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Robert WorthNuclear Graphite Research Group
University of Manchester, UK
Lorraine McDermott, Greg Black, Abbie Jones,Paul Mummery, Barry Marsden, Anthony Wickham
Characterisation and Thermal Treatment of Irradiated PGA Graphite with Investigation into 3H and 14C Behaviour
14th International Nuclear Graphite Specialists MeetingSeattle, USA
15th-18th September, 2013
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OVERVIEW
Irradiated Graphite in the UK Thermal Treatment at Manchester Future Research Conclusions
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IRRADIATED GRAPHITE IN THE UK
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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
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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
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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
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Historically difficult to determine nitrogen content of graphite
~10ppm
~50ppm
NITROGEN SENSITIVITY
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THERMAL TREATMENT AT MANCHESTER
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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
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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 .
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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%
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ISOTOPIC INVENTORY DETERMINATION
Typical determined radioisotope content in Oldbury Magnox installed graphite:
Isotope Activity (Bq/g)3H ~3740014C ~63700
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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
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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:
Time
Temperature
4 5 6 7 8
600oC -- 700oC 800oC -- 900oC -- -- -- -- --
Time
Temperature
4 5 6 7 8
600oC 700oC 800oC 900oC -- -- -- -- --
Argon 1% Oxygen in Argon
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THERMAL TREATMENT EXPERIMENTAL PROGRAMME
Issues with the integrity of the samples post-treatment:
A B C D
A = 800°C in 1% O2/Ar for 5 hoursB = 700°C in 1% O2/Ar for 5 hoursC = 700°C in Argon gas for 5 hoursD = untreated sample
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AUTORADIOGRAPHY
Hotspots
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TIME DEPENDENCETritium, 3H Carbon-14, 14C
3 4 5 6 7 8 90
10
20
30
40
50
60
70
80
90
600°C Ar600°C 1% O2/Ar700°C Ar700°C 1% O2/Ar800°C Ar800°C 1% O2/Ar
Time (h)
Estim
ated
3H
Rele
ase
(%)
3 4 5 6 7 8 90
10
20
30
40
50
60
70
80
90
100
600°C Ar600°C 1% O2/Ar700°C Ar700°C 1% O2/Ar800°C Ar800°C 1% O2/Ar
Time (h)Es
timat
ed 1
4C R
elea
se (%
)
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WEIGHT LOSSTritium, 3H Carbon-14, 14C
0 10 20 30 40 50 60 700
10
20
30
40
50
60
70
80
90
600°C Ar
600°C 1% O2/Ar
700°C Ar
700°C 1% O2/Ar
800°C Ar
800°C 1% O2/Ar
Weight Loss (%)
Estim
ated
3H
Rele
ase
(%)
0 10 20 30 40 50 60 700
10
20
30
40
50
60
70
80
90
100
600°C Ar
600°C 1% O2/Ar
700°C Ar
700°C 1% O2/Ar
800°C Ar
800°C 1% O2/Ar
Weight Loss (%)Es
timat
ed 1
4C R
elea
se (%
)
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GAMMA SPECTROMETRY
SamplePre-
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:
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FUTURE RESEARCH
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FUTURE WORK - PHD
• 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
‘Characterisation and Thermal Treatment of Irradiated PGA Graphite with Investigation into 3H and 14C Behaviour’
Temperature 600 - 900oC
Time 3 - 9 hours
Oxygen content of gas 0.5 - 2% oxygen in argon
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PRE- & POST- TREATMENT ANALYSIS
Porosity• Helium-pycnometry
Weight Loss• 4 d.p. Balance
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
Metrology• Digital Micrometer
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PRE- & POST- TREATMENT ANALYSIS
To determine: Amount of radioisotope loss during treatment Identification of ‘hotspots’ of radioactivity, which might
influence the results
Radioactive Content• Liquid Scintillation Counting• Gamma-spectrometry• Autoradiography
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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
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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 FP7-211333
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THANK YOU FOR LISTENING
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