Patent Number: 056595905
Section: summary

TECHNICAL FIELD This invention relates to boiling water nuclear reactors and specifically, to a dew core shroud and pump deck design which allows for easy removal and/or replacement of these reactor structural components when damaged or otherwise in need of repair or replacement. BACKGROUND Typical boiling water nuclear reactors include a reactor assembly which consists of the reactor vessel and its internal components including the core, core shroud, top guide assembly, core plate assembly, steam separator and dryer assemblies, and jet pumps. Also included in the reactor assembly are the control rods, control rod drive housings and the control rod drives. The reactor vessel is a generally cylindrical pressure vessel (RPV) with a single full diameter removable head. The shroud is a cylindrical stainless steel structure located within the RPV and which surrounds the core, providing a barrier to separate the upward flow through the core from the downward flow in the annulus between the RPV wall and the core shroud. The conventional core shroud is welded to the bottom of the RPV and supports the weight of the top guide, core plate and shroud head along with attached steam separators. Recent discoveries of unexpected circumferential cracks propagating through the thickness of the shrouds in relatively young operating BWR's has prompted a re-design of the core shrouds for future BWR's. The primary cause of the observed cracking has been intergranular stress corrosion in the heat affected zones near many of the horizontal welds of the shroud and shroud supports. There have also been some cracks observed in the mid-belt regions of BWR shrouds, and these have been thought to be caused by irradiation assisted stress corrosion. The current advanced boiling water reactor (ABWR) shroud, like the conventional BWR shroud, is permanently welded to the bottom of the vessel and is not intended to be removed or replaced. The shroud in the ABWR has various horizontal welds similar to the BWR shrouds in current operating plants, and therefore may also be at risk of similar stress corrosion cracking problems. The conventional pump deck section is permanently welded in place between the shroud and the reactor pressure vessel, and it too is not intended to be removed or replaced. As with the shroud welds, the pump deck welds are also susceptible to stress corrosion cracking. SUMMARY OF THE INVENTION This invention relates to a new and improved shroud and pump deck design which allows for the relatively easy removal of both components. In accordance with the invention, both the shroud and pump deck are bolted to the top of the shroud support leg extending upwardly from the bottom of the RPV. Specifically, the shroud has a radially inwardly extending flange at its bottom end for bolting into the upper cylindrical section of the shroud support leg. The bolts restrain vertical loading on the shroud while horizontal loading is restrained by wedges mounted between the shroud and a plurality of wedge support blocks on the pump deck. Thus, the inner diameter portion of the pump deck is vertically sandwiched between the shroud flange and the upper edge of the cylindrical section of the shroud support leg. At the same time, the outer diameter of the pump deck is held in place within a radially inwardly facing groove on the inside wall of the reactor pressure vessel. The pump deck is formed in ten separate segments with a reactor internal pump diffuser adapted to extend vertically through an opening in the middle of each segment. In a further feature of the invention, a keyed pump deck segment locks the remaining nine deck segments of the pump deck in place. It is the last portion of the pump deck to be installed and the first portion of the pump deck to be removed. This keyed segment is very similar in construction to the other nine segments; however, it is bolted into a support ledge on the reactor pressure vessel wall as opposed simply to being held by the groove on the reactor pressure vessel wall as in the case of the other nine pump deck segments. From the above, it will be appreciated that after the shroud bolts have been removed, the shroud may be lifted vertically from the core. The keyed pump deck segment can then be removed, followed by the remaining pump deck segments. Thus, in accordance with one aspect of the subject invention, there is provided in a pressure vessel of a nuclear reactor containing a core assembly enclosed within a core shroud, the core shroud spaced radially inwardly of a side wall of the pressure vessel with an annular pump deck located between the core shroud and the side wall of the pressure vessel, the improvement wherein the shroud is removably secured to an annular support of the pressure vessel. In accordance with another aspect, the subject invention relates to a pressure vessel of a nuclear reactor containing a core assembly enclosed within a core shroud, the core shroud spaced radially inwardly of a side wall of the pressure vessel with an annular pump deck located between the core shroud and the side wall of the pressure vessel, the improvement wherein the annular pump deck is provided in the form of a plurality of removable segments. In accordance with still another aspect, the invention relates to a pressure vessel of a nuclear reactor containing a core assembly enclosed within a core shroud, the core shroud spaced radially inwardly of a side wall of the pressure vessel with an annular pump deck located in an annular radial space between the core shroud and the side wall of the pressure vessel, the improvement wherein the shroud is removably secured to an annular support leg extending upwardly from the bottom of the pressure vessel; and further wherein the annular pump deck is provided in the form of a plurality of removable segments. It will thus be appreciated that the invention provides a shroud and a pump deck capable of being easily removed and reused or replaced. The invention also provides a wedge mechanism to transfer the horizontal loads of a core shroud to the pump deck or other supporting member. The construction in accordance with this invention also results in most horizontal welds being located in otherwise removable elements, facilitating repair and/or replacement of faulty welds. Other objects and advantages of the subject invention will become apparent from the detailed description which follows.