NPP: Power Grid Mutual Safety Assessment - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 424

Figure 16. Hybrid safety assessment method

Table 9. Semantic interpretation of linguistic terms of systems criticality (reactor case study)

Reactor safety levels Physical State Description

Uncontrolled power increase in the reactor core (heat generation), decreased coolant consumption
(heat removal) and increased pressure in the primary coolant circuit. Reactor parameters are
close to the rated values. For fuel elements these are fuel temperature, cladding temperature,
burnout ratio, temperature of physical and chemical processes, heat flow. For the circuit these are
pressure, temperature, brittle fracture ratio, pressure differentials

Criticality state - HIGH (reactor
emergency state)

Criticality state - MEDIUM
(Reactor pre-emergency state)

The state of unstable equilibrium of the reactor. The reactor is in a state of physical and
thermohydraulic stability, which can be upset even by slight disturbances

Criticality state - LOW Normal routine mode of reactor operation

of components operation. Output data are
criticality condition of the system.
2. BBN Block for CI safety Assessment: The
set of NPP systems is divided into two sets:
parent and child systems. When using BBN
parent systems, criticality conditions are
used for determining criticality of the child
systems. The basic data are parameters of
criticality conditions of the parent systems
obtained in the FLI block and conditional
probability table (CPT). CPT determines
the relation between system conditions.
Probabilities can be represented on absolute

and fuzzy scale. Input data are criticality
condition of the child system.
3. Fuzzy Backward Chaining Block: To
obtain predictive estimates of condition
parameters of child system components.
Probability distribution of estimates of
child system criticality obtained using BBN
is used as input data to derive logic equa-
tions. Additional information is the expert
knowledge matrix R. The block's output
data is predictive estimates of component
conditions.