PSI - Issue 14

Ramesh Kumar et al. / Procedia Structural Integrity 14 (2019) 577–583 R. Kumar/ Structural Integrity Procedia 00 (2018) 000–000

578

2

operation. Zirconium alloy pressure tubes subjected to various radiation induced aging mechanisms such as hydride embrittlement, accelerated deformation, delayed hydride cracking (DHC) etc. Hydrogen concentration, neutron fluence and operating nominal stress increases with time due to pressure tube oxidation products and creep induced wall thinning. Sometimes, volumetric flaws may form due to fuel bundle fretting or debris fretting near PT rolled joint or away from it as given in Fig.1. Presence of volumetric flaws in pressure tube leads to stress concentration spots where hydrogen migration takes place and leads to hydride formation and cracking known as DHC.

Fig. 1. Bearing pad fretting flaw at rolled joint burnish mark in a pressure tube by CSA N285.8-15

Several methodologies were evolved to prevent the failure of pressure tube due to DHC at volumetric flaws. Yetisir et al. (1997) have investigated the cause of fretting marks (volumetric flaws) observed at pressure tube inner surface and tried it to correlated with turbulence-induced motion of the fuel elements. McRae et al. (2010) explained that tensile stress at the flaw tip induces a gradient in chemical potential that promotes the diffusion of hydrogen to the flaw tip and hydrides form if the hydrogen concentration reaches the solubility limit for hydride precipitation. Young-Jin et al. (2012) have presented a deterministic flaw assessment method for volumetric flaws present in pressure tube. The method was used to evaluate bounding flaw size envelope for plant cool down transient using CSA N285.8 procedure. Scarth et al. (2002) have explored DHC evaluation of blunt flaws using process-zone model and developed failure assessment diagram (R6) considering flaw geometry parameters. Radu et al. (2007) presented some applications related to the influence of the residual hoop stresses at roll-expanded joint region into stainless steel fittings at both ends on the structural integrity evaluation in the presence of blunt flaws. Radu et al. (2012) have explained a probabilistic approach to estimate CANDU pressure tube failure by DHC mechanisms during cool-down cycles using probabilistic fracture mechanics principles. In this paper, DHC initiation assessment was carried out for typical PHWR during sustained hot condition using CSA N285.8 procedure. The assessment was performed to evaluate the bounding envelop for threshold bulk Heq and volumetric flaw depth for DHC initiation in pressure tube. The length of the volumetric flaws was taken as 30mm which was expected from bearing pad fretting. Several Flaw root radius (r) were used to evaluate the bounding flaw sizes. The analysis assumes hydride ratcheting condition at flaw tip. Four locations, rolled joint inlet/ outlet and PT main body inlet/outlet were selected to assess DHC initiation for volumetric flaws.

2. Safety Assessment of Pressure tube against DHC for sustained hot condition

The assessment was carried out assuming flaw tip hydride ratcheting condition at sustained hot condition. Hydrided region overload evaluation was not carried out for service level A as specified by CSA N285.8 standard. The assessment was performed using peak stress evaluation method. DHC initiation evaluation flow chart was given in Fig.2.