1. Nuclear Graphite Research Group University of Manchester, UK
Robert Worth
Nuclear Graphite Research Group
University of Manchester, UK
Characterisation and Thermal Treatment of Irradiated PGA Graphite with Investigation into 3H and 14C Behaviour
Lorraine McDermott, Greg Black, Abbie Jones,
Paul Mummery, Barry Marsden, Anthony Wickham
14th International Nuclear Graphite Specialists Meeting
Seattle, USA
15th-18th September, 2013

2. Overview Irradiated Graphite in the UK Thermal Treatment at Manchester
Future Research
Conclusions

3. Irradiated Graphite in the UK

4. UK Graphite legacy
Graphite has been used in nuclear power plants worldwide
Historically, the UK has constructed many graphite-moderated reactors
These include power production, plutonium production and research reactors
Some still operational
Graphite contributes to a significant UK waste legacy
The majority of this graphite waste is ILW
Consequently, dismantling and management of radioactive graphite waste is an important issue in the UK
More than 100 NPP worldwide
~96,000 te in UK, ~250,000 te worldwide
44 graphite reactors in UK

5. Why treat graphite?
There is no current disposal route for irradiated graphite in the UK
Geological Disposal Facility (GDF)?
Treatment of irradiated graphite could allow reduction in the volume of ILW (cost-saving)
Utilise GDF space
Allow disposal in current near-surface facilities
This could be achieved by preferential removal of radioisotopes, such as tritium and carbon-14
Goal: Maximise radioisotope removal with minimal weight loss

6. Carbon-14 formation
There are two dominant mechanisms by which 14C is produced in irradiated graphite in a reactor environment:
(1) 13C (n,γ) 14C
(2) 14N (n,p) 14C

7. Historically difficult to determine nitrogen content of graphite
Nitrogen sensitivity
~50ppm
~10ppm
Historically difficult to determine nitrogen content of graphite

8. Thermal treatment at manchester

9. Thermal treatment
A program of thermal treatment work has been conducted at the University of Manchester as part of the collaborative European project ‘CARBOWASTE’
My own research is a continuation of this thermal treatment research:
Investigation of dependent variables, including temperature, time and oxygen
Investigation of 14C and 3H behaviour
Comparison of current world data to UK-irradiated graphite
Optimisation of the process
Using pre- and post-treatment characterisation techniques

10. Isotopic inventory determination
Thermal oxidation has been used as a method for 3H and 14C determination
Graphite samples are placed in a ceramic combustion boat in a Carbolite® MTT Furnace
A suitable cover gas flows past the sample and the temperature is raised
A copper oxide catalyst promotes further oxidation of any gasified 3H and 14C .
Analysis done in duplicate

11. Isotopic inventory determination
HTO and 14CO2 are subsequently trapped in the bubbler system for analysis using liquid scintillation counting (LSC)
Bubblers have a trapping efficiency of 98%
Analysis done in duplicate

12. Isotopic inventory determination
Analysis done in duplicate
Typical determined radioisotope content in Oldbury Magnox installed graphite:
Isotope
Activity (Bq/g) 3H
~37400
14C
~63700

13. Isotopic validation
How do we know we are capturing all of the 3H and 14C?
Regular recovery checks are performed – a known quantity of 3H and 14C labelled sucrose standards are put through the furnace
3H recovery in the range of 88 – 98 %
14C recovery in the range of 85 – 94 %
LSC quenched standard analysis to ensure LSC efficiency
LSC 3H quenched LSC standards 99%. 14C quenched LSC standards 100%.

14. Thermal Treatment Experimental programme
A thermal treatment programme has been designed to determine the effects of time, temperature and oxygen on 3H and 14C release
The following experimental conditions have been applied to samples machined from installed sets retrieved from the Oldbury Magnox power station:
Argon
1% Oxygen in Argon
Time
Temperature 4 5 6 7 8
600oC  --
700oC
800oC
900oC
Time
Temperature 4 5 6 7 8
600oC 
700oC
800oC
900oC --
Comparison of i-graphite samples pre and post treatment,
A = 800°C 1% O2/Ar for 5 hours, B = 700°C 1% O2/Ar for 5 hours, C = 700°C in Argon gas for 5 hours and D = untreated sample
Sample D in figure 23 is the untreated material, it can clearly been seen in the photograph that the effects of a 1% O2 environment at 800°C has had on the material structure. All samples analysed at 800°C in 1% O2 gas become heavily eroded and had a powered surface texture. This affected post thermal treatment analysis handling and unfortunately impacted on the tests that were able to carry out on these samples.

15. Thermal Treatment Experimental programme
Issues with the integrity of the samples post-treatment:
A B C D
Comparison of i-graphite samples pre and post treatment,
A = 800°C 1% O2/Ar for 5 hours, B = 700°C 1% O2/Ar for 5 hours, C = 700°C in Argon gas for 5 hours and D = untreated sample
Sample D in figure 23 is the untreated material, it can clearly been seen in the photograph that the effects of a 1% O2 environment at 800°C has had on the material structure. All samples analysed at 800°C in 1% O2 gas become heavily eroded and had a powered surface texture. This affected post thermal treatment analysis handling and unfortunately impacted on the tests that were able to carry out on these samples.
A = 800°C in 1% O2/Ar for 5 hours
B = 700°C in 1% O2/Ar for 5 hours
C = 700°C in Argon gas for 5 hours
D = untreated sample

16. AUToradiography
Hotspots

17. Time Dependence Tritium, 3H Carbon-14, 14C
The effects of increase in temperature is clearly seen in this graph – this shows the dependence of temperature with tritium migration

18. Weight Loss
Tritium, 3H
Carbon-14, 14C

19. Pre-Treatment Activity (kBq/g) Post-Treatment Activity (kBq/g)
Gamma spectrometry
Sample
Pre-Treatment Activity (kBq/g)
Post-Treatment Activity (kBq/g)
Percent Loss
OM1
8.824
8.473
3.99%
OM14
6.055
5.132
15.26%
OM18
10.093
9.561
5.26%
OM21
6.951
4.689
32.53%
Cobalt-60:

20. Future Research

21. Future work - phd
‘Characterisation and Thermal Treatment of Irradiated PGA Graphite with Investigation into 3H and 14C Behaviour’
Full optimisation of thermal treatment of irradiated Oldbury Magnox reactor graphite with respect to the sensitivity of:
Goal: Maximise radioisotope removal with minimal sample weight loss
Temperature oC
Time
3 - 9 hours
Oxygen content of gas
% oxygen in argon

22. Pre- & post- treatment analysis
Weight Loss
4 d.p. Balance
Metrology
Digital Micrometer
Porosity
Helium-pycnometry
Surface Area
Tristar BET
Laser Confocal Microscopy
To try and determine:
Amount of weight loss during treatment
The typical location of the radioisotopes before removal

23. Pre- & post- treatment analysis
Radioactive Content
Liquid Scintillation Counting
Gamma-spectrometry
Autoradiography
To determine:
Amount of radioisotope loss during treatment
Identification of ‘hotspots’ of radioactivity, which might influence the results

24. Visual representation
Laser Confocal Microscopy
(LCF)

25. Visual representation
LCF Height Mapping

26. Visual representation
LCF
3D Image

27. Conclusions
It has been demonstrated that thermal treatment in an oxidising atmosphere is a potential means of removing 3H and 14C radioisotopes from irradiated graphite
The current data suggests that this treatment technique may be suitable for removing up to ~80% 3H and ~55% 14C from Oldbury Magnox reactor graphite
Further work will be required to optimise this thermal treatment process and to determine the mobility and origin of these radioisotopes

28. acknowledgments
The authors are pleased to acknowledge EPSRC funding under agreement EP/P113315
A portion of this work was carried out as part of the CARBOWASTE Program: Treatment and Disposal of Irradiated Graphite and Other Carbonaceous Waste, Grant Agreement Number FP

29. thank you for listening Any questions?