NERI FINAL TECHNICAL REPORT, DE-FC07-O5ID14647. OPTIMIZATION OF OXIDE COMPOUNDS FOR ADVANCED INERT MATRIX MATERIALS

In order to reduce the current excesses of plutonium (both weapon grade and reactor grade) and other transuranium elements, a concept of inert matrix fuel (IMF) has been proposed for an uranium free transmutation of fissile actinides which excludes continuous uranium-plutonium conversion in thermal reactors and advanced systems. Magnesium oxide (MgO) is a promising candidate for inert matrix (IM) materials due to its high melting point (2827 C), high thermal conductivity (13 W/K · m at 1000 C), good neutronic properties, and irradiation stability However, MgO reacts with water and hydrates easily, which prevents it from being used in light water reactors (LWRs) as an IM. To improve the hydration resistance of MgO-based inert matrix materials, Medvedev and coworkers have recently investigated the introduction of a secondary phase that acts as a hydration barrier. An MgO-ZrO2 composite was specifically studied and the results showed that the composite exhibited improved hydration resistance than pure MgO. However, ZrO2 is insoluble in most acids except HF, which is undesirable for fuel reprocessing. Moreover, the thermal conductivity of ZrO2 is low and typically less than 3 W · m-1 · K-1 at 1000 C. In search for an alternative composite strategy, Nd2Zr2O7, an oxide compound with pyrochlore structure, has been proposed recently as a corrosion resistant phase, and MgO-Nd2Zr2O7 composites have been investigated as potential IM materials. An adequate thermal conductivity of 6 W · m- 1 · K-1 at 1000 C for the MgO-Nd2Zr2O7 composite with 90 vol% MgO was recently calculated and reported. Other simulations proposed that the MgO-pyrochlore composites could exhibit higher radiation stability than previously reported. Final optimization of the composite microstructure was performed on the 70 vol% MgO-Nd2Zr2O7 composite that burnup calculations had shown to have the closest profile to that of MOX fuel. Theoretical calculations also indicated that a homogeneous 70 vol% MgO composite could achieve the desired microstructure that would result in satisfying the dual requirements of good thermal properties