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DOESTD107393
The next task is to select an appropriate method for estimating the component's remaining lifetime. In
some cases, a simple time-in-service approach is sufficient. The remaining lifetime is calculated as the
total lifetime of the component less the time that it has already been in service. For example, if the
vendor indicates that a component should have a total lifetime of 15 years and that component has
already operated at the facility for 11 years, its remaining lifetime is estimated to be 4 years. In other
cases, the time-in-service approach is not adequate for estimating the remaining lifetime. It may be
necessary to take into account the actual service history (e.g., the number of operating cycles and the
stresses associated with each cycle), which may differ from the average service conditions anticipated
by the vendor. An approximation of facility remaining lifetime should include consideration of the
identified aging mechanisms and one or more of the following:
Failure rates (mean time to failure)
Comparison to components with similar materials and environmental history
Straight-line projections utilizing current condition and projected degradation rates
The Arrhenius model (applicable to materials that age as a function of the ambient temperature)
Engineering judgment
Depending on whether future operations of life-limiting components are expected to have stresses that
are similar to, greater than, or less than those experienced in past operations, it may be necessary to
modify simple straight-line projections that begin with current condition and apply observed degradation
rates to estimate remaining lifetime. Existing data that are useful in arriving at this estimate include
operating and maintenance histories, occurrence of severe events that may have significantly stressed
the component, industry experience with similar equipment, vendor specifications, and design
information. This information may reveal a significant difference between the actual operating
conditions and those assumed by the designer or the vendor which may provide the basis for adjusting
the expected total life of a component.
The remaining lifetime is estimated by subtracting the time-in-service (modified as appropriate through
current condition considerations, as discussed) from either qualified life, updated as necessary, or the
expected total life. The remaining lifetime of the subcomponents determines the remaining lifetime of
the component being assessed.
Determination of remaining lifetime should be conservative because of the uncertainties in the
estimating process. The determination should take into account factors such as overall confidence
level of estimated time to failure and frequency of monitoring the limiting age-related characteristics.
The degree of the uncertainty should be estimated and included with the final determination of the
remaining lifetime.
4.2.5 FEASIBILITY OF CONTINUED OPERATIONS AND EXTENDED OPERATIONS
After the previous conclusions concerning the feasibility of continued operations have been either
confirmed or revised, management alternatives similar to those considered in earlier feasibility studies
should be considered during the final feasibility study. The costs, as a function of time, of each
alternative should be determined and presented. Significant break points in the cost factors should be
identified and highlighted. The following cost factors should be considered:
Present operating and maintenance costs (used as a reference for evaluating alternatives)
Costs for continued operations, including those for accommodating both near-term and long-
term continued operations, and any costs related to delays in completing the facility mission
Costs to develop and implement facility upgrades needed for life extension
Costs to enter and maintain standby operations, and then to restart the facility
Costs of decommissioning the facility
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