Doel 3 & Tihange 2 - Some Peer-reviewed Scientific Papers & Reports

Z. Que et al. / Procedia Structural Integrity 13 (2018) 926–931

927

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Z. Que et al. / Structural Integrity Procedia 00 (2018) 000–000

Keywords: Low Alloy Steels; Hydrogen Embrittlement; Dynamic Strain Aging; Environmental Assisted Cracking; Fracture Resistance

1. Introduction RPV is the most critical component in LWR and its structural integrity is of utmost importance regarding operation safety and service lifetime. HTW environment together with hydrogen from radiolysis, intentional additions (to suppress radiolysis in PWRs and to mitigate SCC in BWRs) and absorbed as the by-product of corrosion reactions (oxide film formation & growth, hydrolysis of metal cations from anodic dissolution after film rupture and dissolution of MnS-inclusions), may potentially reduce the fracture resistance of the RPV steel in synergy (or competition) with other degradation mechanisms including irradiation embrittlement (Soneda 2015), EAC (Seifert 2008 & 2012) or DSA (Yoon 1999). There is an ongoing debate about the possibility of such effects and it must be stressed that even minor effects can be critical for long term operation for plants with small margins for irradiation embrittlement. Massive literature on hydrogen effects on the fracture properties of low alloy steels (LAS) exists, particularlly at low temperatures (Šplíchal 1993) and in high strength LAS (Wu 2004). There are only very few studies on HTW (Schellenberger 1994) and hydrogen effects (Krasikov 2012) on the fracture behaviour of low-alloy RPV steels in the LWR operating temperature range. A few wppm of hydrogen may be sufficient to reduce the ductility and upper shelf toughness and increase the ductile brittle transition temperature in RPV steels (Hänninen 1992). Hydrogen concentration in the RPV in case of an intact cladding and under steady-state LWR operation conditions is usually low, therefore the bulk hydrogen embrittlement (HE) appears to be very unlikely. However, hydrogen levels higher than the critical concentration might be reached at stressed and plastically strained bare crack-tips with an aggressive occluded crevice chemistry, and a high hydrostatic tri-axial stress state, e.g., in fracture mechanics tests in HTW or in a loss of coolant accident (LOCA) (Roychowdhury 2016). The main goal of this work is to systematically study the unexplored effects of HTW environments and hydrogen on the fracture behaviours of RPV steels with different DSA & EAC susceptibilities in the upper shelf region in LWR temperature regime. The major influential parameters and the potential interactions with other embrittlement mechanisms like DSA or EAC, which may result in the most pronounced environmental reduction effects, were evaluated.

Table 1: Overview on investigated RPV steels.

Product form

YS 288ºC [MPa]

Remarks

Desig.

Material

S [%]

P [%]

Biblis C BM

22 NiMoCr 3 7 (= SA 508 Cl. 2) 20 MnMoNi 5 5 (= SA 508 Cl. 3)

Moderate DSA & low EAC susceptibility

Forged

0.007

0.008

400

277

Forged

0.004

0.004

418

High DSA susceptibility

508

SA 508 Cl.2

Forged

0.004

0.005

396

High DSA susceptibility

High S & high EAC susceptibility

HSST

SA 533 B Cl. 1

Hot-rolled

0.018

0.006

412

2. Materials and Experimental Procedure The investigated RPV materials and the respective properties are summarised in Table 1. The fracture behaviour of the Biblis C base metal in HTW was systematically characterized in PSI (Que 2017) and thus is the reference material for this study. Biblis C BM, 277 and HSST materials have a similar granular bainitic microstructure, while 508 material has a mixed bainitic-ferritic microstructure. EPFM tests according to ASTM E1820 with air fatigue precracked and side-grooved compact tension specimens were conducted in air and HTW environments, in which the loading rates and crack-tip strain rates were varied between 3E-4 to 3 mm/min and 1E-6 to 1E-2 s -1 , respectively, covering the strain rate for operational transients (1E-6 to 1E-4 s -1 ) and during loss of coolant accident (1E-4 to 1E-2 s -1 ). J integral values at initiation of stable crack growth (J Q ) were calculated corresponding to the intersection point