Microsoft Excel was used to automate the calculation of the two-phase geometry。 The geometry calculations are performed as follows。 The sum of a square area and a thin surrounding perimeter add up to the given volume percentage of the two phase material。 The fuel grain size measurements correspond to the edge length of the square。 Because the UO2/BeO microstructure consists of BeO segregated at the grain boundary of the UO2, the perimeter must have a corresponding thickness based on the grain size to meet the proper volume percentage ratio。 Taking this into consideration, cells were set up to iterate to a grid geometry that has the correct volume percent ratio based on the edge length of the square and the thickness of the outside perimeter。 This then converged so that the total area of a single square in the grid meets this criterion。 The resulting geometry dimensions were introduced into ANSYS to build the two-phase microstructure。

The finite element package ANSYS is capable of model- ing micro-size geometries and has been employed for the stress analysis of the MEMS and micro-shear-stress sensors [14, 15], where the sizes as small as 1 micron are easily accommodated。 The grain sizes used in the present work are in this range of 10–100 microns。

The UO2 was assigned a thermal conductivity curve based on its true density from an empirical equation [6]。 The triangular element, PLANE35, was used to mesh the grid pattern geometry。 The element edge length was set to 36 microns for each case。 The finite element models had approximately the same overall size of 5000 microns long by 2200 microns wide。 A thermal flux

Table 2 Comparison data between ANSYS thermal model and measured data points

FEM Measured

T(K) k (W/mK) T(K) k (W/mK)

BeO 0。6 wt%

278 11。5 278 12。0

478 8。3 479 9。4

688 6。2 688 6。8

902 4。8 903 5。4

1096 4。0 1097 4。6

1277 3。3 1278 3。8

1485 2。8 1486 3。3

1652 2。5 1653 3。0

1860 2。2 1861 2。8

BeO 1。2 wt%

278 14。3 278 13。6

478 10。0 479 10。3

687 7。1 688 7。6

901 5。4 903 5。6

1095 4。8 1097