Application of Structural Integrity Concepts to RF NPP piping and equipment have started in Russia in the beginning of 1990s from requirements to Leak Before Break (LBB) calculation studies adopted by regulatory body. Results of first practical LBB Concept application to RF NPP piping have been reported at Lyon LBB Conference in year 1995.
Experience of LBB Concept practical application in Russia during 1990s formed strong basis for issuing comprehensive guideline RD 95 10547-99 which stated LBB Concept basic principles and requirements applicable to RF NPPs with Light Water Reactors (LWR) at all stages of Life Cycle: design, commissioning and operation. Requirements of RD 95 10547-99 guideline have been in good correspondence to classic LBB principles used in many European countries, USA, Japan and to IAEA Technical Documents as well.
RF NPP piping operation experience analysis have revealed necessity of producing more general methodology — Break Preclusion (BP) Concept — to substantiate structural integrity of piping potentially susceptible to active degradation mechanisms like Flow Accelerated Corrosion (FAC), Intergranular Stress Corrosion Cracking (IGSCC), etc. BP and LBB Concepts are based on the same major principles: (a) quality of design, manufacturing, assembling, (b) monitoring of degradation and operating conditions, (c) enveloping loading conditions with respect to maximum design loads, (d) calculation and experimental substantiation of failure according to LBB scenario. LBB Concept requires full implementation of principles (a)÷(d) and BP Concept allows to achieve balance in implementation of principles (a) and (b) while principles (c) and (d) should be fully implemented.
Memorandum of 8th International Scientific and Technical Conference “Safety of VVER NPPs” in year 2013 stated as a 1st priority goal to substantiate structural integrity of main components for Long-Term Operation (LTO) during 60–80 years. Actual regulatory demands to operating NPPs also produce strong demand to provide structural integrity arguments for safety important piping and equipment as a part of periodic safety justification of NPP Unit needed for license extension.
Peculiarities of structural integrity concepts application to safety important piping and equipment at different stages of NPP Unit Life Cycle (design. commissioning and operation) are discussed in paper with emphasis on technical measures providing conditions for effective degradation monitoring and prevention during LTO. Perspectives of regulatory documents improvement are also discussed.