Patent Number: 
Section: claims

1. A head assembly for a reactor pressure vessel, comprising:a reactor pressure vessel closure head;a seismic support platform spaced from the closure head;an array of control rod drive mechanisms, each control rod drive mechanism including an electro-magnetic coil stack assembly and having a lower end supported by the reactor pressure vessel and an upper end extending through the seismic support platform;a lower shroud surrounding the electro-magnetic coil stack assemblies and having an upper end spaced from the seismic support platform in air flow communication with the atmosphere around the control rod drive mechanisms;a plurality of internal ducts disposed between control rod drive mechanisms within the array of control rod drive mechanisms, each duct having a lower end extending below the electro-magnetic coil stack assemblies and in air flow communication with the lower shroud and each duct having an upper end extending above the seismic support platform;an upper plenum disposed above the seismic support platform having inlet air openings in air flow communication with the upper ends of the internal ducts;a missile shield assembly disposed within the upper plenum, the missile shield having a plate superposed over and spaced from the inlet air openings of the upper plenum in air flow communication with the upper ends of the internal ducts;a plurality of fan assemblies disposed on the upper plenum in air flow communication with the upper plenum; andlift legs connected with the reactor pressure vessel closure head and supporting the seismic support platform, the upper plenum and the missile shield assembly for removal of the head assembly as an integral assembly. 2. A head assembly for a reactor pressure vessel, comprising:a reactor pressure vessel closure head;a seismic support platform spaced from the closure head;an array of control rod drive mechanisms, each control rod drive mechanism including an electro-magnetic coil stack assembly and having a lower end supported by the reactor pressure vessel and an upper end extending through the seismic support platform;a lower shroud surrounding the electro-magnetic coil stack assemblies and having an upper end spaced from the seismic support platform in air flow communication with the atmosphere around the control rod drive mechanisms;a plurality of internal ducts disposed within the array of control rod drive mechanisms, wherein each duct has a lower end extending below the electro-magnetic coil stack assemblies disposed in air flow communication with the lower shroud and each duck has an upper end, and wherein the internal ducts extend within the array of control rod drive mechanisms through the seismic support platform and have internal plates in the section of the ducts disposed in the seismic support platform;an upper plenum disposed above the seismic support platform having inlet air openings in air flow communication with the upper ends of the internal ducts;a missile shield assembly disposed within the upper plenum, the missile shield having a plate superposed over and spaced from the inlet air openings of the upper plenum in air flow communication with the upper ends of the internal ducts;a plurality of fan assemblies disposed on the upper plenum in air flow communication with the upper plenum; andlift legs connected with the reactor pressure vessel closure head and supporting the seismic support platform, the upper plenum and the missile shield assembly for removal of the head assembly as an integral assembly. 3. The head assembly of claim 1, wherein the internal ducts are bolted to the upper plenum and wherein each lift leg comprises an upper leg member attached by a clevis assembly to a lower leg member with the upper plenum supported by the upper leg member and with the seismic support platform supported by the lower leg member. 4. The head assembly of claim 1, wherein the internal ducts are a backfit into an existing head assembly. 5. A head assembly for a reactor pressure vessel, comprising:a reactor pressure vessel closure head;a seismic support platform spaced from the closure head;an array of control rod drive mechanisms, each control rod drive mechanism including an electro-magnetic coil stack assembly and having a lower end supported by the reactor pressure vessel and an upper end extending through the seismic support platform;a lower shroud surrounding the electro-magnetic coil stack assemblies and having an upper end spaced from the seismic support platform in air flow communication with the atmosphere around the control rod drive mechanisms;a control rod drive mechanism plenum disposed between the closure head and the lower shroud;a plurality of internal ducts disposed between control rod drive mechanisms within the array of control rod drive mechanisms, each duct having a lower end disposed in the control rod drive mechanism plenum and each duct having an upper end extending above the seismic support platform;an upper plenum disposed above the seismic support platform having inlet air openings in air flow communication with the upper ends of the internal ducts;a missile shield assembly disposed within the upper plenum, the missile shield having a plate superposed over and spaced from the inlet air openings of the upper plenum in air flow communication with the upper ends of the internal ducts;a plurality of fan assemblies disposed on the upper plenum in air flow communication with the upper plenum; andlift legs connected with the reactor pressure vessel closure head and supporting the seismic support platform, the upper plenum and the missile shield assembly for removal of the head assembly as an integral assembly. 6. The head assembly of claim 5, wherein the internal ducts have an L shaped cross-section within the seismic support platform. 7. The head assembly of claim 5, wherein the internal ducts have a rectangular shaped cross-section.