Status of the Introduction of Digital I&C Systems

The probability of occurrence of a common mode failure in the protection system category I safety I&C system - is estimated conservatively from the available data of the Teleperm XS (TXS) I&C system by means of a model which can be traced objectively. It is seen that the requirements underlying TXS as contained in the nuclear codes and regulations reduce the probability of occurrence of common mode failures to such an extent as to allow this class of events to continue to be evaluated as beyond-design basis events. Additional risk-mitigating measures, such as suitable monitoring facilities or the initiation of so-called vital safety functions, thus can be classed safety level 4. The resultant leeway in using different kinds of equipment, such as existing in-plant I&C equipment, to achieve these additional measures is a welcome way of producing an overall optimum of plant safety.