Chemical characterisation of degraded nuclear fuel analogues simulating the Fukushima Daiichi nuclear accident

The Fukushima Daiichi nuclear accident, caused by the 2011 earthquake and tsunami, resulted in the meltdown of three reactor cores and the release of radionuclides. The resulting molten core-concrete interaction (MCCI) materials, similar to the lava-like fuel-containing materials (LFCM) at Chernobyl, pose significant challenges for retrieval, storage, and disposal. Direct sampling of Fukushima MCCI is currently impossible due to high radioactivity. Previous simulations using VULCANO and VESTA provided insights, but lacked the incorporation of Pu or its surrogates (like Ce and Nd). This study synthesizes and characterizes low-radioactivity simulant Fukushima MCCI materials using Ce as a Pu surrogate and Nd to represent trivalent rare earth fission products. Ce is a suitable surrogate due to similarities in oxidation states, chemical behavior, and ionic radius compared to Pu. However, differences exist, particularly Ce's easier reduction to Ce³⁺ compared to Pu. The synthesis methods are based on those used for Chernobyl LFCM simulants, which closely matched real LFCM. The study aims to understand the chemical distribution and speciation of U and Pu (simulated by Ce) within MCCI and the overall phase assemblage.

Existing literature on MCCI focuses on large-scale simulations (like VULCANO and VESTA), thermodynamic modeling, and analyses of Chernobyl LFCM. These studies provide valuable insights into MCCI formation and microstructure, but often lack the inclusion of Pu or its surrogates. Previous studies incorporating Zr often did so simply as Zr or ZrO₂, neglecting the fact that U, Pu, and Zr would have been in a solid solution at the time of MCCI formation. The limited studies on Pu-surrogate MCCI materials highlight the need for a comprehensive investigation into the behaviour of Pu within this complex system, particularly considering the mixed oxide [(U,Pu)O₂] fuel in Unit 3 of the Fukushima Daiichi plant.

Simulant MCCI materials were synthesized with varying compositions based on estimations of core materials and concrete proportions (Table 1). The concrete-forming oxides (SiO₂, CaO, Al₂O₃) and stainless steel components (Fe₂O₃ and 316 stainless steel) were kept constant. The method of Pu surrogate (Ce) and Zr addition was varied to investigate their impact on the final product. The (U1−x−yCexZry)O2−δ oxide materials were prepared via ammonium hydroxide precipitation, followed by calcination and sintering under a reducing atmosphere (N₂/5% H₂). Bulk characterization involved powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS). Micro-focus X-ray analysis utilized the Swiss Light Source and the National Synchrotron Light Source II to obtain high-resolution, spatially resolved data. Micro-focus X-ray fluorescence (μ-XRF), X-ray absorption near-edge spectroscopy (μ-XANES), and X-ray diffraction (μ-XRD) were combined to identify crystalline phases and determine the local U and Ce chemistry. The average oxidation state at each point was determined by examining the energy position of the μ-XANES spectra, utilizing a suite of uranium-containing compounds of known oxidation state and local coordination for comparison. Extended X-ray absorption fine structure (EXAFS) spectroscopy provided further details on the local coordination environments of U and Ce.

Bulk characterization confirmed the presence of expected phases like uranium-rich cubic c-(U,Zr)O₂, zircon (ZrSiO₄), and anorthite (CaAl₂Si₂O₈). μ-focus X-ray analysis provided high-resolution insights. U was predominantly found as U⁴⁺ in various U-Zr-O phases, with slightly higher oxidation states observed in the glass matrix. Ce was primarily present as Ce³⁺, consistent with the reducing synthesis conditions. The formation of Ce-bearing percleveite, (Ce,Nd)₂Si₂O₇, resulted from the reaction between the U-Zr-O-depleted Ce-Nd-O melt and the silicate melt. The presence of multiple Fe-containing phases (FeCr₂O₄, Fe₂O₃, Fe₃O₄) with differing oxidation states indicated localized variations in oxygen potential during synthesis. The distribution of Ce was closely associated with U and Zr, concentrating in Fe-rich regions and pore interiors. EXAFS analysis provided precise bond lengths for U in different phases, revealing a contraction compared to undoped UO₂ due to Zr⁴⁺ substitution. A small fraction of Ce⁴⁺ was found in regions lacking Fe, suggesting that the presence or absence of metallic iron might influence the local oxygen potential and Ce oxidation state.

The findings demonstrate that the synthesized simulant MCCI materials exhibit microstructures and mineralogy consistent with large-scale simulations and thermodynamic modeling, and closely resemble Chernobyl LFCM. The use of Ce as a Pu surrogate provides valuable insights into the potential behavior of Pu in real MCCI materials, although differences in redox behavior should be considered. The identified phases and their distribution reflect the conditions within the Fukushima Daiichi reactors during the accident. Variations in Zr addition method did not drastically alter the phase assemblage but affected the abundance of U-containing zircon. The localized heterogeneity in oxygen potential, influenced by Fe-containing phases, impacted the oxidation states of both U and Ce. The observation of Ce³⁺ predominantly suggests that Pu may also form silicate phases under sufficiently reducing conditions.

This study successfully synthesized and characterized simulant Fukushima Daiichi MCCI materials. Multi-modal μ-focus X-ray analysis provided detailed insights into the phase assemblages, elemental distributions, and oxidation states of U and Ce. The results are consistent with large-scale MCCI simulations and provide valuable data for understanding the chemical behavior of degraded nuclear fuel. Future research could focus on expanding the simulant compositions to better reflect the actual Fukushima MCCI and investigating the long-term behavior of these materials under various conditions.

The study utilizes Ce as a surrogate for Pu, acknowledging that differences in redox behavior between the two elements exist. The absence of highly radioactive fission products in the simulants limits the ability to perfectly replicate the actual MCCI materials. The relatively small scale of the synthesis compared to the actual meltdown may have subtle influences on the phase formation and distribution.