Reactor Vessel Closure Seals

With a batch process, such as hot isostatic compaction (HIP), heat exchange as used in a continuous reactor is not possible, and it is common practice to provide a furnace within the pressure vessel which is thermally insulated to ensure that the temperature of the vessel does not rise above 300°C. Most HIP operations involve gas pressures in the range 70—200 MPa (10—29,000 psi) and temperatures of 1250—2000°C, occasionally 2250°C (74). The pressure vessel may have a bore diameter from 27 to 1524 mm (75) and is nearly always provided with threaded closures sealed with O-rings made of elastomer provided the temperature is low enough. 

Canned Rotor Pumps With no shaft seals the small LOCA associated with seal failure is eliminated. The motors are relatively small and can be replaced by special provisions in the reactor cavity space without removing the vessel closure head. These provisions include diodes in the core barrel to allow natural circulation with the water level reduced to below the level of the pumps. 

The containment vessel has no penetrations and is designed to be leak tight. It contains the reactor vessel and internals and is sealed with a reactor closure. The inside of the reactor vessel is filled with liquid sodium and a helium cover gas. The helium cover gas is at approximately atmospheric pressure at normal power conditions. A 20 cm annulus between the containment vessel and the reactor vessel is filled with argon at a pressure slightly above the reactor cover gas pressure. 

Containment The primary containment comprises the containment vessel, the reactor head closure and fittings, and the intermediate heat exchangers (Fig. 9.61). There are no penetrations in the reactor vessel below the closure head. During power operation, the reactor is hermetically sealed, all sodium and cover gas service lines (in the head closure) are closed with double isolation valves, and all other penetrations are seal-welded. The pressure of the reactor cover gas is approximately atmospheric during normal power operation. The leak... 

The core, steam generators, and helium circulators are enclosed within a reactor vessel of concrete reinforced by bonded reinforcement steel and prestressed by steel tendons. The top head houses a number of penetration channels which are used for refueling and as housings for the control rod drives. The walls of the vessel have an internal water-cooled liner of carbon steel. The liner forms a gas-tight seal and acts as the primary containment for the reactor, while the secondary containment is provided by the concrete vessel itself. All penetrations of the walls have two independent closures to maintain the principle of the double containment. 

The closure head of the reactor pressure vessel accomodates the control rod drive mechanism nozzles, the in-core instrumentation nozzles, the RPV water level control nozzles and the venting nozzle. The RPV body and closure head are joined together by means of studs and nuts. During refuelling, the RPV closure head is removed, together with the platform, closure head insulation, control drive mechanisms and the seal rings. 

The use of gas cooling, with the consequent high primary circuit pressure compared with a sodium-cooled reactor, increases the probability of a loss-of-coolant accident. With a prestressed concrete pressure vessel however, the only mechanism which could lead to a rapid depressurization is the failure of one of the penetration seals into the vessel. The maximum rate of depressurization can be limited by flow restrictions built into the penetration closures, and the reactivity worth of the helium is typically well below the delayed neutron fraction, so that reactivity transients are not a problem. A reliable emergency heat removal system, independent of the main cooling system, is essential. 

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