JET PUMP RISER CLAMP

Technical field

The present invention relates to a clamp for a jet pump riser in a nuclear reactor pressure vessel and to a method for arranging a clamp over a jet pump riser in a nuclear reactor pressure vessel. More specifically the present invention relates to a clamp as described which holds the pipes together in a jet pump riser in case it would crack.

Description of the prior art

A nuclear reactor has a reactor pressure vessel, which typically has a generally cylindrical shape and is closed in both ends, e.g., by a bottom head and a removable top head, to form a closed container. A core plate and a top guide are arranged inside the reactor pressure vessel with the top guide arranged on top of the core plate. The top guide comprises a number of holes. A reactor core in the form of fuel bundles is arranged inside the reactor pressure vessel with each fuel bundle being arranged through a hole in the top guide and resting on the core plate. Some reactors are provided with a core shroud which surrounds the core, the core plate and the top guide.

The reactor pressure vessel is filled with water covering the core and the shroud. The water in the reactor pressure vessel is arranged to flow through the core in order to cool the core during operation of the nuclear reactor. The flow is usually induced by ciculation pumps arranged between the shroud and the nuclear reactor pressure vessel which makes the water flow downwards in the space between the shroud and the reactor pressure vessel

(RPV) wall. The circulation pumps may be of any of a number of different types. One type of circulation pumps used in nuclear reactors of the above described type is jet pumps, in which water is injected through a narrow nozzle in a jet pump arranged between the shroud and the reactor pressure vessel. Water enters the RPV through an inlet nozzle in the RPV wall. A jet pump riser assembly

is connected to the inlet nozzle, wherein the jet pump riser assembly comprises a thermal sleeve, a lower elbow, a riser pipe and an upper elbow. One end of the thermal sleeve is arranged on the outside of the RPV while the other end of the thermal sleeve is welded to a first end of the lower elbow. The second end of the lower elbow is welded to a first end of the riser pipe. The second end of the riser pipe is welded to the upper elbow. During operation of the nucler reactor substantial amounts of water is forced through the jet pump riser assembly. The large amounts of water flowing through the jet pump riser assembly results in significant hydraulic forces and vibrations exerted on the jet pump assembly. Further, a large amount of heat is generated by the core during operation of the nuclear reactor, resulting in thermal stresses are exerted on the jet pump assembly. The hydraulic forces, vibration stresses and thermal stresse might cause stress corrosion cracking (SCC) in the riser pipe elbow, which might result in the riser pipe elbow being separated from the thermal sleeve. This is of course highly undesirable.

It is desirable to provide an apparatus which provides support for a jet pump riser elbow. Such an apparatus is described in US patent 6,053,652, which is directed towards a clamp assembly which provides support for the riser pipe and the lower elbow. The clamp assembly comprises a lower thermal sleeve clamp, which is arranged to be secured to the thermal sleeve, and a riser clamp which is configured to be secured to the riser pipe. The lower clamp and the riser clamp are coupled together. With such a clamp assembly the stresses on the weld between the thermal sleeve and the lower elbow, as well as the weld between the lower elbow and the riser pipe, are greatly reduced. This minimizes the risk for SCC of the riser pipe assembly at the interface between the lower elbow and the riser pipe, and at the interface between the lower elbow and the thermal sleeve. With a clamp assembly according US patent 6,053,652 it may however be more difficult to inspect the weld between the lower elbow and the riser pipe, and the weld between the lower elbow and the thermal sleeve.

US patent 6,108,391 describes a clamp apparatus for performing jet pump riser pipe repairs. The clamp apparatus described in said patent includes a lower clamp element and an upper clamp element. The upper and lower elements are configured to be positioned at the interface between the thermal sleeve and the jet pump riser elbow. The upper and lower elements include extended ridges that are configured to fit in circumferential grooves precisely machined into the sleeve-elbow assembly on opposing sides of the interface.

One disadvantage of the clamp apparatus described in US patent 6,108,391 is that it requires grooves to be precisely machined into the sleeve-elbow assembly on opposing sides of the interface.

The apparatuses according to the prior art may be used to support the interfaces between the riser pipe, the lower elbow and the thermal sleeve in a riser pipe assembly in a reactor pressure vessel. However, there is a need for an alternative clamp apparatus which solves at least one of the problems of the prior art.

Summary of the invention

An object of the present invention is to provide a clamp apparatus for supporting at least one of the joints between the riser pipe, the lower elbow and the thermal sleeve in a riser pipe assembly in a reactor pressure vessel.

Another object of the present invention is to provide a clamp apparatus for supporting at least one of the joints between the riser pipe, the lower elbow and the thermal sleeve in a riser pipe assembly in a reactor pressure vessel, which solves at least one of the problems of the clamps according to the prior art.

These objects are achieved with a clamp apparatus according to the independent claim.

Further advantages with the present invention are achieved with the features defined in the dependent claims.

According to a first aspect of the present invention a clamp assembly is provided for securing a thermal sleeve and an elbow in a pipe assembly in a nuclear reactor. The pipe assembly comprises a thermal sleeve with a length axis, an elbow and a riser pipe with a length axis, including a first joint between the elbow and the thermal sleeve, and a second joint between the riser pipe and the elbow. The elbow is arranged between a reactor pressure vessel wall, and at least one of a core shroud and a core plate support ring. The clamp assembly comprises a sleeve clamp adapted to be arranged surrounding the elbow at the first joint between the elbow and the thermal sleeve, and a sleeve support means adapted to extend across the first joint between the elbow and the thermal sleeve, and a clamp band adapted to be arranged partly between the pipe assembly and the core shroud fixed in relation to the sleeve clamp, and in contact with the pipe assembly. The clamp assembly is characterized in that the clamp band is adapted to be in contact with the side of the elbow facing the core shroud and in that the clamp assembly also comprises at least one clamp arm with a first end being arranged to rest against at least one of the core shroud and the core plate support ring, and a spring means being arranged between the clamp arm and the sleeve clamp for applying a force on the sleeve clamp away from the core shroud forcing the clamp band to be in contact with the side of the elbow facing the core shroud. The spring means thereby applies a force on the elbow towards the thermal sleeve and fixing the position of the sleeve clamp in relation to the first joint between the elbow and the thermal sleeve.

With a clamp assembly according to the invention a force is applied on the elbow towards the thermal sleeve at the same time as the elbow is hindered from moving in relation to the thermal sleeve perpendicularly to the length axis of the thermal sleeve. Furthermore, the clamp arms provide for a support from the core shroud. By having a spring means between the clamp arms and the

sleeve clamp, the core shroud is allowed to move in relation to the reactor pressure vessel, which may be the case during changes in the operation mode of the nuclear reactor. With a clamp assembly according to the present invention the position of the sleeve clamp is determined by the size of the clamp band, while the force of the clamp band on the elbow is determined by the spring constant of the spring means and the length of the clamp arms.

The spring means may comprise a compressible ring. This is a robust implementation of the spring means. It is, however, possible to implement the spring means in other ways. The spring means may, e.g., be one or more disc springs or coil springs.

The clamp assembly may comprise a spring support adapted to be arranged between the elbow and the sleeve clamp and arranged fixed in relation to said at least one clamp arm, wherein the spring means is arranged between the spring support and the sleeve clamp. By the arrangement of such a spring support between the clamp arms and the spring means, the arrangement of the spring means is facilitated.

The sleeve support means may comprise a number of studs adapted to extend from the sleeve clamp and to be spaced around the thermal sleeve. By the sleeve support means comprising a number of studs future inspection of the first joint between the elbow and the thermal sleeve is facilitated as the space between the studs provide for a camera to be arranged at the first joint between the elbow and the thermal sleeve.

The sleeve clamp may comprise a lower sleeve clamp part and an upper sleeve clamp part, which are attached to each other by means of fastening means. By the sleeve clamp being divided in two parts the arrangement of the sleeve clamp around the elbow may be performed in the reactor in a relatively simple way.

The fastening means may be screws. It is also possible to use any fastening means known to men skilled in the art as fastening

means. However, screws are relatively simple to arrange on the sleeve clamp in the reactor.

The clamp band may be attached to the upper sleeve clamp by means of the fastening means. By the clamp band being attached to the upper sleeve clamp by means of the fastening means a minimum of components are required in the clamp assembly.

The sleeve clamp may be arranged to be in loose contact with the elbow and the thermal sleeve, thereby allowing the elbow and the thermal sleeve to move in relation to the sleeve clamp. In this way the position of the sleeve clamp is defined by the length of the clamp band only.

According to a second aspect of the present invention a nuclear reactor is provided comprising a reactor pressure vessel with a wall, a core arranged inside the reactor pressure vessel, a core plate on which the core is arranged, a core plate support ring supporting the core plate, a core shroud surrounding the core and being arranged within the reactor pressure vessel, and a pipe assembly for transferring cooling water into the reactor pressure vessel for cooling of the core. The pipe assembly comprises a thermal sleeve with a length axis being arranged through the reactor pressure vessel wall, an elbow and a riser pipe with a length axis, including a first joint between the elbow and the thermal sleeve, and a second joint between the riser pipe and the elbow. The riser pipe and the elbow are arranged between the reactor pressure vessel wall, and at least one of the core shroud and the core plate support ring. The nuclear reactor also comprises a clamp assembly for securing the thermal sleeve and elbow, said clamp assembly comprising a sleeve clamp being arranged surrounding the elbow at the first joint between the elbow and the thermal sleeve, which sleeve clamp comprises sleeve support means extending across the first joint between the elbow and the thermal sleeve, and a clamp band which is arranged partly between the pipe assembly and the core shroud fixed in relation to the sleeve clamp, and in contact with the pipe assembly. The nuclear reactor is characterized in that the clamp

band is in contact with the side of the elbow facing the core shroud and in that the clamp assembly also comprises at least one clamp arm with a first end being arranged to rest against at least one of the core shroud and the core plate support ring, a spring means being arranged between the clamp arm and the sleeve clamp for applying a force on the sleeve clamp away from the core shroud forcing the clamp band to be in contact with the side of the elbow facing the core shroud, thereby applying a force on the elbow towards the thermal sleeve and fixing the position of the sleeve clamp in relation to the first joint between the elbow and the thermal sleeve.

With a clamp assembly being arranged in such a way in the nuclear reactor a force is applied on the elbow towards the thermal sleeve at the same time as the elbow is hindered from moving in relation to the thermal sleeve perpendicularly to the length axis of the thermal sleeve. Furthermore, the clamp arms provide for a support from the core shroud. By having a spring means between the clamp arms and the sleeve clamp, the core shroud is allowed to move in relation to the reactor pressure vessel, which may be the case during changes in the operation mode of the nuclear reactor. With a clamp assembly according to the present invention the position of the sleeve clamp is determined by the size of the clamp band, while the force of the clamp band on the elbow is determined by the spring constant of the spring means and the length of the clamp arms.

The features described above in relation to the first aspect of the invention may also be implemented in the nuclear reactor according to the second aspect of the present invention, with the same advantages as has been described above.

The entire clamp band may be arranged below the second joint between the elbow and the riser pipe. The arrangement of the clamp band in this way provides for easy inspection of the second joint between the elbow and the riser pipe.

The nuclear reactor may comprise two clamp arms arranged on opposite sides of a plane being defined by the length axis of the riser pipe and the length axis of the thermal sleeve. By having two clamp arms being arranged in this way provides stability to the clamp assembly.

The fastening means may be arranged essentially parallel to the length axis of the riser pipe. This facilitates the arrangement of the fastening means in the reactor.

The clamp band may essentially be U-formed extending from a fastening means on one side of a plane being defined by the length axis of the riser pipe and the length axis of the thermal sleeve behind the elbow between the elbow and the core shroud and to the fastening means on the other side of said plane.

According to a third aspect of the present invention a method is provided for securing a thermal sleeve and an elbow in a nuclear reactor comprising a reactor pressure vessel with a wall, a core arranged inside the reactor pressure vessel, a core plate on which the core is arranged, a core plate support ring supporting the core plate, a core shroud surrounding the core and being arranged within the reactor pressure vessel, and a pipe assembly for transferring cooling water into the reactor pressure vessel for cooling of the core. The pipe assembly comprises a thermal sleeve with a length axis being arranged through the reactor pressure vessel wall, an elbow and a riser pipe with a length axis, including a first joint between the elbow and the thermal sleeve, and a second joint between the riser pipe and the elbow. The riser pipe and the elbow are arranged between the reactor pressure vessel wall, and at least one of the core shroud and the core plate support ring. The method comprises the steps of arranging a clamp assembly on the pipe assembly by arranging a sleeve clamp surrounding the elbow at the first joint between the elbow and the thermal sleeve, which sleeve clamp comprises sleeve support means extending across the first joint between the elbow and the thermal sleeve, and by arranging a clamp band partly between the pipe assembly and the core shroud

fixed in relation to the sleeve clamp, and in contact with the pipe assembly. The method is characterized in that the clamp band is arranged in contact with the side of the elbow facing the core shroud and in that the method also comprises the steps of arranging at least one clamp arm with a first end being arranged to rest against at least one of the core shroud and the core plate support ring, arranging a spring means between the clamp arm and the sleeve clamp for applying a force on the sleeve clamp away from the core shroud forcing the clamp band to be in contact with the side of the elbow facing the core shroud, thereby applying a force on the elbow towards the thermal sleeve and fixing the position of the sleeve clamp in relation to the first joint between the elbow and the thermal sleeve.

With a method according to the third aspect of the present invention the clamp assembly is arranged in such a way in the nuclear reactor that a force is applied on the elbow towards the thermal sleeve at the same time as the elbow is hindered from moving in relation to the thermal sleeve perpendicularly to the length axis of the thermal sleeve. Furthermore, the clamp arms provide for a support from the core shroud. By having a spring means between the clamp arms and the sleeve clamp, the core shroud is allowed to move in relation to the reactor pressure vessel, which may be the case during changes in the operation mode of the nuclear reactor. With a clamp assembly according to the present invention the position of the sleeve clamp is determined by the size of the clamp band, while the force of the clamp band on the elbow is determined by the spring constant of the spring means and the length of the clamp arms.

In the following preferred embodiments of the invention will be described with reference to the appended drawings.

Short description of the drawings

Fig 1 shows schematically a boiling water nuclear reactor in which a clamp assembly according to the present invention may be arranged.

Fig 2 shows in larger detail a clamp assembly arranged on a pipe assembly in the boiling water reactor of Fig 1 .

Fig 3 is an exploded view of the clamp assembly in Fig 2.

Description of preferred embodiments

In the following description of preferred embodiments of the invention the same reference numeral will be used for similar features in the different drawings, which are not drawn to scale.

Fig 1 shows schematically a boiling water nuclear reactor 1 in which a clamp assembly according to the present invention may be arranged. The nuclear reactor 1 comprises a reactor pressure vessel 2 with a wall 3, a core 4 arranged inside the reactor pressure vessel 2, a core plate 5 on which the core 4 is arranged, a core plate support ring 6 supporting the core plate 5, a core shroud 7 surrounding the core 4 and being arranged within the reactor pressure vessel 2. The nuclear reactor also comprises a number of pipe assemblies, in the form of jet pump assemblies 8, for transferring cooling water into the reactor pressure vessel 2 for cooling of the core 4. The jet pump assemblies 8 comprises a thermal sleeve 9 with an inlet 10 being arranged through the reactor pressure vessel wall 3, an elbow 1 1 and a riser pipe 12 extending upwardly from the elbow 1 1 to a transition piece 13 and two inlet mixers 14 extending downwardly from the transition piece. Water is pumped into the inlet 10 by external pumps (not shown). The water is forced through the elbow 1 1 and the riser pump to the transition piece 13 where the water is forced through a nozzle (not shown) in a jet pump (not shown) to force water through the inlet mixers, thereby creating a flow of water downwards from the transition piece 13 towards the bottom of the reactor pressure vessel 2 and up through the core 4.

Fig 2 shows in larger detail a clamp assembly 19 arranged on a pipe assembly in the boiling water reactor 1 of Fig 1 , which pipe

assembly comprises a thermal sleeve 9, an elbow 1 1 and a riser pipe 12. The thermal sleeve 9 comprises a length axis 15 which is essentially perpendicular to the reactor pressure wall 3. The riser pipe 12 comprises a length axis 16 which is essentially perpendicular to the length axis of the thermal sleeve. The pipe assembly includes a first joint 17 between the elbow 1 1 and the thermal sleeve 9, and a second joint 18 between the riser pipe 12 and the elbow 1 1 . A first connector pipe 20 is arranged between the elbow 1 1 and the riser pipe 12. A second connector pipe 21 is arranged between the elbow 1 1 and the thermal sleeve 9. The riser pipe 9 and the elbow 1 1 are arranged between the reactor pressure vessel wall 3, and at least one of the core shroud 7 and the core plate support ring 6. The clamp assembly 19, for securing the thermal sleeve 9 and the elbow 1 1 , comprises a sleeve clamp 37 being arranged surrounding the elbow 1 1 at the first joint 17 between the elbow 1 1 and the thermal sleeve 9. The sleeve clamp 37 comprises a lower sleeve clamp part 22 and an upper sleeve clamp part 23, which are attached to each other by means of fastening means in the form of screws 24. The screws 24 are arranged essentially parallel to the length axis 16 of the riser pipe 12. The sleeve clamp 37 comprises sleeve support means 25 in the form of studs extending from the sleeve clamp and being spaced around the thermal sleeve 9 extending across the first joint 17 between the elbow 1 1 and the thermal sleeve 9, and a clamp band 26 which is arranged partly between the pipe assembly and the core shroud 7, fixed in relation to the sleeve clamp 37, and in contact with the pipe assembly. The clamp band 26 is attached to the upper sleeve clamp part 23 by means of the screws 24. The clamp band 26 is in contact with the side of the elbow 1 1 facing the core shroud 7. The clamp assembly also comprises two clamp arms 27 arranged on opposite sides of the plane being defined by the length axis 16 of the riser pipe 12 and the length axis 15 of the thermal sleeve 9, with a first end 30 of each clamp arm arm 27 being arranged to rest against at least one of the core shroud 7 and the core plate support ring 6. The clamp assembly also comprises a spring means 28, in the form of a ring. The spring means 28 is arranged between a spring support 29, being fixed in relation to the

clamp arms 27, and the upper sleeve clamp part 23 for applying a force on the sleeve clamp 37 away from the core shroud 7 forcing the clamp band 26 to be in contact with the side of the elbow 1 1 facing the core shroud 7. The spring means 28 thereby applies a force on the elbow 1 1 towards the thermal sleeve 9 and fixes the position of the sleeve clamp in relation to the first joint 17 between the elbow 1 1 and the thermal sleeve 9. The sleeve clamp is arranged in a loose fit with the elbow 1 1 and the thermal sleeve 9, thereby allowing the elbow 1 1 and the thermal sleeve 9 to move in relation to the sleeve clamp. The clamp band 26 is essentially U- formed and extends from the screw 24 on one side of the plane being defined by the length axis 16 of the riser pipe 12 and the length axis 15 of the thermal sleeve 9, behind the elbow 1 1 between the elbow 1 1 and the core shroud 7 and to the screw 24 on the other side of said plane.

Fig 3 is an exploded view of the clamp assembly in Fig 2 showing the lower sleeve clamp part 22, the upper sleeve clamp part 23, the clamp arms 27, the spring support 29, the clamp band 26, the spring means 28 and the screws 24. The clamp arms 27 have a first end 30 which is being arranged to rest against at least one of the core shroud 7 and the core plate support ring 6. The clamp arms 27 also have a length axis 35. The second end of the support arms 27 are to be arranged in the recesses 31 of the lower sleeve clamp part with the support surface 32 of the sleeve clamp arms resting against the lower sleeve clamp part 22. The spring support 29 has two support arms 33 which are arranged to be arranged in corresponding recesses 34 of the clamp arms thus hindering the spring support from moving along the length axis 35 of the clamp arms 27. Support studs 36 on the spring support 29 are arranged to rest against the clamp arms 27. The clamp band 26 is arranged to rest against the over side of the spirng support 29. When the clamp band is fastened to the upper sleeve clamp part 23 and the lower sleeve clamp part 22 by means of the screws 24, the spring support 29 is thus hindered from moving in relation to the clamp arms 27. The spring support 29 and the clamp arms 27 may then only move in along the length axis of the clamp arms 27.

The described embodiments may be amended in many ways without departing from the spirit and scope of the present invention which is only limited by the claims.

It is possible to have only one clamp arm 27 in a clamp assembly according to the present invention.