This invention relates to components for introducing feedwater into a large pressure vessel, and more particularly, to the feedwater nozzles and sparger for a boiling water nuclear reactor vessel.
In a pressurized water boiling reactor for conventional design, feedwater is introduced above the nuclear reactor core through a plurality of feedwater inlet nozzles spaced at a common elevation around the nuclear pressure vessel. The flow through the inlet nozzle is deposited in an annular sparger or header which then distributes the inlet flow uniformly around the inside circumference of the vessel where it can be forced downward outside the core shroud, turned in the lower vessel head and forced upward through the reactor core.
Some years agao, close inspection of the internal surface of the feedwater inlet nozzles revealed cracks that could propagate and threaten the integrity of the nozzle. The area of the nozzle most susceptible to cracking is the region of greatest curvature defining the transition from the feedwater pipe and nozzle entry to the vessel wall. To prevent further propagation of cracks, measures were taken to machine away a portion of the nozzle, ranging from about 0.4 inches of metal removed from the bore to about 0.5 inches removed from the face. Also, attempts have been made to modify the nozzle sleeve.
In the conventional design, a sleeve is disposed between the sparger and the nozzle to provide a flow path therebetween. It is believed, that due to thermal expansion and contraction of the sparger, the sleeve moves back and forth on its nozzle landing surface. Leakage of the inlet water along the outside of the sleeve, coming into contact with the warmer, highly curved region of the nozzle, has created cracks. Thus, efforts have been made to prevent this leakage and to reduce the temperature difference between the nozzle and any feedwater that may come into contact therewith.
One proposed flow shroud or sleeve is disclosed in a paper entitled "Boiling Water Reactor Feedwater Nozzle Cladding Removal", G. A. Diwinski et al., TIS-5362, Combustion Engineering, Inc. (1977). Another proposed solution is shown schematically in a Dutch newspaper article entitled "Dodewaard Werd Gerepareerd Met de Stopwatch Ind Dh Hand", published Jan. 16, 1983, where a new sparger is fixed to a newly machined inner wall in the nozzle, by means of a spring and tumbler mechanism. By means of the mechanism, the sparger can expand and shrink while a new sleeve member can move independently from the nozzle and the sparger.
To the inventor's knowledge, none of the prior art solutions has effectively sealed off the vulnerable region of the nozzle, to prevent a flow of significantly cooler water along the nozzle surface during component expansion and contraction, or during other transients.