Solubility Product of Np(IV) Hydrous Oxide

Fujiwara, Kenso ; Kohara, Yukitoshi*; Mori, Takashi

In support of the performance assessment of geological disposal of high-level radioactive waste, solubility measurement of tetravalent neptunium (Np(IV)) hydroxide in reducing condition was performed. Np stock solution was reduced by using Hgas and platinum black (catalyst). Solubility test were perfomed by oversaturation method. Sample solution of 1.0 M NaClOwas spiked by the Np(IV) solution. The pHc value was adjusted around 3 with NaOH and HClOand the sealed sample solution was standing at room tempreture (23 2C) in the shaded box avoiding photo-oxidation. The concentration of Np(IV) was measured by alpha-spectrometry. The solubility decreased with increasing pHc. It can be seen that the hydrolysis species are predominantly formed with the slope of solubility. It can be seen that the hydrolysis species are predominantly formed with the slope of solubility. From the obtained results, the solubility product (Kof Np hydroxide at I = 1.0 were determined. The experimental values were extrapolated to the standard state (I = 0) by using the specific interaction theory (SIT), and the solubility product of Np hydroxide was determined to be logKsp° = -56.2 0.3.