Component Description

DOESTD107393

ldentify measurements that will monitor significant aging effects.

Make baseline measurements of component material condition.

The methodology, depicted In Figure 43, provides a model that may be used for both Iife-limiting

components and important SSCs that are not life-limiting, but have been selected for detailed MCA

analysis.

4.2.3.1 Component Description

The description of component parts, environment, and functions should be sufficiently detailed to

permit the identification and evaluation of the significant stressors and aging mechanisms. The safety-,

environmental-, or mission-related functions and operation of each component should be described in

terms of design requirements. Components may have multiple functions that are either active or

passive. Each component should be described in a way that makes clear the boundaries between

what is being evaluated and what is not being evaluated. For example, the boundaries of a motor-

operated valve may be at the welds or flanges that connect it to the piping system, at the electrical

breaker that provides the electrical power to the motor operator, and at the connectors for the

instrumentation and control circuits. In this case, the connecting piping, the electrical power distribution

system, and the instrument and control system are outside the component boundary. Interfaces with

other equipment and systems should be described relative to physical, design, and environmental

factors. If the component was qualified for its application by special testing or analysis, the specific set

of functional requirements and environmental conditions that comprise the qualification of the

component should also be described.

Breaking down the component into subcomponents simplifies the task of identifying significant aging

mechanisms and failure modes. Subcomponents are generally divided into those that have a similar

identifiable importance to the overall function of the component/assembly and those that react to

stressors in a similar manner. The breakdown of components into subcomponents often facilitates the

aging degradation evaluations. For example, a battery can be divided into subcomponents consisting

of the container, the plates, the terminals, and the electrolyte. Each has different aging mechanisms

and failure modes. Evaluating each subcomponent separately is easier than evaluating the component

as a unit.

4.2.3.2 Identification of Significant Aging Mechanisms

For each subcomponent, the stressors and aging mechanisms that could lead to failure should be

identified. This process is shown in Figure 44. The descriptions of the components make it possible

to identify the types of stresses and the materials that are affected by each stress. It is important to

identify the degrading effects that the stresses have on the materials to help determine potential failure

modes for the equipment. The Nuclear Regulatory Commission (NRC), the commercial nuclear

industry, and industry standards organizations have performed a number of aging studies. That provide

useful information concerning materials susceptible to aging, the stresses that cause them to degrade,

and resulting degradation mechanisms. Examination of the component, its design, its functions, and

pertinent aging mechanisms, as well as qualification, performance, maintenance, test, and condition--

monitoring data may provide additional information. For example, excessive temperature is a stress to

the insulation of electrical cables that can cause the insulation to become brittle and lose its integrity;

the resulting failure modes are shorts to ground and shorts to other electrical circuits.

Evaluation of the potential aging stresses and the resulting failure modes that have the most significant

effects on facility safety or availability takes into account the severity of the stresses found in the facility

and the rate of progression, or aging rate, to identify each aging mechanism. The magnitude of

stresses in the facility may already have been measured and documented in facility records, or

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