Publication Date： October 28, 2026
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Unraveling High-Temperature Specific Heat Anomalies in Nuclear Materials at the Atomic Scale
－Machine-Learning Simulations Reveal What Happens Before Melting－
Fig. 1 High-temperature simulations of fluorite-structured material using machine-learning molecular dynamics
Thorium dioxide (ThO₂), a nuclear fuel material, and lithium oxide (Li₂O), a candidate tritium breeding material for fusion reactors, have crystal structures known as fluorite-type structures. At high temperatures, these materials exhibit a phenomenon called a λ (lambda) transition, in which their heat capacity increases sharply (Fig. 1b). However, the atomic-scale mechanism underlying this transition has not been fully understood.
We investigated the high-temperature properties of these materials using machine-learning molecular dynamics (MLMD), which enables fast and accurate atomistic simulations by training machine-learning models on quantum-mechanical calculations. MLMD accurately reproduced experimental properties such as lattice constants, thermal expansion coefficients, and melting points. For Li₂O, the simulations also predicted a previously unobserved λ-transition temperature of 1560 K.
In these materials, some atoms (O atoms in ThO₂ and Li atoms in Li₂O) become increasingly mobile within the crystal as the temperature increases. We therefore introduced a new “order parameter” to quantify the degree of disorder in the local octahedral arrangement of these atoms, in which six atoms surround a central atom in an octahedral geometry (Fig. 1c). Our analysis revealed that three types of local structures—symmetry-preserving ordered structures, defect-like structures, and liquid-like structures—coexist near the transition temperature (Fig. 1d). Above the transition temperature, liquid-like structures become dominant, and the symmetry-preserving ordered structures are progressively lost.
This new picture, in which the λ transition arises from local symmetry breaking, reveals atomic-scale structural changes that occur in high-temperature nuclear materials before melting and is expected to contribute to their design and safety assessment.
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This work was supported by JSPS KAKENHI Grant Numbers JP24K08574, JP25K01658, and JP23K04637.
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