Since boron carbide is extensively used in building reactors, it is of scientific and practical interest to investigate radiation defects in it.Radiation Defects. A characteristic feature of boron carbide is that neutron capture results in the formation of helium and lithium, which according to investigations form flat pores [1].The experimental samples of boron carbide are pellets fabricated by hot pressing and irradiated in BN-350 at 370°C to boron burnup 2.8% (26·10 26 captures/m 3 ). The increase of their volume after irradiation, as determined by the change in the diameter, was 1.7-2.6%. The microstructure of the sample was studied in a JEM-1000CX (Japan) electron microscope. The cleaving method was used to prepare the objects for electron microscopy [2].The electron microscope studies showed that irradiation resulted in the formation of 3-20 nm defects with contrast characteristic for dislocation loops or flat precipitates of a second phase and 1-4 nm in diameter spherical pores (Fig. 1a). Besides pores, defects with an irregular spherical shape, 5-15 nm in size, with contrast characteristic of particles of precipitates were found in the structure of the irradiation boron carbide (Fig. 1b) [3]. The relative volume of the defects was 0.3%.Higher magnification also revealed in the electron microscopic photographs, aside from the defects mentioned above, defects 0.3-1 nm in size (Fig. 2a), which, evidently, comprise a complex of helium or lithium atoms surrounded by carbon or boron atoms which have been displaced from their equilibrium positions. The concentration of these defects reaches 10 25 m -3 .A characteristic feature of the microstructure of the neutron-irradiated boron carbide was the formation of 30 nm wide zones, which are free of pores and other radiation defects, near grain boundaries (Fig. 2b). Helium pores which are larger than the pores in the grain bodies and have an average size of 3 nm are found on the boundary itself.The results attest that the irradiation of boron carbide at 370°C results in the formation of several types of defects, which exhibit different contrast under different photographing conditions. In contrast to [4][5][6], where predominately flat pores were observed, irradiation at such a temperature resulted in the formation of spherical pores, which is due to the influence of the irradiation temperature.Effect of High-Temperature Annealing on Swelling and Helium Release. The investigations of the helium content, swelling, and microstructure of boron carbide were performed after irradiation and annealing at 1000-1600°C [7]. The linear dimensions of 3 × 3 × 14 mm samples were measured to within 0.15%. The procedure used to determine the helium content is described in [8].The investigations showed that after irradiation to boron burnup 1.4% and 2.6% the volume of the boron carbide samples increased by 1.5 and 3.3%, respectively. Figures 3 and 4 show that annealing of the irradiated boron carbide resulted in a substantial release of helium and increase of the volume of the sample ...

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