Young Researchers Meeting 2012
Karen Verrall
Date: 27/03/12
Career History
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Berkeley Nuclear Labs (BNFL Magnox Generation) –
Environmental & Effluent Monitoring / Waste Radiochemistry
•
Electric Power Research Institute (EPRI) – PWR Chemistry
Secondment
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Safety Case Management for Berkeley Site (BNFL Research &
Technology)
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Plant Chemistry – Ion Exchange & Filtration, Fuel Storage,
Pond Chemistry, Corrosion Monitoring
•
Project Management (Nexia Solutions / NNL)
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Chemistry & Graphite Team (NNL based at Stonehouse)
•
Seconded to EDF Energy Carbon Deposition Team (Barnwood)
Slide 3
AGR Coolant Chemistry and Carbon
Deposition Overview
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Young Researchers Meeting 2012
Objectives in Setting AGR Coolant Chemistry
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•
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Minimise oxidation of the graphite core
Minimise carbon deposition on fuel, boilers and in
other areas of the circuit
Minimise steel oxidation
Minimise plant dose rates and radioactive discharges
Young Researchers Meeting 2012
Reasons for Selecting CO2 Reactor Coolant
• Radiologically compatible
- low neutron capture cross section
- minimises activation of coolant
- reduces discharges
• Good heat transfer properties at high pressure
• Reasonably cheap and readily available
• Reasonably chemically compatible with steel and graphite
(when Magnox stations were designed it was thought to be
completely compatible!)
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Young Researchers Meeting 2012
Mechanism of Radiolytic Graphite Oxidation
• Under intense gamma and/or neutron irradiation CO2
breaks down to give oxidising species, e.g. CO3-, and
these react with graphite to reduce its density
γ,n
CO2
CO + [O]
γ,n
Cg + [O]
CO
[O] represents ‘oxidising species’
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Young Researchers Meeting 2012
More Protective Coolants for the Graphite Core?
• CH4 added to maintain a target concentration of ~230
vpm
CH4 + 3CO2
2H2O + 4CO
• Increasing CH4 has significant effects for other parts of
the plant
• Radiolysis of H2O results in production of H2, both of
which can lead to steel corrosion
• Limited by gas by-pass plant drier capacity to remove
H2O
• CO oxidised to CO2 by flow of O2 onto a catalyst
• 1.2% CO/280 vpm CH4 achievable, but increases carbon
deposition
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Young Researchers Meeting 2012
Evolution of AGR Coolants – Early Days
• HPB/HNB started with 0.5% CO and natural CH4
• Needed to increase CH4 for moderator protection
• High CH4 (350-415 vpm) and CO (up to 1.7%)
- Significant fuel-pin deposition and heat transfer
impairment
- Down-rating, early fuel discharges, by-pass plant
changes
• Coolant drawn back to 1.0% CO / 230 vpm CH4
• HRA/HYA/HYB/TOR
- 0.9-1.0% CO / 210-230 vpm CH4
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Young Researchers Meeting 2012
Evolution of AGR Coolants – Mid-1990s
• Further coolant optimisation and justification
• No ideal coolant composition
- All coolant compositions capable of depositing, just depends
on nickel catalyst availability in steel components and fuel
cladding
• Coolant chemistry is a compromise!
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Young Researchers Meeting 2012
Deposition Mechanism
radiation
Ethene
(CH2=CH2)
Nickel particle
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Young Researchers Meeting 2012
Methane
(CH4)
Carbon deposit
Role of Nickel in Deposition
Cross-section of oxidised fuel pin
Ni(CO)4
CH2=CH2
CH2=CH2
Ni
Ni does not oxidise under reactor
conditions, unlike other metals in the alloy.
This leaves particles of Ni metal on the fuel
pins surface.
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Young Researchers Meeting 2012
Intrinsic catalysis
Extrinsic catalysis
Unsaturated hydrocarbons are formed from
methane radiolysis. Decomposition to form
carbon is catalysed by intrinsic nickel.
Once some deposit present – further
decomposition can be catalysed by extrinsic
nickel from the coolant.
Evolution of AGR Coolants – Late1990s/Early
2000s
Intervention / Prevention – Carbonyl sulphide (COS)
injection
• Inhibits catalytic deposit formation in whole circuit
- Very effective on boilers (lower temperature)
- Some benefit for fuel, especially lower elements
Intervention / Clean-Up - O2 injection
• Removes deposit
• Limited to upper parts of boilers
- Plans to inject into the cores not pursued
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Young Researchers Meeting 2012
COS Mechanism (1)
100 vpb and 1.0% CO
COS
<460°C
Nickel particle
Nickel sulphide
No deposition
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Young Researchers Meeting 2012
COS Mechanism (2)
100 vpb and 1.0% CO
Sulphur-catalysed
loss of nickel
Some deposition?
COS
>>460°C
≥460°C
Nickel particle
COS
Surface layer of
sulphur
Some deposition
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Young Researchers Meeting 2012
Consequences of Carbon Deposition for Fuel and
Boilers
• Heat Transfer from fuel pin to coolant is impaired
• Clad runs hotter than is predicted and can re-crystallise
forming a new internal structure, leading to clad ductility
changes
• Reduced Heat Transfer from gas to steam in the boilers
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Young Researchers Meeting 2012
Carbon Deposition Strategy Diagram
UNDERSTAND
REMOVE
existing burden
PREVEN
further formation
T
& / or
HARDEN
plant to cope
PREVENT – further formation of carbonaceous deposit to prevent the problem from worsening
REMOVE – deposit already formed within the plant to minimise challenges to operations
HARDEN – modify plant to operate comfortably in the presence of deposit (either instead of or in combination with the above)
UNDERSTAND – deploy further investigation and research to fully underpin possible mitigations for the above 3 areas
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Young Researchers Meeting 2012