1. Field of the Invention
The present invention is generally related to the repair of nuclear pressurizers and more particularly to an assembly for positioning tools used in the repair of heating element penetrations in the lower head of nuclear 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 electric heaters are comprised of a plurality of heating elements that extend through nozzles or sleeves in the wall of the pressurizer. The nozzles extend outward through the curved bottom of the pressurizer to provide support to the heating elements. Due to the operating environment, it is a common requirement that repair work be performed on the heating elements and the nozzles through which they extend.
In its normal operational position, a pressurizer is stood on end and supported by a support skirt attached to the sides of the pressurizer and a base such as a concrete platform. The base must have a hollow cylindrical interior to accommodate the surge line that extends into the bottom of the pressurizer. The outside surface of the pressurizer thus presents a convex surface with a plurality of nozzles and heating elements that extend downward and a centrally located surge line. The variable height surface, obstacles, and open area thus present difficulties in properly positioning and supporting tools used in effecting repairs on the pressurizer.
Known approaches to the positioning of tools for repair purposes have typically been directed to the repair of a predetermined position on the heater penetration matrix. One system uses a set of clamps and brackets attached to adjacent penetration hardware to hold a tooling fixture plate below the pressurizer penetration to be worked on. All positioning is performed manually and the brackets and clamps are normally designed for a single fixed position in the pressurizer head penetration matrix. Since the vertical and horizontal (X/Y) position of each penetration is unique, this approach is limited to use on isolated repairs, with the need for special brackets for different heater penetrations. A second approach uses a support system from below as opposed to being attached to the penetration hardware for support. In this approach, a set of horizontal beams are attached to the pressurizer support skirt at a fixed elevation. Tooling mounts slidably attached to the beams are manually positioned and then clamped in place. This approach is limited to the penetrations that lie along the path above the support beams, that are within the reach of the slidable tool mounts, and that are within the reach of the vertical travel of the tooling. Due to the spherical head arrangement of the penetration locations, the vertical distance from the tooling support to the spherical head surface varies drastically and typically requires vertical repositioning of the support beams upward for work on heater penetrations at the outer periphery of the heater matrix or would require vertical shimming of the tooling in order to narrow the range of vertical travel variation required by the tooling. This approach can theoretically reach an array of penetrations within a rectangular area of approximately 10 inches defined by the beam length in one direction and the stroke of the slidable plates in the other direction. However, this approach can not reach penetrations that are diametrically opposite each other due to the presence of the vertical surge line at the apex of the lower head of the pressurizer. It can be seen that a need exists for a tool positioning assembly with the capability to reach heater penetrations at all locations on the pressurizer head without the need to change the support equipment during positioning operations to accommodate the variety of vertical and horizontal positions encountered.