The effect of surface treatment on reducing the metal release from Ni-base alloy in high temperature water

The effect of surface treatment for Ni-base alloy on Ni release was studied for reduction of Ni release in pressurized water reactor primary water. The surface treatments were performed by three methods: pre-oxidation in humidified H₂, pre-oxidation in high temperature water, and electro-polishing. The Ni release tests were conducted in the titanium inner vessel placed in an autoclave and the test condition was as follows; deaerated solution containing 500 ppm boron ion and 2 ppm lithium ion with dissolved hydrogen of 30 cc/kgH₂O (STP) at temperature of 325 °C for 500 hours. The Ni concentration in the solution was measured by ICP-MS. All the surface treatments showed inhibiting effect on the release of Ni. The pre-oxidation in humidified H₂ was the most effective for reduction of Ni release. The surface characterization suggested that the composition and thickness of oxide layer formed by surface treatment are strongly associated with Ni release resistance. (author)