THE EFFECT OF DISSOLVED HYDROGEN CONCENTRATION ON THE SCC INITIATION OF NI-BASED ALLOY IN THE PRIMARY WATER WITH KOH

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The replacement from lithium hydroxide (LiOH) to potassium hydroxide (KOH) for the pH control of the primary water of pressurized water reactors (PWRs) is under consideration because of an unassured supply and high cost of enriched ⁷Li. In this study, the susceptibility of the stress corrosion cracking (SCC) initiation of a Ni-based alloy in the KOH chemistry was investigated. In addition to the comparison of KOH and LiOH, the influence of the dissolved hydrogen (DH) concentration on the SCC initiation susceptibility in the KOH chemistry was examined. In the experiment, the constant load tests were performed for Ni-based alloy X-750 at 360°C in simulated or modified primary water with LiOH or KOH. In the LiOH environment, the DH concentration was set at 30 cc/kg, a normal DH concentration in PWRs. In the KOH environment, the DH concentration was set at 5, 30, or 45 cc/kg. As a result, the SCC initiation time showed no significant difference between the KOH and LiOH conditions at the DH concentration of 30 cc/kg. Interestingly, the SCC initiation time was significantly extended under the condition of the low DH environment (5 cc/kg). This DH dependence in the KOH environment was consistent with the reported DH dependence in the LiOH environment. The experimental result in this study indicates that the initiation of SCC can be mitigated with a low concentration of DH in the KOH environment as in the LiOH environment.