Accidents with multiple failures application of physical state approach

It is well-known that the event-oriented procedures do not cover every possible accident with any number of instrument or system failures or operator errors. A new approach, based on the relation between the physical state of the reactor and the operator action, has been developed to take into account multiple human and/or system failures, without postulating a time-history transient (1). EDF has applied this new philosophy in two steps. First, the existing event-oriented procedures have been improved and a set of two state-oriented procedures has been introduced, which only interferes in case of failure of the event-oriented procedures. Figure 1 shows, in a severity versus complexity diagramm, how accidental situations are covered by the help of the first generation of state-oriented procedures: a permanent monitoring post-incident procedure (SPI) and an ultimate one (U1). Then, EDF and FRAMATOME (the french manufacturer of the nuclear steam supply system) have verified that it was possible to completely replace event-oriented procedures by a set of state-oriented procedures (figure 1). This second generation of state-oriented procedures, which uses reactor vessel water level instrumentation, is now written and will be implemented at PENLY power station, which will start up in 1989. This paper presents how second generation state-oriented procedures can help the operators to deal with multiple system and/or human failures accidents