Effect of dissolved gas on mechanical property of sheath material of mineral insulated cables under high temperature and pressure water

Abstract

In terms of an applicability of mineral insulated (MI) cables under high temperature and pressure water such as coolants in nuclear power plants, effects of dissolved gases on mechanical properties of the sheath materials of the MI cables were investigated. Slow strain rate testing was performed for AISI 304 and 316 stainless steels under high temperature and pressure water at 325 °C and 15MPa in pure water. At a strain rate of 5 × 10−3mm/min with the condition, oxygen: ∼8.5ppm, hydrogen: < 1ppb, nitrogen: ∼14ppm, fully ductile fractures were observed. The tensile strengths increased with the change in the strain rate from 5 × 10−3 to 5 × 10−4mm/min. On the other hand, with oxygen: < 1ppb, hydrogen: < 1ppb, nitrogen: ∼30ppm by nitrogen bubbling, partial brittle fractures were observed mainly near the edge of the samples. In addition, the change in the strain rate from 5 × 10−3 to 5 × 10−4mm/min increased the rate of the brittle fracture surface and decreased the tensile strengths. The change in the dissolved hydrogen from < 1ppb to 50ppb also increased the rate of the brittle fracture surface and decreased the tensile strengths. The similar brittle behavior was observed by replacing nitrogen by argon bubbling. The results implied the possibility of embrittlement phenomena of the stainless steels in high-temperature and pressure water even with very low dissolved oxygen resulting in high susceptibility to hydrogen embrittlement.