[en] In 1999 WANO issued Significant Operating Event Report 99-01 recommending that the industry verify whether or not currently approved procedures were adequate for a loss or degradation of the electrical grid. A detailed review identified that the procedures available at the time were not adequate to respond effectively to such a disturbance. Operations personnel reviewed operating expectations and the expected plant response to a Loss of Grid and updated the Operating Manual to cover 'Full Load Rejection'. Operations used the simulators to validate the procedure and train crews and Licensed staff. The revised documentation was approved and implemented. (author)

[en] A particle-laden turbulent flow in a square duct is predicted using large eddy simulation (LES). The simulation is performed for a Reynolds number of 35,500, and correctly predicts the existence of secondary flows and their effects on the mean flow. The results are also in good qualitative agreement with experimental data obtained at different Reynolds numbers. One-way coupling is assumed between solid particles and the fluid, and a particle equation of motion, including Stokes drag, lift, buoyancy and gravity force terms, solved using a Lagrangian particle tracking technique. Three sizes of particle (1, 50 and 100 μm) are considered, and results demonstrate that size has a significant effect on particle dispersion and deposition in the duct flow. As particle size increases, therefore, they tend to settle on the floor of the duct, with less dispersion in the fluid phase. The study demonstrates the usefulness of LES for nuclear waste processing applications since secondary flows occur in many practically-relevant flows, and since it is desirable that the two-phase waste mixture is kept as homogeneous as possible to prevent, or at least discourage, the settling out of solid particles to form a bed which can promote pipe blockages. (authors)

[en] The impingement of a fluid jet onto a surface has broad applications across many industries. Within the UK nuclear industry, during the final stages of fuel reprocessing, impinging fluid jets are utilised to mobilise settled sludge material within storage tanks in preparation for transfer and ultimate immobilisation through vitrification. Despite the extensive applications of impinging jets within the nuclear and other industries, the study of two-phase, particle -laden, impinging jets is limited, and generally restricted to computational modelling. Surprisingly, very little fundamental understanding of the turbulence structure within such fluid flows through experimental investigation is found within the literature. The physical modelling of impinging jet systems could successfully serve to aid computer model validation, determine operating requirements, evaluate plant throughput requirements, optimise process operations and support design. Within this work a method is considered, capable of exploring the effects of process and material variables on the flow phenomena of impinging jets. This is achieved on a number of experimental test rigs of varying scale employing both intrusive and non-intrusive measurement techniques Particle image velocimetry (PIV), ultrasonic Doppler velocity profiling (UDVP) and high speed imaging, through to visual observations and direct measurements, are all techniques that can be deployed. The influence of a number of parameters on the erosion characteristics of sediment beds following application of an axisymmetric impinging jet is presented in detail. Bed erosion is found to be enhanced as the jet height above the sediment bed is increased, due to greater turbulence development. Different erosion characteristics, as jet outlet velocity increased, were found for the particulates tested; sand, fine Mg(OH)₂ (test simulant representative of waste sludge, has similar particle size to sand, 200-1000 μm) and coarse Mg(OH)₂ (1000-2000 μm). The crater diameter increased with increasing velocity as expected. However, the effect of the increase in velocity on the crater depth was very different, particularly for the coarse material which was found to re-deposit in the crater when the velocity increased above 1.3 ms^-1, most likely due to enhanced re-circulation at the higher velocities. (authors)

[en] The impingement of a fluid jet onto a surface has broad applications across many industries. Within the UK nuclear industry, during the final stages of fuel reprocessing, impinging fluid jets are utilised to mobilise settled sludge material within storage tanks and ponds in preparation for transfer and ultimate immobilisation through vitrification. Despite the extensive applications of impinging jets within the nuclear and other industries, the study of two-phase, solid loaded, impinging jets is limited, and generally restricted to computational modelling. Surprisingly, very little fundamental understanding of the turbulence structure within such fluid flows through experimental investigation is found within the literature. The physical modelling of impinging jet systems could successfully serve to aid computer model validation, determine operating requirements, evaluate plant throughput requirements, optimise process operations and support design. Within this project a method is illustrated, capable of exploring the effects of process and material variables on flow phenomena of impinging jets. This is achieved via the use of non-intrusive measurement techniques Particle Image Velocimetry (PIV), Ultrasonic Doppler Velocity Profiler (UDVP) and high speed imaging. The turbulence structure for impinging jets, and their resultant radial wall jets, is presented at different jet-to-plate ratios, jet Reynolds numbers and jet outlet diameters. (authors)

[en] A large amount of nuclear waste is stored in tailings ponds as a solid-liquid slurry, and liquid flows containing suspensions of solid particles are encountered in the treatment and disposal of this waste. In processing this waste, it is important to understand the behaviour of particles within the flow in terms of their settling characteristics, their propensity to form solid beds, and the re-suspension characteristics of particles from a bed. A clearer understanding of such behaviour would allow the refinement of current approaches to waste management, potentially leading to reduced uncertainties in radiological impact assessments, smaller waste volumes and lower costs, accelerated clean-up, reduced worker doses, enhanced public confidence and diminished grounds for objection to waste disposal. Mathematical models are of significant value in nuclear waste processing since the extent of characterisation of wastes is in general low. Additionally, waste processing involves a diverse range of flows, within vessels, ponds and pipes. To investigate experimentally all waste form characteristics and potential flows of interest would be prohibitively expensive, whereas the use of mathematical models can help to focus experimental studies through the more efficient use of existing data, the identification of data requirements, and a reduction in the need for process optimisation in full-scale experimental trials. Validated models can also be used to predict waste transport behaviour to enable cost effective process design and continued operation, to provide input to process selection, and to allow the prediction of operational boundaries that account for the different types and compositions of particulate wastes. In this paper two mathematical modelling techniques, namely Reynolds-averaged Navier-Stokes (RANS) and large eddy simulation (LES), have been used to investigate particle-laden flows in a straight square duct and a duct with a bend. The flow solutions provided by these methods have been coupled to a three-dimensional Lagrangian particle tracking routine to predict particle trajectories. Simulation results are shown to be good agreement with experimental data, where available. Based on the LES and RANS-Lagrangian methods, the mean value of the particle displacement in a straight square duct is found to generally decrease with time due to gravity effects, with the rate of deposition increasing with particle size. Using the RANS-Lagrangian method to study flows in a duct bend, there is good agreement between predicted profiles and data, with the method able to simulate particle dispersion, the phenomenon of particle roping and the increase of particle collisions with the bend-wall with particle size. With the LES-Lagrangian method, particle re-suspension from a bed is studied in a straight square duct flow and this process shown to be dominated by secondary flows within the duct, with smaller particles tending to re-suspend in preference to larger ones. Overall, the study demonstrates that modelling techniques can be used to provide insight in to processes that are of relevance to the processing of nuclear waste, and are capable of predicting their transport behaviour. In particular, they are able to provide reliable predictions of particle deposition within flows to form solid beds, the re-suspension of particles from a bed, and the influence of complex flow geometries on particle dispersion. In the latter case, they are also of value to studies of erosion due to particle impact. Such models are therefore of value as engineering tools for use in the prediction of waste behaviour and in cost effective process design. (authors)

[en] Highlights: • A novel generalized modelling approach for multiphase flows has been developed. • Suitable closures are selected as a function of the local interface morphology. • The switch between different closure models depends on the local mesh resolution. • Good predictions obtained for multiphase flows with different interfacial scales. Multiphase flows are ubiquitous both in nature and industry. A broad range of interfacial scales, ranging from fine dispersions to large segregated interfaces, is often observed in such flows. Standard multiphase models rely on either the interface-averaging approach, which is suitable for the modelling of dispersed flows, or on the interface-resolving approach, which is ideal for large segregated interfaces. This results in the inability of such models to deal with complex multiscale flows, and different generalized hybrid modelling approaches having been proposed to overcome this shortcoming. This work presents a novel generalized multifluid modelling approach where large segregated interfaces are identified in the multifluid field from the local interface topology and resolution, avoiding the need for a-priori thresholds of the local volume fraction used in the majority of the models available in the literature. Interface compression and suitable modelling closures for drag and surface tension are activated in the large interfaces regions, whilst the model reverts to a standard multifluid formulation in the regions of small/dispersed interfaces. An assessment against different benchmark cases shows that the approach is as accurate as one-fluid interface-resolving techniques for large/segregated interfaces, while successfully recovering the expected multifluid behaviour for fully dispersed flows. Further, a prototypical multiscale flow has been simulated to demonstrate that the model can effectively switch between large-interface and dispersed-interface mode based on the local flow conditions and mesh size. It is concluded that the present approach represents a promising step towards the development of a comprehensive multiphase model capable of simulating complex multiscale flows of industrial interest.

[en] Particle deposition in fully developed turbulent square duct flows is predicted using large eddy simulation for Reynolds numbers, based on the bulk flow velocity and duct width, equal to 250k, 83k and 10,320. A particle equation of motion, solved in conjunction with a Lagrangian particle tracking technique, and including Stokes drag, lift, buoyancy and gravitational forces is used to analyse the trajectory of 50, 100 and 500 μm particles. Results obtained for the fluid phase show good agreement with experimental data and the predictions of direct numerical simulations. Predictions for particles show that high-inertia particles (Stokes number, St > 12.38) tend to deposit close to the corners of the duct floor, while low-inertia particles (St < 6.43) deposit near the floor centre. Particle deposition in the vertical direction is also found to increase with flow Reynolds number, whilst in the horizontal direction deposition increases with particle size and decreases with Reynolds number. Additionally, and in both the vertical and horizontal directions, the deposition profile, as quantified by the probability density function of deposited particle locations, is more variable for small particles when compared to larger ones, independent of the flow Reynolds number.

[en] Large eddy simulation (LES) of particle-laden turbulent flow is studied for a square duct with a 90° bend and a radius of curvature of 1.5 times the duct width, and for a Reynolds number based on the bulk flow velocity of 100,000. A Lagrangian particle tracking technique is used to study the motion of particles experiencing drag, shear lift, buoyancy and gravitational forces in the flow. LES predictions capture important physical aspects of these flows known to occur in practice, unlike alternative Reynolds-averaged Navier-Stokes (RANS) approaches, such as flow separation in the boundary layers around the bend entrance on the concave wall of the bend, and around the bend exit on the convex wall. The LES predicted flow and particle statistics are generally in good agreement with both experimental data used for validation purposes and RANS solutions, with r.m.s. fluctuating velocity predictions from the LES in particular being superior to values derived using the RANS technique.

[en] Reynolds-averaged Navier-Stokes modelling of particle-laden turbulent flows is studied for circular pipes with simulated bed heights of 0, 0.25 and 0.5 of the pipe diameter. A Lagrangian particle tracking technique is used to predict the deposition of spherical particles with sizes ranging from 5-500 μm. Secondary flows are observed to be present in the circular pipe flows with bed heights of 0.25 and 0.5. For the larger particles the presence of a stationary flat bed is found not to influence the deposition rate. For particles <50 μm an increasing bed height, in general, is seen to lead to a decrease in the particle mean displacement from the pipe walls with time. For the smallest particles, the secondary flows in the pipe with a bed height of 0.5 are found to contribute to some re-circulation of the particles, with the mean displacement of particles from the pipe walls found to decrease with time before a further increase is observed.

[en] A great deal of existing nuclear waste is stored as a solid-liquid slurry, and the effective transportation of such systems is an essential element in the successful implementation of almost all waste treatment strategies involving particulate wastes within the nuclear industry. A detailed knowledge of turbulent, particle-laden liquid flow behaviour is therefore obviously important. However, systematic and detailed studies of solid-liquid flows by experimental investigation are still limited for pipe flows, contrary to the significant amount of work available for channel flows. Research is therefore required to understand the effects of physical parameters, such as particle shape, size and size distribution, and solids concentration, on the properties of solid-liquid systems, particularly in horizontal pipe flows where particles may settle out of the flow and form solid beds which can potentially lead to pipe blockages. The presence of particles in a turbulent pipe flow also modifies the characteristics of the flow, thereby changing its ability to maintain particles in suspension The work described concerns pipe flows over a Reynolds number range of 1,000-10,000, with varying levels of solids concentration within the flow. Measurements of the flow and particle characteristics have been gathered using particle image velocimetry (PIV) and, for high solids concentrations, ultrasound Doppler velocity profiling (UDVP) techniques. This work has demonstrated that the intensity of turbulence within such flows can be significantly affected by the presence of solid particles, with small particles generally attenuating turbulence levels, while large particles often augment turbulence levels from the pipe centre-line to the near-wall region. In addition, the coagulation of particles into larger agglomerates is also of importance, with data demonstrating that whilst turbulence levels are influenced and augmented by such agglomerates at low Reynolds numbers, high turbulence levels at high Reynolds numbers can destroy the agglomerates and reduce their effect on the carrier fluid. Work has also been undertaken to examine the effect of particle size and Reynolds number on particle deposition within the flows, and also to establish the minimum transport velocity required to re-suspend particles from solid beds. All these findings are of importance in enhancing our understanding of flows of particles in pipes which in turn is of value in enabling the design of cost effective and efficient waste treatment processes. (authors)