EP1946329A1

EP1946329A1 - A control device and a nuclear reactor plant

Info

Publication number: EP1946329A1
Application number: EP06784232A
Authority: EP
Inventors: Gunnar STÅHL
Original assignee: Westinghouse Electric Sweden AB
Current assignee: Westinghouse Electric Sweden AB
Priority date: 2005-11-09
Filing date: 2006-09-08
Publication date: 2008-07-23
Also published as: WO2007055645A1; SE529279C2; EP1946329A4; SE0502482L

Abstract

The present invention refers to a control rod device for a nuclear reactor (1) having a core which comprises nuclear fuel and which is arranged to permit through-flowing of water. The control rod device (2) comprises a moveable control rod member (6) and a stationary guide member (7). The control rod member may be displaced axially into the core and comprises a control rod (8) with a neutron-absorbing substance. The control rod member is displaceable in the guide member between a first end position outside the core and a second end position in the core. A retarding device is arranged to retard the control rod member when it approaches the second end position. The retarding device comprises an eddy current brake having a first eddy current unit (31) on one of the control rod member and the guide member and a second eddy current unit (32) on the other of the control rod member and the guide member.

Description

A control device and a nuclear reactor plant

THE BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention refers to a control rod device for a nuclear reactor having a core which comprises nuclear fuel and is arranged to permit through-flowing of moderating and cooling water, comprising a moveable control rod member, which extends along a longitudinal axis and which is displaceable axially into the core, wherein the control rod member comprises a control rod having a neutron absorbing substance, a stationary guide member, in which the control rod member is dis- placeably provided between a first end position, in which the control rod is located substantially outside the core, and a second end position, in which the control rod is located substantially in the core, and a retarding device, which is arranged to retard the control rod mem- ber during its axial displacement towards the second end position immediately before the control rod member reaches the second end position and which comprises an eddy current brake. The invention also refers to a nuclear reactor plant having such a control rod device.

Such control rod devices are used in nuclear reactors of boiling water type (BWR) and of pressure water type (PWR). The control rod members are then arranged in such a way that the control rods may be introduced into the reactor core within a very short time pe- riod (approximately 2-4 seconds) for quick shutdown of the reactor. In a BWR, the control rod members are introduced from beneath by means of a power source, for instance a so-called SCRAM-system. In a PWR, the control rod members are introduced by letting the control rod members fall down from above by means of gravity. In a BWR, the control rod members may also be introduced in a more slowly and controlled manner in order to enable control of the effect of the reactor. Since quick shutdown takes place so quickly, the control rod members must be retarded before they reach their second end position. Today this retarding takes place by means of hydraulic damping. The control rod members are designed in such a way that they permit liquid to be pressed out between the moveable control rod member and the stationary guide member when the control rod member approaches the second end position. This hydraulic damping is, however, in many cases insufficient, which means that the control rod member hits the stationary guide member with a too high kinetic energy, which in turn may lead to damages on included components, for instance the bayonet coupling which forms an essential part in the control members for BWR and which permits releasing of the control rod proper from the rest of the control rod member.

More specifically, the control rod device in a BWR comprises a guide tube and a piston member consisting of a piston and a piston tube, which is moveably provided in the guide tube. By means of the power source mentioned above, the piston member may be displaced upwardly in the guide tube by the supply of a pressurized medium from the power source. The piston and the upper part of the guide tube are designed in such a way that a retarding of the control rod member by hydraulic damping is accomplished when the piston approaches the second, upper end position. The control rod proper is via a rod connected to the upper end of the piston tube by means of a bayonet coupling. This bayonet coupling may be damaged if the piston hits the upper part of the guide tube with a too large force.

US-A-4,487,739 discloses a control rod device for a reactor arranged to be cooled with liquid metal. In the lower end of the control rod device there are means for hydraulic damping, which are designed in such a way that the liquid metal is pressed out between gaps between the lower end of the control rod and a receiving guide member. The known device is supplemented with an eddy current brake comprising a magnet in the receiving guide tube. When the control rod approaches the magnet, eddy currents will arise in the electric conductor, which in this case is formed by the liquid metal. The outward flowing of the liquid metal through the gaps will then be retarded and in such a way the hydraulic damping action is in- creased.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an efficient retard- ing of a control rod member in connection with quick shutdown of a water reactor. This object is achieved by the control rod device initially defined, which is characterized in that the eddy current brake comprises a first eddy current unit provided on one of the control rod member and the guide member and a second eddy current unit provided on the other of the control rod member and the guide member.

By means of such an eddy current brake comprising one unit on the moveable control rod member and another unit on the stationary guide member, an efficient retarding of the movement of the control rod member may be achieved. In such a way possible damages on the control rod member and/or the guide member may be avoided. Especially, the eddy current brake according to this invention may form a supplement to the present hydraulic damping, and in such a way the total braking action may be increased. Furthermore, the eddy current brake according to this invention may operate independently of the cooling medium. The eddy current brake according to this invention is thus suitable to be used in a nuclear reactor arranged to permit through-flowing of a moderating and cooling water. Especially, the control rod device according to the invention may be installed in light water reactors or heavy water reactors. The control rod device according to the invention is in its general form applicable to boiling water reactors as well as pressure water reactors.

According to an embodiment of the invention, the first eddy current unit comprises at least one magnet. The first eddy current unit may advantageously comprise at least one permanent magnet. With such a permanent magnet, it is ensured that the eddy current brake always operates independently of external current supply. Such a design is thus very suitable for being able to shut down the reactor quickly when any undesired event has occurred.

According to a further embodiment of the invention, the first eddy current unit comprises at least one electromagnet. By means of an electromagnet, the applied braking action may be controlled. One such embodiment is especially suitable for testing the quick shut- down system when the reactor is already shut down for revision and when an even more powerful retarding is desired.

According to a further embodiment of the invention, the second eddy current unit comprises at least one electric conductor. It is to be noted here that the electric conductor may be provided on the control rod member or the guide member and that the magnet may be provided on the guide member or the control rod member.

According to a further embodiment of the invention, the second eddy current unit is provided on the control rod member. The magnet will then be provided on the stationary guide member. Such an embodiment is especially suitable in a boiling water reactor since there may be many electric components immediately outside the control rod member, which could be influenced by a magnet in the moveable control rod member.

According to a further embodiment of the invention, the control rod member has an outer peripheral surface, wherein the electric conductor extends around the whole outer peripheral surface. Further- more, the second eddy current unit may comprise a plurality of electric conductors, which extend around the outer peripheral surface and which are provided at an axial distance A from each other.

According to a further embodiment of the invention, the first eddy current unit comprises a plurality of magnets. According to a further embodiment of the invention, the first eddy current unit is provided on the guide member. The guide member may then have an inner peripheral surface, wherein the magnets form at least one set of magnets which are provided at the same axial position and distributed around the inner peripheral surface. Furthermore, the magnets may form several sets of magnets, wherein the magnets in each of the sets are provided at the same axial position and distributed around the inner peripheral surface and wherein the sets are provided at an axial distance B from each other.

According to a further embodiment of the invention, the axial distance A is substantially equal to the axial B. In such a way an eddy current brake having a plurality of magnets in each eddy current unit is achieved, wherein each of these magnets may cooperate with a respective electric conductor and thus together provide a very large braking force.

According to a further embodiment of the invention, each magnet has a north pole and a south pole, wherein the magnets in each of the sets has the north pole turned radially inwardly and the magnets in the adjacent set or sets have the south pole turned radially inwardly.

According to a further embodiment of the invention, the guide member comprises a guide tube and a control rod member a piston member which is moveably provided in the guide tube, wherein the control rod device comprises a power source which is arranged to permit, to the guide tube, supply of a medium acting on the piston member in such a way that the control rod member is displaced to the second end position, and wherein the first eddy current unit is provided on one of the guide tube and the piston member, and the second eddy current is provided on the other of the guide tube and the piston member. The outer peripheral surface may then be lo- cated on the piston member and the inner peripheral surface on the guide tube. Such a design of the control rod device is especially applicable to a boiling water reactor, wherein the magnets may be located on the inner peripheral surface of the guide tube and the electric conductors may extend around the outer peripheral surface of the piston member.

According to a further embodiment of the invention, the control rod member comprises a rod, which connects the control rod and the piston member, wherein the guide member may comprise an outer tube. In this embodiment, the first eddy current unit may be provided on one of the rod and the outer tube whereas the second eddy current unit is provided on the other of the rod and the outer tube.

According to a further embodiment of the invention, the first eddy current unit is provided on one of the control rod and the guide member, and the second eddy current unit on the other of the control rod and the guide member. Such a design is also applicable to a pressure water reactor, wherein one of the eddy current units may be provided at the lower end of the control rod and the other eddy current unit at the lower end of a guide tube of the guide member.

The object is also achieved by a nuclear reactor plant comprising a nuclear reactor and a control rod device as specified above. According to various embodiments, the nuclear reactor may be of boiling water type or pressure water type.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now to be explained more closely through a description of various embodiments and with reference to the draw- ings attached hereto.

Fig. 1 discloses schematically a nuclear power plant having a control rod device.

Fig. 2 discloses schematically a first embodiment of a control rod device according to the invention.

Fig. 3 discloses more closely a part of an eddy current brake of the control rod device in Fig. 2. Fig. 4 discloses a second embodiment of the control rod device according to the invention.

Fig. 5 discloses a third embodiment of a control rod device according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 discloses schematically a nuclear power plant comprising a nuclear reactor 1 and a plurality of control rod devices 2. Fig. 1 merely discloses one such control rod device 2. The nuclear reactor plant also comprises a core 3 which is formed by a large number of fuel elements 4 with nuclear fuel. The nuclear reactor 1 is arranged to permit through-flowing of moderating and cooling water and may for instance be a so-called light water reactor. The nuclear reactor 1 , which is disclosed in Fig. 1 , is a boiling water reactor. The nuclear power plant comprises a very large number of control rod devices 2.

Each control rod device 2 comprises a moveable control rod mem- ber 6 and a stationary guide member 7 in which the control rod member 6 is displaceable along a longitudinal axis x between a first end position substantially outside the core 3 and a second end position substantially in the core 3. Each control rod member 6 comprises a control rod 8, see Fig. 2, which comprises a neutron ab- sorbing substance and which in its entirety is intended to be located substantially in the core 3 in the second end position and substantially completely outside the core 3 in the first end position. The control rod device 2 also comprises a drive member 9 arranged to displace the control rod member 6 between the two end positions in such a way that the position of the control rod 8 in the core 3 may be adjusted in order to control the effect of the reactor 1.

The drive member 9 comprises an electric drive motor 10 which is connected to a drive screw 11. A drive nut 12 is provided on the drive screw 1 1 and will be moved upwardly or downwardly depending on in which direction the drive screw 1 1 is rotated by means of the motor 3. On the drive nut 12 a piston member 13 comprising a piston 14 and a piston tube 15 rests. The guide member 7 comprises an outer tube 16 and a guide tube 17 provided in the lower part of the outer tube 16. The outer tube 16 is pressure-supporting and extends into the reactor 1 proper. The piston member 13 is dis- placeable along the longitudinal axis x in the guide tube 17. The upper end of the piston tube 15 is connected to a rod 18 which in turn is connected to the control rod 8 proper. The rod 18 is connected to the piston tube 15 by means of a bayonet coupling 19. The rod 18, the bayonet coupling 19, the piston member 13 and the guide tube 17 are enclosed in the pressure-supporting outer tube 16. The rotary movement of the drive motor 10 is thus transferred to a vertical translation movement of the control rod member 2 via the drive screw 1 1 , the drive nut 12, the piston member 13 and the rod 18. With this drive member 9, the control rod 8 may thus slowly be displaced into and out of the core for control of the effect of the reactor 1.

The nuclear power plant also comprises a quick shutdown system, which comprises a schematically disclosed power source 21. The power source 21 may for instance comprise a pressure vessel which provides a fluid, for instance water, under a very high pressure. This pressurised fluid may be supplied to the control rod device 2 via a conduit 22 into the drive member 9 via a schematically indicated valve 23. When the fluid is supplied at a very high pres- sure, this pressure will act on the underside of the piston 14 and very quickly press the piston 14, the piston tube 15 and the rod 18 vertically upwardly along the longitudinal axis x and in such a way force the control rod 8 into the core 3.

The introduction of the control rod member 6 with such a quick shutdown system occurs very quickly, in the order of 2-4 seconds. Since the introduction occurs so quickly, the movement of the piston member 13 has to be retarded before the piston 14 reaches its upper end position in the guide tube 17. This retarding is accom- pushed by means of a retarding device comprising hydraulic damping and an eddy current brake. The hydraulic damping is achieved by the fact that the piston 14 at its upper end is designed in such a way that it permits the liquid present between the piston tube 13 and the guide tube 17 to be pressed out between the piston 14 and the guide tube 17 when the piston 14 approaches the upper end position.

The eddy current brake of the retarding device comprises a first eddy current unit 31 and a second eddy current unit 32. In the first embodiment, disclosed in Fig. 2, the first eddy current unit 31 is provided on the guide tube 17 and the second eddy current unit 32 on the piston 14. The first eddy current unit 31 comprises a magnet 33 and the second eddy current unit 32 comprises an electric conductor, see Fig. 3 which discloses more closely how the eddy current brake may be designed. The first eddy current unit 31 comprises in this embodiment a plurality of magnets 33, preferably per- manent magnets. The magnets 33 form in the embodiment disclosed four sets 35 of permanent magnets. Each set 35 comprises in turn a plurality of magnets 33 which are distributed along an inner peripheral surface 36 of the guide tube 17. The sets 35 are provided at a mutual axial distance B from each other.

The second eddy current unit 32 comprises a plurality of electric conductors 34 which extend around an outer peripheral surface 37 of the piston 14. Each electric conductor 34 is annular and thus extends around the whole outer peripheral surface 37. The electric conductors 34 are provided at a mutual axial distance A from each other. The distance A is substantially equal to the distance B so that each set 35 at the same time may cooperate with one of the electric conductors 34. Each magnet 33 has a north pole and a south pole. The magnets in one of the sets 35 has the north pole turned radially inwardly and the magnets in the adjacent set 35 has the south pole turned radially inwardly in such a way that the magnets 33 along the axis x are provided in an alternating order with respect to the poles.

Fig. 4 discloses a control rod device according to a second embodiment, which differs from the first embodiment in that the first eddy current unit 31 is provided on the outer tube 16 and the sec- ond eddy current unit 32 on the rod 18. It is to be noted that the arrangement also may be reversed in such a way that the first eddy current unit 31 is provided on the rod 18 whereas the second eddy current unit 32 is provided on the outer tube 16. The eddy current unit 31 , 32 provided on the outer tube 16 may be located within or outside the reactor proper.

Fig. 5 discloses a third embodiment of a control rod device according to the invention. Fig. 5 illustrates schematically a part of a con- trol rod device 2 in a pressure water reactor. The control rod device 2 comprises a control rod member with a control rod 8, which is arranged, at shutdown of the reactor by means of gravity, to be introduced downwardly into the core in a stationary guide member comprising a guide tube 17. The guide tube 17 is mounted in a bottom plate 40 of the core 3. The disclosed control rod device 2 comprises a retarding device comprising a hydraulic damping and an eddy current brake. The hydraulic damping is achieved in this case by a reduced cross sectional area of the guide tube 17 at its lower end. The eddy current brake comprises a first eddy current unit 31 pro- vided on the guide tube 17 and a second eddy current unit 32 provided on the control rod 8. Also in this case it is possible as an alternative to provide the first eddy current unit 31 on the control rod 8 and the second eddy current unit 32 on the guide tube 17.

The invention is not limited to the embodiments disclosed but may be varied and modified within the scope of the following claims. It should be noted especially that a plurality of various locations of the two eddy current units 31 and 32 are possible within the scope of this invention. It is also possible to vary the provision of the mag- nets on either the stationary or the moveable part of the control rod device 2 with the electric conductor on the other of the stationary and moveable part of the control rod device 2.

Claims

1. A control rod device for a nuclear reactor (1 ) having a core (3) which comprises nuclear fuel and is arranged to permit through- flowing of moderating and cooling water, comprising a moveable control rod member (6), which extends along a longitudinal axis (x) and which is displaceable axially into the core, wherein the control rod member comprises a control rod (8) having a neutron absorbing substance, a stationary guide member (7), in which the control rod member (6) is displaceably provided between a first end position, in which the control rod (8) is located substantially outside the core (3), and a second end position, in which the control rod (8) is located substantially in the core (3), and a retarding device, which is arranged to retard the control rod member (6) during its axial displacement towards the second end position immediately before the control rod member (6) reaches the second end position and which comprises an eddy current brake. characterized in that the eddy current brake comprises a first eddy current unit (31 ) provided on one of the control rod member (6) and the guide member (7) and a second eddy current unit (32) provided on the other of the control rod member (6) and the guide member (7).

2. A control rod device according to claim 1 , characterized in that the first eddy current unit (31 ) comprises at least one magnet (33).

3. A control rod device according to claim 2, characterized in that the first eddy current unit comprises at least one permanent magnet.

4. A control rod device according to any one of claims 2 and 3, characterized in that the first eddy current unit comprises at least one electromagnet.

5. A control rod device according to any one of claims 1 to 4, characterized in that the second eddy current unit (32) comprises at least one electric conductor (34).

6. A control rod device according to claim 5, characterized in that the second eddy current unit (32) is provided on the control rod member (6).

7. A control rod device according to claim 6, characterized in that the control rod member (6) has an outer peripheral surface, wherein the electric conductor extends around the whole outer peripheral surface.

8. A control rod device according to claim 7, characterized in that the second eddy current unit comprises a plurality of electric conductors which extend around the outer peripheral surface and which are provided at an axial distance A from each other.

9. A control rod device according to any one of the preceding claims, characterized in that the first eddy current unit (31 ) comprises a plurality of magnets (33).

10. A control rod device according to claim 2, characterized in that the first eddy current unit (31 ) is provided on the guide member (7).

11. A control rod device according to claims 9 and 10, characterized in that the guide member (7) has an inner peripheral surface (36), wherein the magnets (33) form at least one set (35) of mag- nets which are provided at the same axial position and distributed around the inner peripheral surface (36).

12. A control rod device according to claim 11 , characterized in that the magnets (33) form several sets (35) of magnets, wherein the magnets (33) in each of the sets (35) are provided at the same axial position and distributed around the inner peripheral surface (36) and wherein the sets (35) are provided at an axial distance B form each other.

13. A control rod device according to claims 8 and 12, character- ized in that the axial distance A is substantially equal to the axial distance B.

14. A control rod device according to any one of claim 12 and 13, characterized in that each magnet (33) has a north pole and a south pole, wherein the magnets (33) in one of the sets (35) has the north pole turned radially inwardly and the magnets (33) in the adjacent set or sets (35) have the south pole turned radially inwardly.

15. A control rod device according to any one of the preceding claims, characterized in that the guide member (7) comprises a guide tube (17) and that the control rod member (6) comprises a piston member (13) which is moveably provided in the guide tube (17), wherein the control rod device comprises a power source which is arranged to permit, to the guide tube, supply of a medium acting on the piston member (13) in such a way that the control rod member (2) is displaced to the second end position and wherein the first eddy current unit (31 ) is provided on one of the guide tube (17) and the piston member (13), and the second eddy current unit (32) is provided on the other of the guide tube (17) and the piston mem- ber (13).

16. A control rod device according to claims 7, 1 1 and 15, characterized in that the outer peripheral surface (37) is located on the piston member (13) and the inner peripheral surface on the guide tube (17).

17. A control rod device according to any one of claims 15 and 16, characterized in that the control rod member (6) comprises a rod (18), which connects the control rod (8) and the piston member (13) and that the guide member (7) comprises an outer tube (16).

18. A control rod device according to claim 17, characterized in that the first eddy current unit (31 ) is provided on one of the rod (18) and the outer tube (16), and that the second eddy current unit (32) is provided on the other of the rod (18) and the outer tube (16).

19. A control rod device according to any one of claims 1-14, characterized in that the first eddy current unit (31 ) is provided on one of the control rod (8) and the guide member (7), and the second eddy current unit (32) is provided on the other of the control rod (8) and the guide member (7).

20. A nuclear reactor plant comprising at least one control rod device according to any one of the preceding claims.

21. A nuclear reactor plant according to claim 20. characterized in that the plant comprises a nuclear reactor of a boiling water type.

22. A nuclear reactor plant according to claim 20, characterized in that the plant comprises a nuclear reactor of a pressure water type.