Samples of Chernobyl fuel debris, including massive corium and “lava” were collected inside the Chernobyl “Sarcophagus” or “Shelter” in 1990, transported to Leningrad (St. Petersburg) and stored under laboratory conditions for many years. In 2011 aged samples were visually re-examined and it was confirmed that most of them remained intact, although some evidence of self-destruction and chemical alteration were clearly observed. Selected samples of corium and “lava” were affected by static leaching at temperatures of 25, 90 and 150 °C in distilled water. A normalized Pu mass loss (NL_Pu) from corium samples after 140 days was noted to be 0.5 g/m² at 25 °C and 1.1 g/m² at 90 °C. For “lava” samples NL_Pu was 2.2–2.3 g/m² at 90 °C for 140 days. The formation of secondary uranyl phases on the surface of corium and “lava” samples altered at 150 °C was confirmed. The results obtained are considered as an important basis for the simulation of fuel debris aging at Fukushima Daiichi nuclear power plant (NPP).

Chemical degradation of borosilicate glass doped with ²³⁸Pu was modelled in conditions of a failed underground radwaste repository in granite host rock with bentonite buffer material after penetration of aqueous solutions at temperature of 90 °C. The total duration of the experiment exceeded two years. It is shown that wet bentonite preserved its barrier function and prevents migration of plutonium to the solution. The total amount of plutonium adsorbed on bentonite clay during the experiment did not exceed 0.02% of the initial amount of plutonium in the glass sample. Estimated accumulated dose of self-irradiation of the glass sample after the experiment varies from 3.16 × 10¹⁵ to 3.39 × 10¹⁵ α-decays per gram, which is equivalent to more than 1000 years storage of ²³⁹Pu doped sample with the same Pu content. Beishan granite remained intact, with no evidence of Pu penetration into the granite matrix along mineral grain boundaries.