The hydrolysis behaviour of the NpOi ion has been investigated at 25 by solubility experiments in the pH ränge 7 to 14 in 0.1 M. 1.0 M and 3.0 M NaC104 solutions under pure Ar atmosphere. By measuring the time dependent equiiibrium concentration of Np(V), the chemical State of Np(V) hydroxide precipitatesin each Solution isconfirmed. In 0.1 M NaClO« the precipitate remains amorphous over several months, while in 1.0 M NaClOi the precipitate changes from an amorphous to a more stable aged State within a relatively short time. In 3.0 M NaCiOt the aged modification of NPO2OH is formed from the beginning. Applying specific ion interaction theory (SIT), the thermodynamic constants at 1 = 0 are found to be: log/r,p= -8.76±0.05 for amorphous Np020H(s). logA:,p=-9.44±0.10 for aged Np020H(s), log/?i=2.7±0.2 for the hydrolysis species NpO^OH and iog^2 = 4.35±0.15 for Np02(0H)2".

Hydrolysis of Np(V) in Ι.ΟΛί NaCIO, was investigated by radiometric pH-titration of 23, NpOj under C0 2 -free Ar atmosphere. From the resulting solubility curve the first and second hydrolysis constant (3, and ß 2 as well as the solubility product Ksp were evaluated: log ß 1 = 2.3, log ß 2 = 4.89 and log Ksp = -8.81.

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Solubilities of Pu from leaching of a simulated HLW glass (C31–3-EC) spiked with 5 wt % Pu and from PuO2 are determined in salt solutions of different NaCl concentration and in groundwaters which are representative for the Gorleben area. Th.e leaching experiment is carried out either in an autoclave at 200 °C or under normal conditions (1 atm, 25°C). Solubilities of Pu in all investigated solutions are observed to be considerably higher than the values known from theoretical estimates. The predominant Pu species in solutions are found to be microcolloids, which are characterized by means of ultrafiltration and pulsed laser-induced photoacoustic spectroscopy.

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