In order to investigate a corrosion behavior of stainless steel in the typical nuclear fuel reprocessing plant, corrosion tests using nitric acid solution with neptunium were conducted under atmospheric and the reduced pressure conditions in the laboratory where radioactive substance could be handled. An ultra low carbon type of SUS304ULC stainless steel was used. Obtained results were as follows: Under the reduced pressure condition, corrosion of the stainless steel was accelerated in the nitric acid solution with neptunium than in pure nitric acid solution. Under an atmospheric condition, the corrosion rate of the stainless steel increased with increase of neptunium content. The higher solution temperature enhanced the corrosion rate of the stainless steel in the nitric acid solution with neptunium. Thermodynamic data showed that Np(V) oxidized to Np(VI) by concentrated and elevated nitric acid solution. The corrosion potential of the stainless steel shifted to nobler direction in the nitric acid solution with neptunium. It is estimated that the electrochemical reaction of Np(VI)/Np(V) is reversible and the rate constant of the reaction has great values. And it is considered that the dominance of Np(VI) in nitric acid solution accelerated the corrosion rate of the stainless steel in the solutions with neptunium.

We have studied corrosion of a stainless steel in nitric acid solution containing neptunium. Using type 304L stainless steel, corrosion tests in boiling neptunium nitrate solution were conducted under immersion and heattransfer condition. By the weight loss measurement of stainless steel and the quantitative analysis of metallic ions dissolved in solution, the corrosion rates of stainless steel were obtained. The surface morphology was observed by , scanning electron microscopy. The corrosion acceleration mechanism was investigated by polarization measurement and spectrophotometry. The corrosion rate of stainless steel was accelerated by addition of neptunium in nitric acid solution. Preferential intergranular corrosion was observed. The corrosion of stainless steel was promoted under heattransfer condition compared to. immersion condition. In polarization measurements, the cathodic over-voltage was decreased; the cathodic current was increased by addition of neptunium. Spectrophotometric measurements showed the oxidization of neptunium in boiling nitric acid. The corrosion mechanism in nitric acid solution containing neptunium suggested the re-oxidation of neptunium.

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Radiolysis calculations of simulated seawater were conducted using reported data on chemical yields and chemical reaction sets to predict the effects of seawater constituents on water radiolysis. Hydrogen, oxygen, and hydrogen peroxide were continuously produced from simulated seawater during γ-ray irradiation. The concentration of H 2 exceeded its saturation concentration before it reached the steady-state concentration. The production behavior of these molecules was significantly promoted by the addition of bromide ions (Br −) because of the high reactivity of Br − with the hydroxyl radical, an effective hydrogen scavenger. It is also shown that the concentrations of these molecules were effectively suppressed by diluting seawater constituents by less than 1%.

The effect of phosphorous on intergranular corrosion of type 310 stainless steel at transpassive potential was investigated using phosphorous added ultra high purity type 310 stainless steel. Intergranular corrosion depth was measured by scanning electron microscopic observation. Grain boundaries were observed using three dimensional atom probe. Intergranular corrosion was observed on phosphorous added stainless steel. The sample heat treated at 600 showed severe integranular corrosion morphology. Observation by three dimensional atom probe revealed that grain boundary with high dissolution rate had segregated zone of chromium and phosphorous about 5nm in width, on the other hand, grain boundary with low dissolution rate had no segregated zone. From these results, it is suggested that intergranular corrosion of phosphorous added type 310 stainless steel at transpassive potential is affected by segregation of phosphorous and chromium along grain boundaries.