The effect of sulfate ion which is one component of seawater or concrete on the corrosion behavior of SUS316L was investigated in high active liquid waste tank environment. The corrosion tests were conducted using surrogate high active liquid waste containing some oxidizing metallic ions and nitric acid. The results showed that sulfate ion decreased the corrosion rates. The XPS analysis indicated that the compound formed from sulfate ion on material surfaces suppressed the anodic reaction.

In order to study an effect of sulfate on stress corrosion cracking (SCC) initiation in primary coolant environments of boiling water reactors, creviced bent-beam tests were conducted for sensitized Type 304 stainless steel specimens in sodium sulfate water at 288℃. The addition of 30～500 ppb sulfate ion to purity water increased both numbers of all cracks more than 1 µm in depth and steady-propagating cracks more than 50 μm in depth. Spatial distribution of initiated cracks deeper than 50 μm was approximated to a Poisson stochastic process under the condition of [SO₄^2-] ≤ 100 ppb, whereas it was not approximated to the process under the condition of [SO₄^2-] ≥ 300 ppb. A new approximation method based on Poisson distribution was presented as an estimation method of cumulative probability for crack initiation and the availability of it was confirmed. Sulfate ion concentration dependency on lower-limit values, a _est , for distribution of the SCC initiation life was given to be a_est = 683/[SO₄^2-] by an analysis based on the Poisson stochastic process model. For example, the addition of 40 ppb sulfate ion to purity water can shorten a lower-limit value of distribution of the SCC initiation life by one tenth without changing stochastic character of it. Shortening of a_est by the addition of sulfate probably depended not on solution conductivity but on sulfate ion itself.

In spent fuel pool （SFP） under loss-of-cooling or loss-of-coolant severe accident condition, the spent fuels would be exposed to air and heated by their own residual decay heat. Integrity of fuel cladding tube is crucial for SFP safety therefore study on oxidation of fuel cladding tube in air at high temperature is thought important. Zircaloy-2 （Zry2） and Zircaloy-4 （Zry4） were used for thermogravimetric analyses （TGA） in different temperatures in air at different flow rates to evaluate oxidation behavior. Oxidation rate increased with testing temperature. In a range of flow rate of air which is predictable in spent fuel lack during a hypothetical SFP accident, influence of flow rate was not clearly observed below 950°C for the Zry2, or below 1050°C for the Zry4. In higher temperature, oxidation rate was higher in high flow rate condition, and this trend was seen clearer when temperature increased.

Oxide layers were carefully examined after the TGA analyses and compared with mass gain data to investigate detail of oxidation process in air. It was revealed that the mass gain data in pre-breakaway regime reflects growth of dense oxide film on specimen surface, meanwhile in post-breakaway regime, it reflects growth of porous oxide layer beneath fracture of the dense oxide film.