Patent Number: 
Section: claims

1. In a pressurized water nuclear reactor having a containment building containing ice condenser compartments for absorbing heat in the event of a loss of coolant accident, a reactor pressure vessel (RPV) having a head assembly, a missile shield superposed over the head assembly and a ventilation system for directing cooling air from within the containment building toward the head assembly; the head assembly including a RPV head, a plurality of control rod drive mechanisms (CRDMs) extending upwardly from the RPV head, a seismic support platform above the RPV head laterally supporting the CRDMs, a plurality of lift connections extending upwardly of the seismic support platform for connecting with a detachable lift rig, and a CRDM cooling shroud surrounding a portion of the CRDMs and extending upwardly from the RPV head to a terminal end spaced from the seismic support platform; and the ventilation system including ductwork extending from a ventilation fan and a heat exchanger to an outlet located adjacent the head assembly between the RPV closure head and the seismic support platform; a method of retrofitting the RPV head assembly comprising the steps of:  removing at least a portion of the ventilation ductwork extending below the seismic support platform;  extending the CRDM cooling shroud to the seismic support platform;  mounting a plenum on the seismic support platform and in air flow communication with interior portion of the extended CRDM cooling shroud and under the missile shield;  connecting one end of a removable spool piece to the plenum and a second end of the removable spool piece to the ductwork such that the spool piece extends between the lift connections, the connection of the one end of the removable spool piece being above the seismic support platform and the connection of the second end of the removable spool piece being adjacent to the seismic support platform; and  reversing the flow of cooling air through the ventilation system for directing cooling air from the head assembly. 2. A pressurized water nuclear reactor comprising: a containment building containing ice condenser compartments for absorbing heat in the event of a loss of coolant accident and having a reactor cavity;  a reactor pressure vessel (RPV) disposed in the reactor cavity, the RPV having a head assembly; the head assembly including a RPV head, a plurality of control rod drive mechanisms (CRDMs) extending upwardly from the RPV head, a seismic support platform above the RPV head laterally supporting the CRDMs, a plurality of lift connections extending upwardly of the seismic support platform, and a CRDM cooling shroud having an interior portion in which the CRDMs are located and extending upwardly from the RPV head to the seismic support platform;  a concrete missile shield superposed over the head assembly;  a plenum mounted on the seismic support platform and in air flow communication with the interior portion of the CRDM cooling shroud and disposed under the concrete missile shield;  a ventilation system including ductwork extending from an outlet adjacent the head assembly to an inlet of a ventilation fan; and  a removable spool piece having a first end connected to the plenum and a second end connected to the outlet of the ductwork with the spool piece extending between the lift connections, the connection of the first end of the spool piece being above the seismic support platform and the connection of the second end of the spool piece being adjacent to the seismic support platform. 3. The nuclear reactor of  claim 2  wherein the ventilation fan has an outlet and the ventilation system includes a heat exchanger connected by ductwork with the outlet of the ventilation fan. claim 2 4. The nuclear reactor of  claim 2  wherein the ventilation system includes a heat exchanger disposed in the ductwork between the outlet adjacent to the seismic support platform and the inlet of the ventilation fan. claim 2 5. A RPV head assembly in a pressurized water nuclear reactor backfitted by the method of  claim 1 . claim 1