Densification of mixed oxide fuel in Liquid Metal Fast Breeder Reactor (LMFBR) during irradiation is demonstrated to be predictable through knowledge of original pore size distribution which is controllable in fabrication. Quantitative measurements of pore size distributions before and after irradiation cover the range of fuel structure and irradiation conditions typical of the Fast Flux Test Facility (FFTF). Pore diameter change is accurately described by:

ΔD =−(4.37 ± 0.49) × 10−5 I
+ (9.45 ± 0.62) × 10-13 l2 T
in the range 1000°K < T < 1450°K and to 30 000 MWd/MTM, the current extent of test bumup, I. Fuel column length change due to in-reactor sintering will be <1.3% in FFTF.