Operational reliability of the safety system to detect abnormal heating of fuel assemblies in the Phenix reactor

In fast reactors, the surveillance of safety barriers requires a large number of temperature measurements, so that any anomalous heating of fuel assemblies can be detected very rapidly. The data can be handled within the time available only by means of computers. For dealing with overheating, the Phenix reactor has a safety system to which information is transmitted from the thermocouples on the fuel assemblies; the information passes through industrial mini-computers to a relay system which triggers the control rod drop if necessary. During normal operation the computers serving each safety circuit operate in pairs in a coincidence regime - in other words, the information from the two thermocouples on a fuel assembly must coincide, as must the information passed on by the two computers, if an emergency shut-down is to be triggered. In view of the importance of this safety system and the use of small computers for surveillance and initiation of safety measures, knowledge of the operational history of the system components is essential. Since a record has been kept of breakdowns affecting Phenix safety components, operational reliability estimates covering a period of over 18 months have been made for the components of the safety system for detection of anomalous heating of fuel assemblies. On the basis of these estimates it has been possible to verify the reliability and availability of the system in real operating conditions. The results of this study will be used in designing and constructing the corresponding safety system for the next French fast reactor, Super Phenix