Effect of fast neutron irradiation on the properties of boron carbide pellet

Boron carbide pellets were irradiated in the experimental fast reactor ''JOYO'' to B-10 burnup of up to 170 x 10(26) cap/m(3), fluences of 2x10(26)/m(2) (E>0.1 MeV), and maximum temperatures of about 1,200 degrees C. Post irradiation examinations were made of microstructural changes, helium release, swelling, and thermal conductivity. Boron carbide pellets irradiated to high burnups developed extensive cracking. Helium release from the pellets was initially low, but enhanced helium release was observed at high burnups and high temperatures. The swelling linearly increased with burnup, and when boron carbide was irradiated at high temperatures, the swelling rate began to decrease corresponding to the beginning of enhanced helium release. The correlation between swelling and the helium release was studied and the swelling was interpreted in terms of accumulation of helium in the boron carbide pellet. The thermal conductivity of the boron carbide pellets decreased rapidly by neutron irradiation accompanied with loss of temperature dependence.