Patent Number: 06163588&
Section: summary

BACKGROUND OF THE INVENTION This invention relates generally to nuclear reactors and more particularly, to core plate and reactor internal pump differential pressure lines for a boiling water reactor. A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core plate and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. The core center axis is substantially coaxial with the center axis of the shroud, and the shroud is open at both ends so that water can flow up through the lower end of the shroud and out through the upper end of the shroud. The shroud, top guide, and core plate limit lateral movement of the core fuel bundles. The shroud, due to its large size, is formed by coupling a plurality of stainless steel cylindrical sections together, typically by welding. Shroud welds, however, increase the susceptibility of the shroud material to a detrimental effect known as inter-granular stress corrosion cracking (IGSCC). Typically, cracking may occur in the heat affected zone of the shroud welds. Currently, volumetric inspections are performed to detect and evaluate the extent of cracking. If the cracking is determined to be significant, repairs may be performed to re-establish the integrity of the weld joint, or the shroud is replaced. The RPV also includes reactor internal pumps located in the annulus between the shroud and the pressure vessel wall. The internal pumps provide circulation of water in the RPV. Typically differential pressure lines are used to measure the reactor internal pump flow and the flow of water through the reactor core located inside the shroud. The pressure lines are usually constructed using pipe and pipe fittings. The pressure lines enter the RPV through penetrations in the bottom head. The pressure lines extend along the inside of the shroud and are supported by brackets welded to the shroud. The brackets are required to prevent flow induced vibrations in the pressure lines. One reactor internal pump differential pressure line penetrates the shroud above pump impellers and the other terminates below the impellers inside of the shroud. The core differential pressure lines terminate above and below the core plate. Because the differential lines are welded to the shroud, replacement of shroud sections is difficult and time consuming. The pressure lines must first be removed from the shroud section before replacement of the shroud section. Also the differential pressure lines must be reinstalled, i.e., welded to the new shroud section. It would be desirable to provide a shroud that includes easily replaceable shroud sections. Particularly, it would be desirable to provide a shroud that includes replaceable shroud sections that can be removed without cutting pressure lines or pressure line supports, and that does not require welding pressure lines and/or pressure line supports to install a shroud section. BRIEF SUMMARY OF THE INVENTION These and other objects may be attained by a modular differential pressure measuring system for a boiling water nuclear reactor pressure vessel. The differential pressure measuring system permits the use of replaceable shroud sections because the modular pressure system does not require cutting the pressure lines or pressure line supports for replacement of the replaceable shroud sections. Additionally, the modular differential pressure system does not require welding of pressure lines and/or pressure line supports during installation of a replaceable shroud section. The modular differential pressure system includes a plurality of pressure lines with each pressure line including a plurality of pressure line sections. The modular system also includes a shroud having at least one replaceable shroud section. Each shroud section includes at least one pressure line section configured to connect to and disconnect from corresponding pressure line sections in adjacent shroud sections without welding. A shroud section may include at least one pressure line coupled to support brackets welded to the shroud section. The pressure line remains coupled to the shroud section and is removed or installed with the shroud section as a modular component. Connections of the pressure lines of one shroud section to an adjacent section are located at the flanged interface between shroud sections. Therefore, a separate flanged joint for the pressure line sections are not required, and welding of the connection is also not required. The modular system also includes a reactor bottom head petal section. The bottom head petal section is configured to support the shroud sections and includes a shroud support flange. The bottom head petal section also includes a plurality of bores defining pressure line sections. At least one pressure line section of the bottom head petal is configured to couple with a corresponding pressure line section of an adjacent shroud section. Particularly, a short vertical bore extends from the end of the horizontal bore to an outside surface of the shroud support flange. This vertical bore is configured to couple to a vertical bore in the lower shroud section, sometimes referred to as the shroud support. The vertical bore may extend vertically through several shroud sections, or the lower shroud vertical bore may include a short horizontal bore extending from the end of the bore to an outside surface of the lower shroud section. This horizontal bore may in turn be coupled to a vertical or horizontal pipe section of the pressure line extending along the inside surface of the shroud. In operation, the modular differential pressure system measures the pressure at two separate points within the reactor pressure vessel. The pressure differential is an indication of the flow between the two points within the reactor. Typically, the core flow is measured by measuring the pressure above and below the reactor core plate. Also the flow in the annulus of the reactor may be measured by measuring the pressure above and below the reactor internal pump impellers. Because the pressure system is modular, when a shroud section is removed for replacement, the modular pressure lines are removed with the shroud section. No cutting is required to disconnect a pressure line section from the corresponding pressure line section of an adjacent shroud section. The removed shroud section is then replaced with a shroud section that also includes integral pressure lines which re-couple to the pressure lines of an adjacent shroud section without welding. The above described modular differential pressure system permits the replacement of shroud sections without having to cut the differential pressure lines from the shroud. The modular differential pressure system also permits the installation of a replacement shroud section without having to reinstall the pressure lines by welding the lines to the core shroud. The modular system also simplifies and speeds up the process of replacing the core shroud in a nuclear reactor.