Surveillance of nuclear-thermohydraulic stability in BWR operation

A concept of a survey of the nuclear-thermohydraulic stability during operation of large modern boiling water reactors is presented. The concept incorporates detailed measurements of the stability boundary as well as accompanying evaluations of the stability margin during the operating reactor cycle. An automatic system to avoid unstable power flow conditions by immediate control rod insertion is an additional element of the concept. The proper functioning of this system in a practical event was demonstrated. Thus, inadvertent unstable reactor operating conditions could be avoided in all completed cycles by means of reactor and core loading design together with the enforced surveillance concept.