Sintering and Thermal Behavior of Uranium Dioxide in Beryllium Oxide Matrix
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The primary fuel used in light water nuclear reactors (LWRs) is uranium dioxide (UO2), which has a low thermal conductivity that causes a large thermal gradient across the fuel pin during operation. One proposed method to improve the thermal conductivity of the fuel is to insert a thermally conductive additive such as beryllium oxide (BeO) or other ceramic materials into the fuel structure. This study is focused on a particular fuel design developed at Purdue University wherein large UO2 microspheres are dispersed within a continuous BeO matrix. The BeO has a relatively high thermal conductivity for an oxide material and the interconnected matrix is intended to enable higher heat removal from the fuel. Therefore it is of interests to characterize the effective thermal conductivity of this UO2-BeO fuel concept and consider variations of this property with BeO composition of 2.5, 5, 7.5, and 10 volume % BeO. As part of this study, the basic pellet manufacturing procedures were improved over previous work to create samples suitable for characterization that did not crack or have a large volume of porosity. The pellet pressing and sintering methods were reproduced and then modified; densification measurements were performed to track the pellet status prior to, during, and after sintering using geometric measurements, immersion mass-based measurements, and LVDT dilatometery. The pellet samples were prepared using ball milled UO2 powder that was compacted with high pressure (680MPa). The compacted pellets were crushed then self-milled to create green spherical granules. These granules were mixed with jet-milled BeO powder, and the mixture was pressed and sintered. The pellet processing parameters were modified from literature values to achieve pellet with less than 8% porosity and minimal cracking. The optimal parameters determined for this study include: 1) pre-compaction pressure of 680 MPa, 2) pellet final compaction pressure of 200 MPa, 3) sintering temperature of 1600°C, 4) sintering time from 4 to 6hr, and 5) sintering atmosphere of flowing Ar-5%H2. The thermal conductivity was measured by Light Flash Analysis (LFA) at temperatures from 25 to 250°C. From this study it was found that the thermal conductivity of the baseline UO2 was improved approximately 10% for each 1 volume percent BeO over the measured temperature range.
Light Water Reactor
Garcia, Chad (2014). Sintering and Thermal Behavior of Uranium Dioxide in Beryllium Oxide Matrix. Master's thesis, Texas A & M University. Available electronically from