GB2307330A

GB2307330A - Control rod guide tube

Info

Publication number: GB2307330A
Application number: GB9622019A
Authority: GB
Inventors: Franklin Delano Obermeyer; James C Buker
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1995-11-14
Filing date: 1996-10-22
Publication date: 1997-05-21
Also published as: JP2931260B2; JPH09166681A; IT1287976B1; ITPD960264A1; FR2741185A1; GB9622019D0

Abstract

A control rod guide tube (130) adapted to reduce vibration of the rods, comprises a housing (140) surrounding the control rods (80). The housing has an inlet opening (150) opposite a hole formed in an upper core plate (90), which hole has coolant flowing therethrough. The opening of the housing is elevated above the hole to define a gap (155), so that a first predetermined portion of coolant flows laterally out the gap and avoids the opening. The housing also has an aperture (175a/175b) for exit of a second predetermined portion of coolant. Because of the opening and the aperture, very little turbulent coolant flows over the rods and in consequence vibration of the control rods is reduced.

Description

CONTROL ROD GUIDE TUBE BACKGROUND This invention generally relates to control rod guide tubes and more particularly relates to a control rod guide tube adapted to reduce vibration of a plurality of control rods disposed therein, which control rod guide tube is suitable for use in pressurized water nuclear reactor pressure vessels.

Although devices for reducing vibration of control rods are known in the prior art, it has been observed that these prior art deices have certain limitations associated with them. However, before these limitations can be appreciated, some background is necessary as to the structure and operation of a typical pressurized water nuclear reactor pressure vessel and its associated control rod guide tubes.

In this regard, a nuclear reactor pressure vessel is a device for producing heat by controlled fission of neutron-producing fissionable material contained in fuel assemblies. A plurality of the fuel assemblies are grouped in the reactor pressure vessel to define a nuclear reactor core therein. Pressurized liquid moderator coolant is caused to circulate through the pressure vessel and thus through the fuel assemblies for assisting in the fission process and for removing the heat produced by fission of the fissionable material contained in the fuel assemblies.

However, the neutrons produced by the fission process in the reactor core must be suitably controlled for safety reasons. Therefore, disposed in the pressure vessel are a plurality of control rod guide tubes, each control guide tube being vertically aligned with its associated fuel assembly. The control rod guide tube houses a plurality of movable control rods belonging to a control rod cluster assembly. Each control rod cluster assembly is slidably movable so that the control rods belonging to the control rod cluster assembly are capable of being slidably inserted into and withdrawn from each fuel assembly to control the fission process.Moreover, disposed in each guide tube are a plurality of guide plates having a plurality of bores for slidably receiving respective ones of the control rods therethrough so that the control rods are suitably guided and laterally supported by the guide plates.

As the previously mentioned liquid moderator coolant upwardly circulates through each fuel assembly, it continues its upwardly flow path and enters the guide tube associated with that fuel assembly. The liquid moderator coolant is allowed to exit the guide tube through a plurality of apertures piercing the sides of the guide tube. After exiting the guide tube, the coolant exits the pressure vessel and flows to a steam generator for providing steam that ultimately produces electricity by means well known in the art.

However, applicant has observed that as the coolant is admitted into the guide tube to flow upwardly therein, a portion of the coolant will develop turbulent flow vibrating the control rods. This is so because, as the coolant flows upwardly within the guide tube, a portion of the coolant will change direction in order to flow out the aperture piercing the guide tube, thereby causing turbulent flow in the guide tube. This portion of the coolant will flow out the aperture because of a pressure gradient existing between the interior and the exterior of the guide tube. Moreover, as the coolant impacts the guide plates within the guide tube, it will change direction causing further turbulent flow within the guide tube, thereby causing further vibration of the control rods.In addition, an alignment plate capping the top end of the guide tube results in additional turbulence in the guide tube as the coolant impacts the alignment plate and changes direction to flow out the aperture, thereby resulting in additional vibration of the control rods. In other words, as the coolant changes direction, it will impinge the control rods at a velocity sufficient to cause the control rods to vibrate. This vibration is undesirable because such vibration may ultimately cause the control rods to wear against one or more of the guide plates through which they are slidably received. This wear may potentially interfere with the ability of the control rods to suitably slide in the bores of the guide plates and thus may potentially interfere with the ability of the control rods to suitably control the fission process.Therefore, a problem in the art is to reduce vibration of the control rods to prevent control rod wear.

Devices for reducing vibration of control rods are known. A guide tube comprising tube sheet tube carrying a control rod surrounded by an inner tube mounted within the tube sheet tube is disclosed in U.S. Patent 4,584,168 titled "System For Controlling Destructive Vibration Of A Nuclear Control Rod" issued April 22, 1986 in the name of Frank J. Formanck. The inner tube disclosed by this patent is fitted closely about the control rod. Mounted connections between the upper and lower ends of the inner tube are provided to route coolant for purposes of controlling vibration of the control rod.

Although this patent discloses a guide tube capable of controlling vibration of a control rod carried within it, this patent requires mounted connections to route the coolant. It appears that the mounted connections of the Formanck device may give rise to at least the possibility of loose parts migrating in the reactor vessel, which loose parts may damage the internal components in the reactor vessel should the mounted connections become disassociated from the inner tube. This would be undesirable for economic and safety reasons.

Therefore, an object of the present invention is to provide a control rod guide tube adapted to suitably reduce vibration of a plurality of control rods disposed therein.

SUMMARY Disclosed herein is a control rod guide tube adapted to reduce vibration of a plurality of control rods disposed therein. The guide tube comprises a housing surrounding the control rods. The housing has an inlet opening opposite a hole formed in an upper core plate, which hole has a reactor vessel coolant flowing therethrough. The coolant is capable of inducing vibration of the control rod as the coolant flows through the hole and is received through the opening and into the housing. The opening of the housing is elevated a predetermined distance above the hole to define a gap between the opening and the hole, so that a first predetermined portion of the coolant flows laterally out the gap and avoids the opening. The housing also has a lower side wall portion thereof pierced by an aperture located near the opening for exit of a second predetermined portion of the coolant from the housing. Vibration of the control rods is reduced as the first predetermined portion of the coolant avoids the inlet opening and as the second predetermined portion of the coolant exits the aperture.

In its broad form, the invention is, for use in a pressure vessel having a plate member therein defining a hole for allowing a fluid therethrough, a control rod guide tube adapted to reduce vibration of a control rod disposed therein, comprising a housing surrounding the control rod and having an inlet opening disposed opposite the hole for receiving the fluid flowing through the hole, the fluid capable of inducing vibration of the control rod as the fluid is received through the opening and into said housing, the opening being elevated a predetermined distance above the hole to define a gap between the opening and the hole, so that a first predetermined portion of the fluid flows out the gap and avoids the opening, said housing having a lower portion thereof pierced by an aperture located near the opening for exit of a second predetermined portion of the fluid from said housing, whereby vibration of the control rod is reduced as the first predetermined portion of the fluid avoids the inlet opening and as the second predetermined portion of the fluid exits the aperture.

A feature of the present invention is the provision of elongated bolts connecting the housing to an upper core plate disposed in the reactor pressure vessel and also the provision of an elongated pedestal depending from the housing and engaging the upper core plate, such that a gap is defined between the upper core plate and the housing.

An advantage of the present invention is that turbulent fluid flow within the housing is reduced in order to reduce vibration of the control rods.

These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described illustrative embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the invention, it is believed the invention will be better understood from the following description, taken in conjunction with the accompanying drawings wherein: Figure 1 shows in partial elevation, a typical pressurized water nuclear reactor pressure vessel with parts removed for clarity, the pressure vessel having a plurality of control rod guide tubes disposed therein mounted on an upper core plate; Figure 2 illustrates in partial elevation, a guide tube having an inlet opening and a plurality of apertures formed in the lower portion thereof near the inlet opening; Figure 3 illustrates in partial elevation, the lower portion of the guide tube; Figure 3A is a view taken along 3A-3A of Figure 3; and Figure 4 is a view taken along section line 4-4 of Figure 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Fig. 1, there is shown a typical pressurized water nuclear reactor pressure vessel, generally referred to as 10. Pressure vessel 10 includes a shell 20 open at its top end and having a plurality of inlet nozzles 30 and outlet nozzles 40 attached thereto (only one of each nozzle is shown). A closure head 50 is mounted atop shell 20 and is sealingly attached to shell 20 by a plurality of studs 55.

Still referring to Fig. 1, disposed in pressure vessel 10 are a plurality of nuclear fuel assemblies 60 containing fissionable material capable of emitting fission neutrons in the course of generating heat. Penetrating the top of closure head 50 are a plurality of control rod drive mechanism (CRDM's) 70, each CRDM 70 comprising a plurality of elongate slidable control rods 80 (see Fig. 3). Each control rod 80 includes absorber material for absorbing the previously mentioned neutrons in order to suitably control the fission process occurring in fuel assemblies 60.

Referring again to Fig. 1, disposed in pressure vessel 10 is a horizontally-oriented upper core plate member 90 having a plurality of transverse bores 100.

Bores 100 allow passage of the reactor coolant upwardly therethrough and also allows passage of control rods 80 into and out-of fuel assembly 60. Moreover, spaced above upper core plate 90 is an upper support plate 110 having a plurality of bores 120 therethrough for reasons described presently. Interposed between upper core plate 90 and upper support plate 110 are a plurality of elongate guide tubes, generally referred to as 130, for guiding control rods 80 vertically into and out-of fuel assembly 60. That is, each guide tube 130 is coaxially aligned with its respective fuel assembly 60, so that control rods 80 may penetrate and withdraw from fuel assembly 60 to control the fission process therein.

Referring to Figs. 2, 3, 3A and 4, guide tube 130 comprises a vertically-oriented, elongate and generally tubular housing 140 surrounding control rods 80.

Housing 140 has an inlet opening 150 at a lower end portion 160 thereof for receiving the coolant flowing upwardly through bore 100 and for movement of control rods 80 through opening 150. For reasons disclosed hereinbelow, inlet opening 150 is elevated a predetermined distance above upper core plate 90, so as to define a gap 155 therebetween. Surrounding lower end portion 160 is an outwardly-extending annular flange 165 having a plurality of leaf spring bolts 170 extending through flange 165.

Each bolt 170 matingly engages an associated bolt hole 180 formed through upper core plate 110 for connecting guide tube 140 to upper core plate 90. In addition, flange 165 includes two depending pedestals 185 engaging upper core plate 90 for supporting the weight of housing 140 on upper core plate 90. In this manner, pedestals 185 substantially support the weight of housing 140 and consequently bolts 170 do not carry the full weight of housing 140. Thus, the likelihood of bolts 170 bowing or fracturing under the weight of housing 140 is reduced.

Further, lower end portion 160 is pierced by a plurality of rectangularly-shaped apertures 175a and 175b for exit of the coolant from housing 140. The lower most aperture 175a and the upper-most aperture 175b each has a predetermined area 107.42 cm2 (e.g., about 16.65 in2) and a center located a predetermined distance of approximately 219.20 milimeters and approximately 384.3 milimeters (approximately 8.63 inches and approximately 15.13 inches), respectively, above upper core plate 90, for reasons disclosed hereinbelow. Moreover, inlet opening 150 is coaxially aligned with bore 100 to allow a predetermined portion of the coolant flowing through bore 100 to enter through inlet opening 100 and into housing 140. In addition, housing 140 has an upper end portion 190 received in the previously mentioned bore 120 of upper support plate 110 for laterally supporting the upper end portion 190 of housing 140. There may be a total of eight equally-spaced and horizontally aligned apertures 175a and 175b circumscribing housing 140. Also formed around housing 140 are a plurality of slots 200 for reasons disclosed hereinbelow.

As best seen in Fig. 4, disposed in housing 140 are a plurality of spaced-apart coaxially aligned and horizontally-oriented guide plates 200, each of the guide plates 200 having a marginal edge extending therearound that generally conforms to the inside surface of housing 140. Moreover, engaging the marginal edge of each guide plate 200 are a plurality of spaced-apart pins 210, each pin 210 being sized to matingly fit within its respective slot 220 in order to connect each guide plate to housing 140. Each guide plate 200 also includes a plurality of bores 230 therethrough for slidably receiving respective ones of the control rods 80 in order to guide control rods 80 through housing 140 and into fuel assembly 60. Moreover, formed transversely through each guide plate 210 are a plurality of flow passages 240 for allowing the coolant to pass through guide plate 210.Guide plate 210 further includes a centrally disposed opening 250 for allowing space for passage of a control rod spider assembly (not shown) connected to control rods 80 as well as for allowing the coolant to pass through guide plate 210.

OPERATION During operation of pressure vessel 10, the liquid moderator coolant enters inlet nozzle 30 eventually to flow upwardly through bores 100 that are formed through upper core plate 90. As the coolant flows through bores 100, it flows towards inlet opening 150. However, a first predetermined portion (e.g., approximately 20% by volume) of this coolant will flow laterally out gap 155 and will thus avoid inlet opening 150, such as shown by the direction of the arrows in Fig. 3. The amount of coolant avoiding inlet opening 150 is therefore not available to cause turbulent flow in housing 140 which would otherwise lead to undesirable vibration of control rods 80. The remaining portion of the coolant will enter housing 140 to flow upwardly therein.However, a second predetermined portion (e.g., approximately 80% by volume) of the coolant will immediately exit apertures 175a/175b as the remaining coolant passes into housing 140. The coolant exiting apertures 175a/175b is thus also not available to cause turbulent flow in housing 140 which would otherwise lead to undesirable vibration of control rods 80. Any remaining coolant continues its upwardly travel in housing 140 and will encounter the alignment plate that caps housing 140. The coolant flow stream then changes direction to flow downwardly out apertures 175a/175b.

Although some turbulence may occur in housing 140 as the coolant flows upwardly therein, such turbulence, if any, is nonetheless significantly reduced due to the size of gap 155 and the size and location of apertures 175a/175b.

By way of example only and not by way of limitation, housing 140 may have a total length of approximately 96 inches for completely surrounding the length of control rods 80 when control rods 80 are withdrawn into housing 140. Each rectangularly-shaped aperture 175 may have an flow area of approximately 421.64 milimeters (approximately 16.6 inches) for allowing the liquid coolant to flow out housing 140. The lower-most aperture 175a has a center located a predetermined distance of approximately 219.20 milimeters (approximately 8.63 inches) above the top surface of upper core plate 90. The upper-most aperture 175b has a center located a prede termined distance of approximately 384.3 milimeters (approximately 15.13 inches) above the top surface of upper core plate 90.Inlet opening 150 is elevated at approximately 20.32 to 25.4 milimeters (approximately 0.80 to 1.0 inches) above the top surface of upper core plate 90 in order to define a relatively large gap 155 for allowing a desired amount of the coolant to avoid inlet opening 150.

Thus, it is appreciated that applicant's elevation of inlet opening 150 a predetermined distance above the top surface of upper core plate 90 defines a relatively large sized gap 155 which allows a predetermined portion (e.g., approximately 20% by volume) of the coolant to avoid inlet opening 150. Less coolant entering inlet opening 150 results in less coolant entering housing 140, thereby resulting in less turbulent flow within housing 140. Less turbulent flow within housing 140 in turn results in less vibration of control rods 80. It will also be appreciated from the discussion hereinabove that locating apertures 175a/175b in the lower portion of housing 140 results in more coolant leaving housing 140 sooner than by locating apertures 175a/175b in the intermediate or upper portion of housing 140, as in previous guide tube designs. This results in less coolant being available to cause vibration of control rods 80.

Although the invention is fully illustrated and described herein, it is not intended that the invention as illustrated and described be limited to the details shown, because various modifications may be obtained with respect to the invention without departing from the spirit of the invention or the scope of equivalents thereof. For example, there may be an additional number of apertures piercing the length of housing 140 for allowing even more coolant to exit housing 140 in order to reduce vibration of control rods 80.

Therefore, what is provided is a control rod guide tube adapted to reduce vibration of a plurality of control rods disposed therein, which control rod guide tube is suitable for use in pressurized water nuclear reactor pressure vessels.

Claims

What is claimed is:

1. For use in a pressure vessel (10) having a plate member (90) therein defining a hole (100) for allowing a fluid therethrough, a control rod guide (130) tube adapted to reduce vibration of a control rod (80) disposed therein, characterized by a housing (140) surrounding the control rod and having an inlet opening (150) disposed opposite the hole for receiving the fluid flowing through the hole, the fluid capable of inducing vibration of the control rod as the fluid is received through the opening and into said housing, the opening being elevated a predetermined distance above the hole to define a gap (155) between the opening and the hole, so that a first predetermined portion of the fluid flows out the gap and avoids the opening, said housing having a lower portion thereof pierced by an aperture located near the opening for exit of a second predetermined portion of the fluid from said housing, whereby vibration of the control rod is reduced as the first predetermined portion of the fluid avoids the inlet opening and as the second predetermined portion of the fluid exits the aperture.

2. The guide tube of claim 1, further characterized by guide plate (200) disposed in said housing, said guide plate defining a bore (230) therethrough for receiving the control rod to guide the control rod through said housing.

3. The guide tube of claim 1, further characterized by: (a) a flange (165) surrounding the lower portion of said housing, said flange having a bore therethrough; and (b) a bolt (170) extending through the bore of said flange and engaging the upper core plate for connecting said housing to the upper core plate.

4. The guide tube of claim 3, further characterized by a pedestal (185) depending from said flange and engaging the upper core plate for supporting said housing on the upper core plate.