Nano-sized precipitated formations in irradiated NaCl

The interest in the formation of radiation damage in alkali halides and in particular, in NaCl, is stimulated by the fact that rock-salt in stable geological formations is a prominent candidate medium for storage of high-level waste (HLW) of nuclear power plants. Since the 1950s, scientists and engineers have promoted the idea that rock salt is one of the most promising geological media to host radioactive waste repositories. Laid down by evaporating seas over a long time period, natural salt is rock-solid and essentially impermeable. Energy is stored in crystalline rock salt by introducing radiolytic products, among which colloidal sodium and molecular chlorine, as a result of the interaction between the ionizing radiation emitted by HLW, and the rock salt crystal. Most recent experiments on heavily irradiated NaCl samples have shown that in natural rock salt and synthetic pure and doped NaCl samples the concentrations of precipitated sodium and chlorine have been found to increase with increasing dose without any sign of saturation. The amounts of radiolytic products may reach very high values. In NaCl samples irradiated to an extremely high dose of 2×103 GRad (2×1010 Gy or 8×1019 e–/cm2) the concentration of colloidal sodium in the NaCl sample was found to be as high as 20 at%, while it was still increasing without any sign of saturation as a function of the dose. Accumulation of very large amounts of the radiolytic products in the crystalline material leads to a significant decrease of the chemical and mechanical stability of the rock salt and ultimately to explosive decomposition of NaCl crystal as a result of spontaneous exothermic recombination of radiolytic Na and Cl2. Eventual instability of irradiated rock salt is certainly not acceptable for the above mentioned applications. Good understanding of the radiolysis process in alkali halides and its consequences for the materials, which are exposed to ionizing radiation for a very long time period, is crucial for important industrial and environmental applications related with the safe storage of high-level nuclear waste in salt mines. The main purpose of this thesis is to provide an insight into the radiolysis process in crystalline NaCl in very advanced stages, where extremely high concentrations of radiolytic products have been accumulated. A detailed analysis of the structure and properties of the radiolytic sodium and chlorine precipitates systems and the host crystalline matrix at very high irradiation doses is required to explain the behavior of extremely heavily irradiated NaCl crystals. Comprehensive experimental observations of the natural rock salt and synthetic pure and doped NaCl samples, which have been irradiated to extremely high doses by means of electron beams, have been carried out in combination with the application of newly developed theoretical models describing the radiolysis process and the development of the main extended defects in NaCl. The high capacity of NaCl crystals to accumulate very large amounts of precipitated sodium has been explained by the formation of highly organized structures of radiolytic particles in heavily irradiated rock salt crystals.