KR20090098979A - Permanent seal ring for a nuclear reactor cavity - Google Patents

Abstract

A permanent cavity seal ring that replaces the function of the temporary cavity seal ring typically used in narrow thermal expansion gap pressurized water reactors, to seal the gap between the reactor cavity well and the reactor during refueling. The permanent seal ring uses a C-shaped flexure that is shielded by a rigid cantilevered support arm from any accidentally dropped equipment from above in the refueling canal. The construction accommodates the thermal expansion of the reactor vessel while permitting the reactor cavity cooling air to exit the annulus between the vessel and the reactor cavity wall during plant operation, without a significant increase in pressure drop.

Description

PERMANENT SEAL RING FOR A NUCLEAR REACTOR CAVITY}

FIELD OF THE INVENTION The present invention relates to nuclear reactor containment arrangements, and more particularly, to a permanent seal ring that extends across an annular thermal expansion gap between the surrounding wall and the containment wall of the reactor vessel. This provides a water tight seal across the thermal expansion gap to allow lateral and vertical translation of the reactor vessel to the containment wall.

Fuel refueling of pressurized water reactors is an established routine operation performed with high reliability. Under normal load conditions, fuel reloading is performed approximately every 12 to 22 months. A full fuel reload usually takes about four weeks.

In many reactor containment structures, the reactor vessel is located in a concrete cavity having an upper annulus above the vessel and defining a refueling canal. The canal remains dry during reactor operation, but canal is filled with water during the fuel reload of the nuclear power plant. The level is high enough to provide adequate shielding to keep the radiation level within acceptable limits when the fuel assembly is completely removed from the container. Boric acid is added to the water to ensure subcritical conditions during fuel reloading. At the beginning of the fuel reload operation, the reactor vessel flange is sealed to the underside of the fuel reload canal before the fuel reload canal is filled with water. Originally, such sealing is achieved by clamped gasket seal rings that prevent leakage of fuel reload water to the wells where the reactor vessel rests. These gaskets are fastened and sealed after cooling the reactor before filling the canal with water. Typically, such removable seal rings consist of four large diameter elastomeric gaskets that are prone to leaking and must be replaced at every fuel reload operation.

The annular thermal expansion gap between the reactor vessel and the concrete wall surrounding the reactor vessel also accommodates thermal expansion of the vessel and other movements of the vessel, such as in the event of an earthquake, and also cooling of the cavity wall and embedding in the concrete cavity wall. It is provided to allow cooling of the excore detector. Pressurized water reactor plants have two basic expansion gap sizes: wide and narrow. Wide gaps tend to range from 2 feet to 3 feet, while narrow gaps tend to range from 2 inches to 4 inches. For all power plants, the gap area must be sealed during fuel reload. The upper part of the cavity, i.e. the fuel reload canal, may be filled with water, but no water is allowed in the lower part of the cavity. Typically, the reactor vessel has a horizontally extending flange and the containment wall surrounding the reactor cavity is a ledge extending horizontally to the floor of the fuel reloading canal at approximately the same height as the flange or A shelf is provided, in which a temporary seal ring is placed during fuel reload.

In power plants with a wider thermal expansion gap, permanent seal rings, such as those disclosed in U.S. Pat. Was adopted. However, the permanent seal ring needs to allow thermal expansion of the reactor vessel, which reduces the gap between the containment wall in the region of the reactor well and the surrounding wall of the reactor vessel, and ideally also somewhat of the reactor vessel relative to the containment wall. It is necessary to accommodate vertical and lateral movements. In addition, the seal ring must be able to withstand heavy blows from objects such as fuel assemblies that accidentally fall during fuel reload. The permanent seal ring must also pass cooling air from the lower reactor cavity to the fuel reloading canal along the wall surrounding the vessel. Therefore, the seal ring is resistant to (1) strength to hold large amounts of water used for fuel reloading operations, (2) flexibility to accommodate movement of the reactor vessel within the containment wall, and (3) resistance to damage from falling objects. To have structural integrity, and to have air passages between the lower reactor cavity and the fuel reload canal to cool the cavity walls during reactor operation. The above US patents disclose various designs that achieve these goals for a wide expansion gap power plant, but fail to meet all of these goals for a narrow expansion gap power plant. If a design is available that can achieve all of the above goals, there are approximately 40 power plants in the United States with a narrow expansion gap that can utilize permanent seal fixtures.

It is therefore an object of the present invention to provide a design that satisfies all of the above objectives for a narrow expansion gap power plant.

This object is achieved by sealingly coupling a fuel reloading ledge adjacent to the reactor vessel flange and a containment wall on the floor of the fuel reloading canal and employing an annular stainless steel ring attached to them and extending therebetween. do. The annular seal ring has a first end secured to either the floor of the fuel reloading canal or the fuel reloading ledge and extends over the expansion gap over the expansion gap, preferably the floor or fuel reloading of the fuel reloading canal. A rigid cantilevered annular support extending over at least a portion of the other of the ledges. One end of the C-shaped bending member is generally attached to the distal end of the rigid cantilevered annular support. The other end of the C-shaped flexure member is fixed to the floor of the fuel reload canal or the other of the fuel reload ledge.

In a preferred embodiment of the invention, the rigid cantilevered annular support comprises a substantially horizontal foot secured to either the floor of the fuel reload canal or the fuel reload ledge. A leg, one end of which is connected to the foot, extends generally vertically from the foot and is attached at a height away from the foot to an arm or top plate that extends generally radially outwardly across the expansion gap. Preferably, the arm or top plate extends in the radial horizontal direction. Preferably, the foot extends radially from the leg toward the expansion gap and is bolted to the surface of either the floor of the fuel reloading canal or the fuel reloading ledge. A seal weld is provided around the intersection between the rigid cantilever annular support and the fuel reload canal floor or one of the fuel reload ridges. Alternatively, the foot may be attached by structural welds.

In a variant embodiment, the end of the arm opposite the distal end extending over the expansion gap extends radially past the leg and is attached to the distal end of the L-shaped flexure member, the other end of the L-shaped flexure member being fueled. Reload Locks to either the floor of the canal or the fuel reload ridge. In yet another embodiment, the first end of the generally C-shaped flexure member is attached to the rigid cantilevered annular support via a flexure link extending substantially vertically.

According to an embodiment of the present invention, a hatch opening can be arranged in the arm or the top plate of a rigid cantilever annular support of a sufficient size, so that in a narrow gap power plant structure without sacrificing the structural strength and flexibility of the seal. No further restriction on the cooling air flow of is added.

The present invention may be better understood upon reading the following detailed description with reference to the accompanying drawings.

1 is a partial cross-sectional schematic of a nuclear reactor containment embodying the present invention,

2 shows a preferred embodiment of the annular seal ring of the present invention, a side view showing a portion of the reactor well portion around the thermal expansion gap,

3 is a side view showing a portion of the reactor weld shown in FIG. 2, showing an air path through the seal of one embodiment of the present invention;

4 is a plan view of the permanent cavity seal ring of the present invention showing an air hatch arrangement;

Fig. 5 shows a second embodiment of the present invention, which is a side view showing a part of the reactor enclosure around the thermal expansion gap;

FIG. 6 shows a third embodiment of the present invention, which is a side view showing a part of the reactor enclosure around the thermal expansion gap;

FIG. 7 is a side view showing a portion of the reactor enclosure shown in FIG. 5, showing a cooling air path through the permanent cavity seal ring of the present invention. FIG.

The present invention provides a seal structure between a reactor vessel and a cavity that forms a permanently flexible seal between a reactor fuel reload canal floor and a reactor vessel that can be used in a nuclear power plant with a narrow expansion gap. The seal design of the present invention provides a watertight seal for fuel reloading during fuel reloading operations, while allowing sufficient coolant air flow and without disrupting the watertight integrity of the seals during normal reactor operation. Accommodates material expansion and contraction that occurs.

The environment to which the present invention is applied can be best understood with reference to a partial cross-sectional side view of the reactor containment unit shown in FIG. 1, and FIG. nuclear steam generating system). The pressurized container 10 is shown to form a pressurized container when sealed by its head assembly 12. The vessel has a cooling flow suction nozzle 20 and a cooling flow discharge nozzle 14 formed integrally with and through the cylindrical wall. As is known in the art, the vessel 10 is a nuclear reactor core whose main construction is a plurality of clad nuclear fuel elements that generate a significant amount of heat depending mainly on the location of the control means. (Not shown), a pressure vessel housing 18 thereof is shown. The heat generated by the reactor core is transferred from the core by the coolant flow which is introduced through the suction nozzle 20 and exits through the discharge nozzle 14.

The flow exiting the discharge nozzle 14 is sent to the heat exchange steam generator 22 through the hot leg conduit 28. The steam generator 22 is of a type in which a heated coolant stream is conveyed through a tube (not shown) that is in heat exchange relationship with water used to produce steam. The steam produced by the steam generator 22 is typically used to drive a turbine for power generation (not shown). Steam flows from the steam generator 22 through the cold leg conduit 24 and the pump 26, from which steam flows through the cold leg conduit to the suction nozzle 20. Thus, it can be seen that the closed circulating first or steam generating loop has coolant piping for communicatively coupling the vessel 10, the steam generator 22 and the pump 26. The steam generation system shown in FIG. 1 has three such closed fluid flow systems or loops. It should be understood that the number of such systems depends on the power plant, usually two to four are employed.

Within containment 42, reactor vessel 10 and head seal 12 are maintained in a separate reactor cavity surrounded by concrete wall 30. The reactor cavity is divided into a lower portion 32 which completely surrounds the vessel structure and an upper portion 34 which is commonly used as a fuel reloading canal. During reactor operation, air flow communication is maintained between the lower reactor vessel well 32 and the fuel reload canal 34, such that the concrete wall of the reactor cavity and the excore detector 44 embedded within the concrete wall are located. Helps cooling. This air flow is facilitated by an exhaust fan located in containment 42 outside the concrete barrier 30. During the fuel reload operation, the reactor vessel flange 36 is sealed to the reactor cavity shelf 40, which is the floor of the fuel reload canal. In Figure 1, a wide expansion gap 46 is shown for clarity. In power plants with such a wide gap 46, permanent seal rings of the type described above are usually employed. By placing the fixture over gap 46, the heat dissipated around the reactor vessel is limited. In a wide gap power plant with an annular width of about 3 feet, the permanent seal fixation design generally uses a flexible steel member welded on the side with several large hatches on the top steel plate. Prior to refueling, the hatch is locked to form a watertight seal. After the fuel reload canal is drained, the hatches are removed and stored to allow air flow during standard operation.

Since the wide gap is wide, it is relatively easy to size and deploy the hatch to allow proper air flow. However, since the narrow gap has a small width, the hatch becomes a design limiting factor. If the hatch is not large enough, the air flow during the reactor operation is greatly limited. Of particular importance is the structural strength of the permanent seal fixture. One wide gap seal fixture that is commonly used is disclosed in US Pat. No. 4,904,442, where two steel legs are used as the seal fixtures that support the rigid top plate. The upper plate is attached with two L-shaped flexures on both sides. The main structure supports the load but is not fixed to the refueling ledge or compartment floor. The bend, which is made of thin sheet metal, is only partially fixed to the fuel reload ledge and the fuel reload canal floor, forming a watertight seal, allowing the seal structure to deflect to the extent necessary to accommodate the movement of the container. The two legs are not welded to the ledge. Instead, two flexures are welded to the top plate and the ledge. This allows the seal structure to have the required support performance in case of drop loads such as catastrophic drop of fuel assemblies. However, the seal structure may also move as expansion is required due to loads such as operating temperature changes and seismic activity.

The biggest problem in applying a wide gap permanent fixed seal to a narrow gap power plant is that of heat flow. The two legs and the top plate trap the heat. To alleviate this problem, the design of the present invention eliminated one leg. In doing so, the remaining legs and top plate were modified to accommodate the design structural load. In addition, new flexural designs have been developed that withstand the movement caused by the required design load case and allow more areas to accommodate the cooling air flow.

In the design of the present invention, the hatch is also a structural concern because it occupies a large volume of the top plate. Similar to the wide gap design, hatches are used to allow air flow through the structure during normal operation. However, the hatch for the wide gap structure occupies a smaller portion of the top plate than is required for the narrow gap structure. Typically, the hatch plate is not thicker than the top plate to be attached. This means that a significant amount of support material is lost in the narrow expansion gap design resulting in more stress reactive sections.

The narrow expansion gap seal design of the present invention installed in a power plant fuel reload structure, ie a structure equipped with a hatch cover plate, is shown in FIG. 2. The reactor vessel 10 is shown centered at the lower portion 32 of the reactor cavity surrounded by the thermal insulator 68. The fuel reload ledge 66 is shown as an extension of the reactor vessel flange 36 and the cavity thermal expansion gap 46 between the buried plate 64 and the fuel reload ledge 66 fixed to the containment wall 30. Extend radially from the flange 36 toward the containment cavity wall 30, which defines. Steel liner 70 covers the exterior of the concrete containment wall. The permanent reactor cavity seal ring 78 of the present invention is shown secured to the embedding plate 64 by bolts 60 passing through a foot 48 of the cantilever portion of the seal 78. The foot 48 is welded to one end of the leg portion 50 of the cantilever portion, and the leg portion 50 is welded to an arm or top plate 52 extending horizontally, the top plate 52 having an expansion gap. Over 46, preferably over at least a portion of the fuel reload ledge 66. Top plate 52 has a hatch 54 held in place by bolts 56 that compress seal ring 58 to form a watertight seal. The seal weld 80 surrounds the intersection of the foot 48 and the embedding plate 64. The bending member 62 has one end connected to the arm 52 near the distal end 82 of the arm 52. The flexure member 74 is a generally C-shaped member with the other end connected to the upper surface of the fuel reload ledge 66.

The thin thickness of the C-shaped flexure, such as less than about 0.2 inches (0.51 cm), is designed to accommodate the radial and vertical thermal expansion of the reactor vessel. This combines the function of the L-shaped bends used in the wide gap permanent cavity seal ring disclosed in US Pat. No. 4,747,993. The thin C-shaped bends are protected from accidental fuel assembly drops by the strong top plate 54. Preferably, the permanent reactor cavity seal is made of stainless steel.

When the used fuel canal 34 is drained after the fuel reload, the air hatch 54 is removed to provide a flow path for the reactor cavity cooling air. 3 shows the cooling air flow path with the hatch cover plate removed during power plant operation. 4 is a plan view showing a typical orientation of the hatch 54 opening.

A variant design is shown in FIG. 5, which is a C-shaped outer flexure 74 welded to the fuel reload cavity embedding ring 64 and a short weld welded to the fuel reloading ledge 66 of the reactor vessel. L-shaped inner flexures 86 with long legs 90 secured to legs 88 and extensions 84 of arms 54 are used. In this embodiment, a cantilever annular support 47 consisting of a leg 50 and an arm or top plate 52 is seated on the fuel reload ledge 66 and the lower link of the C-shaped outer flexure 62. The bottom link 72 is fixed to the embedding plate 64. The same reference numerals as those in FIG. 2 are used for the corresponding components shown in FIG. 5 for the components common to FIGS. 2 and 5. The embodiment shown in FIG. 5 differs from the embodiment shown in FIG. 2 in several other respects. For example, the C-shaped bend 62 in the embodiment shown in FIG. 5 is located away from the reactor vessel 10 while the C-shaped bend in FIG. 2 faces the reactor vessel 10 side. have. In addition, the arm or top plate 52 in the embodiment shown in FIG. 5 has a segment 84 extending beyond the leg 50 in a direction away from the C-shaped bend 62. The L-shaped inner flexure functions similarly to that disclosed in US Pat. No. 4,904,442. The design shown in FIG. 5 shares many of the same characteristics as described above with respect to FIG. 2, but also provides additional flexibility due to an increase in the overall length of the bend material associated with the combination of the two bends 86, 62. This design uses a support leg 50 that rests directly on the reactor fuel reload ledge so that the fuel reload cavity provides the support performance required for the seal ring during the water refill operation. Watertight seals are achieved by the fully welded inner and outer bends 86, 62.

6 shows an alternative design for the outer bend 62. This design has an additional flexural link 92 inserted between the distal end 84 of the horizontal arm 52 and the upper link 76 of the C-shaped flex 62. The shape of the bend 62 is modified to allow a large hatch 54 to allow a large air opening in the top plate 52. The modified flexure shape shown in FIG. 6 retains the same flexural properties as the C-shaped design shown in FIG. Large openings may be required in some applications to compensate for the increased pressure drop in the reactor cavity air cooling system associated with the seal ring. FIG. 7 shows the reactor cavity air passage through the two bending seal designs shown in FIGS. 5 and 6 during power plant operation.

Although specific embodiments of the present invention have been described in detail, it will be apparent to those skilled in the art that various modifications and variations to these details may be made in light of the overall subject matter herein. Accordingly, the specific embodiments disclosed are by way of illustration only and are not limiting of the scope of the invention, which is to be given the full scope of the appended claims and all any equivalents thereof.

Claims (1)

In the vortex suppressing device, Comprising a plurality of spaced apart and generally parallel porous panels Vortex Suppressor.

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