Young Researchers Meeting 2012

Transcription

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2 Karen Verrall Date: 27/03/12

3 Career History Berkeley Nuclear Labs (BNFL Magnox Generation) Environmental & Effluent Monitoring / Waste Radiochemistry Electric Power Research Institute (EPRI) PWR Chemistry Secondment Safety Case Management for Berkeley Site (BNFL Research & Technology) Plant Chemistry Ion Exchange & Filtration, Fuel Storage, Pond Chemistry, Corrosion Monitoring Project Management (Nexia Solutions / NNL) Chemistry & Graphite Team (NNL based at Stonehouse) Seconded to EDF Energy Carbon Deposition Team (Barnwood) Slide 3

4 AGR Coolant Chemistry and Carbon Deposition Overview 4

5 Objectives in Setting AGR Coolant Chemistry 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 5

6 Reasons for Selecting CO 2 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!) 6

7 Mechanism of Radiolytic Graphite Oxidation Under intense gamma and/or neutron irradiation CO 2 breaks down to give oxidising species, e.g. CO 3-, and these react with graphite to reduce its density CO 2 γ, n CO + [O] C g + [O] γ, n CO [O] represents oxidising species 7

8 More Protective Coolants for the Graphite Core? CH 4 added to maintain a target concentration of ~230 vpm CH 4 + 3CO 2 2H 2 O + 4CO Increasing CH 4 has significant effects for other parts of the plant Radiolysis of H 2 O results in production of H 2, both of which can lead to steel corrosion Limited by gas by-pass plant drier capacity to remove H 2 O CO oxidised to CO 2 by flow of O 2 onto a catalyst 1.2% CO/280 vpm CH 4 achievable, but increases carbon deposition 8

9 Evolution of AGR Coolants Early Days HPB/HNB started with 0.5% CO and natural CH 4 Needed to increase CH 4 for moderator protection High CH 4 ( 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 CH 4 HRA/HYA/HYB/TOR % CO / vpm CH 4 9

10 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! 10

11 Deposition Mechanism radiation Ethene (CH 2 =CH 2 ) Methane (CH 4 ) Nickel particle Carbon deposit 11

12 Role of Nickel in Deposition Cross-section of oxidised fuel pin Ni(CO) 4 CH 2 =CH 2 CH 2 =CH 2 Ni Intrinsic catalysis Extrinsic catalysis Ni does not oxidise under reactor conditions, unlike other metals in the alloy. This leaves particles of Ni metal on the fuel pins surface. 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. 12

13 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 - O 2 injection Removes deposit Limited to upper parts of boilers - Plans to inject into the cores not pursued 13

14 COS Mechanism (1) 100 vpb and 1.0% CO COS <460 C Nickel particle Nickel sulphide No deposition 14

15 COS Mechanism (2) 100 vpb and 1.0% CO COS >>460 C Sulphur-catalysed loss of nickel Some deposition? Nickel particle COS 460 C Surface layer of sulphur Some deposition 15

16 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 16

17 Carbon Deposition Strategy Diagram UNDERSTAND REMOVE PREVEN further formation T existing burden & / 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 17