Strategic management of electric power supply has been one of the most important issues all over the world. The operation of nuclear power plant has been changed according to the demands from the society. In the advanced operations of nuclear power plants, the degree of burnup of UO2 based fuels has been and will be increased up to ~60GWd/t for the efficient usage and energy extraction of UO2 fuel. MOX fuel is also planned to be used to promote the plutonium thermal use (Pluthermal). New types of spent fuels will be produced and should be reprocessed in near future. For the encapsulation of high-level radioactive nuclear wastes (HLW) from these spent fuels, vitrification process should also be advanced, because these HLWs are estimated to contain the chemical elements like Mo, minor actinoids, noble metals etc, in higher concentration than those from the previously produced spent fuels. In this research, a new feeding form of glass matrix, Spherical Briquet (BQT), has been developed for the vitrification of HLW from spent fuels of high-burnup UO2 and/or MOX.
BQT feeding form consisting of borosilicate glass powder has been fabricated intending to have following functionalities; high strength to ensure stable transportation for feeding operation, and high reactivity with HLW solution to form slurry immediately (homogeneous mixture of glass powder and wastes). In this research, the preparation condition of BQTs and their chemical reactions examination with simulated HLW solution from high-burnup and MOX spent fuels including the melting test in small scale melter have been carried out to satisfy the required functionalities,

Glass powder was prepared from present borosilicate glass beads (Li2O-Na2O-CaO-ZnO-Al2O3-B2O3-SiO2), which will be conventionally used, by pulverizing followed by ball milling process. After dried once, appropriate amount of water and the selected inorganic binder were added and mixed well to form paste. A pair of hemispherical molds were used to mold paste into spherical shape. After drying at 85 oC in maximum, spherical BQTs were obtained as shown in Figure 1.
Mechanical strength was evaluated using uniaxial pressing (crushing test) by autograph, and crushing strength was determined from yield stress as a function of concentration of binder. Chemical reaction of BQT with simulated HLW solution has been done using dipping test at room temperature. Behaviors of soaking of HLW into BQT and collapsing of BQT were monitored and evaluated.

Figure 2 shows the strain-stress curve of BQT using 2.5% Borax (sodium tetraborate decahydrate) as an inorganic binder, and high crushing strength was obtained. Increasing with the concentration of binder, crushing strength of BQT increased as shown in Figure 3.
The appearance of BQT soaked in simulated HLW was shown in Figure 4. Collapse of BQT in a few minutes was clearly observed in case of Borax while no collapsing was observed in case of waster glass as binder although HLW solution soaked inside well.

Chemical reaction was checked by confocal Raman spectroscopy. Binder components were found to be concentrated around the BQT surface due to the transportation from inside with water during drying. Fortunately, condensing of binder component near surface is effective to increase crushing strength with the minimum addition. In case of Borax BQT, Raman signals assigned to BO4 tetrahedral units disappeared after the immersion of HNO3 solution. Extraction of Na2O from Borax by strong acid is considered to decrease strong binding function among glass powder in BQT. Borax binder satisfies the required functionalities, and BQT form is one of the alternative feeding forms to the present glass beads and increases effective contact of glass surface with HLW, which is expected to incorporate MoO3 component into glass melt earlier and faster to prevent formation of Yellow Phase.

In the presentation, the results of feeding experiments of BQT with simulated HLW solution (high-burnup and/or MOX) will be shown using the compact glass melter with the capacity of 0.3-0.5L.

This work was carried out as a part of the basic research programs of vitrification technology for waste volume reduction（JPJ010599）supported by the Ministry of Economy, Trade and Industry, Japan.