The application of redundancy is an attempt to decrease the probability that an accident will occur.

3.3.1 Redundancy

In reactor systems, a component or system which is vital to the safety of the plant is made redundant; that component or system is doubled in the design, so that in a fault condition one of the components or systems could fail and there would still be one to do the vital job. Thus the probability of failure of the component or system is reduced. There are, however, three main drawbacks to the principle of redundancy.

(a) There is a design inhibition against redundancy; the incorporation of a system or component which may never be used. Some designers may tend to believe one Webster definition of the word redundancy: “an act of needless repetition.”

(b) Mere redundancy does not ensure greater safety if the redundant components or systems share vital components or lines, interfere with each other so that a failure of one could cause the other to fail, or are subject to common mode failure (failure from the same fault). Thus true redundancy also implies independence.

(c) The cost of doubling up some systems and components is often considerable and may be more than the plant can economically allow.

Thus the application of redundancy requires a certain engineering and safety judgment to obtain optimum safety and operational capability.