$SiB_4$ is a possible burnable poison that is added in various amounts to slightly enriched $UO_2$ to develop higher burnup or extend cycle CANDU fuel.
Effects of additions to $UO_2$ of 645, 5000, 10,000, 50,000, 100,000 ppm burnable poison $SiB_4$ on the green density, densification, interdependence of density - grain growth and microstructure of sintered $UO_2$ were studied. Since loss of during sintering is due to oxidation of $SiB_4$ by oxygen coming from initially hyperstoichiometric $UO_2$ (O/U ratio = 2.10), $UO_2$ pellets were sintered in flowing 100% hydrogen, at temperature 1200, 1350, 1500, and 1680℃ for 3 hours and at 1680℃ for 0, 1, 3, and 10 hours, respectively.
Green densities obtained were in the range of about $4.5-5.4 g/cm^3$ and they decreased as the amount of $SiB_4$ addition increased when green pellets were fabricated with use of a double action press at $1 ton/cm^2$.
The density of sintered $UO_2$ pellets did not change significantly as the amount of $SiB_4$ addition increased to a limit of addition of 10,000 ppm and it was around 92 - 94 % pellets of the theoretical density. With an addition of $SiB_4$ more than 10,000 ppm, however, the density of sintered pellets decreased with the increasing addition of $SiB_4$.
Grain growth in $UO_2-SiB_4$ pellets was characterized by two stage growth : Grain growth occurred with the increasing density in the first stage whereas the second stage growth was characterized by grain growth without increasing of density. A liquid phase existing at grain boundaries and grain edges was observed in the microstructure of sintered $UO_2$ pellets with 5000 ppm $SiB_4$ and 10,000 ppm $SiB_4$. This liquid, that may be formed at about 1680℃, did not enhance the shrinkage of pellets but possible accelarated grain growth. It seems that the second stage grain growth was due to the presence of high pressure trapped insoluble gas in isolated pores.
