Electrochemical purification of waste salt from pyro-processing of spent nuclear fuels

•Boron can be used as a solid cathode to remove the Cs+, La3+, Nd3+, Sm3+, and Ce3+ to purify waste molten salt.•The removal rate of Sr2+ can reach nearly 90% at 3.4 V and 720 °C for 10 h in NaCl-KCl molten salts.•Highly efficient removal of La3+, Nd3+, Sm3+, and Ce3+ has been achieved, and the removal rate of rare-earth elements reaches >99%.•The highly stable metal borides are conducive to the permanent geological disposal of spent nuclear fuels. Electrorefining spent fuels accumulates large amounts of fission elements in molten salt electrolytes. Thus the purification of waste molten salt is needed to improve the utilization and the stability of molten salt electrolytes. Herein, boron (B) is used as a solid cathode to remove active fission products from waste molten salt. Benefiting from the stronger interaction of active fission products with the B, B is used as a cathode to selectively allow the deposition of active fission products to produce borides. The percentage removal of Sr2+ can be as high as nearly 90% at 3.4 V and 720 °C for 10 h. Under the above optimal electrolytic conditions, the percentage removal of La3+, Nd3+, Sm3+, and Ce3+ can reach more than 99% from the waste NaCl-KCl, and the percentage removal of Cs+ only reaches 47.67% because of the relatively weak interaction of B and Cs. The metal borides produced on the B cathode possess high stability, which is conducive to permanent geological disposal. Therefore, B is a promising material to effectively separate active fission elements from waste molten salt, enabling the reuse of waste molten salt and minimizing the generation of hazardous nuclear wastes.