The importance of rheological assessment in the mobilisation, mixing and transport of nuclear waste sludges Dr Neil Alderman Senior Consultant, Applied Rheology and Slurry Handling Email: nalderman@bhrgroup.co.uk 13 September 2012 © BHR Group 2012

Introduction Ultimate disposal of nuclear waste sludge - still a highly emotive subject - often called an unsolved problem - the Achilles’ heel of nuclear power Resolution of this problem actively pursued worldwide Most viable approach - Long term storage of waste after immobilisation (cementation/vitrification) in deep repositories located in stable geological formations Most waste sludges currently stored in tanks, ponds or silos located on same site where nuclear reactors situated

Hanford Site Contains 177 single and double-walled underground storage tanks with 53 million US gallons of high level radioactive waste

Sellafield Site B30 – old reactor parts and fuel rods Holds leftovers from first Magnox plants in ageing ponds B30 – old reactor parts and fuel rods B38 – radioactive cladding from fuel rods Sludge bed formed from metal corrosion products over few decades

Dounreay Site - failed/redundant equipment - fuel element cans Used a 65m, 4.6m diameter vertical shaft and a 720 m3 underground silo for storage of ILW Waste consisted of - failed/redundant equipment - fuel element cans - fuel element breakdown debris - plastic - glass - paper and filters

Mobilisation, Mixing and Transport of Nuclear Waste Sludges Key component for removal of radioactive waste from - tanks on the Hanford site - ponds on the Sellafield site - shaft/silo on the Dounreay site is the mobilisation of consolidated sludge bed Long storage times of sludges in tanks/pond/silos caused sludge to become consolidated leading to formation of high yield stresses. Mobilisation of sludge bed requires agitation to break up structure of consolidated bed before it can be transported in a pipeline.

Typical pneumatically-operated pulse jet mixing system Because of lack of moving mechanical parts, pulsed jet mixers (PJMs) currently being put forward as best means of agitating consolidated sludge bed in tanks or ponds

Typical pneumatically-operated pulse jet mixing system Vacuum first applied to break up sludge in vicinity of PJM nozzle and move this sludge into PJM until a certain sludge height is reached. Compressed air then supplied to force sludge inside PJM through PJM nozzle. This process is repeated continuously, causing either the structure of consolidated sludge to break down or the dilution of consolidated sludge by incorporation of supernatant water leading to a significant lowering of yield stress.  

Typical pneumatically-operated pulse jet mixing system To avoid settling-out of heavy uranium/plutonium particles during agitation stage and thus any possible criticality, attention must be paid to rheological requirements necessary for particle support. Once sludge is mobilised, it can be pumped through a pipeline to treatment plant. Sizing and selection of pump dependent on pressure drop and volumetric flowrate calculations performed on sludge flowing in pipeline. These calculations require appropriate rheological model parameters for sludge flowing in pipeline.

Sludge Rheology Understanding sludge rheology is key to successful design for mobilisation and transfer of waste sludge from tanks, ponds or silos to treatment plant. But Because of their highly radioactive nature, rheological characterisation of waste sludge is extremely difficult.

Sludge Rheology Limited datasets of actual waste sludge in ponds obtained show thixotropic and shear-thinning, non-Newtonian rheological properties. flow curves akin to those for a thixotropic fluid consisting of a distribution of different types of structure. existence of separate dynamic and static yield stresses in the same material

Sludge Rheology

Sludge Rheology This behaviour explained by assuming there can be more than one type of structure in a thixotropic fluid. Some would be more sensitive and are broken down by the least shear whereas others are more robust and can survive moderate to high shear rates. The robust structure varies with shear rate and determines equilibrium flow curve, extrapolation of which gives dynamic yield stress. At rest, the weak structure will build up over a certain period of time. The combined structure gives static yield stress.

Sludge Rheology Because of scarcity of actual data, recourse often made with either making flow curve measurements of simulant sludges having a similar chemical composition to that of actual waste sludges or to assume published rheological data for a given slurry type is applicable for actual waste sludge.

Rheology and Pipeline Flow Relevant rheological property : Equilibrium flow curve with yield stress being the dynamic yield stress. NB Model fit made over relevant shear rate window

Rheology and Particle Support within an Unsheared Sludge Bed Relevant rheological property : Static yield stress. With this yield stress, sludge bed acting as a viscoplastic medium in unsheared condition has the capacity to support weight of embedded uranium/plutonium particles for an indefinite (or sufficiently long) period of time. At point of motion/no motion of sphere in a viscoplastic medium, two relevant forces are due to yield stress and buoyant weight of sphere.  

Rheology and Particle Support within an Unsheared Sludge Bed A measure of relative magnitudes of these two forces defined by dimensionless group Published data indicated that YG falls into one of two groups, taking values of between 0.04 and 0.08 a value of 0.212. Five-fold variation in YG attributed to different values of yield stress obtained from different measurement methods.  

Measurement Methods for obtaining Yield Stress

Rheology and Particle Support within a Sheared Sludge Bed Particle settling in a sheared flow field is referred to as dynamic settling. Not well understood and cannot be readily predicted. First approximation for estimating particle support in a sheared sludge bed is use dynamic yield stress, not static yield stress, in calculation of YG. This will give minimum condition that will ensure uranium/plutonium particles will remain in suspension once sludge bed is sheared by PJM.  

Sludge Rheological Measurement Correct usage of rheology data important in engineering design for transfer of sludges from tanks to treatment plant Rheological characterisation of sludges using conventional rotational viscometry Difficult Very expensive  

Sludge Rheological Measurement Barring problems of handling radioactive nature of sludges, issues that make viscometer selection difficult Sludge - a range of non-Newtonian behaviour due to wide variation in solids concentration within sludge bed and across tank/silo/pond. Height of free space above sludge bed significant. Viscometer robustness. Wall slip may occur when sludge at higher concentration is sheared. Sample representiveness.  

Laboratory Viscometers Cone-and-plate Rotating disc Coaxial cylinder Untruncated cone Truncated cone Parallel plate Bob in cup “Infinite sea” All rely on samples being taken from sludge bed for remote flow curve measurement

On-line Viscometers Rotational Coaxial Cylinder Rotating Bob Nutating Cylinder Rotating Disc

On-line Viscometers Tube Moving Cylinder

On-line Viscometers None of these on-line viscometers are suitable Drag on Blade Squeeze Flow Vibrational Moving Blade None of these on-line viscometers are suitable

Vane Rheometry Assume consolidated sludge bed exhibit plastic behaviour since Bingham plastic viscosities are very small.   Actual Assumed Shear stress (Pa) Shear rate (s-1)

Vane Rheometry Preferable to conventional rotational viscometry: it is an in-situ measurement it is a direct measurement of static yield stress of the unsheared sludge bed, not an extrapolated value of dynamic yield stress from flow curve measured using coaxial cylinder viscometry it can measure yield stress at much higher solid contents of sludge well beyond the limit for a coaxial cylinder viscometer  

Vane Rheometry Yield stress measurements previously made to obtain in-situ measurements of sludge in tanks using a field vane tester But Serious bowing and buckling problems encountered. Significant portion of rotating shaft in frictional contact with sludge. Prototype developed at BHR Group Ltd overcame these problems

BHR Vane Rheometer Prototype

BHR Vane Rheometer Prototype

Use of Simulated Sludges Rheological measurement of simulated sludges probably only viable means for obtaining data for engineering design. Not an easy task. Prepare sludge with similar formulation in terms of Chemical consitutents Particle size distribution Probably satisfactory for sludge pipeflow but not for consolidated sludge in tanks/silos/ponds

Use of Published Rheological Data A desk study could be carried out to obtain rheological information from the published literature as a function of Solids concentration Particle size Other physical /chemical composition Through comparison on chemical constituent make-up, an indication of rheological properties can be inferred

Summary of Rheological Options Actual Sludge Flow curve measurement using laboratory viscometers Coaxial cylinder viscometer only viable option but must ensure shear rate window for measurement is relevant for application In-situ flow curve measurement using on-line viscometry None of seven generic types suitable In-situ yield stress measurement using vane rheometry Probably best option but it relies on assumption of sludge exhibiting plastic behaviour being valid 33

Summary of Rheological Options Simulated Sludge Same conclusions for actual sludge apply here Probably satisfactory for sludge pipeflow but not for consolidated sludge bed Desk Study using published data Least reliable 34

Thank you Neil Alderman, Nigel Heywood. email: nalderman@bhrgroup.co.uk nheywood@bhrgroup.co.uk Enquiries: contactus@bhrgroup.co.uk Direct dial: +44 (0)7785 621660 Water, Environment & Power (WEP) Fluid Systems Academy Process