Patent Number: 047132116
Section: description

EXAMPLE A high temperature reactor with a power capacity of 500 MW.sub.el, is operated with spherical fuel elements in a single passage mode such that an axial neutron flow density is established with a maximum in the upper third of the core, which then declines in an approximately exponential manner. The core reactor has two different shut-down installations: (1) core rods for long term shut-downs and, (2) reflector rods for scram. The heat produced in the core is removed by several principal cooling systems. In case of an accident involving the failure of the principal cooling systems, the decay heat is removed by several auxiliary cooling systems. The total shut-down reactivity of the reflector rods is dimensioned so that the reactor may be rendered subcritical, in the case of any incident and beginning with any operating state, and maintained in a subcritical condition until a temperature level clearly less than the normal operating value is attained. Reactivity estimates for a high temperature reactor with a power capacity of 500 MW.sub.el yields the following efficiency of all of the reflector rods (less the failure of the most effective rod) for the individual conditions of the plant: ______________________________________ normal operation (100% full load) 2.6% .DELTA.K/K cold start 2.1% .DELTA.K/K load cycle 2.3-3.1% .DELTA.K/K ______________________________________ The maximum accident reactivity (incursion of water) amounts to approx. 1.1% .DELTA.K/K, so that, depending on the initial situation, by the insertion of the reflector rods following a reactivity accident, a subcriticality of 1.0 to 2.0% .DELTA.K/K is obtained. Without such an accident, a subcriticality of roughly 2 to 3% .DELTA.K/K is attained. In cases such as operational shut-downs and incidents without an increase in reactivity, therefore, the number of rods employed for the rapid shut-down may be reduced. Consequently, the shut-down process according to the invention requires that all of the reflector rods be inserted only in the case of reactivity accidents. Following a scram and removal of the decay heat by means of the principal or auxiliary cooling systems, the reactor again becomes critical at a reduced temperature level and with a low power output, as a function of the shut-down reactivity introduced and the initial operational state. This state is permissible for the reactor installation over an arbitrary period of time. The reactor is operated for extended periods of time cold and subcritically if conditions are not conducive for a rapid resumption of its normal operation, as the result of the onset of the decay of iodine and the concentration of xenon associated with it, or by the insertion of core rods. The above description describes a preferred embodiment of the invention. It is to be understood however, that the invention is not limited to any single embodiment or feature, but should be construed to cover all modifications and alternative embodiments falling within the scope of the invention as defined by the claims which follow.