Abstract: A quenching element of the size and shape of a graphite nuclear fuel ball is introduced to the stacked balls of a stacked ball nuclear reactor core to release a neutron absorbing substance upon being heated to a predetermined temperature, thereby controlling the reactivity of the reactor by causing this substance, which may be a gadolinium compound, to deposit upon the graphite surfaces of the fuel element balls.

Abstract: A method of operating a gas-cooled nuclear reactor having graphite fuel elements in which, to reduce the reactor, a quenching element is introduced which takes a particle of a reaction-reducing substance in a sheath which will melt or release the substance in vapor form so that the substance can penetrate in gaseous form through the surrounding graphite body and deposit upon fuel elements.

Abstract: A gas-cooled nuclear reactor whose nuclear-fuel elements are constituted by graphite-encapsulated nuclear fuel particles and have graphite coatings is shut down in accordance with the present invention by introducing into the primary coolant gas circulation, a gadolinium-containing substance which is dispersed in the primary cooling gas before the latter enters the space between the fuel elements within the reactor core so that the gadolinium-containing substance is at least partly deposited on the free graphitic surfaces of the fuel elements.