F.EN08: Scientific Basis for Nuclear Waste Management

Olivia Roth, Studsvik Nuclear AB

Investigation of Secondary Phases Formed During Long Term Aqueous Leaching of Spent Nuclear Fuel

Written by Emma Perry

In 1982 researchers at Studsvik placed one BWR spent fuel segment (42  MWd/kgU) into 200 ml of deionized water and another into 200 ml of synthetic ground water under oxic conditions.  Unbeknownst to them, this experiment would run for 37 years!

Most spent fuel rods have been kept in interim storage facilities without any problem. However some are damaged and their fuel has been exposed to water in oxic conditions for several years or decades. In order to understand how these damaged fuel rods may dissolve in a geological disposal facility, the 37-year experiment was brought to an end. The surface of samples stored in simulant ground water mostly comprised of uranite, with some metastudite. The surface of samples in deionized water mostly comprised of studite with some lanthanide and metashoepite and a very small amount of uranite.

The fuel segments were then subjected to a further leaching study in bicarbonate water for 119 days. The white/yellowish secondary phase on the fuel segments stored in deionized water  disappeared from the surface but remained in the cracks. In this leaching experiment, fuel segments stored in deionized water had a faster release of radionuclides than the fuel segments stored in synthetic ground water.