JP5106344B2 - Fuel assembly - Google Patents

Description

  The present invention relates to a fuel assembly for a water-cooled nuclear reactor, and more particularly to a fuel assembly for making a void reactivity negative.

  In general, the core of a water-cooled nuclear reactor is composed of a large number of fuel assemblies in which a large number of fuel rods loaded with fissile material (fuel) are bound, and cooling for removing heat generated by the fissile material. Water is used as a material.

  Because water has a large neutron moderating ability for hydrogen atoms contained in water, high-energy neutrons generated by nuclear fission are greatly decelerated in conventional water-cooled nuclear reactors with a large proportion of water, and thermal neutrons with low energy are large neutrons. The part is tightened.

  When a fissile material absorbs neutrons with low energy, the ratio of the capture reaction that does not cause fission and is captured in the nucleus increases rather than the fission reaction that generates about three neutrons. That is, the number of neutrons generated per neutron absorption is reduced by fission by low energy neutrons.

  On the other hand, with high-energy neutrons, the rate of capture reaction is small, so the average number of neutrons generated per absorption can be 2 or more, including the effect of capture, and one is used to maintain the chain reaction. The remaining one can be absorbed by a parent material such as U-238 to efficiently produce a fissile material. If the ratio of generation and extinction of this fissile material is 1 or more, it means that the fuel has been propagated, and breeding reactors have been developed in various countries from the viewpoint of securing resource energy.

  However, in a conventional water-cooled nuclear reactor, the ratio of water to fuel is large and neutrons have low energy. Therefore, breeding cannot be performed, and the ratio of generation and extinction of fissile material (referred to as the proliferation ratio. If it is 1 or less, it is referred to as a conversion ratio, but here it is simply referred to as a growth ratio.) Was 1 or less (about 0.5). For this reason, in the case of a breeder reactor, in principle, only about 1% of uranium resources that can be converted to 100% heat energy can be used.

  However, if the high energy spectrum is used, the reactivity due to boiling of the coolant (void reactivity) may be positive as in a conventional large fast reactor. In water-cooled nuclear reactors, it is important to make the void reactivity negative in terms 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 the fuel assembly to reduce the 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 number of fuel rods are aligned in a cylindrical channel box having a regular hexagonal cross section, a neutron leak tube is arranged along the center axis, and cooling water flows upward from below. It is configured to flow to. The void fraction of the coolant flowing inside the neutron leak tube increases as the power of the core rises, so neutron leakage increases. This effect reduces the void reactivity.

  However, in a conventional water-cooled nuclear reactor, a fuel assembly having a neutron leakage tube in the channel box to increase the proliferation ratio and at the same time have a negative void reactivity is used for the activation of water molecules and the cladding tube. Hydrogen is generated by the reaction with water, and this hydrogen is trapped inside the neutron leak tube. In the worst case, there is a risk of explosion.

  In addition, in the conventional neutron leak tube, the corner at the lower end is acute, so a large amount of vapor accumulated in the neutron leak tube suddenly or unevenly escapes. Adversely affects sex. At the time of transient change, the thermal integrity of the core deteriorates, 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 perform neutron leakage, it is necessary to increase the area ratio of the neutron leakage tube to the entire cross-sectional area composed of the coolant channel and the neutron leakage tube. In the conventional configuration in which the coolant channel is an annular part and the neutron leak tube is provided in the center, the inner wall of the coolant channel in the annular part (the outer wall of the neutron leak tube is increased when the cross-sectional area ratio of the neutron leak tube is increased. ) And the outer wall are narrowed, the diameter of the flow path is narrowed, the flow resistance is increased, the flow rate flowing through the coolant flow path is decreased, and the flow velocity is increased to cause a problem of hydrodynamic vibration. .

  Furthermore, as the core characteristics of the dense bundle, the output increases, the void ratio increases, and the void reactivity (reactivity increase ratio with respect to the void ratio increase) tends to be positive. Therefore, it is necessary to increase the void rate of the core and increase the neutron leakage rate in the neutron leakage tube.

  The present invention has been made in view of such circumstances, and a fuel assembly for a cooled nuclear reactor whose performance has been improved in a water-cooled nuclear reactor having an increased proliferation ratio and negative void reactivity at the same time. The purpose is to provide.

Another object of the present invention is to provide a fuel assembly that can effectively store voids inside the neutron leak tube when the output is increased without impairing the function of the neutron leak tube in the operating state.
It is another object of the present invention 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.

The invention corresponding to claim 1 is a channel box, a plurality of fuel rods arranged in the channel box and loaded with fissile material, and arranged between the plurality of fuel rods and connected in the height direction. A neutron leak tube composed of an upper neutron leak tube and a lower neutron leak tube, small holes respectively provided on the upper surfaces of the upper neutron leak tube and the lower neutron leak tube, and connected to each small hole inside An inner tube in which a vertical coolant channel is formed; a void accumulation portion formed between each inner tube and the upper neutron leak tube and lower neutron leak tube; and the lower neutron leak tube of a heating element by neutron absorption or gamma ray absorption having a plurality of flow path hole provided in the lower, a fuel assembly provided with the diameter of the coolant channel of the neutron leakage tube of the upper of the lower Characterized in that less than the diameter of the coolant channel of the neutron leakage tube.

  According to the present invention, the neutron leakage tube having a cross-sectional structure in which the coolant channel is arranged in the central portion and the high void ratio region having a neutron leakage function is an annular portion is arranged in two stages in the axial direction. In order to increase the void ratio in the void accumulation region prior to the increase in the void ratio of the coolant, the diameter of the coolant channel in the central portion is larger at the upper portion and smaller at the lower portion.

  The invention corresponding to claim 2 is characterized in that the coolant flow path of the upper neutron leakage tube is formed by a narrow flow path and a wide flow path.

  According to the present invention, the upper coolant channel is made narrower and the venturi-type channel continues and the flow channel continues, so that the void ratio in the coolant channel is increased and the upper annular void accumulation portion is High void area.

  According to the present invention, voids can be accumulated inside the neutron leak tube when the output is increased without impairing the function of the neutron leak tube in the operating state. Furthermore, the neutron leakage tube of the present invention can obtain a void cross-sectional area ratio 1.5 times or more that of the prior art, and can effectively accumulate voids.

  As a result, an abnormal increase in dissolved oxygen concentration in the coolant can be suppressed, and the void reactivity can be prevented from changing rapidly in time, and adverse effects on the soundness of the cladding tube can be eliminated. In addition, the void reactivity can be reduced even during a transient change without affecting the normal operation, the core output can be reduced, and the thermal margin of the core can be further improved.

  Therefore, not only can the proliferation be increased in water-cooled reactors, but the utilization rate of uranium resources can be greatly increased as compared to the conventional one, and the void reactivity should be negative with a size equivalent to the radial size of the conventional core. Therefore, a nuclear reactor that can simultaneously satisfy environmental protection, safety, and economic efficiency can be configured.

  A first embodiment of a fuel assembly according to the present invention will be described with reference to FIGS.

  As shown in FIG. 1, the present embodiment is 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. . Although an example of a 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 neutron leakage tube 3 is closed at both upper and lower ends by an upper end plug 4 and a lower end plug 6, and has a plurality of tapered coolant outlet holes 5 whose upper ends are constricted at the upper end plug 4. A coolant inflow hole 7 is provided in the vicinity of 6. Thereby, when the void ratio rises in the neutron leakage tube, the flow resistance of the outflow hole 5 at the upper end portion increases, making it difficult to remove the void in the neutron leakage tube.

  FIG. 3 shows the state of the coolant inside the neutron leakage tube 3. The inflow coolant 8 that has flowed in from the inflow hole 7 generates heat due to radiation such as gamma rays. Therefore, by appropriately setting the sizes of the inflow hole 7 and the outflow hole 5, as shown in FIG. Voiding becomes possible.

  In that case, since the heat generation amount changes depending on the output of the core, the void ratio is low in the neutron leakage tube 3 as shown in FIG. 3A at the time of low output, but FIG. ) And the void ratio inside the neutron leak tube 3 increases. As the void ratio increases, the flow resistance of the tapered coolant outflow hole 5 increases, so that it is difficult to escape the void in the neutron leakage tube 3. Therefore, the effect of reflecting neutrons passing through this space is reduced and the leakage of neutrons is increased, so that the void reactivity can be reduced.

  Next, a second embodiment according to the present invention will be described with reference to FIGS.

  This embodiment is obtained by improving the upper end plug 4 in the first embodiment. Since other parts are the same as those in the first embodiment, the description of the overlapping parts is omitted. That is, as shown in FIG. 4 (b), the surface of the tapered coolant outflow hole 5 provided in the upper end plug 4 is roughened, and the step-like groove that resists the flow of the coolant flowing through the inside is provided. 13 is provided as a step-like tapered outflow hole.

  According to the present embodiment, by increasing the flow resistance at the time of void outflow, void outflow inside the neutron leakage tube at high output is further suppressed, thereby further increasing neutron leakage and increasing the void 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 the present embodiment, as shown in FIG. 5, the coolant flows upward in the neutron leak tube 3a shown in the first embodiment, and the inside of the neutron leak tube 3 is The structure of the closed thin tube bundle 14 in which the thin closed thin tubes 15 whose both ends are closed is bundled, and other parts are the same as those in the first embodiment.

  According to the present embodiment, since the thickness of the sealed narrow tube 15 is small, it can be reduced to about 0.2 mm. Therefore, the absorption of neutrons by the sealed capillary 15 can be suppressed, and the hollow in the neutron leakage tube can be stably maintained regardless of the flow conditions, so the effect of reflecting neutrons passing through this space is reduced, Since neutron leakage increases, void reactivity can be reduced.

Next, a fourth embodiment according to the present invention will be described with reference to FIG.
This embodiment conforms to the third embodiment, and FIG. 6 shows the longitudinal sectional shape of the neutron leakage tube 3b according to the present embodiment. That is, in FIG. 6, since the inside of the neutron leak tube 3b has a thin-walled honeycomb sealing structure 16, neutron absorption can be suppressed and the hollow in the neutron leak tube 3b can be stably maintained regardless of the flow conditions. Since the effect of reflecting neutrons passing through this space is reduced and leakage of neutrons is increased, void reactivity can be reduced.

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

  In the present embodiment, as shown in FIG. 7, a heating element 19 having a neutron leak tube 3c and a flow path hole 18 formed of a material that generates heat by neutron or gamma ray absorption, for example, hafnium, stainless steel, or the like below. And a shape memory alloy tube 20 made of a shape memory alloy at the upper part of the neutron leakage tube 3c. At normal temperature, the inside and outside of the neutron leakage tube 3 c communicate with each other through a shape memory alloy tube 20.

  If there is no shape memory alloy tube 20 as in the prior art, it cannot be removed because there is no passage through which the air 23 trapped inside the neutron leak tube 3c escapes. For this reason, the air 23 dissolves in the coolant, and the dissolved oxygen concentration of the coolant increases, which adversely affects the fuel cladding tube.

  In the present embodiment, the shape memory alloy pipe 20 is communicated at normal temperature, and the shape memory alloy pipe 20 is closed in the operating state (that is, the coolant temperature is high). That is, all the air 23 goes out through the shape memory alloy pipe 20 at room temperature.

  When the temperature becomes high, the shape memory alloy tube 20 is sealed, so that the steam 21 generated by heating by the heating element 18 flows into the neutron leakage tube 3 c until the inside of the neutron leakage tube 3 c is filled with the vapor 24. . On the other hand, after the inside of the neutron leakage tube 3 c is filled with the vapor 24, most of the generated vapor 21 passes through the outer flow path 22 between the fuel rod cladding tube 25.

Next, a sixth embodiment of the present invention will be described with reference to FIGS.
Fig.8 (a) is a longitudinal cross-sectional view of the neutron leak tube in this Embodiment, (b) is AA arrow sectional drawing of (a).

  In this embodiment, a metal cylinder 27 is provided at the center of the upper cover 26 of the neutron leakage tube 3d, and a metal cylinder 28 made of a metal having a larger thermal expansion coefficient than the metal of the metal cylinder 27 is installed inside the metal cylinder 27. In addition, a plurality of plates 29 are installed above and below the metal cylinder 29 as shown in FIG.

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

  That is, all the air 23 goes out through the gap 30 in the normal temperature state. When the temperature is high, the gap 30 is sealed, so that the steam 21 heated and generated 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 leakage tube 3d is filled with the steam 24, most of the generated steam 21 passes through the outer flow path 22.

Next, a seventh embodiment of the present invention will be described with reference to FIGS.
Since this embodiment conforms to the sixth embodiment, the same parts in FIG. 9 as those in FIG.

  In this embodiment, the upper lid 26 of the neutron leakage tube 3d is made of Ni. In the operating state, as shown in FIG. 9, hydrogen 31 is generated by the activation of water molecules and the reaction between the cladding tube and the water, and the hydrogen 31 is trapped inside the neutron leakage tube 3d and, in the worst case, explodes. There is a risk. In this embodiment, the upper lid 26 of the neutron leak tube 3d is made of Ni. In this configuration, since the hydrogen atoms are small, the hydrogen 31 can be removed from the Ni lid 26 and removed.

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

  The basic configuration of the present embodiment is the same as that of the embodiment shown in FIGS. In the neutron leakage tubes 3c and 3d described above, as shown in FIG. 10 (b), since the lower end has an acute angle, a large amount of vapor bubbles 32 accumulate and suddenly or unevenly escape. May suddenly change and adversely affect the soundness of the cladding. Therefore, in the present embodiment, as shown in FIG. 10A, the corners of the neutron leak tube are rounded and formed on the curved surface 33. According to the present embodiment, it is possible to smooth out the escape of the vapor and to alleviate the rapid time change of the void reactivity described above.

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 in the lower part of the neutron leakage tube 3c or 3d. According to the present embodiment, it is possible to smooth out the escape of steam and to alleviate the rapid time change of the void reactivity described above.

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 leakage tube for explaining the present embodiment.
Although the basic configuration of the present embodiment is the same as that of the embodiment of FIGS. 7 to 11, the present embodiment is that the upper lid 35 of the neutron leakage tube 3c or 3d has a large diameter. That is, the outer flow path 22 is throttled at the exit of the neutron leak tube 3c or 3d by the large-diameter upper lid 35 having an enlarged outer diameter of the upper lid.

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

  According to the present embodiment, the portion of the end portion B of the upper lid 35 is narrowed down in order to increase the void ratio of the outer flow path 22 and suppress the void reactivity. At the time of transient change, the amount of generated voids increases compared to normal. If the upper part is narrowed, the amount of voids passing therethrough increases during a transient change, resulting in an increase in pressure loss. That is, it becomes difficult for the vapor to escape, and as a result, the void ratio of the outer flow path 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).
In this embodiment, in the tenth embodiment, the large-diameter upper lid 35 is provided with a plurality of vertical holes 36 as shown in FIG. 13 (b). In the configuration shown in FIG. 12, during normal times, voids accumulate at the corners and abruptly escape, and the change in the voids 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 voids during normal operation.

Next, a twelfth embodiment of the present invention will be described with reference to FIG.
FIG. 14 schematically shows the neutron leakage tube 3e in the present embodiment in a bird's-eye view. That is, in the present embodiment, an annular void accumulating portion 37 that becomes a high void ratio region when the water-steam two-phase flow flows into the neutron leakage pipe 3e, and the coolant that flows into and out of the water-steam two-phase flow. It is composed of a flow path 38, a heating element 39 for supplying heat to supply a water-steam two-phase flow, and an inner tube 40.

  When the core power is low and the heat generation amount of the heating element is small, the water-steam two-phase flow with a relatively small void rate flows in, so that the steam flowing into the annular part accumulates in the annular part and has a high void rate region. It becomes. As for the distribution of steam (void) in the cross section of the inlet, the void spatial distribution generally has a so-called saddle-type distribution under the subcooled void condition in which the water-steam two-phase flow is still under the condition, and the void ratio tends to increase at the outer periphery.

  Since the proportion of voids supplied to the annular part is high, there is an advantage that voids can be effectively accumulated in the annular part. Further, the void flowing into the central coolant channel flows out through the coolant channel 38. As the void rate flowing from the inlet increases, both the void accumulation part 37 and the coolant channel have a high void rate, and the neutron leakage rate increases.

In the conventional neutron leakage tube 41 shown in FIG. 15, in the case where the cylindrical void accumulating portion 42 for accumulating voids is provided at the center, the following is compared to the case of the annular void accumulating portion 37 of the present embodiment. Thus, the area ratio of the void ratio can be increased. The diameter of the outer tube of the neutron leakage tube 41 in FIG. 15 is D, the flow path of the conventional technology is a concentric flow path, the width with the outer wall is d, and the flow path diameter at the center of this embodiment is 2d. Comparing the area ratio of the void accumulation part, the prior art is 1-4d / D and the present invention is 1-4d ² / D ² .

  For example, when d / D = 0.1 (10%), the cross-sectional area ratio occupied by the void is 60% in the conventional technique and 96% in the present embodiment, and the neutron leakage tube of the present embodiment is compared with the conventional technique. It can be seen that a void cross-sectional area ratio of 1.5 times or more can be obtained, and voids can be accumulated effectively.

Next, a thirteenth embodiment of the present invention will be described with reference to FIG. 16 and FIGS. 17 (a) and 17 (b).
FIG. 16 is a bird's-eye view of the neutron leakage tube 43 for explaining the present embodiment. In this embodiment, the neutron leakage tube 44 at the upper part of the neutron leakage tube 43, the neutron leakage tube 45 at the lower part, the upper annular void accumulating part 46 that becomes a high void ratio region by the flow of the water-vapor two-phase flow, the water -Coolant flow path 47 of the upper neutron leak pipe that flows in and out of the steam two-phase flow, the lower annular void accumulating section 48 that becomes a high void ratio region by the flow of water-steam two-phase flow, water-steam Consists of a coolant flow path 49 in the lower neutron leak pipe that flows in and out of the two-phase flow, a small hole 50 on the upper surface of the neutron leak pipe, and a heating element 39 that supplies heat to supply the water-vapor two-phase flow Has been.

  When the core power is low and the heat generation amount of the heating element is small, the water-steam two-phase flow with a relatively small void rate flows in, so that the steam flowing into the annular part accumulates in the annular part and has a high void rate region. It becomes. As for the distribution of steam (void) in the cross section of the inlet, the void spatial distribution generally has a so-called saddle-type distribution under the subcooled void condition in which the water-steam two-phase flow is still under the condition, and the void ratio tends to increase at the outer periphery.

  There is a merit that the proportion of voids supplied to the annular part increases, and voids can be effectively accumulated in the annular part. In the lower annular void accumulation part 48 in the lower neutron leakage tube 45, a high void ratio region is obtained by the above-described function under the condition that the coolant becomes a two-phase flow.

  On the other hand, in the upper part of the neutron leak tube 44, the path through which the flow containing the vapor (void) flows into the upper void accumulation part 46 is the coolant channel diameter d2, of the upper coolant channel 47 and the lower coolant channel 49. An annular portion (channel width (d2-d1) / 2) due to the difference in d1.

  Therefore, if the core power increases and the void rate in the coolant in the lower coolant channel 49 increases, the amount of void inflow into the void accumulation section increases and forms a high void rate region, so the neutron leakage rate Becomes higher.

  17 (a) and 17 (b) show the void ratio in the neutron leak tube 43 of the present embodiment in the reactor operation state. FIG. 17A shows a state in which the core power is low and the void ratio in the coolant is low, and FIG. 17B shows a state in which the core power is high and the void ratio in the coolant is high.

  In the lower neutron leak tube 45, since the void accumulation part 48 is in a high void ratio region, both the high core output state and the low core output state have high void ratios. Due to the difference in the void ratio flowing into the upper void accumulation part 46 from the annular flow path (flow path width (d2-d1) / 2) due to the difference between the upper and lower coolant flow path diameters, The void ratio of the void accumulation part increases.

  In the case of a boiling water reactor, subcooled water enters from the inlet, so the lower void ratio is relatively low. Accordingly, the neutron leakage effect is high when the lower void accumulating portion 48 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 explaining the present embodiment.
This embodiment includes a neutron leakage tube 51, an upper neutron leakage tube 44, a lower neutron leakage tube 45, an upper annular void accumulation part 46 that becomes a high void ratio region by the flow of water-steam two-phase flow, water-steam two Phase flow enters and exits upper neutron leak pipe coolant channel 47, upper coolant channel narrowing channel 52, upper coolant channel widening channel 53, water-steam two phase flow in In order to supply the lower annular void accumulating section 48, which becomes a high void ratio region, the water-steam two-phase flow flows in, the coolant channel 49 of the lower neutron leakage pipe that flows out, and the water-steam two-phase flow And a heating element 39 for supplying heat.

  When the core power is low and the heat generation amount of the heating element is small, the water-steam two-phase flow with a relatively small void rate flows in, so that the steam flowing into the annular part accumulates in the annular part and has a high void rate region. It becomes. The distribution of steam (voids) in the cross section of the inlet is generally a so-called saddle type distribution under the subcooled void condition in which the water-steam two-phase flow is still sub-void, and the void ratio tends to increase at the outer periphery. Since there is a high ratio of supplying voids, there is an effect that voids can be effectively accumulated in the annular portion.

  In the void accumulation part of the lower neutron leakage tube 45, a high void ratio region is obtained by the above-described function under the condition that the coolant has a two-phase flow. On the other hand, in the upper part of the upper neutron leakage tube 44, the coolant flow in the lower part of the upper coolant channel 53 flows into the upper coolant channel 47 under the condition that the void ratio in the coolant is low. Therefore, there is almost no void accumulation.

  When the void ratio increases, the flow with a high void ratio becomes a large flow resistance at the throat (minimum channel diameter dt) of the venturi-type flow path where the narrow flow path 52 and the wide flow path 53 continue, and the flow rate decreases. A flow containing (voids) flows into the upper void accumulation part 46, forms a high void rate region, and increases the neutron leakage rate.

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

  19 (a) and 19 (b) show the void ratio in the neutron leakage tube of the present invention in the reactor operating state. FIG. 19 (a) shows a state where the core power is low and the void ratio in the coolant is low, and FIG. 19 (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, the void accumulation portion is in a high void ratio region, so that the void ratio is high in both the high and low core power.

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

  In the case of a boiling water reactor, subcooled water enters from the inlet, so the lower void ratio is relatively low. Therefore, the neutron leakage effect is high when the lower void accumulating portion is in a high void ratio region even in a state where the core power is low.

Next, a fifteenth embodiment of the present invention will be described with reference to FIGS.
FIG. 21 is a bird's-eye view of the neutron leakage tube 54 for explaining the present embodiment.
In the present embodiment, an annular void accumulation part 55 is provided in the lower part of the neutron leakage tube 54, a coolant channel 56 is provided in the annular void accumulation part 55, a fuel region 57 is provided in the upper part, and the annular void accumulation part 55 A heating element 39 is provided below.

  Therefore, in the case of a core in which the coolant region is reduced in order to achieve a core configuration with a high neutron spectrum, for example, in which fuel rods are densely arranged, in order to effectively eliminate water, it is necessary to place the core in the lower part of the core where the water density in the coolant is large. A neutron leakage tube with a neutron leakage function will be provided. Providing the neutron leak tube is not advantageous in terms of fuel cycle cost because the space for arranging the 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 portion of the neutron leakage tube 54 as the fuel region 57. By providing such a neutron leakage tube 54, it is possible to provide a reactor core with reduced fuel cycle cost and reduced void reactivity.

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

1 is a cross-sectional view for explaining a first embodiment of a fuel assembly according to the present invention. The longitudinal cross-sectional view which expands and shows the neutron leak tube in FIG. (A) is a schematic longitudinal cross-sectional view for demonstrating the behavior of the refrigerant | coolant at the time of the low output of the neutron leak tube in FIG. 1, (b) is a schematic longitudinal cross-sectional view for demonstrating the time of the same high output. (A) is a longitudinal cross-sectional view which shows the principal part of 2nd Embodiment of the fuel assembly which concerns on this invention, (b) is a longitudinal cross-sectional view which expands and shows the A section in (a). The longitudinal cross-sectional view which shows the principal part of 3rd Embodiment of the fuel assembly which concerns on this invention. The longitudinal cross-sectional view which shows the principal part of 4th Embodiment of the fuel assembly which concerns on this invention. The longitudinal cross-sectional view which shows the principal part of 5th Embodiment of the fuel assembly which concerns on this invention. (A) is a longitudinal cross-sectional view which shows the principal part of 6th Embodiment of the fuel assembly which concerns on this invention, (b) is a cross-sectional view cut | disconnected and shown to the AA arrow direction in (a). (A) is a longitudinal cross-sectional view which shows the principal part of 7th Embodiment of the fuel assembly which concerns on this invention, (b) is a cross-sectional view cut | disconnected and shown to the AA arrow direction in (a). (A) is a longitudinal cross-sectional view which shows the principal part of 8th Embodiment of the fuel assembly which concerns on this invention, (b) is a longitudinal cross-sectional view of the prior art example for comparing the effect | action of (a). The longitudinal cross-sectional view which shows the principal part of 9th Embodiment of the fuel assembly which concerns on this invention. FIG. 20 is a longitudinal sectional view showing a main part of a tenth embodiment of a fuel assembly according to the present invention. (A) is a longitudinal cross-sectional view which shows the principal part of 11th Embodiment of the fuel assembly which concerns on this invention, (b) is a top view of the large diameter upper cover in (a). The longitudinal cross-sectional view which shows the principal part of 12th Embodiment of the fuel assembly which concerns on this invention. The perspective view for demonstrating an effect | action compared with the prior art example in the neutron leak tube in FIG. The perspective view which shows the principal part of 13th Embodiment of the fuel assembly which concerns on this invention. (A) is a void ratio distribution diagram when the core power in the thirteenth embodiment is low (the void ratio in the coolant is low), and (b) is also in the high core power (void ratio in the coolant). Is a void ratio distribution map. The perspective view which shows the principal part of 14th Embodiment of the fuel assembly which concerns on this invention. (A) is a void ratio distribution diagram in the 14th embodiment when the core power is low (the void ratio in the coolant is low), and (b) is also in the high core output (void ratio in the coolant). Is a void ratio distribution map. FIG. 23 is a characteristic diagram showing a change in the void ratio of the upper void accumulation part of the neutron leakage tube in the fourteenth embodiment. A longitudinal sectional view showing a main part of a fifteenth embodiment of a fuel assembly according to the present invention. FIG. 18 is a curve diagram showing a relationship between a void ratio 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 leak pipe, 4 ... Upper end plug, 5 ... Tapered coolant outlet, 6 ... Lower end stopper, 7 ... Coolant inflow hole, 8 ... Inflow coolant, 9 ... Outflow coolant, 10 ... Coolant (liquid phase), 11 ... Coolant (bubbles), 12 ... Coolant (steam), 13 ... Stepped groove, 14 ... Sealed capillary bundle, 15 ... Sealed capillary, 16 ... Thin honeycomb structure, 17 ... cavity, 18 ... channel hole, 19 ... heating element, 20 ... shape memory alloy tube, 21 ... generated steam, 22 ... outer channel, 23 ... air, 24 ... inside neutron leak tube 25 ... Fuel rod cladding tube, 26 ... Upper lid, 27 ... Metal cylinder, 28 ... Metal cylinder, 29 ... Plate, 30 ... Gap, 31 ... Hydrogen, 32 ... Steam bubble, 33 ... Curved surface (R), 34 ... Horizontal hole, 35 ... Large diameter top cover, 36 ... Vertical hole, 37 ... Annular void accumulation part, 38 ... Outflow coolant channel, 39 ... Heating element, 40 ... Inner tube, 41 ... Conventional neutron leak tube, 42 ... Cylindrical void storage 43 ... Neutron leak tube, 44 ... Upper neutron leak tube, 45 ... Lower neutron leak tube, 46 ... Upper annular void accumulator, 47 ... Upper coolant channel, 48 ... Lower annular void accumulator, 49 ... lower coolant channel, 50 ... small hole, 51 ... neutron leak tube, 52 ... narrow coolant channel in upper coolant channel, 53 ... wide channel in coolant channel in upper part, 54 ... neutron leak tube 55 ... annular void accumulating part, 56 ... coolant flow path, 57 ... fuel region.

Claims (2)

A channel box, a plurality of fuel rods arranged in the channel box and loaded with fissile material, an upper neutron leak tube and a lower neutron arranged between the fuel rods and connected in the height direction A neutron leakage tube composed of a leakage tube, small holes respectively provided on the upper surface of the upper neutron leakage tube and the lower neutron leakage tube, and a vertical coolant channel connected to each of the small holes is formed. and the tube another, wherein a void accumulating portions formed respectively between the neutron leakage tube and the lower neutron leakage tube of the upper and the inner tube, a plurality of flow provided in the lower portion of the neutron leakage tube of the lower A fuel assembly comprising a neutron absorbing or gamma ray absorbing heating element having a passage hole ,
The fuel assembly according to claim 1, wherein a diameter of a coolant channel of the upper neutron leak tube is smaller than a diameter of a coolant channel of the lower neutron leak tube.   2. The fuel assembly according to claim 1, wherein the coolant flow path of the upper neutron leakage tube is composed of a narrowed flow path and a widened flow path.

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