Abstract: In the method of shutting down a high temperature nuclear reactor having a negative temperature coefficient of reactivity, such as a gas cooled pebble-bed nuclear reactor, while the core is operating in the critical state with the core having a predetermined critical average core temperature and with the heat generated in the critical state being removed by a coolant, effecting the shut down by reducing or discontinuing the removal of heat from the reactor core and increasing the average core temperature by an amount above the critical average core temperature for rendering the core hot sub-critical due to the negative temperature coefficient of reactivity. The core can be maintained in the hot sub-critical state by a controlled removal of after shut-down heat from the core.

Abstract: A reaction vessel which holds a charge of spherical reaction elements, for example a pebble bed nuclear reactor having a charge of spherical fuel elements, has the charge formed of at least two groups of spherical elements with all of the elements in each group being of equal diameter, but each group of elements having a different diameter from the other. The elements of both groups are uniformly mixed together and the different diameters are chosen in such a way that when the elements are continuously circulated through the reaction vessel during operation, the circulation does not cause the two groups of elements to segregate during the intended number of circulation cycles.