Driven by major advances in deep drilling technology and the geological understanding of the deep continental crust over the past 70 years, disposal in deep boreholes has moved from being technically unachievable to the point that it now offers a viable solution for the most hazardous nuclear wastes that could effectively be implemented “tomorrow”—i.e., within a few years. Moreover, disposal in deep boreholes is arguably superior in almost every respect to the mined and engineered repositories being pursued for high level waste by most countries. During the first 50 years of their evolution, almost all deep borehole disposal concepts shared five key aspects: (i) the hole was as deep as possible, (ii) it was vertical, (iii) it was fully cased, and (iv) it was in “hard” basement rock (v) saturated with aqueous fluid (groundwater). Technical advances in drilling over the last 20 years have encouraged proposed versions of the concept which depart from one or more of these aspects, but it is our contention that all five fundamental aspects should be retained. This paper summarises the more important arguments supporting this view. In order to meet the necessary post-closure (radiological) safety requirements, engineer out possible operational problems during construction and waste-package deployment, and capitalise on the main benefits of borehole disposal, the hole itself must be over 3 km deep, vertical, fully cased, and in suitably hard (ideally granitic) host rock saturated with aqueous fluid.

We report molecular simulations using a relatively soft interatomic potential to determine the nature of melting under extreme conditions. At temperatures and pressures above the triple point of normal materials, the density range of two-phase solid and fluid equilibria is bounded by freezing and melting curves. We address the unresolved issue of the termination of these boundaries, i.e., whether the melting curve of a solid terminates in a critical point, exhibits a maximum, goes to an asymptotic limit, or continues indefinitely. Significantly, we observe a negative change in volume upon melting at high pressures, which is normally observed only for water. We provide unequivocal evidence that the densities of the meeting and freezing lines can merge at a melting temperature maximum point. This could be a general feature of "soft" atomic fluids at extreme pressures.

We develop a sequential multi-scaling framework for studying the problem of fatigue crack propagation due to liquid droplet impingement. The scope is limited to a hypothetical material and a hypothetical liquid. The multi-scaling is achieved by handshaking the atomistic scale molecular dynamics (MD) simulations with the continuum scale smoothed particle hydrodynamics (SPH). The handshaking, in turn, is performed by evaluating the material, the fracture and the loading properties from MD simulations, and using them as inputs in the continuum scale SPH model. Due to the qualitative agreement of the pressure developed in the fluid and the substrate between the MD simulations and already published results, the liquid droplet impact in SPH is simulated through appropriate surface stresses. Further, we incorporate the pseudo-spring approach within the SPH model to develop a methodology for studying mixed-mode fatigue crack propagation. Our methodology provides good agreement with the existing literature for several cases. Lastly, we calculate the fatigue life of an edge-cracked specimen due to liquid droplet impingement.

The existing smoothed particle hydrodynamics (SPH) approaches for propagating fatigue cracks involve either the deletion of the crack front particle or stopping all its interactions in the total Lagrangian form. Here, we adopt the pseudo-spring-based Eulerian form of SPH to model mode-I fatigue crack propagation. For modeling fatigue crack growth, only the interactions between the crack front particle and its neighbors, which display the largest axial stresses in the connected pseudo-springs, are stopped. We show that our framework can determine accurately the mode-I stress intensity factors (SIFs) and capture both the fatigue crack path and the fatigue life of different specimens.

We study the problem of a projectile impacting on a target at different length scales employing two widely used techniques — molecular dynamics (MD) simulations at atomistic scales and smooth particle hydrodynamics (SPH) at continuum scales. At the atomistic scale, the impact problem is modeled through a short-ranged pair potential whereas, at the continuum scale, the partial differential equations related to the different conservation equations are solved using SPH. SPH is parametrized in a manner that the equation of state and the elastic constants match those of the underlying MD simulations. We evaluate a number of recent improvements to the SPH framework – artificial viscosity, XSPH, Jaumann stress rotation, tensile instability, and mass initialization – by making a systematic comparison between an SPH model with varying degrees of these corrections, and a model based on MD, which acts as the pseudo experiment. We show that if all corrections are incorporated, the SPH results agree well with those from MD simulations. This has been ascertained by comparing the deformation response, the radial distribution function, and the shear strain.

We consider the practicalities of defining, simulating, and characterizing “Liquids” from a pedagogical standpoint based on atomistic computer simulations. For simplicity and clarity we study two-dimensional systems throughout. In addition to the infinite-ranged Lennard-Jones 12/6 potential we consider two shorter-ranged families of pair potentials. At zero pressure one of them includes just nearest neighbors. The other longer-ranged family includes twelve additional neighbors. We find that these further neighbors can help stabilize the liquid phase. What about liquids? To implement Wikipedia’s definition of liquids as conforming to their container we begin by formulating and imposing smooth-container boundary conditions. To encourage conformation further we add a vertical gravitational field. Gravity helps stabilize the relatively vague liquid-gas interface. Gravity reduces the messiness associated with the curiously-named “spinodal” (tensile) portion of the phase diagram. Our simulations are mainly isothermal. We control the kinetic temperature with Nosé-Hoover thermostating, extracting or injecting heat so as to impose a mean kinetic temperature over time. Our simulations stabilizing density gradients and the temperature provide critical-point estimates fully consistent with previous efforts from free energy and Gibbs ensemble simulations. This agreement validates our approach.

We report molecular dynamics simulations of multiple radiation damage cascades in the pyrochlores Gd2Ti2O7 and Gd2Zr2O7 and the solid solution Gd-2(Zr (x) Ti1-x )(2)O-7 (x = 0.25, 0.50, 0.75). Using a simulation cell of 360,448 atoms, for each compound 2200 decay events are simulated over a total time of 10 ns, with each recoiling uranium atom (primary-knock-on atom) assigned initial kinetic energy of 5 keV. The structures generated are analysed using Steinhardt local order parameters. There is a large increase in volume for the Ti pyrochlore associated with a transition to an amorphous structure which resembles the melt while preserving the local environment of the Ti. The calculated dose for amorphisation is 20 eV atom(-1) which compares well with experiment overlap of cascade and damage accumulation drives the amorphisation suppressessing the healing mechanisms. The behaviour of the zirconate is different - the substantial anion disorder produced by each recoil event is followed by healing and reversion to the parent pyrochlore. In the solid solution the onset of amorphisation is delayed to later times on increasing the Zr concentration and overall swelling reduced. Our simulations highlight the importance of ion mobility, associated with the weaker Zr-O bonds, in healing.

This Special Issue of the Energies Journal on Deep Borehole Disposal of Nuclear Waste has delivered a timely update on the science and technology of borehole disposal and the types of radioactive wastes it could potentially accommodate. The Special Issue papers discuss (i) circumstances under which a national waste management programme might wish to consider deep borehole disposal; (ii) a status report of deep borehole disposal options in Germany; (iii) the analysis of corrosion performance of engineered barrier systems; (iv) a review of the potential cementing systems suitable for deep borehole disposal; (v) the thermal evolution around heat-generating waste for a wide range of material properties and disposal configurations; (vi) a geochemical analysis of deep brines focussed on fluid-rock interactions; (vii) post-closure performance assessment calculations for deep borehole disposal of Cs/Sr capsules and an example safety case for (viii) horizontal and (ix) vertical deep borehole disposal of nuclear wastes.

The Lennard-Jones (LJ) spline potential is a truncated LJ potential so that both the pair potential and the force continuously approach zero at rc≈1.74σ. We present a systematic map of the thermodynamic properties of the LJ spline model from molecular dynamics and Gibbs ensemble Monte Carlo simulations. Results are presented for gas/liquid, liquid/solid and gas/solid coexistence curves, the Joule-Thomson inversion curve, and several other thermodynamic properties. The critical point for the model is estimated to be Tc∗=0.885±0.002 and Pc∗=0.075±0.001, respectively. The triple point is estimated to Ttp∗=0.547±0.005 and Ptp∗=0.0016±0.0002. The coexistence densities, saturation pressure, and supercritical isotherms of the LJ spline model were fairly well represented by the Peng-Robison equation of state. We find that Barker-Henderson perturbation theory works less good for the LJ spline than for the LJ model. The first-order perturbation theory overestimates the critical temperature and pressure by about 10% and 90%, respectively. A second-order perturbation theory is not much better. Our assessment is that the mean compressibility approximation gives a poor representation of the second-order perturbation term. Our main conclusion is that we at the moment do not have a theory or model that adequately represents the thermodynamic properties of the LJ spline system.

Cementitious grouts are being developed for use as sealing and support matrices (SSMs) in deep borehole disposal (DBD) where temperatures do not exceed 190 °C. They will seal radioactive waste containers into the bottom 2 km of holes drilled up to 5 km deep into the crystalline basement. The temperature and pressure is likely to be similar to those in hydrocarbon and geothermal energy wells, where grout placement and durability are affected. This paper reviews the potential cementing systems suitable for this application and explains why a single solution of a formulation of Class G oil well cement, silica flour, water, fluid loss additive, and retarding admixture has been selected. This type of formulation has been used extensively for over 100 years in well cementing. It should provide the short-term performance and durability required for an SSM, maintaining the seal around the waste packages within the disposal zone long after the boreholes are sealed back to the surface, and thus augmenting the safety case for DBD.

The safety case for deep borehole disposal of nuclear wastes contains a safety strategy, an assessment basis, and a safety assessment. The safety strategy includes strategies for management, siting and design, and assessment. The assessment basis considers site selection, pre-closure, and post-closure, which includes waste and engineered barriers, the geosphere/natural barriers, and the biosphere and surface environment. The safety assessment entails a pre-closure safety analysis, a post-closure performance assessment, and confidence enhancement analyses. This paper outlines the assessment basis and safety assessment aspects of a deep borehole disposal safety case. The safety case presented here is specific to deep borehole disposal of Cs and Sr capsules, but is generally applicable to other waste forms, such as spent nuclear fuel. The safety assessments for pre-closure and post-closure are briefly summarized from other sources; key issues for confidence enhancement are described in greater detail. These confidence enhancement analyses require building the technical basis for geologically old, reducing, highly saline brines at the depth of waste emplacement, and using reactive-transport codes to predict their movement in post-closure. The development and emplacement of borehole seals above the waste emplacement zone is also important to confidence enhancement.

We demonstrate that the main features of DPD may be obtained using molecular dynamics employing a deterministic thermostat. This apparent isomorphism holds as long as the MD pair potentials are sufficiently smooth and short ranged, which gives rise to a quadratic equation of state (pressure as a function of density). This is advantageous because it avoids the need to use stochastic forces, enabling a wider choice of integration algorithms, involves fully time reversible motion equations and offers a simpler algorithm to achieve the same objective. The isomorphism is explored and shown to hold in 2 and 3 physical dimensions as well as for binary and ternary systems for two different choices of pair potential. The mapping between DPD and Hildebrand’s regular solution theory (a consequence of the quadratic equation of state) is extended to multicomponent mixtures. The procedure for parametrization of MD (identical to that of DPD) is outlined and illustrated for a equimolar binary mixture of SnI4 and isooctane (2,2,4-trimethylpentane).

We present a new method of characterizing damage arising from alpha-recoil cascades in amorphous materials including glasses. The approach taken is topological, yielding information on atom connectivity and utilizing complete sets of orthogonal functions (spherical harmonics and Hermite functions) to compute order parameters. The utility of our new approach is demonstrated by first applying it to models of radiation-damaged crystalline zircon) enabling validation against the standard defect counting method (Wigner-Seitz). We then apply it to a simple model of a glass, obtained by supercooling a Lennard-Jones liquid, for which defect counting methods are inapplicable. The method shows great promise for use in characterizing damage in more complicated glasses, particularly those of interest in the immobilization of nuclear waste, and when used in conjunction with nonequilibrium computer simulation could be a powerful tool to elucidate experimental data on the radiation tolerance of such wasteforms.

The performance of grouts made using oilwell cement is markedly different above 90 degrees C than at lower temperatures, and the rapidity with which grouts thicken can cause failures in well cementing. One grouting application in which such temperatures are encountered is deep borehole disposal (DBD). DBD is a concept for disposing of high-level radioactive wastes where the temperature and pressure will be 90-140 degrees C and 30-50 MPa, respectively. In developing DBD grouts, a number of issues have been identified that will be of interest to well-cementing organizations. (1) The type of retarder used to delay grout thickening above 90 degrees C is of extreme importance, and should be selected based on local temperature, pressure, and geochemical environment. Addition level might vary considerably depending on the retarder used. (2) Temperature and pressure will shorten the time for grouts to thicken, particularly the former. Water content will also affect grout properties such as consistency, viscosity, and flow. (3) The retarder may not influence hardened grout composition, which suggests that only the time at which the cement hydration reactions occur is influenced.

Cementitious grouts are being developed for use as sealing and support matrices (SSMs) in deep borehole disposal (DBD) of high-level radioactive waste. These SSMs will seal the waste containers within a disposal zone and provide physical support during package placement. The ambient temperature and pressure will be 80–130°C and 30–50 MPa, respectively, depending on depth and local geological conditions. This will accelerate grout thickening, so retardation is required to facilitate slurry placement. Certain inorganic materials suppress cement hydration and retard thickening under ambient conditions. However, their retardation characteristics are unknown at elevated temperature and pressure. Inorganic compounds of borate, phosphate, tin and zinc were investigated as retarders, and a summary of the work is provided in this paper. All retarders were found to delay thickening, with only borate providing sufficient retardation, but only at 90°C (0·75% addition); retardation was nearly sufficient at 120°C (1% addition). The effect of all the retarders on wet paste properties was not consistent but, generally, plastic viscosity and yield stress peaked at specific addition levels. None of the retarders influenced phase composition, and each retarding ion was evenly distributed throughout the hardened microstructure. The results of this study led to the conclusion that no inorganic retarder is appropriate for use across the whole DBD temperature range.

The release of radionuclides from spent fuel in a geological disposal facility is controlled by the surface mediated dissolution of UO2 in groundwater. In this study we investigate the influence of reactive surface sites on the dissolution of a synthesized CeO2 analogue for UO2 fuel. Dissolution was performed on the following: CeO2 annealed at high temperature, which eliminated intrinsic surface defects (point defects and dislocations); CeO2-x annealed in inert and reducing atmospheres to induce oxygen vacancy defects and on crushed CeO2 particles of different size fractions. BET surface area measurements were used as an indicator of reactive surface site concentration. Cerium stoichiometry, determined using X-ray Photoelectron Spectroscopy (XPS) and supported by X-ray Diffraction (XRD) analysis, was used to determine oxygen vacancy concentration. Upon dissolution in nitric acid medium at 90 °C, a quantifiable relationship was established between the concentration of high energy surface sites and CeO2 dissolution rate; the greater the proportion of intrinsic defects and oxygen vacancies, the higher the dissolution rate. Dissolution of oxygen vacancy-containing CeO2-x gave rise to rates that were an order of magnitude greater than for CeO2 with fewer oxygen vacancies. While enhanced solubility of Ce(3+) influenced the dissolution, it was shown that replacement of vacancy sites by oxygen significantly affected the dissolution mechanism due to changes in the lattice volume and strain upon dissolution and concurrent grain boundary decohesion. These results highlight the significant influence of defect sites and grain boundaries on the dissolution kinetics of UO2 fuel analogues and reduce uncertainty in the long term performance of spent fuel in geological disposal.

Deep borehole disposal (DBD) is being increasingly seen as a viable and potentially superior alternative to comparatively shallow mined repository concepts for disposal of some high-level radioactive wastes. We report here details of proof-of-concept investigations into the use of cementitious grouts as sealing/support matrices for use in low temperature DBD scenarios. Using the cementitious grout to fill annular space within the disposal zone will not only support waste packages during placement, but will also provide a low permeability layer around them which will ultimately enhance the safety case for DBD. Grouts based on Class G oil well cement are being developed. The use of retarders to delay the accelerated onset of thickening and setting (caused by the high temperature and pressure in the borehole) is being investigated experimentally. Sodium gluconate and a polycarboxylate additive each provide sufficient retardation over the range 90-140 degrees C in order to be considered for this application. Phosphonate and sulphonate additives provide desirable retardation at 90 degrees C. The additives did not affect grout composition at 14 days curing and the phases formed are durable at elevated temperature and pressure. (C) 2016 The Authors. Published by Elsevier Ltd.

A new method is outlined for constructing realistic models of the mesoporous amorphous silica adsorbent, MCM-41. The procedure uses the melt-quench molecular dynamics technique. Previous methods are either computationally expensive or overly simplified, missing key details necessary for agreement with experimental data. Our approach enables a whole family of models spanning a range of pore widths and wall thicknesses to be efficiently developed and yet sophisticated enough to allow functionalisation of the surface necessary for modelling systems such as self-assembled monolayers on mesoporous supports (SAMMS), used in nuclear effluent clean-up.The models were validated in two ways. The first method involved the construction of adsorption isotherms from grand canonical Monte Carlo simulations, which were in line with experimental data. The second method involved computing isosteric heats at zero coverage and Henry law coefficients for small adsorbate molecules. The values obtained for carbon dioxide gave good agreement with experimental values.We use the new method to explore the effect of increasing the preparation quench rate, pore diameter and wall thickness on low pressure adsorption. Our results show that tailoring a material to have a narrow pore diameter can enhance the physisorption of gas species to MCM-41 at low pressure. (C) 2016 The Authors. Published by Elsevier Inc.