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

Z. Que et al. / Procedia Structural Integrity 13 (2018) 926–931 Z. Que et al. / Structural Integrity Procedia 00 (2018) 000–000

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of the regression line (J = C1ꞏ  a C2 ) and the 0.2 mm exclusion line. EPFM tests were conducted in HTW with different simulated LWR environments. Reducing BWR hydrogen water chemistry (HWC) was simulated by hydrogenated (dissolved hydrogen (DH) content of 2.0 ppm) neutral high purity water (pH 288°C = 5.7, inlet conductivity = 0.055  Sꞏcm -1 ) with the corresponding electrochemical corrosion potentials (ECP) at 288 °C of -590 mV SHE . PWR primary water was simulated by mildly alkaline borated and lithiated, hydrogenated high-purity water (1000 ppm B as H 3 BO 3 , 2.3 ppm Li as LiOH, pH 288 °C = 6.9, inlet conductivity of 24  Sꞏcm -1 , DH content of 2.2 ppm) with the corresponding ECP at 288 °C of -735 mV SHE . Selected additional tests were performed in nitrogenated (oxygen- and hydrogen-free, -400 mV SHE ) or oxygenated (2 ppm dissolved oxygen, +100 mV SHE ) neutral high purity water. To clearly elucidate the effect of hydrogen, the hydrogen concentration in the HWC tests was a magnitude higher than in real BWR. The higher oxygen content was selected to simulate a realistic ECP on the RPV cladding.

Figure 1: Summary of fracture initiation resistance JQ of 277 material (a), 508 material (b) and HSST material (c).

3. Results and Discussion 3.1. 277 and 508 materials with high DSA susceptibility and low S content

Investigations on fracture behavior of 277 material with a high DSA susceptibility and low sulphur content in HTW are shown below. The effect of loading rates was evaluated by EPFM tests on 277 specimens in hydrogenated HTW and air with loading rates of 0.35, 35 and 3500 µm/min at 250°C. In Figure 1(a), the increasing fracture initiation resistance (J Q ) at decreasing loading rates in air and hydrogenated HTW were due to the hardening and negative strain rate sensitivity by DSA. For the EPFM tests in air and HTW with loading rates of 3.5-3500  m/min at 250°C, the fracture mode was mainly ductile with microvoid coalescence (MVC) while at slow loading rate of 0.35  m/min in hydrogenated HTW the fracture mode was significantly different, in which the crack first initiated in a stable ductile mode by MVC but then propagated by sub-critical strain-induced corrosion cracking (SICC) with “high-sulphur” EAC crack growth rates at higher J levels, as shown in Figure 2(a). The apparent stronger reduction in tearing resistance and increase in crack growth rate at the slow loading rate of 0.35  m/min in HTW was mainly caused by sub-critical SICC crack growth. As shown in Figure 1(a), the environmental reduction in fracture initiation resistance at 288 °C was slightly stronger than at 250 °C with 35  m/min. Therefore the effects of HTW environments on reduction of fracture resistance of 277 material was evaluated in air, hydrogenated, oxygenated and nitrogenated HTW with 35  m/min at 288 °C. The environmental reduction of fracture initiation and tearing resistance and the environmental acceleration of ductile crack growth rate were most pronounced in the hydrogenated HTW with 35  m/min at 288 °C, which indicated clear HTW and hydrogen effects in synergy with DSA. The Biblis C base metal material with low DSA susceptibility, but otherwise similar bainitic microstructure and mechanical properties, did not reveal any environmental effects on fracture initiation resistance under these conditions. As indicated in Figure 2 (c)-(g), the fracture morphology of 277 specimens tested in oxygenated HTW resulted in lower roughness and smaller amount of secondary cracks and quasi-cleavage facets compared to hydrogenated HTW, while the fracture surface after the EPFM test in air was purely ductile by MVC. The stronger fracture resistance reduction in hydrogenated vs.