2.1 Pressure vessel

The pressure vessel, without head, steam generators and internal components, weights 100 tons.

The internal arrangement of the vessel is dominated by the in-vessel disposition of steam generators. Hydraulic control rod drives are also included inside the vessel. For refueling, control rod drives must be dismantled. Steam generators can be replaced during a longer than normal maintenance outage.

The reactor is self pressurized, so no prressurizer is needed. A steam volume, in the upper part of the vessel, accommodates volume changes during operation.

The following "in-vessel" variables must be monitored during operation:

— Water level

— Pressure

— Core inlet and outlet temperatures

— Neutron flux

— Control rod position

— Coolant flow

In a natural convection reactor, coolant flow measurement is not straightforward. Different methods are being evaluated; correlation of flow with pressure differences across steam generators is the most promising one.

The first four variables can be measured without difficulties specific to this type of reactor. Control rod drive position is more difficult to measure as a consequence of drive design. Ultrasonic methods are being studied for solving this problem.

Control drive Control dr. ve rod structure Vote/- level Steer» generotor SG water inlet SG steam outlet Absorbing element Fuel element Core

Core support structure