which is open at the inlet, closed at the outlet or has fine holes, and an annular void accumulation portion is formed outside the inner tube.

According to the present invention, the coolant flow path is arranged at the center, and the high void ratio region having the neutron leakage function is formed as the annular portion. And the flow rate can be secured to reduce the possibility that the problem of the flow force vibration due to the increase in the flow velocity will occur.

According to a thirteenth aspect of the present invention, in a fuel assembly having a plurality of fuel rods loaded with fissile material and a neutron leak tube, the neutron leak tube has a two-stage structure in a height direction. An upper neutron leak pipe and a lower neutron leak pipe, wherein a coolant flow path diameter of the upper neutron leak pipe is smaller than a coolant flow path diameter of the lower neutron leak pipe; And a void accumulation mechanism in which the void accumulation portion of the outer annular portion of the upper neutron leakage tube quickly becomes a high void ratio region as soon as the void ratio in the coolant flow path increases.

According to the present invention, the coolant flow path is disposed at the center, and the neutron leak pipe having a cross-sectional structure in which the high void ratio region having the neutron leak function is formed as the annular portion is provided in two axial stages. Since the void ratio of the void accumulation region increases prior to the increase of the void ratio of the coolant, the diameter of the coolant flow passage at the center is large at the top and small at the bottom.

According to a fourteenth aspect of the present invention, the coolant flow path of the upper neutron leakage pipe comprises a narrowed flow path and a widened flow path, and the two-phase flow flowing through the coolant flow path of the upper neutron leakage pipe. Has a void accumulation mechanism in which the annular void accumulation portion of the upper neutron leak tube becomes a high void ratio region as the void ratio increases.

According to the present invention, a neutron leak pipe having a cross-sectional structure in which the coolant flow path is arranged at the center and the high void ratio region having a neutron leak function is an annular portion is provided in two stages in the axial direction. In order to increase the void ratio of the void accumulation region prior to the increase of the void ratio of the coolant, the upper coolant channel is narrowed and widened to form a Venturi-type channel followed by a widened channel. As a result, the void ratio in the coolant passage increases, and the upper annular void accumulation portion becomes a high void region.

According to a fifteenth aspect of the present invention, in a fuel assembly including a plurality of fuel rods loaded with fissile material and a neutron leak tube, a plurality of fuel rods are provided with an upper part in the neutron leak tube as a fuel region. The neutron leakage tube is provided with an annular void accumulating portion at a lower portion thereof, and a coolant flow path is provided in the annular void accumulating portion.

According to the present invention, in the case of a boiling water reactor, the coolant in the subcooled state at the reactor core inlet rises in temperature due to heat transfer from the fuel assembly and boils. It becomes a steam-water two-phase flow.
Therefore, the lower part of the core can have a relatively lower void fraction than the upper part.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of a fuel assembly according to the present invention will be described with reference to FIGS.

In this embodiment, as shown in FIG. 1, a fuel assembly having a structure in which a fuel rod 2 and a neutron leak tube 3 having a hollow inside are provided in a regular hexagonal rectangular channel box 1 to reduce void reactivity. Body. Although the embodiment of the hexagonal fuel assembly is shown here, the shape of the fuel assembly is not particularly limited, and for example, a square fuel assembly may be used.

FIG. 2 shows a longitudinal section of the neutron leak tube 3 in FIG. That is, the upper and lower ends of the neutron leak tube 3 are closed by the upper end plug 4 and the lower end plug 6, and the upper end plug 4 is closed.
It has a plurality of tapered coolant outlet holes 5 whose upper ends are narrowed, and a coolant inlet hole 7 near the lower end plug 6.
As a result, when the void ratio in the neutron leak tube increases, the flow resistance of the outlet hole 5 at the upper end increases, making it difficult for the void in the neutron leak tube to escape.

FIG. 3 shows the state of the coolant inside the neutron leak tube 3. The inflow coolant 8 flowing from the inflow hole 7 generates heat due to radiation such as gamma rays. Therefore, by appropriately setting the size of the inflow hole 7 and the outflow hole 5, as shown in FIG. Enables voiding.

In this case, since the calorific value changes depending on the power of the reactor core, the neutron leakage tube 3 has a low percentage of voids inside the neutron leak tube 3 as shown in FIG. 3 (b), and the void ratio inside the neutron leak tube 3 increases. Since the flow resistance of the tapered coolant outflow hole 5 increases due to the increase in the void ratio, it becomes difficult for the void in the neutron leak tube 3 to escape. Therefore, the effect of reflecting neutrons passing through this space is reduced, and leakage of neutrons is increased, so that void reactivity can be reduced.

Next, a second embodiment according to the present invention will be described with reference to FIGS. 4 (a) and 4 (b).

The present embodiment is obtained by improving the upper end plug 4 in the first embodiment, and the other parts are the same as those of the first embodiment.
Since the embodiment is the same as that of the first embodiment, the description of the overlapping part will be omitted. That is, as shown in FIG.
The surface of the tapered coolant outlet hole 5 provided in the above is made rough, and a step-shaped groove 13 is formed to provide resistance to the flow of the coolant flowing inside, thereby forming a step-shaped tapered outlet hole. .

According to the present embodiment, by increasing the flow resistance at the time of void outflow, the void outflow inside the neutron leak tube at the time of high output is further suppressed, thereby further increasing the neutron leakage, and The reactivity can be reduced.

Next, a third embodiment according to the present invention will be described with reference to FIG. FIG. 5 shows a vertical cross-sectional shape of a neutron leak tube according to the third embodiment of the present invention.

In this embodiment, the neutron leak tube 3 shown in the first embodiment flows upward in the neutron leak tube 3 as shown in FIG. The inside of 3 has a structure of a closed thin tube bundle 14 in which thin closed thin tubes 15 with both ends closed are bundled, and the other portions are the same as in the first embodiment.

According to the present embodiment, the thickness of the sealed narrow tube 15 can be reduced to about 0.2 mm because the tube diameter is small. Therefore, the absorption of neutrons by the sealed thin tube 15 can be suppressed, and the hollow inside the neutron leak tube can be stably maintained regardless of the flow conditions, so that the effect of reflecting neutrons passing through this space is reduced, Void reactivity can be reduced due to increased neutron leakage.

Next, a fourth embodiment according to the present invention will be described with reference to FIG. This embodiment is based on the third embodiment, and FIG. 6 shows a vertical cross-sectional shape of a neutron leak tube 3b according to the present embodiment. That is, in FIG. 6, the inside of the neutron leak tube 3b is a thin honeycomb sealed structure 16.
In this case, the absorption of neutrons can be suppressed, and the hollow inside the neutron leak tube 3b can be stably maintained regardless of the flow conditions. Therefore, the effect of reflecting neutrons passing through this space decreases, and neutron leakage occurs. , The void reactivity can be reduced.

Next, a fifth embodiment according to the present invention will be described with reference to FIG. This embodiment is another example of the neutron leak tube 3 in the first embodiment, and FIG. 7 shows a neutron leak tube 3c of the present embodiment and a state of a flow of a steam flow around the neutron leak tube 3c.

As shown in FIG. 7, the present embodiment has a neutron leak tube 3c and a flow passage hole 18 formed under the neutron leak tube 3c and made of a material which generates heat by neutron or gamma ray absorption, for example, hafnium, stainless steel, or the like. A heating element 18 is arranged, and a shape memory alloy tube 20 made of a shape memory alloy is provided above the neutron leak tube 3c. At normal temperature, the inside and the outside of the neutron leak tube 3c are communicated by the shape memory alloy tube 20.

When the shape memory alloy tube 20 is not provided as in the prior art, the air trapped inside the neutron leak tube 3c
Since there is no passage through which 23 can pass, it cannot be removed. For this reason, this air 23 dissolves in the coolant and the dissolved oxygen concentration of the coolant increases, which has a bad influence on the fuel cladding tube.

In this embodiment, the shape memory alloy tube 20 is in communication at normal temperature, and the shape memory alloy tube 20 is closed in the operating state (that is, when the coolant temperature is high). I have. In other words, all the air 23 goes out through the shape memory alloy tube 20 at normal temperature.

When the temperature becomes high, the shape memory alloy tube 20 is closed, so that the steam 21
Flows into the neutron leak tube 3c until the inside of the neutron leak tube 3c is filled with steam 24. On the other hand, after the inside of the neutron leak tube 3c is filled with steam 24, the generated steam 21
Most of the outer flow passage 22 between the fuel rod cladding tube 25 and
Will pass through.

Next, a sixth embodiment of the present invention will be described with reference to FIGS. 8 (a) and 8 (b). FIG. 8A is a longitudinal sectional view of the neutron leak tube according to the present embodiment, and FIG.
3 is a sectional view taken along the line AA of FIG.

In this embodiment, the upper lid 26 of the neutron leak tube 3d is used.
A metal cylinder 27 made of a metal having a larger coefficient of thermal expansion than the metal of the metal cylinder 27 is installed inside the metal cylinder 27 at the center of the metal cylinder 27.
That is, a plurality of plates 29 are provided as shown in FIG.

At room temperature, there is a gap 30 between the metal cylinder 27 and the metal cylinder 28, so that the air 23 inside the neutron leak tube 3d can escape from the gap 30. In the present embodiment, since the metal cylinder 28 is made of a metal having a larger thermal expansion coefficient than the metal cylinder 27, the gap 30 is eliminated in the operating state (that is, when the coolant temperature is high). .

That is, the air 23 passes through the gap 30 at normal temperature.
All go outside. When the temperature becomes high, the gap 30 is closed, so that the steam 21 generated by heating by the heating element 19 flows into the neutron leak tube 3d until the inside of the neutron leak tube is filled with steam as shown in FIG. . On the other hand, after the inside of the neutron leak tube 3d is filled with the steam 24, most of the generated steam 21 passes through the outer flow path 22.

Next, FIG. 9A and FIG.
Seventh embodiment will be described. Since the present embodiment is based on the sixth embodiment, the same reference numerals in FIG. 9 denote the same parts as in FIG. 8, and a description of the same parts will be omitted.

In this embodiment, the upper lid 26 of the neutron leak tube 3d is used.
Is made of Ni. In the operating state, as shown in FIG. 9, the activation of water molecules and the reaction between the cladding tube and water generate hydrogen 31, which is trapped in the 3-1 neutron leak tube 3d. Risk of explosion. In the present embodiment, the neutron leak tube 3d has
It is formed with. In this configuration, since the hydrogen atoms are small, the hydrogen atoms can be removed from the NI upper lid 26 and the hydrogen 31 can be removed.

Next, an eighth embodiment of the present invention will be described with reference to FIG. FIG. 10 (a) is a longitudinal sectional view of a neutron leak tube for explaining the present embodiment, and FIG. 10 (b) is a comparative view of FIG.

FIG. 7 shows the basic configuration of this embodiment.
9 is the same as the embodiment shown in FIG. In the above-mentioned neutron leak tubes 3c and 3d, as shown in FIG.
Since the lower end is sharp, a large amount of vapor bubbles 32 accumulate and exit suddenly or unevenly, so that the void reactivity changes abruptly with time, which may adversely affect the integrity of the cladding tube. . Therefore, in the present embodiment, as shown in FIG. 10A, the neutron leak tube is formed to have a rounded curved surface 33 at the corner. According to the present embodiment, it is possible to smooth out the vapor and to alleviate the above-mentioned rapid change in the void reactivity.

Next, a ninth embodiment of the present invention will be described with reference to FIG. In the present embodiment, a large number of lateral holes 34 are formed below the neutron leak tube 3c or 3d. According to the present embodiment, it is possible to smooth out the vapor and ease the above-mentioned rapid change of the void reactivity.

Next, a tenth embodiment of the present invention will be described with reference to FIG. FIG. 12 is a vertical cross-sectional view of a neutron leak tube explaining the present embodiment. The basic configuration of this embodiment is the same as that of the embodiment shown in FIGS. 7 to 11, but this embodiment is characterized in that the upper lid of the neutron leak tube 3c or 3d has a large diameter 35. That is, the outer flow path 22 is separated from the neutron leakage pipe 3c or 3c by the large-diameter upper lid 35 having a larger outer diameter.
It has been squeezed at the exit of d.

For example, when the pump stops, the heating element 18
The amount of generated steam increases because the amount of coolant to the heater decreases. Since the inside of the neutron leak tube is already filled with the steam 24, the generated steam 21 flows through the outer flow path 22 and increases the void fraction of this part, thereby decreasing the void reactivity. it can.

According to the present embodiment, in order to increase the void ratio of the outer flow path 22 and suppress the void reactivity,
The part of end B of 35 is squeezed. At the time of a transient change, the generated void amount increases as compared with the normal case. If you keep the top narrow,
During a transient change, the amount of voids passing therethrough increases, resulting in an increase in pressure loss. That is, it becomes difficult for steam to escape, and as a result, the void ratio of the outer channel 22 increases, and the void reactivity can be further suppressed.

Next, an eleventh embodiment of the present invention will be described with reference to FIGS. 13 (a) and 13 (b). This embodiment is different from the tenth embodiment in that a plurality of vertical holes 36 are provided in the large-diameter upper lid 35 as shown in FIG. 13B. In the configuration shown in FIG. 12, the voids collect in the corners during normal times and suddenly escape,
Void change in the side channel is severe. Therefore, by providing a plurality of holes 36 in the large diameter 35, it is possible to smoothly remove the void in a normal state.

Next, a twelfth embodiment of the present invention will be described with reference to FIG. FIG. 14 schematically shows the neutron leak tube 3e in the present embodiment in a bird's-eye view. That is, in the present embodiment, the annular void accumulating portion 37 which becomes a high void fraction region when the water-steam two-phase flow flows into the neutron leak pipe 3e, and the coolant that the water-steam two-phase flow flows into and out of Channel 3
8, heating element that supplies heat to supply water-steam two-phase flow
39 and an inner tube 40.

Under conditions where the core power is low and the amount of heat generated by the heating element is small, the water-steam two-phase flow having a relatively small void ratio flows, so that the steam flowing into the annular portion accumulates in the annular portion and becomes high. It becomes a void fraction region. Regarding the distribution of steam (voids) in the cross section of the inlet, the space distribution of voids generally becomes a so-called saddle-shaped distribution under the subcooled void condition in which the water-steam two-phase flow is still present, and the void ratio tends to increase in the outer peripheral portion.

Since the rate at which voids are supplied to the annular portion is high, there is an advantage that voids can be effectively accumulated in the annular portion. The void that has flowed into the central coolant flow path flows out through the flow path 38. As the void ratio flowing from the inlet increases, the void accumulation portion 37 and the coolant channel both have a high void ratio, and the neutron leakage rate increases.

In the conventional neutron leak tube 41 shown in FIG. 15, when the cylindrical void accumulating portion 42 for accumulating voids is provided at the center, compared with the case of the annular void accumulating portion 37 of the present embodiment. Thus, the area ratio of the void ratio can be increased as follows. The diameter of the outer tube of the neutron leak tube 41 of FIG. 15 is D, the width of the outer wall is d as a concentric flow channel of the prior art, and the flow channel diameter of the central part of the present embodiment is 2 d. Comparing the area ratios of the void accumulation portions, the prior art is 1-4 d / D, and the present invention is 1-4 d ² / D ² .

For example, when d / D = 0.1 (10%), the cross-sectional area ratio occupied by voids is 60% in the prior art and in the present embodiment.
It is 96%, which indicates that the neutron leak tube of the present embodiment has a void cross-sectional area ratio of 1.5 times or more as compared with the conventional technology, and can effectively accumulate voids.

Next, a thirteenth embodiment of the present invention will be described with reference to FIGS. 16 and 17A and 17B. FIG. 16 is a bird's-eye view of the neutron leak tube 43 for explaining the present embodiment. In the present embodiment, a neutron leakage tube 44, a lower neutron leakage tube 45, a lower neutron leakage tube 45, an upper annular void accumulating portion 46 which becomes a high void fraction region when a water-steam two-phase flow flows therein, -The coolant flow path 47 of the upper neutron leakage pipe into and out of which the vapor two-phase flow flows in, and the lower annular void accumulation part 4 that becomes a high void fraction region when the water-vapor two-phase flow flows in
8. The coolant flow path 49 of the lower neutron leak pipe, the small hole 50 on the upper surface of the neutron leak pipe and the water-
And a heating element 39 that supplies heat to supply a two-phase steam flow.

Under conditions where the core power is low and the amount of heat generated by the heating element is small, the water-steam two-phase flow having a relatively small void ratio flows, so that the steam flowing into the annular portion accumulates in the annular portion and becomes high. It becomes a void fraction region. Regarding the distribution of steam (voids) in the cross section of the inlet, the space distribution of voids generally becomes a so-called saddle-shaped distribution under the subcooled void condition in which the water-steam two-phase flow is still present, and the void ratio tends to increase in the outer peripheral portion.

The rate at which voids are supplied to the annular portion is increased, and there is a merit that voids can be effectively accumulated in the annular portion. In the lower annular void accumulating portion 48 in the lower neutron leak tube 45, a high void fraction region is formed by the above-described function under the condition that the coolant becomes a two-phase flow.

On the other hand, at the upper part of the neutron leak pipe 44, the path through which the flow containing steam (void) flows into the upper void accumulating part 46 depends on the coolant flow path diameter of the upper coolant flow path and the lower coolant flow path 49. It is an annular portion (flow path width (d2-d1) / 2) due to the difference between d2 and d1.

Therefore, when the core power increases and the void fraction in the coolant in the lower coolant passage 49 increases, the amount of voids flowing into the void accumulation section 4-12 increases, and the high void fraction region is increased. As a result, the neutron leakage rate increases.

FIGS. 17A and 17B show the void fraction in the neutron leak tube 43 of the present embodiment in a reactor operating state. Fig. 17
17A shows a state where the core power is low and the void ratio in the coolant is low, and FIG. 17B shows a state where the core power is high and the void ratio in the coolant is high.

In the lower neutron leak tube, since the void accumulation portion is in a high void ratio region, a high void ratio is obtained regardless of whether the core power is high or low. Due to the difference in the void ratio flowing into the upper void accumulation section from the annular flow path (flow path width (d2-d1) / 2) due to the difference in the diameter of the coolant flow path between the upper and lower parts, the upper The void ratio of the accumulation section increases.

In the case of a boiling water reactor, the lower part has a relatively low void ratio because subcooled water enters from the inlet. Therefore, the neutron leakage effect is high when the lower void accumulation portion is set to the high void ratio region even when the core power is low.

Next, a fourteenth embodiment of the present invention will be described with reference to FIG. FIG. 18 is a bird's-eye view of a neutron leak tube for describing the present embodiment. In this embodiment, the neutron leak tube 51, the upper neutron leak tube 44, the lower neutron leak tube 45,
The upper annular void accumulation part 46 which becomes a high void fraction region due to the inflow of the steam two-phase flow, the water-steam two-phase flow flows in,
The coolant channel 47 of the upper neutron leaking tube flowing out, the narrow channel 52 of the upper coolant channel, the widening channel 53 of the upper coolant channel,
The lower annular void accumulation part 48 which becomes a high void fraction region due to the inflow of the water-steam two-phase flow, the coolant passage 49 of the lower neutron leak pipe into which the water-steam two-phase flow flows in and out, and the water- And a heating element 39 for supplying heat to supply a two-phase steam flow.

Under the condition where the core power is low and the heat generation amount of the heating element is small, the water-steam two-phase flow having a relatively small void ratio flows, so that the steam flowing into the annular portion accumulates in the annular portion and becomes high. It becomes a void fraction region. The distribution of steam (voids) in the cross section of the inlet is generally a so-called saddle-shaped distribution under the subcooled void condition where the water-steam two-phase flow is still in the subcooled void condition. Since the rate at which voids are supplied is high, there is an effect that voids can be effectively accumulated in the annular portion.

In the void accumulating portion of the lower neutron leak tube 45, a high void ratio region is formed by the above-described function under the condition that the coolant flows in two phases. On the other hand, at the upper part of the upper neutron leakage pipe 44, the coolant flow at the lower part of the spreading flow path 53 of the upper coolant flow path flows into the upper coolant flow path 47 under the condition that the void ratio in the coolant is low. Only little accumulation of voids.

The flow rate having a high void rate becomes large flow resistance at the throat (minimum flow path diameter dt) of the venturi-type flow path where the narrow flow path 52 and the widening flow path 53 are continued, and the flow rate is reduced. Then, a flow containing steam (voids) flows into the upper void accumulating portion 46 to form a high void fraction region, and the neutron leakage rate increases.

The pressure is recovered by setting the narrowing angle and the widening angle of the venturi-type flow path to, for example, an angle (about 10 ° or less) at which flow separation does not occur on the flow path wall, so that resistance to the flow can be suppressed. In addition, the channel area of the throat
Assuming that the area of the inlet passage is half, the throat diameter dt is dt =
It should be 0.7d2.

FIGS. 19A and 19B show the void fraction in the neutron leak tube of the present invention in the reactor operating state. FIG. 19A shows a state in which the core power is low and the void ratio in the coolant is low.
(B) shows a state where the core power is high and the void ratio in the coolant is high. In the lower neutron leak tube, since the void accumulation portion is in a high void ratio region, a high void ratio is obtained regardless of whether the core power is high or low.

When the core power is high and the void fraction of the coolant flow path is high, the two-phase flow flowing from the lower coolant flow path overflows and flows into the upper void accumulation part, and the void fraction is accumulated. Therefore, as shown in FIG. 20, prior to the increase in the void ratio in the coolant flow path, the upper void accumulation portion forms a high void ratio region.

In the case of a boiling water reactor, the lower part has a relatively low void ratio because subcooled water enters from the inlet. Therefore, the neutron leakage effect is high when the lower void accumulation portion is set to the high void ratio region even when the core power is low.

Next, FIG. 21 and FIG. 22 show the fifteenth embodiment of the present invention.
An embodiment will be described. FIG. 21 is a bird's-eye view of the neutron leak tube 54 for explaining the present embodiment. In the present embodiment, an annular void accumulating portion 55 is provided in a lower portion of the neutron leak tube 54, a coolant flow path 56 is provided in the annular void accumulating portion 55, and a fuel region 57 is provided in an upper portion. A heating element 39 is provided below.

Therefore, in the case of a core in which the coolant region is reduced to form a core having a high neutron spectrum and, for example, the fuel rods are densely arranged, the water density in the coolant is large in order to effectively remove water. A neutron leak tube with a neutron leak function will be provided below the core. Providing a neutron leak tube is not advantageous in terms of fuel cycle cost because the space for arranging fuel rods is reduced.

Therefore, as in the present embodiment, the fuel region is increased by arranging a plurality of fuel rods with the upper part of the neutron leakage pipe 54 as the fuel region 57. By providing such a neutron leak tube 54, it is possible to provide a reactor core having reduced fuel cycle cost and reduced void reactivity.

FIG. 22 shows a normal void fraction distribution in the fuel assembly height direction in a reactor operating state. In the case of a boiling water reactor, the lower part has a relatively low void fraction because subcooled water enters from the inlet. Therefore, it is possible to effectively remove water by providing a neutron leak tube at the bottom. Further, the fuel cycle cost is improved by arranging a plurality of fuel rods with the upper portion as a fuel region.

[0085]

According to the present invention, it is possible to easily bleed the air inside the neutron leak tube without impairing the function of the neutron leak tube in the operating state, and to accumulate voids inside the neutron leak tube when the output is increased. In addition, it is possible to smoothly escape the steam from the lower part of the neutron leak tube. Further, the neutron leak tube of the present invention has a void cross-sectional area ratio of 1.5 times or more as compared with the prior art, and can effectively accumulate voids.

As a result, an abnormal increase in the dissolved oxygen concentration in the coolant can be suppressed, and furthermore, it is generated by the activation of water molecules and the reaction between the cladding tube and water, and is trapped inside the neutron leak tube. Hydrogen can be removed, the danger of explosion can be eliminated, and the void reactivity does not change abruptly with time, so that a bad influence on the soundness of the cladding tube can be eliminated. In addition, it is possible to reduce the void reactivity and reduce the core power even during a transient change without affecting the normal operation, thereby further improving the thermal margin of the core.

Thus, not only can the breeding be increased in the water-cooled reactor and the utilization rate of uranium resources can be significantly increased, but also the void reactivity has a negative value at the same size as the radial size of the conventional core. Therefore, it is possible to construct a nuclear reactor that simultaneously satisfies environmental protection, safety and economy.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view for explaining a first embodiment of a fuel assembly according to the present invention.

FIG. 2 is an enlarged longitudinal sectional view showing a neutron leak tube in FIG. 1;

FIG. 3A is a schematic longitudinal sectional view for explaining the behavior of a coolant at a low output of the neutron leak tube in FIG. 1;
Is a schematic longitudinal sectional view for explaining a high output state.

4A is a longitudinal sectional view showing a main part of a second embodiment of a fuel assembly according to the present invention, and FIG. 4B is an enlarged longitudinal sectional view showing a portion A in FIG.

FIG. 5 is a longitudinal sectional view showing a main part of a third embodiment of the fuel assembly according to the present invention.

FIG. 6 is a longitudinal sectional view showing a main part of a fourth embodiment of the fuel assembly according to the present invention.

FIG. 7 is a longitudinal sectional view showing a main part of a fifth embodiment of the fuel assembly according to the present invention.

8A is a longitudinal sectional view showing a main part of a fuel assembly according to a sixth embodiment of the present invention, and FIG. 8B is a sectional view taken along the line AA in FIG. 8A. Cross-sectional view.

9A is a longitudinal sectional view showing a main part of a fuel assembly according to a seventh embodiment of the present invention, and FIG. 9B is a sectional view taken along the line AA in FIG. 9A. Cross-sectional view.

FIG. 10A is a longitudinal sectional view showing a main part of an eighth embodiment of the fuel assembly according to the present invention, and FIG. 10B is a longitudinal sectional view of a conventional example for comparing the operation of FIG. FIG.

FIG. 11 is a longitudinal sectional view showing a main part of a ninth embodiment of a fuel assembly according to the present invention.

FIG. 12 is a longitudinal sectional view showing a main part of a tenth embodiment of a fuel assembly according to the present invention.

13A is a longitudinal sectional view showing a main part of an eleventh embodiment of a fuel assembly according to the present invention, and FIG. 13B is a top view of the large-diameter upper lid in FIG.

FIG. 14 is a longitudinal sectional view showing a main part of a twelfth embodiment of a fuel assembly according to the present invention.

FIG. 15 is a perspective view for explaining the operation of the neutron leak tube in FIG. 14 in comparison with a conventional example.

FIG. 16 is a perspective view showing a main part of a thirteenth embodiment of a fuel assembly according to the present invention.

FIG. 17 (a) is a void ratio distribution diagram in a state where the core power is low (the void ratio in the coolant is low) in the thirteenth embodiment, and FIG. 17 (b) is a state where the core power is high (coolant) (Void ratio is high in the middle).

FIG. 18 is a perspective view showing a main part of a fuel assembly according to a fourteenth embodiment of the present invention.

FIG. 19 (a) is a void ratio distribution diagram in a state where the core power is low (the void ratio in the coolant is low) in the fourteenth embodiment, and FIG. 19 (b) is a state where the core power is high (coolant) (Void ratio is high in the middle).

FIG. 20 is a characteristic diagram showing a change in a void ratio of an upper void accumulation part of a neutron leak tube according to a fourteenth embodiment.

FIG. 21 is a longitudinal sectional view showing a main part of a fifteenth embodiment of the fuel assembly according to the present invention.

FIG. 22 is a curve diagram showing a relationship between a void fraction distribution in a nuclear reactor and a height direction of a neutron leak tube in a fifteenth embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Channel box, 2 ... Fuel rod, 3 ... Neutron leakage pipe, 4 ... Upper end plug, 5 ... Tapered coolant outlet, 6
... lower end plug, 7 ... coolant inflow hole, 8 ... inflow coolant, 9 ...
Outflow coolant, 10: coolant (liquid phase), 11: coolant (bubbles), 12: coolant (steam), 13: step-shaped groove, 14: sealed thin tube bundle, 15: closed thin tube, 16: thin honeycomb Structure, 17 hollow part, 18 passage hole, 19 heating element, 20 shape memory alloy tube,
21 ... generated steam, 22 ... outside flow path, 23 ... air, 24 ... neutron leakage pipe, 25 ... fuel rod cladding tube, 26 ... top lid, 27 ... metal cylinder, 28 ... metal cylinder, 29 ... plate, 30… Gap,
31 ... hydrogen, 32 ... vapor bubbles, 33 ... curved surface (R), 34 ... lateral hole, 35 ... large diameter lid, 36 ... vertical hole, 37 ... annular void accumulation part, 38 ... coolant flow path, 39 ... Heating element, 40 ... inner tube,
41: Conventional neutron leak tube, 42: Cylindrical void accumulation part, 43
... neutron leak pipe, 44 ... upper neutron leak pipe, 45 ... lower neutron leak pipe, 46 ... upper annular void accumulation section, 47 ... upper coolant flow path, 48 ... lower annular void accumulation section, 49 ... Lower coolant channel, 50: missing number, 51: neutron leakage tube, 52: upper coolant channel narrowing channel, 53: upper coolant channel spreading channel, 54: neutron leak tube, 55 … Annular void accumulation part,
56: coolant passage, 57: fuel area.

Continued on the front page. (72) Inventor Nao Hayashi, Narabayashi, Yokohama, Kanagawa Prefecture, 8-8 Shinsugita-cho, Inc. Inside the Toshiba Yokohama Office (72) Inventor Toru Mitsutake, 8-8 Shinsugitacho, Isogo-ku, Yokohama, Kanagawa, Japan Yokohama office

Claims (15)

[Claims] 1. A fuel assembly comprising a channel box, a plurality of fissile material-loaded fuel rods disposed in the channel box, and a neutron leak tube disposed between the plurality of fuel rods. The neutron leakage pipe has a plurality of tapered coolant outflow holes whose upper and lower ends are closed by an upper end plug and a lower end plug, and the upper end plug has an upper end narrowed, and the lower end plug or its vicinity. A fuel assembly having a coolant inlet hole in a fuel assembly. 2. The fuel assembly according to claim 1, wherein a stepped groove is provided on an inner surface of the tapered coolant outflow hole. 3. A fuel assembly comprising a plurality of fuel rods loaded with fissile material and a neutron leak tube.
A fuel assembly comprising: a flow passage through which a coolant flows from a lower portion to an upper portion inside the neutron leak tube; and a sealed bundle of a plurality of small tubes whose both ends are closed. 4. The fuel assembly according to claim 3, wherein a thin honeycomb sealing structure is provided inside the neutron leak tube. 5. A fuel assembly comprising a plurality of fuel rods loaded with fissile material and a neutron leakage tube, wherein the neutron leakage tube has an open lower end, an upper end closed with an upper lid, and a lower end open inside. The coolant flows from the upper part to the upper part, the upper lid is constituted by joining a tube formed of a shape memory alloy, and a heating element by neutron absorption or gamma ray absorption is provided below the neutron leak tube. Fuel assembly. 6. A metal cylinder is attached to the upper lid in place of the tube made of the shape memory alloy, and a metal cylinder made of a metal having a higher thermal expansion coefficient than the metal of the metal cylinder is placed in the metal cylinder. The fuel assembly according to claim 5, wherein a plurality of plates are provided on an upper portion and a lower portion of the metal cylinder. 7. The fuel assembly according to claim 5, wherein the upper lid is made of a nickel plate. 8. The fuel assembly according to claim 5, wherein a corner of a lower end opening of the neutron leak tube is chamfered into a spherical shape. 9. The fuel assembly according to claim 5, wherein a plurality of holes are provided near a lower end opening of the neutron leak tube. 10. The apparatus according to claim 5, wherein the upper lid is formed to have a larger diameter than the outer diameter of the neutron leakage pipe, and a flow path outside the neutron leakage pipe is narrowed near the large diameter upper lid. 10. The fuel assembly according to claim 9, wherein 11. The fuel assembly according to claim 10, wherein a plurality of holes are provided in the large-diameter upper lid. 12. A fuel assembly comprising a plurality of fuel rods loaded with fissile material and a neutron leak tube.
The neutron leak tube is provided with an inner tube that annularly surrounds a vertical coolant flow path inside, the inner tube is open at an inlet, has a closed or fine hole at an outlet, and has an annular shape outside the inner tube. A fuel assembly wherein a void accumulation part is formed. 13. A fuel assembly comprising a plurality of fuel rods loaded with fissile material and a neutron leak tube.
The neutron leak tube has an upper neutron leak tube and a lower neutron leak tube in a two-stage structure in the height direction, and a coolant flow path diameter of the upper neutron leak tube is smaller than that of the lower neutron leak tube. The void accumulation portion of the outer annular portion of the upper neutron leakage tube is smaller than the diameter of the coolant passage, and the void ratio in the coolant passage of the lower neutron leakage tube is quickly increased. A fuel assembly comprising a void accumulation mechanism. 14. The coolant flow path of the upper neutron leak pipe comprises a narrowed flow path and a widened flow path, and the two-phase flow flowing through the coolant flow path of the upper neutron leak pipe increases as the void fraction increases, and 14. The fuel assembly according to claim 13, wherein the annular void accumulating portion of the neutron leak tube has a void accumulating mechanism that becomes a high void ratio region. 15. A fuel assembly comprising a plurality of fuel rods loaded with fissile material and a neutron leak tube.
A plurality of fuel rods are arranged with an upper portion in the neutron leak tube as a fuel region, an annular void accumulation portion is provided in a lower portion of the neutron leak tube, and a coolant channel is provided in the annular void accumulation portion. Fuel assembly.