Thermophysical Property of MgO-Nd2Zr2O7 Composite Ceramic Inert Matrix Prepared by One-step Method

Abstract

Inert matrix fuel (IMF) can effectively burn hazardous nuclides (such as Pu and minor actinides) as transmutation targets in reactors, making it an attractive option for nuclear waste management. IMF is a dispersion type of fuel that diffuses fissile phase, PuO2, in a neutron-transparent inert matrix (IM). MgO-Nd2Zr2O7 (M-NZO) composite ceramic is a potential IM that can replace UO2 in (U, Pu)O2 mixed oxide fuel (MOX) and to produce IMF by dispersing PuO2 in it. In this work, MgO, Nd2O3, ZrO2 powders were chosen as the raw materials, and series of ωM-(1-ω)NZO (ω=0.3, 0.4, 0.5, 0.6, 0.7 within mass percent) composite ceramics were prepared by one-step method at 1 500 ℃ for 24 h. Based on the analysis of phase composition and morphology, the thermophysical properties such as thermal conductivity and the coefficient of thermal expansion (CTE) for as-prepared M-NZO composite ceramics were systemically investigated. It is indicated that the as-prepared M-NZO composite ceramics show the remarkable densification and the mass fractions of MgO and Nd2Zr2O7 phases are almost the same as the designed nominal chemical composition. The average grain size of MgO phase and Nd2Zr2O7 phase is about 0.75 μm and 0.70 μm, respectively. The thermophysical properties results show that the thermal conductivity of all as-prepared M-NZO composite ceramics is 2.1-3.8 times that of UO2 ceramics at 1 400 ℃, suggesting the excellent thermal conductivity. And the thermal conductivity of M-NZO composite ceramics prepared by one-step method is higher than that of M-NZO composite ceramics fabricated by two-step method in the literatures within the tested temperature range, and the highest difference value is about 2 W·m-1·K-1 at high temperature (800 ℃). In terms of thermal expansion, the CTE value of as-prepared M-NZO composite ceramics ranges from 12.3×10-6-14.1×10-6/K and is comparable to that of UO2 ceramics. In addition, an incremental quantity of the thermal conductivity and CTE for ωM-(1-ω)NZO composite ceramics will merge with increasing MgO content. Combining the minimum thermal conductivity results for M-NZO composite materials, 0.5M-0.5NZO is the optimal sample composition, in which the mass percent of MgO in the composites stays the same as that of Nd2Zr2O7. In conclusion, M-NZO composite ceramics prepared in this work exhibits good thermal conductivity and stable thermal expansion property. And these thermal physical properties of M-NZO composite ceramics are superior to that of traditional UO2 ceramics. It is suggested that M-NZO composite ceramics can meet the thermal physical performances of IMF.

Keywords

• mgo-nd2zr2o7 composite ceramic
• inert matrix
• one-step method
• thermal conductivity
• coefficient of thermal expansion