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Abstract
The internal production of helium gas and point defects in neutron-irradiated boron carbide causes a significant swelling, the size of which is roughly linear with the irradiation dose and weakly temperature dependent when the irradiation temperature is lower than 1500°C. Extensive intergranular and intragranular microcracking also causes destruction of this neutron-absorbing ceramic. Lenticular bubbles filled with helium gas (penny-shaped cracks) are responsible for both problems.
Careful consideration of the equilibrium conditions of the bubbles and of their growth in the presence of point defects and gas atoms permits us to estimate the size of the swelling. Balancing the fraction of helium which accumulates in bubbles and the fraction which precipitates at the grain boundaries enables us to calculate the threshold dose for intergranular microcracking. The problem of gas release from the material is also examined qualitatively.
The very unusual behaviour of boron carbide under neutron irradiation is attributed to the absence of point-defect clusters and dislocations below 1500°C and to the fact that it is impossible for the bubbles to reach their equilibrium shape at these temperatures, even in the presence of point defects and diffusion.