Boron carbide, in nuclear reactor neutron absorber applications, generates large quantities of helium, some of which is trapped in small bubbles within the grains. Transmission electron microscopy was used to examine the size distribution, shape, and density of these bubbles in boron carbide irradiated, at 540" to 1850"C, to burnups of 62 X 1W0 captures/cm3. Bubble density was reduced but bubble diameter was increased at high temperatures. Bubbles were flat disks at low temperatures and more equiaxed at higher temperatures.

An instrumented absorber experiment i n the Experimental Breeder Reactor-II yielded data on the helium release of boron carbide a t exposures up t o 53x1020 neutron captures/cm3 and temperatures from 766 t o 952OC. Helium release r e s u l t s demonstrated a high rate of heliwn release i n i t i a l l y , and a much lower rate i n a secondary region. In addition, the experiment demonstrated the effectiveness of vents i n releasing gas pom a plenwn during irradiation.

Boron carbide pellets, used as the neutron absorber material in fast breeder reactors, were observed to crack in two distinctly different ways. Radial thermal gradients are produced as a result of internal heat generation. In larger-diameter pellets, these thermal expansion gradients are large and it was observed that pellet cracking occurred early in operation. It was also observed that swelling gradients were established at high burnup levels. These pellets were also cracked but in a different manner.

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