CONTROL ROD DRIVE MECHANISM ("CRDM") ASSEMBLY FOR A NUCLEAR REACTOR

Abstract

The present invention relates to a control rod drive mechanism assembly for a nuclear reactor having a nuclear reactor vessel, a nuclear reactor core, a reactor vessel head, a latch housing nozzle, a latch housing, a rod travel housing, a latch assembly, a drive rod assembly and a rod control cluster assembly. The latch housing is integrated with the latch housing nozzle, the rod travel housing is welded to the latch housing, and the latch assembly is connected to the rod travel housing. The latch assembly includes the drive rod assembly and the rod control cluster assembly which is attached to the drive rod assembly.

Claims

1. A control rod drive mechanism assembly for a nuclear reactor having a reactor vessel, a reactor core, a reactor vessel head, a latch housing nozzle, a latch housing, a rod travel housing, a latch assembly, a drive rod assembly and a rod control cluster assembly, comprising: a first end of the latch housing nozzle attached to the reactor vessel head; a first end of the latch housing integrated with a second end of the latch housing nozzle; a first end of the rod travel housing welded to a second end of the latch housing; and a first end of the latch assembly connected to a second end of the rod travel housing wherein the latch assembly includes the drive rod assembly and the rod control cluster assembly attached to the drive rod assembly.

2. The control rod drive mechanism assembly of claim 1, wherein the reactor vessel is a pressurized reactor vessel.

3. The control rod drive mechanism assembly of claim 1, wherein the first end of the latch housing is welded to the second end of the latch housing nozzle to form a single integrated component.

4. The control rod drive mechanism assembly of claim 1, wherein the first end of the latch housing and the second end of the latch housing nozzle are welded with a full penetration bimetallic weld.

5. The control rod drive mechanism assembly of claim 1, wherein the first end of the latch housing and the second end of the latch housing nozzle are fabricated to form a single component.

6. The control rod drive mechanism assembly of claim 1, wherein the first end of the latch housing is integrated with the second end of the latch housing nozzle to form a single component and is installed into the reactor vessel head as an integrated unit.

7. The control rod drive mechanism assembly of claim 1, wherein the second end of the latch housing and the first end of the rod travel housing are connected with a threaded connection and welded with a canopy seal weld.

8. The control rod drive mechanism assembly of claim 1, wherein the reactor control rod assembly is attached to the drive rod assembly such that as the drive rod assembly is moved upwardly and downwardly, the reactor control rod is correspondingly retracted and inserted into the nuclear reactor core to control a rate of nuclear reaction in the reactor core.

9. A method for controlling the reactivity of a nuclear reactor having a reactor vessel, a reactor core and a reactor vessel head, comprising: inserting a control rod drive mechanism into the reactor vessel to decrease the rate of power output from the nuclear reactor; retracting a control rod drive mechanism out of the reactor vessel to increase the rate of power output from the nuclear reactor, wherein the control rod drive mechanism includes a latch housing nozzle, a latch housing, a rod travel housing, a latch assembly, a drive rod assembly and a rod control cluster assembly, wherein a first end of the latch housing nozzle is attached to the reactor vessel head, a first end of the latch housing is integrated with a second end of the latch housing nozzle, a first end of the rod travel housing is welded to a second end of the latch housing, the latch assembly is connected to the rod travel housing, and wherein the latch assembly includes a drive rod assembly and a rod control cluster assembly attached to the drive rod assembly.

10. The method of claim 9, wherein the first end of the latch housing is welded to the second end of the latch housing nozzle to form a single component.

11. The method of claim 9, wherein the first end of the latch housing and the second end of the latch housing nozzle are welded with a full penetration bimetallic weld.

12. The method of claim 9, wherein the first end of the latch housing and the second end of the latch housing nozzle are fabricated to form a single component.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with accompanying drawings in which:

[0017] FIG. 1 is an elevational view of a CRDM assembly in accordance with the prior art. FIGS. 1A, 1B and 1C are detailed elevational views of the CRDM assembly in FIG. 1.

[0018] FIG. 2 is an elevational view of a CRDM assembly in accordance with an embodiment of the present invention. FIGS. 2A, 2B and 2C are detailed elevational views of the CRDM assembly in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention relates to a control rod drive mechanism (“CRDM”) assembly for a nuclear reactor. The CRDM assembly can be used in various nuclear reactor designs, such as, but not limited to, pressurized water reactors (“PWRs”) and boiling water reactors (“BWRs”). For ease of description, the CRDM assembly of the present invention will be described in accordance with an embodiment wherein the nuclear reactor is a PWR and, in particular, an AP1000 PWR which is designed and manufactured by Westinghouse Electric Company. The PWR includes a reactor pressure vessel which contains a nuclear reactor core, fuel rods, control rods, RCCAs, CRDM assemblies and related components. The CRDM assemblies are mounted onto the head of the reactor pressure vessel.

[0020] In the present invention, each of the CRDM assemblies include a latch housing nozzle, a latch housing, a rod travel housing, a latch assembly, a drive rod assembly, and a RCCA. One end of the latch housing nozzle is attached or connected to the reactor vessel head. The other end of the latch housing nozzle is attached or connected to one end of the latch housing. The latch housing nozzle and the latch housing are attached or connected in the absence of a threaded connection and/or in the absence of being seal welded with, for example, a canopy seal weld. In one embodiment, the latch housing nozzle and the latch housing are fabricated and/or manufactured to form a single integrated component. This nozzle/latch housing assembly can be installed into the reactor vessel head as an integrated unit. In this embodiment, the latch housing nozzle and the latch housing can be welded together by, for example, a full penetration bimetallic weld. This eliminates the need for a canopy seal weld which has shown a tendency to leak and/or corrode. The other end of the latch housing is attached or connected to one end of the rod travel housing. These two components are typically attached or connected by a threaded connection and/or a seal weld, such as but not limited to, a canopy seal weld. The other end of the rod travel housing is attached or connected to one end of the latch assembly. These two components are typically attached or connected by a threaded connection and/or a seal weld. The latch assembly includes a drive rod assembly and a RCCA. One end of the drive rod assembly is attached or connected to the other end of the latch assembly and the other end of the drive rod assembly is attached or connected to one end of the RCCA.

[0021] The latch housing compresses a cylindrical tube serving to house various gripping or latching mechanisms which are operatively associated with the drive rod assembly. The rod travel housing includes a cylindrical tube which houses and guides the vertically movable drive rod assembly. The drive rod assembly is connected to the RCCA such that the RCCA is moved either upwardly or downwardly to control reactor operations.

[0022] To actuate or control the gripping or latching mechanisms associated with the drive rod assembly, each CRDM assembly can be provided with a coil stack assembly which can include a one-piece integrated package of three, vertically spaced, but connected, electromagnetic assemblies. The various electromagnetic assemblies can be sequentially energized and de-energized so as to elevate or lower the drive rod assembly and therefore, the RCCA controlled thereby. In one embodiment, the latch assembly can include a coil stack having coils of copper wire (e.g., thousands of feet each), wrapped in fiberglass insulation, wound on an epoxy/fiberglass bobbin, and potted using a high temperature silicone and sand mixture. The RCCA is attached or connected to the drive rod assembly such that when the drive rod assembly is moved upwardly, the RCCA is correspondingly retracted from the nuclear reactor core. When the drive rod assembly is moved downwardly, the RCCA is correspondingly inserted into the nuclear reactor core. When the RCCA is retracted, the rate of nuclear reaction and power output of the core is increased. When the RCCS is inserted, the rate of nuclear reaction and power output of the core is decreased. In one embodiment, wherein it is necessary to shutdown the nuclear reactor in response to an emergency event, the latch assembly releases the drive rod assembly and the RCCA is allowed to drop by gravity to the bottom of the reactor core.

[0023] In accordance with the present invention, the latch assembly can be removed or installed with the rod travel housing which can significantly simplify latch assembly repair at the nuclear power plant.

[0024] The latch assembly can be upgraded to Seismic Category 1 in accordance with Nuclear Regulatory Commission (NRC) Regulatory Guide 1.29 using the recommendations of NRC Regulatory Guide 1.100, Revision 2, and the recommended practices for Seismic Qualification of equipment contained in IEEE Standard 344-1987. The latch assembly is not an IEEE Class 1E component. However, IEEE Standard 344 includes methodology that is applicable for qualification of mechanical components, and is the methodology recommended by NRC Regulatory Guide 1.100, Revision 2.

[0025] The CRDM assembly of the present invention can optionally include a hex shape machined onto the end of the rod travel housing that attaches to the latch housing to assist in the torquing or untorquing of the rod travel housing into the latch housing. Further, the CRDM assembly of the present invention can eliminate the conventional top vent screw in the rod travel housing.

[0026] In the art, it is typical to fabricate the CRDM assembly, for example, in one facility, and then install the CRDM assembly onto the reactor head to produce a finished component. In accordance with the present invention, the CRDM latch assembly can be fabricated, for example, in one facility, and the ASME Code pressure boundary housing can be fabricated in another different facility where the reactor head is fabricated. The CRDM assembly of the present invention allows the manufacturer of the reactor pressure vessel head to make the complete ASME Code Pressure Boundary under its N-Stamp Program and the CRDM manufacturer to make solely the non-ASME Code latch assembly. It is believed that the fabrication and manufacture steps employed in the present invention, as compared to the prior art, can result in significant cost savings.

[0027] Further advantages associated with the CRDM assembly of the present invention can include one or more of the following: [0028] a reduced number of canopy seals which can result in reduced maintenance and leakage concerns; [0029] integral venting and draining features incorporated into existing canopy seals that can significantly prolong service life; and [0030] ease of latch assembly replacement in remote field service operations.

[0031] FIG. 2 shows generally a CRDM assembly 40 in accordance with an embodiment of the present invention. The CRDM assembly 40 includes a reactor vessel head 30, a latch housing nozzle 32, a latch housing 34, a rod travel housing 36 and a coil stack assembly 39. FIGS. 2A, 2B and 2C show details of the CRDM assembly 40 as shown in FIG. 2, including a canopy seal interface, a bi-metallic interface and a latch housing nozzle interface, respectively. As shown in FIG. 2C, the latch housing nozzle 32 interfaces with and is connected to the reactor vessel head 30. As shown in FIGS. 2 and 2B, one end of the latch housing 34 and the latch housing nozzle 32 are connected by a full penetration bimetallic weld 33. As shown in FIGS. 2 and 2A, the other end of the latch housing 34, is attached or connected to an end of the rod travel housing 36. As shown in FIG. 2A, the rod travel housing 36 is threaded into the latch housing 34 end and is seal welded with a canopy seal weld 35. This canopy seal weld 35 serves as a maintenance joint for factory or field installation welding services. Further, as shown in FIG. 2A, one end of a latch assembly 38 is attached to another end of the rod travel housing 36. This configuration allows the latch assembly 38 to be removed or installed with the rod travel housing 36 and therefore, simplifies repair or replacement of the latch assembly 38. In one embodiment, these two components can be attached in a field service operation. Further, FIGS. 2A and 2B show a drive rod assembly 42. As shown in FIG. 2A, one end of the drive rod assembly 42 is connected to the latch assembly 38 and the other end is connected to a RCCA (not shown).

[0032] In one embodiment, the latch housing 34 and the latch housing nozzle 32 (as shown in FIG. 2B) can be integrated such that these two components are fabricated and/or manufactured to form a single integrated component. In this embodiment, one end of the integrated component (the end or portion which contains the latch housing nozzle 32) is attached or connected to the reactor vessel head 30 (as shown in FIG. 2C). The other end of the integrated component (the end or portion which contains the latch housing 34) is attached or connected to an end of the rod travel housing 36 as shown in FIG. 2A.

[0033] As shown in FIG. 2, the CRDM assembly 40 in accordance with an embodiment of the present invention eliminates the need for an upper canopy seal weld (17, as shown in FIG. 1C) and a lower canopy seal weld (13, as shown in FIG. 1A).

[0034] In one embodiment, the latch housing nozzle 32 is a thermally treated nickel alloy. The latch assembly can be fabricated of Type 304 and 410 stainless steels, stellite hard facing materials, and hard chromium plating. Typically, the hard chromium plating is applied to or deposited on wear surfaces.

[0035] Whereas particular embodiments of the invention have been described herein for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details may be made without departing from the invention as set forth in the appended claims.