1. Field of the Invention
The present invention is generally related to nuclear system pressurizers and in particular to the replacement of nozzle or heater sleeve penetrations in the pressurizers.
2. General Background
The pressurizer in a nuclear reactor coolant system establishes and maintains the reactor coolant system pressure within the prescribed limits of the system. It provides a steam surge chamber and a water reserve to accommodate reactor coolant density changes during operation. A typical pressurizer is a vertical, cylindrical vessel with replaceable electric heaters in its lower section. The electric heaters are positioned below the normal water line and are actuated to restore normal operating pressure when the pressure in the reactor coolant system has decreased. The pressurizer also contains a plurality of nozzles at various locations for purposes such as sensing the liquid level or temperature in the pressurizer.
The electric heaters are comprised of a plurality of heating elements that extend through nozzles or sleeve penetrations through the wall of the pressurizer. Support plates inside the pressurizer are provided with holes in coaxial alignment with the holes in the pressurizer wall and the sleeves for receiving and supporting the heating elements. The sleeves extend outward from the pressurizer to provide exterior support to the heating elements. Due to the operating environment, it is a common requirement that heating elements and the sleeves through which they extend be replaced. Because alignment between the support plate holes and the sleeve is critical, it has previously been required that the replacement sleeve be fabricated to original design dimensional specifications and installed into the original bore in the pressurizer wall to insure proper alignment of the sleeve inner diameter with the corresponding support plate hole after welding. This process required that the removal of the original sleeve weld and installation of the repair weld be 10 performed from inside the pressurizer because the bore through the pressurizer could not be enlarged for tooling access without potentially altering the alignment of the heater penetration with respect to the holes in the support plates. It is also a common requirement that nozzles used for penetration of level sensing and thermowell equipment into the pressurizer be replaced. Replacement of such nozzles presents very similar considerations and problems encountered when replacing heater sleeves. Since pressurizer components in nuclear power plants become radioactive after they have been in operation, performing such work inside the pressurizer is difficult and hazardous to personnel and thus impractical. Related patents that applicants are aware of include the following.
U.S. Pat. No. 4,255,840 discloses a method for replacing a defective immersion heater in a pressurizer. However, this patent is limited to the replacement of the heater and does not address the replacement of a nozzle.
U.S. Pat. No. 5,094,801 and 5,091,140 disclose an apparatus and method for replacing a heater sleeve. The original sleeve is removed and the original bore in the pressurizer is enlarged. An outer sleeve is installed in the bore with its upper end being seal welded to the cladding on the interior of the pressurizer. An inner sleeve in installed in the outer sleeve to extend into the pressurizer and is welded to the lower end of the pressurizer.
The known art addresses the replacement of the heater or the entire sleeve but does not address the replacement of that portion of a heater sleeve or nozzle penetration that extends beyond the exterior of the pressurizer.