A process for operating a fuel fired reactor includes introducing fuel into the reactor and burning the fuel in the reactor by means of at least one main burner. Relevant parameters of the process are monitored. Within a predetermined critical operating range for an enforced shut down, a secondary, more stringent operating range is implemented as shut down criteria. The main burner is shut down upon one or more of the relevant parameters leaving the secondary operating range while at least one pilot burner continues to operate as long as the relevant parameters are maintained within the critical operating range.