Leaching of actinides and other radionuclides from matrices of Chernobyl “lava” as analogues of vitrified HLW

Synthesis, characterisation and corrosion behaviour of simulant Chernobyl nuclear meltdown materials

“…This is somewhat unusual behaviour for alkali-aluminosilicate glasses (where the release of alkali elements to solution from the corroding glass and subsequent complexation with alkali elements raises the pH to alkaline values), and points to the formation of pH-influencing secondary alteration phases, such as those that incorporate hydroxide ions (removal of which may lower pH), or those that complex with carbonate ions. Within the Chernobyl sarcophagus, the carbonate concentration was found to vary between 370-2900 mg L −1 , and uranyl carbonate mineral phases were observed to form, 1,16,18,24,25 which may effectively lower the pH. However, the present experiments contained significantly less carbonate (only that in equilibrium with CO 2 in the air), and such phases were neither observed, nor predicted by geochemical modelling.…”

“…Other studies of fission product (e.g., Pu, Am, Cs and Eu) leaching from real LFCM have shown no real difference in the corrosion rate between the two compositions. 18 These crudely obtained uranium release rates should be treated with caution and no extrapolations to longer durations should be made, however, it is clear that the values obtained are approximately one order of magnitude greater than those for uranium leached from spent nuclear fuel under oxidative conditions, 33 but are comparable with those for U 3 O 8 -doped borosilicate glass. 34 The normalised mass loss values of Na and Si from the simulant LFCM compositions are also of the same order of magnitude as borosilicate high-level nuclear waste glass compositions such as MW25 35 or the non-radioactive simplified surrogate of French high-level waste glass, the International Simple Glass.…”

“…Many of these have been concerned specifically with the leaching rate of various fission products (e.g., 106 Ru, 137 Cs, etc.) 12,16,18 , however, relatively few have focused on the corrosion rate of other elements in the LFCM, including uranium-the source of the radioactive dust hazard. Furthermore, the release of uranium and other radionuclides to the ground surrounding and below the reactor, may have resulted in the generation of a significant quantity of contaminated soil that will require eventual remediation, in addition to the contamination of groundwater.…”

“…Samples of the material are limited due to the difficulties and dangers associated with their collection (unstable building, high radiation levels), and only a few specimens of the lava that were sampled from within the reactor building have been studied. Despite recent studies, 9,17,18 little is known about the condition of Chernobyl LFCM, especially now, 33 years after the accident. To overcome this lack of information, past attempts to simulate lowactivity LFCMs (i.e., containing no fission products, only depleted uranium) have been made to help understand the conditions within the reactor during the accident, however, this work was unable to accurately approximate the microstructure, morphology and mineralogy of LFCM.…”

“…It is known that, due to the condensation of water inside the roof of the original Chernobyl sarcophagus (as a result of the temperature differential within and outside the structure), and the presence of holes in the sarcophagus roof, a significant proportion of water has dripped onto the LFCM causing it to corrode. 16,23 This is apparent from the presence of yellow secondary alteration products formed on the surface of LFCM, known to include paulscherrerite (UO 2 (OH) 2 ); studtite (UO 4 1,18,24,25 These phases have potential to generate significant amounts of radioactive, uranium-bearing dust when the humidity within the sarcophagus falls below 85%. 23,26,27 Understanding the corrosion behaviour of LFCM, and developing a detailed evaluation of the kinetics and mechanisms of dissolution, are vital to support ongoing decommissioning efforts-both at Chernobyl and also at the Fukushima Daiichi Nuclear Power Plant, where LFCM-type materials are thought to have formed, which largely remain submerged in water used to cool the melted core.…”