Abstract: A gas-cooled, and preferably helium cooled, graphite-moderated high temperature nuclear reactor of the kind having a pressure vessel containing a reactor core with fuel elements, graphite components and a primary coolant gas circuit including a steam generator and coolant gas blowers, has the steam generator disposed outside the pressure vessel and the steam generator includes a heat exchanger at least a portion of which is located above the level of the reactor core. This situation of the heat exchanger ensures that when the reactor is shut down, either intentionally or owing to a fault, a natural convection flow takes place through the coolant gas circuit and this natural convection flow is in the same direction as it is when the blowers are functioning so that the flow conducts away the decay heat from the core to the steam generator in the same way as when the blowers are in operation.

Abstract: A method of removing the decay heat of radioactive fission products from a gas-cooled high temperature nuclear reactor in an emergency situation wherein a fault or a combination of faults results in the normal operational systems of the reactor not being able to remove the decay heat satisfactorily, comprises introducing water into the reactor core. Preferably the water is completely desalinated water which is introduced into the core from above. Absorber material may also be mixed with the water before it is introduced into the core to effect permanent shutdown of the reactor by absorbing free neutrons within the core.

Abstract: In a gas-cooled solid-moderated high-temperature reactor where fuel elements are introduced into the core through feed pipes in the ceiling reflector, the ceiling is cooled after reactor shutdown by introducing cooled gas through centrally arranged feed pipes and by removing the gas through the other feed pipes located radially outwardly of the inlet feed pipes. This arrangement effects a flow of the gas generally radially outwardly over the ceiling reflector. After the gas is removed from the core it is circulated through a closed circuit, cooled and blown back through the inlet feed pipes into the core.