Gas-water separator and boiling-water nuclear reactor

Abstract

Provided are a gas-water separator and a boiling-water nuclear reactor which make it possible to reduce pressure loss while preserving gas-water separation performance of the gas-water separator.　In the gas-water separator, cooling water flowing into a reactor core is heated by a nuclear reaction of a fuel assembly to be a gas-liquid two-phase flow, which then flows into a gas-water separator through a standpipe, and the gas-liquid two-phase flow flowing through is separated into moisture-containing steam (gas phase) and water (liquid phase). A swirler having a hub and a plurality of swirl vanes is configured to have a constant hub diameter in a position of 0.5-2.0 D (D: hydraulic equivalent diameter) situated in the upstream direction from the surface of the hub at the downstream side of the flow of the gas-liquid two-phase flow.

Description

Steam separator and boiling water reactor

[0001] The present invention relates to a steam-water separator and a boiling water reactor.

[0002] In a boiling water reactor, a steam separator and other equipment, such as a steam dryer, are used to separate a mixed fluid (also called a gas-liquid two-phase fluid) of hot water (water (liquid phase)) and steam (gas phase) at the reactor core outlet into hot water (water (liquid phase)) and steam (gas phase). By reducing the pressure loss while maintaining the steam-water separation performance of the steam-water separator, it is possible to reduce the number of steam-water separators and fuel cycle costs, and also contribute to improving the stability of the reactor core.

[0003] A steam-water separator has swirl vanes installed in a narrow inlet or first inner tube called a standpipe, or a diffuser. The swirl vanes rotate a mixed fluid (gas-liquid fluid) of water (liquid phase) and steam (gas phase) passing through the steam-water separator, generating centrifugal force. Centrifugal force separates the steam (gas phase) and water (liquid phase) downstream of the two-phase flow of the swirl vanes. The water (liquid phase) is collected on the wall of the inner tube, separated by a pickoff ring, and redirected toward the water (liquid phase) outlet. In other words, the water flow toward the downstream side of the two-phase flow is reversed, redirecting the water (liquid phase) toward the upstream side. Increasing the centrifugal force generated by the swirl vanes improves steam-water separation performance, but is likely to increase pressure loss. Conversely, reducing pressure loss is likely to degrade centrifugal force and steam-water separation performance. In order to reduce pressure loss while maintaining the steam-water separation performance of the steam-water separator, it is necessary to adjust the geometric characteristics of the steam-water separator.

[0004] For example, Patent Document 1 describes a steam-water separator that reduces pressure loss by installing a swirler consisting of a hub and multiple swirl vanes in the first-stage inner cylinder, where the flow velocity is relatively low, but because separation performance deteriorates, it provides a separation performance-enhancing component (impingement plate) at the upper outlet of the steam-water separator. Patent Document 1 describes that separation performance improves when the surface diameter downstream of the hub constituting the swirler is increased. Pressure loss in a steam-water separator increases as the hub diameter increases. Because pressure loss in a steam-water separator is affected not only by the surface diameter downstream of the hub but also by the diameter upstream, a steam-water separator with a small hub diameter upstream and a large hub diameter downstream has been disclosed. Patent Documents 2 and 3 also describe steam-water separators that reduce pressure loss while maintaining the steam-water separation performance by changing the shape or installation position of the swirler or extending the hub downstream of the swirler.

[0005] JP 2000-153118 A JP 2003-114293 A JP 2012-154654 A

[0006] However, in the steam-water separator described in Patent Document 1, it is necessary to provide a component (impingement plate) for improving separation performance at the upper outlet of the steam-water separator, and although this can reduce pressure loss, there is a risk that the steam-water separation performance cannot be maintained and may actually deteriorate. Moreover, in the steam-water separators described in Patent Documents 2 and 3, no consideration is given to changing the shape of the hub between the upstream and downstream surfaces of the swirl vanes that form the swirler.

[0007] Therefore, the present invention provides a steam-water separator and a boiling water reactor that can reduce pressure loss while maintaining the steam-water separation performance of the steam-water separator.

[0008] In order to solve the above problems, the steam-water separator of the present invention is a steam-water separator in which cooling water flowing into a reactor core is heated by nuclear reactions in fuel assemblies, becoming a gas-liquid two-phase flow, which flows into the steam-water separator via a standpipe and separates the flowing gas-liquid two-phase flow into moisture-containing steam (gas phase) and water (liquid phase), and is characterized in that a swirler having a hub and a plurality of swirling vanes has a constant diameter of the hub at a position 0.5D or more and 2.0D or less (D: hydraulic equivalent diameter) upstream from the surface of the hub on the downstream side of the gas-liquid two-phase flow.

[0009] Furthermore, a boiling water reactor according to the present invention is a boiling water reactor comprising: a cylindrical core shroud provided within a reactor pressure vessel; a core loaded with a plurality of fuel assemblies installed within the core shroud; and a shroud head covering the core, a steam-water separator attached to the shroud head via a standpipe and extending upward; and a steam dryer disposed above the steam-water separator, wherein the steam-water separator has a hub and a swirler having a plurality of swirling vanes, and the diameter of the hub of the swirler is constant at a position 0.5D or more and 2.0D or less (D: hydraulic equivalent diameter) upstream from the surface of the hub on the downstream side of the gas-liquid two-phase flow.

[0010] According to the present invention, it is possible to provide a steam-water separator and a boiling water reactor that can reduce pressure loss while maintaining the steam-water separation performance of the steam-water separator. Problems, configurations, and effects other than those described above will become apparent from the description of the following embodiments.

[0011] FIG. 1 is a schematic configuration diagram of an advanced boiling water reactor according to one embodiment of the present invention. FIG. 1 is a diagram showing a steam-water separator according to a first embodiment of the present invention, in which a swirler is installed in a first-stage inner cylinder. FIG. 2 is a simplified diagram showing the swirler installed in the first-stage inner cylinder. FIG. 2 is a simplified diagram showing the swirler installed in the first-stage inner cylinder. FIG. 1 is a longitudinal sectional view showing the shape of a hub with a non-constant diameter gradient that constitutes a swirler according to a first embodiment of the present invention. FIG. 1 is a diagram showing the relationship between pressure loss performance and the length in which the hydraulic equivalent diameter is constant in the upstream direction from the downstream surface of the hub of a gas-liquid two-phase flow. FIG. 1 is a longitudinal sectional view showing the shape of a hub with a constant diameter gradient that constitutes a swirler according to a first embodiment of the present invention. FIG. 2 is a simplified diagram showing the relationship between pressure loss performance and the length in which the hydraulic equivalent diameter is constant in the upstream direction from the downstream surface of the hub of a gas-liquid two-phase flow. 10 is a longitudinal cross-sectional view of a steam-water separator according to a third embodiment of the present invention, showing a hub shape in which the position where the hub diameter first becomes constant on the hub downstream side of the gas-liquid two-phase flow is located inside a first-stage inner cylinder. 11 is a longitudinal cross-sectional view of a steam-water separator according to a third embodiment of the present invention, showing a hub shape in which the position where the hub diameter first becomes constant on the hub downstream side of the gas-liquid two-phase flow is aligned with the height of a diffuser.

[0012] In this specification, the term "boiling water reactor" refers to a normal boiling water reactor (BWR) that is equipped with a recirculation pump and circulates cooling water as a moderator by passing the cooling water outside the reactor pressure vessel and then flowing it back into the downcomer inside the reactor pressure vessel; an advanced boiling water reactor (ABWR) that has an internal pump; a resource-renewable boiling water reactor (RBWR); and an economical simplified boiling water reactor (Economic Simplified Boiling Water Reactor) that uses a natural circulation system of cooling water using a chimney, thereby eliminating the need for a recirculation pump in a BWR or an internal pump in an ABWR. These include the Advanced Boiling Water Reactor (ESBWR), the small, highly economical light water reactor BWRX-300, etc. In the following, an Advanced Boiling Water Reactor (ABWR) that is equipped with an internal pump and circulates the coolant within the reactor pressure vessel will be described as an example.

[0013] 1 is a schematic diagram of an advanced boiling water reactor (ABWR) according to one embodiment of the present invention. As shown in FIG. 1 , an AWR 10 equipped with a steam-water separator according to one embodiment of the present invention includes a reactor pressure vessel 11, a cylindrical core shroud 13, and a core 12, an initial core containing a plurality of fuel assemblies (not shown), installed within the core shroud 13. Also installed within the reactor pressure vessel 11 are a shroud head 20 covering the core 12, a steam-water separator 15 (described in detail below) attached to the shroud head 17 via a standpipe 22 and extending upward, and a steam dryer 16 disposed above the steam-water separator 15. An upper lattice plate (not shown) is disposed within the core shroud 13 below the shroud head 17, and is attached to the core shroud 13 and positioned at the upper end of the core 12. A core support plate (not shown) is located at the lower end of the core 12, disposed within the core shroud 13, and installed in the core shroud 13. A plurality of fuel support fittings (not shown) are also installed on the core support plate (not shown). Also provided within the reactor pressure vessel 11 are control rod guide tubes (not shown) that enable a plurality of control rods (not shown) with a cross-shaped cross section to be inserted into the core 12 to control the nuclear reaction of the fuel assemblies. A control rod drive mechanism (not shown) is provided within a control rod drive mechanism housing (not shown) installed below the bottom of the reactor pressure vessel 11, and the control rods are connected to the control rod drive mechanism (not shown).

[0014] A plurality of internal pumps 18 are installed on a bottom head 19, which is the bottom of the reactor pressure vessel 11, so as to penetrate into the reactor pressure vessel 11 from below. The internal pumps 18 are arranged annularly at predetermined intervals outside the outermost peripheries of a plurality of control rod guide pipes (not shown). This prevents the internal pumps 18 from interfering with the control rod guide pipes (not shown). The impellers of each internal pump 18 are positioned in an annular downcomer 14 formed between a cylindrical core shroud 13 and the inner surface of the reactor pressure vessel 11. Cooling water in the reactor pressure vessel 11 is supplied from the bottom head 19 side to the core 12 via the downcomer 14 by the impellers of each internal pump 18. The cooling water flowing into the core 12 is heated by nuclear reactions in the fuel assemblies (not shown) and becomes a gas-liquid two-phase flow, which then flows into the steam-water separator 15 via a standpipe 22. The gas-liquid two-phase flow passing through the steam separator 15 is separated into moisture-containing steam (gas phase) and water (liquid phase), and the liquid phase descends again to the downcomer 14 as cooling water. Meanwhile, the steam (gas phase) is introduced into the steam dryer 16, where moisture is removed, and then supplied to a turbine (not shown) via the main steam pipe 20. Cooling water flowing into the reactor pressure vessel 11 via the feedwater pipe 21 via a condenser or the like flows downward (descending) through the downcomer 14. In this manner, the internal pump 18 forcibly circulates cooling water to the core 12 to efficiently cool the heat generated in the core 12. Below, an embodiment of the present invention, particularly the details of the steam separator 15, will be described using the drawings. Each figure is merely a schematic illustration to enable a sufficient understanding of the present invention. Therefore, the present invention is not limited to the illustrated example. Furthermore, in each figure, common or similar components are designated by the same reference numerals, and redundant description thereof will be omitted.

[0015] FIG. 2 is a diagram showing a steam-water separator according to a first embodiment of the present invention, in which a swirler is installed in a first-stage inner cylinder. FIG. 3 is a simplified diagram of FIG. 2 in which the swirler installed in the first-stage inner cylinder is clarified.

[0016] 2 , the steam-water separator 15 according to this embodiment includes, along the flow direction of a mixed fluid (gas-liquid two-phase fluid) of water (liquid phase) and steam (gas phase), a diffuser 62, a swirler 63, a first-stage inner cylinder 64, a first-stage outer cylinder 66, a first-stage pick-off ring 65, a first-stage annular plate 67, a second-stage inner cylinder 68, a second-stage outer cylinder 70, a second-stage pick-off ring 69, a second-stage annular plate 71, a third-stage inner cylinder 72, a third-stage outer cylinder 74, a third-stage pick-off ring 73, and a third-stage annular plate 75. In the example shown in FIG. 2 , a hub 26 and a swirler 63 composed of a plurality of swirl vanes are installed in the first-stage inner cylinder 64.

[0017] A mixed fluid (gas-liquid two-phase fluid) of water (liquid phase) and steam (gas phase) flows into the diffuser 62 via the standpipe 61, and the flow velocity of the mixed fluid (gas-liquid two-phase fluid) is reduced by the expansion of the flow path area by the diffuser 62. Thereafter, centrifugal force is applied by the swirler 63 installed in the first-stage inner cylinder 64, and the mixed fluid is separated into steam (gas phase) containing liquid droplets and water (liquid phase) within the first-stage inner cylinder 64. A portion of the separated water (liquid phase) is separated by the first-stage pick-off ring 65 and the first-stage annular plate 67, and flows down the first-stage drainage channel 50 formed between the first-stage inner cylinder 64 and the first-stage outer cylinder 66 by gravity to be drained.

[0018] The water (liquid phase) passing through the first-stage pick-off ring 65 flows into the second-stage inner cylinder 68, is separated by the second-stage pick-off ring 69 and the second-stage annular plate 71, and flows down the second-stage drainage channel 51 formed between the second-stage inner cylinder 68 and the second-stage outer cylinder 70 by gravity, and further flows down along the outer surface of the first-stage outer cylinder 66. On the other hand, the steam (gas phase) rises outside the steam-water separator 15.

[0019] Water (liquid phase) passing through the second-stage pickoff ring 69 flows into the third-stage inner casing 72, is separated by the third-stage pickoff ring 73 and the third-stage annular plate 75, and flows down by gravity through the third-stage drainage channel 52 formed between the third-stage inner casing 72 and the third-stage outer casing 74 to be drained. Meanwhile, steam (gas phase) rises outside the steam-water separator 15. Note that a small amount of water (liquid phase) flowing out of the steam-water separator outlet 55 flows into the steam dryer 16. Here, FIG. 3 is a simplified diagram of FIG. 2 to clarify the swirler installed in the first-stage inner casing. As shown in FIG. 3, this embodiment is characterized by the shape of the hub 26 that constitutes the swirler 63.

[0020] FIG. 4 is a longitudinal cross-sectional view showing the shape of a hub with a non-constant diameter gradient that constitutes the swirler according to this embodiment. As shown in FIG. 4 , the swirler 63 includes a hub 224 and multiple swirl vanes 225 and is installed within the first-stage inner cylinder 126. The diameter of the hub 224 gradually increases in accordance with the flow direction of a mixed fluid (gas-liquid two-phase fluid) of water (liquid phase) and steam (gas phase). The diameter of the hub 224 is constant from the downstream surface of the mixed fluid (gas-liquid two-phase fluid) in the upstream direction (position indicated by the dotted line). Here, the distance (length) along the flow direction of the mixed fluid (gas-liquid two-phase fluid) where the diameter of the hub 224 is constant from the downstream surface of the mixed fluid (gas-liquid two-phase fluid) in the upstream direction is between one-half and two times the hydraulic equivalent diameter D. In other words, it is technically significant that the diameter of the hub 224 is constant at a position 0.5D or more and 2.0D or less (D: hydraulic equivalent diameter) upstream from the surface on the downstream side of the flow of the mixed fluid (gas-liquid two-phase fluid). This point will be explained below.

[0021] Figure 5 shows the relationship between the length of the constant hydraulic equivalent diameter from the downstream surface of the hub in the gas-liquid two-phase flow in the upstream direction and the pressure loss performance. In Figure 5, the horizontal axis represents the length (distance) of the constant hydraulic equivalent diameter from the downstream surface of the hub in the gas-liquid two-phase flow in the upstream direction, and the vertical axis represents the combined performance (X) = pressure loss (X) / swirl force, which is used to simultaneously evaluate the gas-water separation performance and pressure loss performance. ０．５The calculated results are shown as (X). First, the air-water separation performance and pressure loss performance were evaluated using three-dimensional computational fluid dynamics (CFD). X is the distance (length) from the downstream surface of the hub in the upstream direction of the flow of the mixed fluid (gas-liquid two-phase fluid) where the hub diameter is constant. The pressure loss performance when X is changed was evaluated using CFD and fitted with a quadratic polynomial. In addition, the swirl force of the gas-liquid two-phase flow, which is related to the air-water separation performance when X is changed, was also evaluated using CFD and fitted with a quadratic polynomial. Since the water flow rate at the swirler outlet is proportional to the 0.5 power of the swirl force, in order to simultaneously evaluate the air-water separation performance and pressure loss performance, the combined performance (X) = pressure loss (X) / swirl force ０．５ The combined performance (X) was calculated and visualized as shown in Figure 5. Reducing pressure loss reduces the combined performance (X). Furthermore, improving swirl force, i.e., air-water separation performance and pressure loss performance, reduces the combined performance (X). In Figure 5, when the combined performance (X) is approximately 1.3D, the combined performance (X) is smallest, and therefore the air-water separation performance and pressure loss performance are the best. Furthermore, in the range of 0.5D to 2.0D, the improvement in combined performance (X) is within one-third of that at the best performance, confirming that there is technical significance in keeping the diameter of the hub 224 constant within the range of 0.5D to 2.0D.

[0022] Returning to Figure 4, the rate at which the diameter of the hub 224 increases along the direction of the gas-liquid two-phase flow is not constant upstream from the portion where the diameter becomes constant (the portion indicated by the dotted line), as shown in Figure 4. In other words, the diameter of the hub 224 along the direction of the gas-liquid two-phase flow gradually increases from the most upstream position toward the downstream position. However, the rate of increase increases rapidly (nonlinearly) near the upstream side of the portion where the diameter of the hub 224 becomes constant (the portion indicated by the dotted line).

[0023] Figure 6 is a longitudinal cross-sectional view showing the shape of a hub 224 with a constant diameter gradient, which constitutes a swirler according to this embodiment. As shown in Figure 6, the diameter of the hub 224 along the direction of gas-liquid two-phase flow increases continuously from the portion where the diameter becomes constant (the dotted line portion) upstream, and the rate of increase in the diameter of the hub 224 is constant. In other words, the diameter of the hub 224 along the direction of gas-liquid two-phase flow increases continuously from the most upstream portion to the downstream portion. The rate of increase is constant (linear) up to the portion where the diameter of the hub 224 becomes constant (the dotted line portion).

[0024] As described above, according to this embodiment, it is possible to provide a steam-water separator and a boiling water reactor that can reduce pressure loss while maintaining the steam-water separation performance of the steam-water separator.

[0025] Fig. 7 is a diagram showing a steam-water separator according to a second embodiment of the present invention, in which a swirler is installed in a diffuser, and Fig. 8 is a simplified diagram of Fig. 7, in which the swirler, which is at the same height as the diffuser, is clearly shown. While the first embodiment described above has a configuration in which the swirler is installed in the first-stage inner cylinder 64, this embodiment differs from the first embodiment in that the swirler is installed in the diffuser.

[0026] 7 and 8 , a mixed fluid (gas-liquid two-phase fluid) of water (liquid phase) and steam (gas phase) flows into a diffuser 62 via a standpipe 61, and the flow velocity of the mixed fluid (gas-liquid two-phase fluid) is reduced by the expansion of the flow path area by the diffuser 62. Thereafter, centrifugal force is applied to the mixed fluid (gas-liquid two-phase fluid) by a swirler 63 installed in the diffuser 62, and the mixed fluid is separated into droplet-containing steam (gas phase) and water (liquid phase) within the diffuser 62. The other configuration is the same as the configuration shown in FIG. 2 described in the first embodiment above, and therefore description thereof will be omitted.

[0027] FIG. 9 is a longitudinal cross-sectional view showing the shape of a hub having the same height as the swirler and diffuser according to the second embodiment. As shown in FIG. 9 , the swirler is composed of a hub 224 and multiple swirl vanes 225 and is installed in the diffuser 122. As shown in FIG. 9 , the diameter of the hub 224 along the direction of the gas-liquid two-phase flow continuously increases from the most upstream to the downstream side. The rate of increase is constant (linear) up to the point where the diameter of the hub 224 becomes constant (the dotted line portion). The diameter of the hub 224 becomes constant in the upstream direction (the position indicated by the dotted line). Here, as in the first embodiment, the diameter of the hub 224 is constant from the surface on the downstream side of the gas-liquid two-phase flow to a position 0.5D or more and 2.0D or less (D: hydraulic equivalent diameter) in the upstream direction.

[0028] As described above, according to this embodiment, it is possible to achieve the same effects as in the first embodiment.

[0029] Fig. 10 is a simplified vertical cross-sectional view of a steam-water separator according to a third embodiment of the present invention, showing the shape of a hub in which a swirler is installed over a portion of the diffuser and the first-stage inner cylinder, and Fig. 11 is a vertical cross-sectional view of a steam-water separator according to the third embodiment of the present invention, showing the shape of the hub in which the position downstream of the hub in the gas-liquid two-phase flow where the diameter of the hub first becomes constant is located inside the first-stage inner cylinder. While the first embodiment described above has a configuration in which the swirler is installed inside the first-stage inner cylinder 64, this embodiment differs from the first embodiment in that a swirler is installed over a portion of the diffuser and the first-stage inner cylinder.

[0030] 10 and 11 , the upstream surface of the swirler, which is made up of the hub 224 and the multiple swirl vanes 225, in terms of gas-liquid two-phase flow, is installed inside the diffuser 122, while the downstream surface of the swirler is installed inside the first-stage inner cylinder 126. In other words, the swirler according to this embodiment is installed across a portion of the diffuser 122 and the first-stage inner cylinder 126.

[0031] As in the first and second embodiments, the diameter of the hub 224 along the direction of the gas-liquid two-phase flow increases continuously from the most upstream to the downstream side. The rate of increase is constant (linear) up to the point where the diameter of the hub 224 becomes constant (the dotted line portion). The diameter of the hub 224 becomes constant upstream at the first-stage inner cylinder 126 (the position indicated by the dotted line). As in the first embodiment, the diameter of the hub 224 is constant at a position 0.5D to 2.0D (D: hydraulic equivalent diameter) upstream from the surface on the downstream side of the gas-liquid two-phase flow.

[0032] 12 is a longitudinal cross-sectional view of the steam-water separator according to this embodiment, showing the shape of the hub in which the position downstream of the hub in the gas-liquid two-phase flow where the diameter of the hub first becomes constant is aligned with the height of the diffuser. As shown in Fig. 12, the diameter of the hub 224 along the direction of the gas-liquid two-phase flow continuously increases from the most upstream side to the downstream side up to the boundary between the diffuser 122 and the first-stage inner cylinder 126. The rate of increase is constant (linear) up to the point where the diameter of the hub 224 becomes constant (the dotted line portion: the boundary between the diffuser 122 and the first-stage inner cylinder 126).

[0033] As described above, according to this embodiment, it is possible to achieve the same effects as in the first embodiment.

[0034] The present invention is not limited to the above-described embodiments, but includes various modifications. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those including all of the described configurations. Furthermore, it is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment.

[0035] 10...Advanced boiling water reactor 11...Reactor pressure vessel 12...Core 13...Core shroud 14...Downcomer 15, 15a...Steam-water separator 16...Steam dryer 17...Shroud head 18...Internal pump 19...Lower head 20...Main steam pipe 21...Feedwater pipe 22, 61, 121...Standpipe 26, 124, 224...Hub 50...First stage drainage passage 51...Second stage drainage passage 52...Third stage drainage passage 55...Steam-water separator outlet 62, 122...Diffuser 63, 123...Swirler 64, 126...First stage inner cylinder 65...First stage pick-off ring 66...First stage outer cylinder 67...First stage annular plate 68...Second stage inner cylinder 69...Second stage pick-off ring 70... Second stage outer cylinder 71... Second stage annular plate 72... Third stage inner cylinder 73... Third stage pick-off ring 74... Third stage outer cylinder 75... Third stage annular plate 125, 225... Swirl vanes

Claims

1. A steam-water separator in which cooling water flowing into a reactor core is heated by the nuclear reaction of fuel assemblies, becoming a two-phase gas-liquid flow, which flows into a steam-water separator via a standpipe and separates the passing two-phase gas-liquid flow into steam (gas phase) containing moisture and water (liquid phase), wherein a swirler having a hub and multiple swirling vanes has a constant diameter of the hub at a position 0.5D to 2.0D (D: hydraulic equivalent diameter) upstream from the surface of the hub on the downstream side of the flow of the two-phase gas-liquid flow.

2. A steam-water separator as claimed in claim 1, characterized in that the diameter of the hub along the flow of the gas-liquid two-phase flow increases gradually or continuously from the most upstream position to the downstream position up to a point where the diameter of the hub becomes constant.

3. A steam-water separator as claimed in claim 2, characterized in that the diameter of the hub along the flow of the gas-liquid two-phase flow gradually increases from the most upstream position towards the downstream position up to a point where the diameter of the hub becomes constant, and the rate of increase of the diameter of the hub increases abruptly upstream of the point where the diameter of the hub becomes constant.

4. A steam-water separator as claimed in claim 2, characterized in that the diameter of the hub along the flow of the gas-liquid two-phase flow increases continuously from the most upstream position to the downstream position up to a point where the diameter of the hub becomes constant, and the rate of increase of the diameter of the hub is constant up to the point where the diameter of the hub becomes constant.

5. A steam-water separator according to claim 4, characterized in that the swirler is installed in the first stage inner cylinder or in the diffuser.

6. A steam-water separator according to claim 4, characterized in that the swirler is installed over a portion of the diffuser and the first stage inner cylinder.

7. A steam-water separator as claimed in claim 6, characterized in that the diameter of said hub along the direction of flow of said gas-liquid two-phase flow increases continuously from the most upstream position to the downstream position up to the boundary between said diffuser and said first stage inner cylinder.

8. A boiling water reactor comprising a cylindrical core shroud provided within a reactor pressure vessel, a core loaded with a plurality of fuel assemblies installed within the core shroud, and comprising within the reactor pressure vessel a shroud head covering the core, a steam-water separator attached to the shroud head via a standpipe and extending upward, and a steam dryer disposed above the steam-water separator, wherein the steam-water separator has a hub and a swirler having a plurality of swirling vanes, and the swirler has a constant diameter of the hub at a position 0.5D to 2.0D (D: hydraulic equivalent diameter) upstream from the surface of the hub on the downstream side of the gas-liquid two-phase flow.

9. A boiling water reactor as claimed in claim 8, characterized in that the diameter of the hub along the flow of the gas-liquid two-phase flow increases gradually or continuously from the most upstream position to the downstream position until the diameter of the hub becomes constant.

10. A boiling water reactor as claimed in claim 9, characterized in that the diameter of the hub along the flow of the gas-liquid two-phase flow gradually increases from the most upstream position to the downstream position up to a point where the diameter of the hub becomes constant, and the rate of increase of the diameter of the hub increases abruptly upstream of the point where the diameter of the hub becomes constant.

11. A boiling water reactor as claimed in claim 9, characterized in that the diameter of the hub along the flow of the gas-liquid two-phase flow increases continuously from the most upstream position to the downstream position until the diameter of the hub becomes constant, and the rate of increase of the diameter of the hub is constant until the diameter of the hub becomes constant.

12. A boiling water reactor according to claim 11, characterized in that the swirler is installed in the first stage inner cylinder or in the diffuser.

13. A boiling water reactor according to claim 11, wherein the swirler is installed over a portion of the diffuser and the first stage inner cylinder.

14. A boiling water reactor as claimed in claim 13, characterized in that the diameter of the hub along the direction of the gas-liquid two-phase flow increases continuously from the most upstream position to the downstream position up to the boundary between the diffuser and the first stage inner cylinder.

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