APPLICATION OF SEISMIC WALKDOWN PROCEDURE TO PERFORM FITNESS-FOR-SERVICE EVALUATIONS

摘要

Confirming system, structure, or component operability for safety related systems is a continuous process required to keep a nuclear plant in compliance to its design and license requirements. A need for verification of operability can arise if a degraded or nonconforming condition is identified during normal plant operations or through review activities. Early hints of such degradations can be noticed visually from condition of the support elements and anchorage. The locations to experience the first damage are generally those supports that are highly loaded, hence referred to as the critical supports. The damage typically initiates at a weak spot such as structural welds, anchor bolts and supporting concrete. Generally, to dispose off any deficiency found in the field requires modelling of piping between equipments or other form of pipe anchors for a detailed analysis using advanced methodologies, or a major repair work (or a combination thereof) that would require a shut-down or delay to restart of the plant if it is already in a shutdown state. Developing efficient in-service inspection program that would give an advance indication of degradation mechanisms challenging the pressure boundary integrity coupled with a limited analysis of the systems under 'as-found' condition is a far more prudent method. Therefore, in-service inspections involving walk-downs to detect visible damage to equipment as well the interconnecting piping systems have been considered as the most efficient approach. Typically, the health report of piping supports is used as forensic indicators of abnormal behaviour of the system during plant operations. Based on the results from the analysis of 'as-found' systems, subsequent assessment can be made as to the integrity of the entire system, and thus system's 'fitness-for-service' (FFS) to operate under the existing condition. This paper describes details of an approach that may be utilized at any time during plant operation, even during outages, to confirm structural and pressure boundary integrity of process systems. The approach starts with a walkdown inspection of pipe supports and equipment anchorage, and the discrepancies gathered are used in selection of an appropriate methodology to perform FFS evaluations. System integrity and operability is evaluated for the plant under normal operating conditions as well as design basis earthquake and accident conditions. System upgrades to new seismic requirements can also be included in the walkdown inspection scope. The walkdown inspection worksheets are prepared using those used during seismic walkdowns, but modified based on system specific experience in design and OPEX. Examples of discrepancies recorded on pipe supports and guiding procedures used to select an appropriate approach utilized for their disposition are discussed.