Abstract: In a system for detecting abnormal movement of a part of a gas-cooled nuclear reactor, for example abnormal tilting of a boiler unit caused by failure of a boiler support, a tracer material is released into the gas coolant path in response to the movement. The releasing of the material may be effected by causing a gas canister to be pierced as a result of the movement. The presence of the tracer material in the coolant is detected by the coolant monitoring system.

Abstract: In a shell and tube boiler for use, in particular, in a liquid-metal-cooled fast-breeder nuclear reactor, the shell is formed as two vertical side-by-side units joined at their upper ends by a chamber. This "folding" of the unit considerably shortens the overall height of the boiler. Each tube is formed in two sections, each comprising a vertical limb with a curved portion at its upper end. The sections are joined by welding together the ends of the curved portions. This reduces the tube lengths which must be manufactured and transported to the site. In use of the boiler, the liquid sodium level in the chamber is maintained below the level of the tube welds. The chamber region above the sodium level may be filled with a blanket gas, such as argon.