Microstructural features of SIMFUEL — Simulated high-burnup UO2-based nuclear fuel

FRAPCON-3: Modifications to fuel rod material properties and performance models for high-burnup application

“…These data are suplplemented by results from other Halden instrumented tests involving small-gap rods operated to significant burnup with surviving centerline thermocouples. The degradation rate initially reported (Kolstad et al 1991) is qualitatively consistent with the results of laser-flash diffusivity measurements aln unirradiated simulated high-burnup fuel performed at Chalk River National Laboratory (CIWL), Ontario, Canada (Lucuta et al 1991;Lucuta et al 1992). These two experimental programs are described below.…”

“…For urania-rare earth mixtures simulating fission product accumulations for burnups up to 10 at.%, Matsui found a remarkable "excess heat capacity" (above that of undoped urania) for temperatures from 500 to 1200°C. However, Lucuta et al (1991) failed to find a similar increase in specific heat in the same temperature and simulated burnup ranges with SIMFUEL samples, which also simulate burnup via rareearth additions to urania before sintering. Lucuta's samples simulated burnup up to 8 at.%.…”

“…G. Lucuta at CRNL measured thermal diffusivity in pellets with simulated burnups of 0,3, and 8 atom % @e., 0,28, and 75 CiWdh41RJ). For the simulation, he added a mixture of 11 rare earth oxides to the urania before pressing and sintering, as described in Lucuta et al (1991). He selected the mixture on the basis of ORIGEN code calculations for water reactor fuel, and the resulting sintered material was thoroughly characterized microstructurally (see Lucuta et al 1992 and1995).…”

BEHAVIORS OF MOLYBDENUM IN UO2FUEL MATRIX

“…The molybdenum oxide powder was mixed thoroughly with small size urania particles to ensure homogeneity on a submicrometer scale, and this mixture was then heated to a temperature sufficient to achieve diffusion rates that ensure homogeneity on an atomic level [1]. To understand the status of molybdenum in UO2 and/or the microstructural transformation of UO2 by molybdenum, a visual inspection of the prepared pellets was performed by EPMA.…”

“…The fission products could be categorized by groups: elements that form solid solutions with UO2, those that precipitate into a UO2 matrix in the forms of oxides or metals, and volatile elements [1,2]. Among the elements that precipitate into a UO2 matrix, molybdenum is the most abundant fission product since its fission yield is equivalent to that of xenon [2,3].…”