Feasibility of Continued Operations and Extended Operations

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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