Magnox control rods

A wide range of auxiliary equipment has been developed for gas-cooled reactors, including appropriate control and instrumentation systems, control rods and drives, and coolant handling and purification systems. The developments in these areas are too varied to report in detail here. However, it may be of interest to note some of the principal developments in control equipment and coolant purification equipment for the magnox, AGR, HTGR, and related plants. 

The control rods used in the magnox reactor plants are solid boron steel rods sheathed in stainless steel. They are suspended by flexible steel cables and are lifted from the core by control-drive drums and motors or, in the case of the concrete pressure vessels, above the reactor core but inside the vessel. The cable-driven rods depend on gravity for shutdown insertion, and, therefore, the clearance between the control rod... 

The reasons for this lay in the shutdown mechanism. UnUke the magnox reactors, the AGR design had a second gas baffle pressure vessel within the main pressure vessel. This was a consequence of the compUcated gas-flow system. The shutdown mechanism used control rods held out of the reactor by electromagnets, and if the power to these were to fail, the rods could drop back under gravity. Unfortunately, the channels for these rods had to pass through both the main pressure vessel and the baffle pressure vessel. The Inspectorate felt that a major incident involving the reactor would quite likely produce a misaUgnment between the holes in the main pressure vessel and the baffle pressure vessel. The report went on to say ... 

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