Within the frame of a project sponsored by the Ministry of Research and Technology (BMFT) we have investigated the influence of hydroxylapatite (HAP) added to the backfill material of a radioactive waste repository located in a salt dome. The time dependence of the sorption and precipitation of radionuclides has been studied in three different praxisrelevant salt brines under variation of temperature.Performing batch experiments uranium and thorium phosphates were precipitated from U0 2 (N0 3 ) 2 and Th(N0 3 ) 4 solutions at 25°C, 75°C and 120°C, the amounts of the relevant actinide compounds exceeding that of HAP. The isolated precipitates were investigated by use of X-ray-powder-diffractometry and SEM. For the various uranium systems two mineral phases were obtained. From solution II the mineral saleeite Mg(U0 2 )2(P0 4 ) 2 · 9H 2 0 was formed as the major component at 25 °C and 75 °C. From solution III mainly metaautunite Ca(U0 2 )2(P0 4 ) 2 · 6H 2 0, a dehydration product of autunite, was precipitated.

In cooperation with the Ministry of Research and Technology we investigated the influence of hydroxylapatite (HAP) added to the backfill material of a radioactive waste disposal located in a salt dome. The time dependence of the sorption and precipitation of radionuclides has been studied in three different praxisrelevant salt brines under variation of temperature and pH-value.Performing batch experiments uranium and thorium phosphates were precipitated from UO2(NO3)2 and Th(NO3)4 solutions at 75°C, the amounts of the relevant actinide compounds also exceeding that of HAP. The isolated precipitates were investigated by use of X-ray-powder-diffractometry and SEM. For the various uranium systems two mineral phases were obtained. From solution II and also from Q-brine the mineral saléeite Mg(UO2)2 (PO4)2 · 9H2O was formed. From solution III meta-autunite Ca(UO2)2(PO4)2 · 6H2O, a dehydration product of autunite, was precipitated.

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