The data presented in this study confirm that the synthesis of realistic low-activity (i.e., containing depleted uranium as the only radioactive isotope) simulants for Chernobyl LFCM is achievable at a small, batch scale, in the laboratory. The microstructure and mineralogy of the resulting materials were the same as found in real Brown and Black LFCMs, containing the same range of phases, including the high uranium–zircon, chernobylite, which previous studies of simulants have been unable to form.19,20,21 It was previously proven possible to synthesise this phase through high-temperature heat treatment of U–Zr–Si–O gels,31 however, to our knowledge, this study is the first to successfully crystallise this phase from a glass melt. The LFCM simulants synthesised here additionally contained Zr–U–O phases, UO 2 , ZrO 2 and Fe-, Ni-, Cr- and Mo-bearing metallic inclusions, also found in real LFCM. The identification of small inclusions of ZrO 2 at the centre of chernobylite grains confirms previous hypotheses that this phase is the product of reaction between Zr–U–O and the silicate melt.1,7

One advantage of being able to create low-activity simulants is the ability to perform measurements that are otherwise difficult to conduct within a hot-cell environment. In this study, we have been able to further elucidate the thermal characteristics of LFCM, determining the glass-phase transition, liquidus temperature and the crystallisation temperatures of zircon and Zr–U–O-containing phases. The latter values are consistent with those measured for natural minerals, and the former may differ slightly from real LFCM due to the use of ZrO 2 as a precursor rather than zircaloy cladding. The preparation of low-activity samples is also advantageous for analysis at user facilities, for example, synchrotron beamlines. This study confirmed, by XANES, that the oxidation state of uranium in the simulant samples, which was initially added to the batch as UO 3 , was U(IV), as observed in real LFCM.17 In the absence of zircaloy cladding, the melting of which is thought to have partially reduced UO 2 during the accident, our study required the application of a reducing atmosphere during synthesis to create the desired oxidation state.

There is a further benefit of being able to synthesise small (25 g) batches of realistic simulant LFCM. The use of large-scale demonstrators, where experiments are conducted to understand the interaction of molten fuel and cladding with structural building materials, such as the VULCANO facility in France,32 are expensive and hazardous to operate. Having a reliable, small-scale synthesis procedure may be helpful in screening compositions and experimental matrices so that the use of the large-scale demonstrator can be reduced to only those most important experiments; this may prove highly useful in the ongoing assessment of molten corium–concrete interaction (MCCI) products at the Fukushima Daiichi Nuclear Power Plant in Japan, ahead of fuel retrieval operations.

The corrosion behaviour of LFCM is of concern with regards to decommissioning of the Chernobyl reactor. This is due to the large amounts of radioactive dust generated from the secondary alteration products when the humidity within the sarcophagus falls; the removal of water from the LFCM surfaces can significantly intensify UO 2 oxidation and dehydrate hydrous uranium-containing oxidation products, forming dust.27 Some estimates suggest that the maximum rate of dust formation in the last 33 years was on the order of 1–10 kg y−1.27 Several studies have been performed to elucidate the corrosion rate of LFCM and to ascertain the nature of the secondary alteration phases that are formed during corrosion. Many of these have been concerned specifically with the leaching rate of various fission products (e.g., 106Ru, 137Cs, 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.

The only reported corrosion rate of uranium from real LFCM pertains to monoliths of Brown material.12 A dissolution rate of 1.8 × 10−3 g m−2 d−1 was calculated by a simple extrapolation of the normalised mass loss over a 22-day period, with samples leached in a solution of 0.4 mol L−1 NaCl (initial pH 6.5), at room temperature.12 Using the same method (between 3 and 28 days, to avoid the possible influence of dissolved fines on the corrosion rate) for simulant Brown LFCM, we obtain a uranium corrosion rate of 1.05 ± 0.06 × 10−4 g m−2 d−1 (Supplementary Fig. 7). This is somewhat lower than the real LFCM. Similarly, the corrosion rate of uranium for simulant Black LFCM was found to be 3.42 ± 0.20 × 10−5 g m−2 d−1. 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 MW2535 or the non-radioactive simplified surrogate of French high-level waste glass, the International Simple Glass.36 The presence of a silicate alteration layer observed after corrosion (Supplementary Fig. 6) suggests that the mechanism of corrosion of LFCMs is, at least in part, similar to that of silicate glasses.

Comparing the corrosion of simulant LFCM with available data in the literature for real LFCM, the pH of the solutions of both were observed to decrease by 1–2 pH units, relative to the blank, over the 28-day duration of leaching. 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. It is possible that colloidal complexes containing carbonate may have formed, which were not detectable using the analytical techniques applied here, however, it seems that the removal of OH− ions into secondary phases is the most plausible explanation for the observed pH decrease in this study.