Abstract: A high temperature pebble bed nuclear reactor having a medium power capacity of 300 to 500 MW.sup.e is equipped with two different shut-down arrangements comprising reflector rods used exclusively for scram. The total shut-down reactivity of the reflector rods is proportioned in order to prevent the excessive cooling of the reactor core folowing scram. In this manner, the reactor core is rendered subcritical (in the event of accidents, for example, or at the beginning of any operating state) yet capable of returning to criticality at a reduced level of temperature and power output following the removal of heat. The use of all of the reflector rods for scram is effected only in the event of reactivity accidents. For all other scram incidents, only a portion of the reflector rods are used.