Electron-microscopic investigation of boron carbide irradiated in a fast reactor

Abstract

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 ...

Helium behaviour in implanted boron carbide

“…The results we obtained here were coherent with previous ones [10][11][12][13][14][15]. However, implanting helium in a thin slab led to different behaviours as compared to those observed in homogeneously implanted or neutron irradiated materials.…”

“…Helium accumulates in flat, high pressure and parallel bubbles (mainly parallel to the (111) planes [10][11][12][13] and also to the (100) and (110) planes [12][13][14][15] of the rhombohedral structure). In fast neutron reactors, the combination of heat release and helium production induces strong radial thermal gradients and extensive cracking of the absorber pellets [4,16,17] as shown in Figure 4.…”

Impact scenarios in boron carbide: A computational study

“…When boron carbide itself is irradiated with thermal neutrons, lithium and helium ions are produced by neutron capture events of 10 B, which cause bubbles to form in the structure. This does not affect nano-crystallinity 15,16 , although micropores are observed. Amorphisation is possible using H + ion radiation, which alters the structure chemically 17 , but 1MeV electron impacts again do not affect the gross crystal structure 18 ; 2MeV impacts are required to induce amorphisation through impact.…”

Experimental determination of intragranular helium diffusion rates in boron carbide (B4C)

“…Unfortunately, this also means that the diffusion coefficients obtained from 1550°C are greatly influenced by B 4 C vaporization (and by bubble migration) resulting in data acquired from 1550°C being no longer exploitable (except for the determination of the total amount of implanted He). _ Tarasikov 44 investigated by TEM the evolution of He bubbles in B 4 C after tens of hours of annealings. It was observed that after annealing 60 h at 1400°C, the intragranular bubbles shapes evolved but not their volume suggesting no bubble merging and consequently no bubble diffusion in the material.…”