Patent Number: 041815726
Section: summary

BACKGROUND OF THE INVENTION This invention relates to closure heads for nuclear reactors and more particularly to closure heads for nuclear reactors having a sealing and lubricating system. In nuclear reactor designs well known in the art, a reactor vessel with fuel assemblies disposed therein and having an inlet and an outlet for circulation of a coolant in heat transfer relationship with the fuel assemblies, is sealed by a closure head located on top of the reactor vessel. In certain designs, the closure head comprises one or more rotatable plugs. These rotatable plugs which may be of varying sizes disposed eccentrically within each other, serve at least two purposes. One purpose is, of course, to seal the reactor internals inside the reactor vessel. Another purpose is to support refueling machines. The rotation of the rotatable plugs positions the refueling machines in appropriate relationship to the fuel assemblies in the reactor vessel to facilitate the refueling process. Since the rotatable plugs must be able to rotate relative to each other, the plugs are mounted so as to define an annulus between them. The annulus, while allowing the rotation of the plugs, also establishes a path for the release of radioactive particles located in the reactor vessel. Accordingly, seals are provided at various locations across the annulus to prevent this release of radioactive particles. The seals also function to prevent oxygen in the atmosphere outside the reactor vessel from passing through the annulus to the reactor coolant, which in a liquid metal fast breeder reactor may be liquid sodium, because contact of liquid sodium with oxygen may result in the formation of impurities in the liquid sodium. To further prevent oxygen leakage into the reactor vessel, a cover gas is provided that fills the space from the top of the reactor coolant pool to the bottom of the closure head and up the annulus to the seals across the annulus. One type of closure head seal well known to those skilled in the art is a liquid dip seal. In a liquid dip seal, the annulus between the closure head plugs is contoured so that a trough is formed by the annulus itself. A liquid such as liquid sodium is placed in the trough thereby dividing the annulus into two sections, one above the liquid and one below thereby forming a dip seal. The cover gas, inside the reactor, containing radioactive particles, then extends from the top of the reactor coolant pool up through the annulus to the liquid sodium in the dip seal. The liquid dip seal under normal conditions provides an effective seal against cover gas migration out of the annulus and against oxygen migration into the reactor vessel while allowing the rotatable closure head plugs to rotate relative to each other. However, it is generally considered advisable to utilize another type of seal in conjunction with the dip seal in order to prevent contact between the liquid metal and air. Another type of closure head seal well known in the art is the inflatable seal wherein single or multiple inflatable seals in series are placed across the annulus. During reactor refueling, the inflatable seals are slightly deflated to allow better rotation on the rotatable closure head plugs while during reactor operation the seals are inflated to increase their sealing capability. Examples of these types of seals may be found in U.S. Pat. No. 3,514,115 to S. Gallo, issued May 26, 1970 and in U.S. Pat. No. 3,819,479 to R. Jacquelin, issued June 25, 1974. Still another seal well known in the art is a type of labyrinth seal in which a piece of metal is bolted to one of the closure head plugs so as to extend across the annulus between the plugs to within close proximity to the other plugs. The purpose of this seal is to effectively lower the leak path area to thus limit leakage. Of course, since the piece of metal does not contact both rotatable components it does not provide an effective seal. In addition, the other commonly known types of seals such as O-rings, bellows, etc., while possibly being effective under certain conditions, do not allow for effective rotation of the closure head plugs. SUMMARY OF THE INVENTION A closure head for a nuclear reactor comprising a stationary outer ring integral with the reactor vessel with a first rotatable plug disposed within the stationary outer ring and defining an annulus therebetween. A bearing is disposed in the annulus and attached to the stationary outer ring and the first rotatable plug for rotatably supporting the first rotatable plug from the stationary outer ring. A sealing system is disposed in the annulus and around the bearing for sealing the annulus against in leakage of oxygen and out-leakage of radioactive contaminants from within the reactor vessel. The sealing system comprises a tubular seal element disposed in the annulus and capable of contacting both the stationary outer ring and the first rotatable component under the actuation of load springs. In addition, the sealing system comprises a mechanism for pumping a lubricating fluid around the bearing, through the annulus, and around the tubular seal element thereby compressing the load springs and allowing the passage through the annulus of the lubricating fluid. The lubricating fluid in conjunction with the tubular seal element provides a seal across the annulus.