Improvement of Integrity Concepts for NPP Primary Circuit Piping

Operating NPPs license extension activities in Russia produced strong demand for safety improvement of plants build according to earlier standards. Installation of additional supports as pipe whip restraints is one of requirement in acting regulatory documentation which should be followed or compensated by appropriate measures like Leak Before Break (LBB) analyses and improvement of In-Service Inspection (ISI) and Leak Detecting System (LDS). Basic document for LBB concept application to Russian NPP piping is RD 95 10547-99. Its requirements correspond to classical LBB principles used in many countries in Europe, USA and Japan. In many real cases requirements of RD 95 10547-99 could not be applied to safety important NPP piping systems due to the presence of specific features of operational degradation due to some corrosion mechanisms: for example, erosion-corrosion (E-C) for carbon steel piping and intergranular stress corrosion cracking (IGSSC) for heat affected zones of austenitic piping weldments. For special case of RBMK piping with outer diameter 325 mm (potentially susceptible to IGSCC) special Break Preclusion Concept has been developed in Russia after IAEA Extrabudgetary Program in 2000–2002. Contrary to LBB Concept demanding for all four basic principles to be completely fulfilled BP Concept accepts some principles to be fulfilled in a balanced way with demonstration of monitored degradation effectively achieved in operation. Special BP Concept is being developed now to support integrity assessment of RBMK carbon steel steam and feed water piping potentially susceptible to E-C which requires another set of measures to demonstrate principle of controlled degradation in operation then in case of austenitic steel piping. General scheme of piping integrity analyses according to LBB and BP Concepts is discussed and examples of specific approaches to achieve controlled degradation are illustrated in paper. As result of LBB and BP Concepts application it is possible to substantiate reject of additional piping whip restraints implementation on-site. Examples of similar safety methodology development in other countries have been reported at IAEA Specialists Meeting on LBB in Kiev, Ukraine in November 2006.