High-level nuclear waste glasses must maintain their mechanical and chemical stability over very long time scales. Self-irradiation damage in these glasses induces bond-breakage and atomic displacements by two distinct mechanisms: radiolysis (principally from energetic beta-decay electrons) and ballistic mechanisms involving collision cascades initiated by energetic fission nuclei and recoil of alpha-emitting actinide nuclei. This study investigates collision-cascade-induced alteration of the glass network in a simplified sodium borosilicate model nuclear waste glass, using molecular dynamics (MD) simulation and efficient topological assessment algorithms. Network topologies of the initial and resulting altered glass structures were determined by enumerating the primitive-ring-based local cluster atom complements at each atom site. The topological description is seen to provide a revealing assessment of network structural changes in the simulated radiation environment.Highlights► Radiation damage in sodium borosilicate glasses modeled with MD collision cascades. ► Glass network models erected using 2- and 3-body Born–Mayer–Huggins potentials. ► Topological connectivity changes assessed using efficient ring-finding algorithms. ► Ring-size changes correlated with changes in global density and boron co-ordination. ► Radiation-induced larger primitive rings associated with sodium segregation.

We analyze the structure of a-GeO2, pressure-densified from a starting density of 3660 kg/m3 to a final density of 6000 kg/m3, using a combination of molecular dynamics simulation and topological analysis employing efficient local cluster ring counting algorithms. The initial modeled configuration is dominated by fourfold germanium co-ordination and rings of six or seven germanium atoms (as in cristobalite- or tridymite-like a-SiO2). The first response to increasing density is a change to larger ring (e.g. 8-ring) configurations (as in quartz-like a-SiO2) that pack more compactly. At still higher pressure, an intermediate fivefold germanium co-ordination appears that progressively converts into almost entirely sixfold (octahedral) germanium at higher pressures, accompanied by growth of the 3- and 4-rings that characterize the rutile-GeO2 crystalline polymorph. The topology of the final densified structure does not entirely resemble a rutile-like glass analogue, however, because of the retention of 6-rings and almost 10% 5-co-ordinated Ge. Overall, the present study shows how pressure (or density) affects the structure of the glass at a range of length scales.

The deposition of biological apatite and subsequent formation of bone on hydroxyapatite implants depends on the partial dissolution of the implant surface and the reprecipitation of carbonated apatite from the biological milieu. Previous investigations in vitro have shown that the degree of dissolution and reprecipitation decreases as the coating crystallinity increases. These findings prompted the current study of the effects of coating crystallinity on the mechanism of bone bonding. The process of mineralization of bone associated with a hydroxyapatite coating was compared to the normal process of ossification.Plasma-sprayed hydroxyapatite (PSHA) coated titanium alloy (6% Al–4% V) rods as received and annealed for 0.7 h at 600°C in air to increase the coating crystallinity were implanted in the proximal and distal femora and proximal tibiae of adult mongrel dogs for 3 h, 3 and 10 days. Bony sites containing the implant were prepared for ultramicrotomy and transmission electron microscopy using an anhydrous embedding procedure: fixation in ethylene glycol and embedment in Spurr's resin.The results demonstrated the precipitation of biological apatite crystallites on non-annealed PSHA coatings in vivo within 3 h of implantation. After 3 and 10 days there were differences in the ultrastructure of the mineral phase on the surfaces of non-annealed and annealed surfaces. Observations showed that there was little difference in the mechanism of mineralization of bone associated with HA-coated prostheses and the normal process of ossification.

Silica irradiation cascade structures, simulated using topological modeling approaches, have been refined using molecular dynamics simulation techniques. Major structural reconstruction was observed when silica is equilibrated in this way at and above a glass transition temperature. Below the glass transition, irradiated cascades were found to have largely retained their original topological structures, and in this way several fully connected metastable silicas with substantially different medium-range structures were obtained. Analysis of these structures further revealed that their total correlation functions were remarkably insensitive to changes in their medium-range ring complement. Information in the first sharp diffraction peak (FSDP) is shown instead to provide some insight into the topologies of irradiated silicas.