Data obtained in our experiment showed a steep increase in enthalpy from around 1,800 ℃. Along with these results, heat capacity was analyzed from the enthalpy data, and significant variation was observed. We propose that this trend arises from the sudden increase in concentration of the lattice defect, specifically electrons and holes, from around 1,800 ℃.

To perform temperature analysis on uranium-plutonium mixed oxide (MOX) fuel, accurate understanding of the fuel properties is required. The MOX fuel pellet reaches high temperatures exceeding 2,000 ℃; however, its fuel properties at such high temperatures are still not known with sufficient accuracy. Therefore, it is important to expand such property data, clarify the underlying mechanism, and improve the reliability of the properties. In this study, we measured the enthalpy of MOX, UO₂, and PuO₂ at temperatures approaching 2,000 ℃, which is experimentally achievable, and determined the heat capacity from those measurements.

The measured enthalpy data (on the left), along with the analysis results of heat capacity (on the right) determined from those enthalpy values, are shown in Figure 1. For all uranium-plutonium compositions, a steep increase in enthalpy was observed starting around 1,800 ℃. The results indicate that composition of the fuel itself has little effect on this trend. Heat capacity is obtained as a temperature derivative of enthalpy. Consistent with the enthalpy trend, heat capacity also showed a steep increase as the very high temperatures are reached.

Concerning the experimental data for enthalpy at temperatures approaching 2,000 ℃, this study is the first to experimentally compare MOX, UO₂, and PuO₂. The data obtained in this study can contribute to more accurate temperature analysis on MOX fuel.

Paper URL: https://doi.org/10.1016/j.jnucmat.2024.155188

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