This is odd; a simple term “computation of plant reliability factor” gives no good resource website when searched using Google. Anyway, reliability factor is just the complement of Forced Outage Rate (FOR). That is;

Reliability factor = (1 - FOR)

where;

FOR = (Forced outage hours)/(Force outage hours + Service Hours)

Service Hours is the number of hours a unit was in the in-service state.
Forced Outage Hours is the number of hours a unit was in a Class 0,1,2, or 3 unplanned outage state.

Ref:
IEEE Standard Definitions for Use in Reporting Electric Generating Unit Reliability, Availability, and Productivity.

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