PWR1次冷却材模擬環境中のほう酸および水酸化リチウムがステンレス鋼の粒界型応力腐食割れに及ぼす影響

摘要

軽水炉の1次冷却材と接している配管，容器等の主要な構成材料は耐食性に優れている必要があり，ステンレス鋼やニッケル基合金が広く用いられている。耐食性に優れたステンレス鋼やニッケル基合金ではあるが，これまでに多くの軽水炉で応力腐食割れ（stress corrosion cracking，SCC）を経験しており，SCCは軽水炉の高経年化における最重要課題の1つとして位置付けられている。%In order to understand systematically the influence of an environmental factor on the intergranular stress corrosion cracking (IGSCC) susceptibility of cold-worked 316 stainless steel, constant extension rate tensile tests were carried out under simulated pressurized water reactor (PWR) primary water conditions while varying the boric acid, lithium hydroxide, and dissolved hydrogen concentrations and temperature. It was found that IGSCC susceptibility increased for a low boric acid concentration and high lithium hydroxide and dissolved hydrogen concentrations, and a high temperature. The influences of the water chemistry changes were recognized as the changes in pH and magnetite solubility in a manner that IGSCC was promoted for higher pH and magnetite solubility. On the other hand, IGSCC crack growth rates measured using compact-tension-type specimens revealed that boric acid, lithium hydroxide, and dissolved hydrogen did not affect IGSCC crack growth rate significantly.