Aging of Class 1E Modules

Prevention of or elimination of common mode failure mechanisms which may compromise the independence of redundant safety systems for nuclear power generating stations is a design objective. Within the last two years degradation of safety equipment performance due to continuous long term exposure to service environment (aging) has been identified as a potential common mode failure mechanism. Consequently placing equipment in a condition representative of advanced age as part of a type test program is now required by IEEE 323, 1974. This paper attempts to highlight some of the technology regarding placing equipment in an aged condition as well as some of the limitations of that technology as it applies to the nuclear power industry. These limitations are particularly important with regard to the extent of commitment to IEEE 323, 1974 possible by electric utilities. A variety of technical information regarding accelerated life testing of individual components appears readily available. However, considerable uncertainty in the state of the art still clouds one´s ability to translate this component information into a scheme for simulating advanced age of many different components assembled into a module. Note Module is any assembly of interconnected components which constitute an identifiable device, instrument or piece of equipment. A module can be disconnected, removed as a unit and replaced with a spare. It has definable performance characteristics which permit it to be tested as a unit. May range in size from a tiny sensor to complete cabinets or racks of equipment.