1. Thermal Treatment of Magnox Sludge Intermediate Level Nuclear Waste through Vitrification
Sean T. Barlow BSc (Hons) AMInstP
Martin C. Stennett1, Russell J. Hand1, Sean P. Morgan2 & Neil C. Hyatt1
1. Department of Materials Science & Engineering, The University of Sheffield, Sheffield S1 3JD, UK
2. Sellafield Ltd., Hinton House, Risley, Warrington WA3 6GR, UK
Scientific Basis for Nuclear Waste Management Symposium 2017, October 29 – November 3 | Sydney, Australia
Good morning everyone, as you’ve just heard name is Sean Barlow and I’m a PhD student at the University of Sheffield’s Immobilisation Science Laboratory and my presentation this morning is on the work I have done on formulating glass compositions for the vitrification of UK Magnox Sludge.

2. Worlds first civil nuclear power station
1956 – Calderhall
Worlds first civil nuclear power station
1950’s Queen visited Sellafield to open the Calderhall reactors – worlds first. Americans had proven it first, Russians first to connect to a grid (not civil use).
Pic at bottom shows Queen inspecting the top of the reactor core, machine in middle used to de-fuel and re-fuel the reactor.
Overall pic of the site under operation. 4 reactors in separate buildings and each with a dedicated cooling tower (away from the coast).
Design went through several variations over the decades resulting finally in the last to be built at Wylfa in North Wales.

3. Wylfa 2015 Research impetus
Various designs of Magnox plant, 11 station and 26 reactors in UK + exports
Magnox fleet of reactors used unenriched uranium metal clad in a Magnesium non-oxidising (Magnox) alloy (99% Mg, 1% Al)
Magnox fuel elements
Magnox design of power stations went through various different iterations during the 60 years of operation. Pictured on the top is Wylfa which was last Magnox reactor to shut down at the very end of No cooling towers needed as it’s by the sea, both reactors in one building.
What is the problem… Almost 60 years of atomic power in the UK has created large quantities of nuclear waste a significant quantity of which is from the Magnox fleet of reactors.
These reactors used natural uranium metal fuel clad in a magnesium alloyed called Magnox which is 99% Mg and 1% Al.

4. Background First Generation Magnox Storage Pond (FGMSP)
Spent fuel from all 26 Magnox reactors in the UK was sent to be reprocessed at Sellafield but was first stored underwater in huge ponds such as the FGMSP pictured
The material was stored underwater to reduce the radiological hazard however due to industrial action in the 1970s and 1980s, a build-up of nuclear fuel for reprocessing occurred which meant that the material was stored underwater for longer than anticipated.
Sites are aging and it is a priority to empty the silos asap. Seagull in pond potentially spreading contamination

5. Background Magnox Swarf Storage Silos (MSSS)
Spent fuel from all 26 Magnox reactors in the UK was sent to be reprocessed at Sellafield but was first stored underwater in huge ponds such as the FGMSP pictured on the top.
After reprocessing, the sheared Magnox cladding, called swarf, was also stored underwater in the MSSS.
The material was stored underwater to reduce the radiological hazard however due to industrial action in the 1970s and 1980s, a build-up of nuclear fuel for reprocessing occurred which meant that the material was stored underwater for longer than anticipated.

6. Over 3148 m3 of consolidated sludge to treat at Sellafield
Magnox corrosion
Magnox sludge retrieval at the Sellafield site
Over 3148 m3 of consolidated sludge to treat at Sellafield
Current plan
Baseline treatment plan is encapsulation in cement matrix
3m3 box used for cement encapsulation
Cross-section of cement ILW
So when you add metal to water corrosion occurs, and in the case of Magnox material the corrosion product is magnesium hydroxide which is a sludge like material lining the bottom of the ponds.
Metallic uranium has also partially corroded meaning both U metal and U oxides are incorporated into this sludge making it one of the most contaminated sites in Western Europe.
The baseline plan for dealing with this waste is encapsulation in a cement matrix, but this has 2 key drawbacks.
Firstly, there is difficulty in predicting the long-term durability of the cemented waste and secondly there is a significant increase in volume of waste to deal with.
So an alternative is needed.

7. Current plan Cementation has two key drawbacks:
Difficulty predicting long-term durability
Significant increase in waste volume
Alternative wasteform needed to improve safety case & save on cost of geological disposal

8. Vitrification Proven technology for immobilisation of high level waste
2-D Schematic network of an alumino-borosilcate glass
Natural analogue, Obsidian
Proven technology for immobilisation of high level waste
Radiation resistant and potential for high waste loadings
Can incorporate wide spectrum of elements
Long-lived natural analogues (obsidian)
In this project we propose vitrification since it is already a proven technology for the immobilisation of HLW.
Glass wasteforms can also incorporate most of the elements in the periodic table, are resistant to radiation damage and have the potential for high waste loadings.
However the most important feature is its longevity. The natural analogue, obsidian, dates back to the Cretateous period millions of years ago, so we can be therefore be confident that our nuclear waste glass will remain intact over hundreds of thousands of years.
Vitrified HLW

9. Previous work O Between 33-37 wt% waste loadings
RLB: (1.40 ± 8.3) x 10-2 g/m2/d
RLU: (3.45 ± 0.01) x 10-5 g/m2/d
Previous work
MAS glass with corroded waste
MAS glass with metallic waste
MBS glass with corroded waste
MBS glass with metallic waste
MBS-M O Mg Si U
Vitrification can save up to £82 million ($100 million)
Between wt% waste loadings

10. Surrogate compatibility
Use of other elements to simulate uranium
Looked into neodymium and mischmetal (lanthanide alloy)
Replaced U in MBS and MAS samples on a molar basis
Same melting conditions
1250 °C for 3 hours (MBS)
1500 °C for 5 hours (MAS)
Nd glass under incandescent light
Nd glass under fluorescent light

11. U, Nd, Mm Uptake Sample Mol% oxide batched Mol% oxide in glass
Uptake into glass (oxide) %
MBS-M (U)
14.5
6.9
47.6
MBS-M (Nd)
5.2
4.9
94.2
MBS-M (Mm)
7.8
4.2
53.8
Sample
Mol% oxide batched
Mol% oxide in glass
Uptake into glass (oxide) %
MBS-C (U) 1
0.7
70.0
MBS-C (Nd)
0.9
0.4
44.4
MBS-C (Mm)
0.5
80.0

12. X-Ray diffraction U samples: Oxide crystals
Nd samples: Mix of oxide and borate crystals
Mm samples: Borate crystals

13. Scanning electron microscope
Nd samples: ~15 µm B Nd Si
Mm samples: ~15 µm B Ce La
U samples: Small crystallites ~5 µm
Fractal pattern

14. Long-term durability All samples show similar pH behaviour
MAS samples higher pH than MBS

15. Long-term durability
Nd shows similar trend to U most likely pH dependent.
Mm samples…
More oxidation states possible – more aqueous phases ∴ more dissolution

16. Conclusions & Further Work
A single glass formulation (MBS) can be used to treat both extremes of the waste expected in Sellafield’s Magnox sludge – previous study.
Mischmetal comparable to Uranium for uptake into glass structure
Neodymium and Mischmetal for borate phases – larger crystal size than uranium
Neodymium comparable for dissolution studies, Mm larger amount of OS possible
Determine the oxidation state of Nd and Mm samples
Raman spectroscopy – structure of the glass
This research serves to provide more effective, detailed knowledge into vitrified glass as a potential wasteform for the disposal of Sellafield's Magnox sludges

17. Thank you for listening!
Special thanks to:
Prof Neil C. Hyatt
Prof Russell J. Hand
Dr Martin C. Stennett
Dr Claire L. Corkhill
Dr Sean P. Morgan
Mr Adam J. Fisher
Mr Daniel J. Bailey
& the ISL team
Thank you for listening!
The authors would like to thank the EPSRC (Grant EP/G037140/1) for funding this research which was performed in part at the MIDAS Facility, University of Sheffield, established with support from the Department of Energy and Climate Change.