1. 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
13 September 2012
© BHR Group 2012

2. 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

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

4. 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

5. 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

6. 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.

7. 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

8. 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.

9. 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.

10. 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.

11. 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

12. Sludge Rheology

13. 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.

14. 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.

15. 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

16. 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.

17. 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
Five-fold variation in YG attributed to different values of yield stress obtained from different measurement methods.

18. Measurement Methods for obtaining Yield Stress

19. 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.

20. 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

21. 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.

22. 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

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

24. On-line Viscometers
Tube
Moving Cylinder

25. 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

26. 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)

27. 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

28. 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

29. BHR Vane Rheometer Prototype

30. BHR Vane Rheometer Prototype

31. 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

32. 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

33. 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

34. 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

35. Thank you Neil Alderman, Nigel Heywood.
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