PSI - Issue 16

Peter Trampus / Procedia Structural Integrity 16 (2019) 161–168 Peter Trampus / Structural Integrity Procedia 00 (2019) 000 – 000

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4.7. Small-bore piping

Experience demonstrate that failure of socket welds at small bore piping (Ø < 50mm) is a recurring problem and it needs an increasing attention for safe operation. Mechanical or thermal fatigue, weld flaws (lack of fusion), SCC are the usual ageing effects. Although ASME Inspection Code requires surface examination only, UT was introduced but due to limited accessibility the conventional UT was later replaced by PA UT. In VVER plants similar degradations appeared. Some of the cases happened in DMWs (level measurement and blowdown pipes on SG), where bimetallic corrosion on the low-alloyed steel was detected. To mitigate, the ISI program was amended by radiographic examination (RT). Formerly, requirements using deterministic methods and detailed standards composed the basis for ISI/NDE. Nowadays, the inspection philosophy placed the focus on performance based and risk based/oriented approach instead of detailed regulation. The capability demonstration of ISI/NDE systems was grown up, and also the application of probabilistic safety assessment (PSA) for passive components started to opening the door for risk informed ISI (RI ISI). An effective ISI is characterized by ISI programs based on capability demonstration and risk considerations. Inspection qualification (European approach) or performance demonstration (US approach) is an organized process to establish confidence by a systematic and independent assessment that the NDE procedure, equipment and personnel are capable of meeting the inspection requirements in real circumstances, Whittle (2009). RI-ISI is a sort of ISI focusing the inspection efforts and resources on the high-risk locations and by this means increases or at least maintains the overall plant safety measured by risk, ENIQ (2005). ISI effectiveness means that there is an optimal balance between NPP safety and ISI/NDE costs in terms of the fundamental ISI parameters (scope and interval) and the NDE system capability. Effective ISI is a substantial part of ageing management with its need for a reliable diagnosis of the condition of the components and the prediction of their future status. Fig. 3 summarizes this evolutionary process and the main features of ISI effectiveness as described above. 5. Moving towards an effective ISI

Traditional ISI program • deterministic approach (semi-probabilistic) • detailed NDE standard requirements

Driving forces • ageing management (life extension) • increase of productivity

Experiences • UT performance deficiencies (results of large round robbin programs)

Need for improve / optimize ISI program

Effective ISI program • probabilistic methods (risk concept) • performance based NDE requirements

Fig. 3. Overview of ISI effectiveness.