High-level liquid waste (HLLW), which is produced from reprocessing of the spent nuclear fuel and contains actinides, 90Sr, 137Cs, 99Tc, 129I, etc., engenders permanent hazard to environment. The safe treatment and disposal of HLLW are crucial to sustainable development of nuclear energy. The TRPO (a mixed trialkylphosphine oxide, commercial name Cyanex 923) process for removal actinides from HLLW has been studied in our laboratory since 1980s. In the TRPO process, the control of the oxidation states and the removal of Neptunium is one of the important problems. Neptunium has three readily accessible oxidation states, IV, V and VI, which can coexist under certain conditions. Therefore, careful control of Np chemistry is needed. There are lots of studies on Np chemistry in the ambience of nitric acid, perchloric acid, or other aqueous solutions. However, there is few study on Np chemistry in the HLLW, which contains lots of different salts.
In this work, we studied the reduction of Neptunium in synthetic HLLW to maintain its oxidation state at IV so that it can be extracted by TRPO and removed from the HLLW. Different reductants were tested and the kinetic of the reduction action was studied. The influence of the elements in HLLW was also studied. The reduction speed of Np was highly enhanced by some elements in the HLLW. Finally, a hybrid reductant was suggested and good reduction effect was obtained.
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