The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A reactor pressure vessel (RPV) of a nuclear reactor such as a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the reactor pressure vessel. A core shroud, or shroud, typically surrounds the reactor core and is supported by a shroud support structure. The shroud has a generally cylindrical shape and surrounds and supports both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically-shaped shroud.
Heat is generated within the core and water circulated up through the core is at least partially converted to steam. Steam separators located over the top of the shroud separate the steam and the water. Residual water is removed from the steam by steam dryers located above the separator assembly. The de-watered steam exits the reactor pressure vessel through a steam outlet near the vessel top head.
Typically, a reactor pressure vessel includes a top head secured to the reactor pressure vessel with vessel closure bolts. A conventional steam dryer is positioned within the reactor pressure vessel for removing residual water entrained in steam exiting the steam separators. It is positioned by its dryer support skirt, which rests on four to six dryer support brackets attached to the vessel shell. During loss of coolant accident (LOCA) conditions, upward displacement of the steam dryer is also limited by a steam dryer hold down assembly. The steam dryer hold down assembly often includes a plurality of RPV steam dryer hold down brackets connected to the steam dryer via corresponding steam dryer lifting rod eyes attached to steam dryer lifting rods. A typical steam dryer hold down assembly for conventional BWR steam dryers includes four RPV steam dryer hold down brackets attached to the inside of the top head of the reactor pressure vessel. During reactor construction, the corresponding steam dryer lifting rod eyes are typically adjusted so that their upper surfaces have from about a one-half inch to one inch clearance to the bottom surface of the steam dryer hold down brackets to allow for differential thermal expansion between the reactor pressure vessel and the steam dryer lifting rod.
However, conventional BWR design steam dryers can exhibit vibratory motion during operation, which manifests itself as unusual wear or, in some cases, cracking on upper bearing surfaces of steam dryer support brackets which support the steam dryer within the reactor pressure vessel. The wear and cracking indicates that the dryer is rocking due to uneven contact with its support brackets. The current dryer hold down assembly is generally intended to function only during a steam line break accident (i.e., loss-of-coolant accident (LOCA)), where the dryer lifts completely off of its support brackets, which support dryer support ring of the steam dryer. Thus, the dryer hold down assembly is not effective as a stabilizing support to prevent the vibratory motion of the steam dryer during normal operation.
There is additional concern that operating reactor plants implementing up-rated power operations (power operations in excess of rated power output) may be more susceptible to steam dryer vibratory motion due to increased steam flow and pressure drop across the dryer. The steam dryer assembly and/or its support bracket may become worn or otherwise damaged during such vibratory motion. When this occurs, the steam dryer vibratory motion is further enhanced and the vibratory motion and damage increases with further reactor operation due to uneven positioning of the steam dryer and/or the hold down brackets.
Typically, the steam dryer assembly is inspected during routine refueling outage maintenance on the reactor. Where damage or uneven contact with the support brackets is observed, a steam dryer rocking test has been used to evaluate required corrective action. The amount and location of the non-uniformity at the interfaces which support the dryer and the dryer support ring between the respective mating steam dryer support brackets which are attached to the RPV upper shell is determined by test. The test is performed by inducing rocking of the dryer that can include inserting a shim in one support bracket location, and applying a tipping weight on the top of the dryer at one eccentrically positioned location, then the diagonally opposite location. The amount of tipping is measured using dial indicators that record the relative movement of the tops of the dryer lifting rods. This testing procedure is repeated with various shim and weight positions. As the measured values include effects of sagging due to dryer flexibility and non-uniform spacing of the support brackets, adjustment calculations are required to determine the amounts of wear or non-uniformity.
If the tests indicated wear or non-uniform seating of the dryer assembly to the support brackets, adjustment is typically made by grinding the bottom of one or more dryer support ring contact or bearing surfaces. Generally during the testing and grinding processes, no further reactor maintenance or correction can be performed within the reactor pressure vessel and the steam dryer cannot be removed until the testing is complete. As such, the testing and modification to the dryer assembly and support brackets is a critical path item during reactor maintenance and therefore increases the amount of time the reactor is off line.