Open Access

ARTICLE

Structural Integrity of Main Heat Transport System Piping of AHWR

K.K. Vaze¹

1 Reactor Structures Section, Reactor Safety Division, Bhabha Atomic Research Centre, Mumbai

Structural Longevity 2010, 3(2), 87-109. https://doi.org/10.3970/sl.2010.003.087

Abstract

Advanced Heavy Water Reactor (AHWR) is a 920 MWth, 300 MWe vertical pressure tube type reactor, which uses boiling light water as a coolant in a high-pressure main heat transport (MHT) system. Structural integrity considerations have been a part of the design process right from selection of material. Among the various degradation mechanisms, low temperature sensitization and low temperature embrittlement were considered to be the life limiting material degradation mechanisms: In view of the proposed 100 year life of AHWR, these issues need to be addressed in a thorough manner. SS 304LN has been chosen based on its satisfactory low temperature sensitization behaviour and superior low temperature embrittlement behaviour. The material specification was optimized to gain maximum advantage in respect of intergranular stress corrosion cracking. The IGSCC behaviour can be further improved by adopting Narrow Gap Welding technique. The beneficial effect of this on residual stresses was demonstrated by measurements on conventionally and narrow-gap-welded pipes. The defect tolerance of the piping was demonstrated by carrying out a test programme showing compliance with Leak-before-break criterion. Fatigue tests were carried out on notched pipe and pipe weld under cyclic loading with different stress ratio. The results of fatigue tests show that for the typical stress range expected in the piping of AHWR, the number of cycles to crack initiation and growth (through thickness) is very large compared to the expected number of cycles. Aspect ratio (2C/a) at the point of through thickness lies in the range of 3 to 4 irrespective of the initial notch aspect ratio, thus favouring application of LBB. The use of the fatigue crack growth curve given in ASME Section XI will produce a conservative result. Fracture tests were carried out on through-wall cracked fatigue tested pipe and pipe weld under monotonic loading. The results of the fracture resistance properties of the pipe and pipe welds prepared by GTAW are comparable whereas that of pipe welds prepared by GTAW+SMAW is on lower side. A combination of narrow gap welding technique and GTAW will provide an added assurance against failure due to IGSCC/fatigue/fracture. Tests carried out with large amplitude bending loads address new failure mechanisms such as fatigue-ratchetting and cyclic tearing whereas tests with fixed-fixed boundary conditions consider the effect of compliance on fracture integrity.

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