Aging Degradation Mechanism Evaluations

DOESTD107393

4.1.2 AGING DEGRADATION MECHANISM EVALUATIONS

During the preliminary MCA phase, the major aging degradation mechanisms are identified for each

potentially life-limiting component. These mechanisms vary for different types of components, but may

include fatigue, corrosion/erosion, stress corrosion cracking, and irradiation. This preliminary evaluation

is not intended to be a thorough analysis of all aging stressors, their effects, and failure modes. Rather,

it is to be based on available data, initial inspections, and engineering judgment.

To provide a basis for an estimate of the facility's remaining lifetime, the current material condition of

the components is determined. The aging degradation mechanisms most likely to cause -failure

should be emphasized, and any previous aging evaluations that have been performed should be used

in this process. Walkdowns may be useful for visually identifying unexpected degradation, and

interviews with cognizant personnel from the operations, maintenance, and systems engineering

organizations may provide insight into the current material condition of each component. In addition,

senior facility personnel who were involved in the construction and initial operation of the facility may be

able to provide useful information regarding historical perspectives, operating practices, maintenance

practices, and previous findings and conditions.

4.1.3 ESTIMATION OF FACILITY REMAINING LIFETIME

The preliminary estimate of a facility's remaining lifetime is not expected to be precise; rather, it should

place components in lifetime categories: 0-2 years, 2-5 years, 5-10 years, and more than 1 0 years.

Unless better information is available, it should be presumed that the stresses on the potentially life-

limiting components involved In operations and operating environments will be the same in the future as

in the past.

The estimated remaining lifetime of the facility equals the shortest of the estimated remaining lifetime of

the facility's potentially life-limiting components, provided that life extension techniques are not applied.

The facility remaining lifetime should be estimated conservatively to compensate for the uncertainties

involved. To ensure that users of the estimated remaining lifetime have some understanding of its

accuracy limitations, the amount of uncertainty involved in the remaining lifetime should be estimated

using engineering judgment.

4.1.4 FEASIBILITY OF CONTINUED OPERATIONS AND EXTENDED OPERATIONS

Only in certain situations are the feasibility of continued operations and the feasibility of extended

operations addressed during the preliminary MCA phase. The feasibility of continued operations should

be addressed when the preliminary estimate of facility remaining lifetime is very short and there may be

questions about the advisability of continuing operations at all. The feasibility of extended operations

should be addressed (1) when the estimated remaining lifetime is less than the DOE desired lifetime,

and (2) when the desired lifetime is comparable to the remaining lifetime, due to the uncertainties

expected to be involved in the estimates.

These feasibility studies involve (1) identifying management alternatives for continued operations or

extended operations, (2) estimating the costs for each alternative as a function of time, and (3)

developing recommendations regarding facility continued and extended operations. Management

alternatives may include the following: operate the facility until the end of its estimated remaining

lifetime; develop and apply facility life extension techniques when the desired lifetime is greater than the

estimated remaining lifetime; or place the facility in a standby mode at a specified time, in anticipation of

future operations. Cost estimates for each alternative need not be precise, but they should indicate

where significant changes in costs would occur. Recommendations regarding continued operations

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