Design of Western BWR plants

The central part of the plant is the reactor pressure vessel housing the reactor core with its components as well as the water-steam separators and the steam dryers (see Fig. 2.2.). The dimensions of this component (inner diameter 6.62 m, height 22.35 m, total weight 785 Mg) exceed the dimensions of a PWR reactor pressure vessel considerably. The reactor pressure vessel is made from the high-alloy steel 22NiMoCr37 with an austenitic weld overlay on the inner surface. In its cylindrical section, the wall thickness of the vessel is 16.3 cm plus a weld overlay of 0.8 cm. 

The control rod drive mechanisms are installed in the lower part of the reactor pressure vessel, so that the control rods are inserted from the bottom into the reactor core. There are different reasons for this layout, among others the fact that the moderator water shows a higher density in the lower part of the core which is counteracted by the partly inserted control rods, thus resulting in a better homo-genity of load distribution in the core axial direction. Movement of control rods during normal operation periods is performed by electrical or hydraulic drive mechanisms in order to initiate a reactor trip, the control rods are forced into the core by a pressurized water drive mechanism. The load control by the control assemblies which are inserted more or less into the reactor core, depending on the demand, is supplemented by a load control which is effected by the speed of the main coolant pumps. A reduction of speed results in a longer residence time of the coolant in the reactor core and, as a consequence, in an increase of the void fraction in the water-steam mixture. As a result, neutron moderation is reduced and 

1) Refuelling machine 2) Reactor wet well 3) Reactor pressure vessel 4) Control rod drive mechanisms S) Main coolant pumps 6) Containment 7) Air recirculation system 8) Pipe floors 9) Fuel pool cooling heat exchanger 10) Pressure suppression pool II) Residual heat removal cooler 12) Lock (By courtesy of Siemens/KWU) 

Pumps force the recirculation of the coolant. Originally, an external recirculation line system was installed which, for larger plants, was then supported by internal jet pumps. In the KWU 69 and 72 designs, external recirculation was replaced by internal axial pumps operating inside the reactor pressure vessel. The KWU 72 reactor is equipped with 8 internal axial pumps at rated speed, each of these pumps has a recirculation capacity of about 8700 mVh. 

The upper part of the reactor pressure vessel houses the water-steam separation facilities. In the moisture separator, the bulk of the water is removed from the steam flow by centrifugal forces down to a residual moisture content of 10%. 

---