how safe is that nuclear reactor?
TRANSCRIPT
HOW SAFE IS THAT NUCLEAR REACTOR?
George D. W. Smith Emeritus Professor, Dept. Materials, University of Oxford
Outline • The global nuclear energy scene • Major nuclear incidents and alerts • The U.K. power generation requirements • How to optimise nuclear safety • The key role of materials research in nuclear safety • Field ion microscopy & atom probe tomography (APT) • Fission reactors: pressure vessels, cooling systems • Fusion materials research ((and Gen IV fission)) • UK Policy issues: how to rebuild nuclear research and
development, and nuclear engineering skills, in the UK
Acknowledgements • Alfred Cerezo, Christopher Grovenor, Samuel Humphry-
Baker, Rong Hu, Karen Kruska, Sergio Lozano-Perez, James Marrow, Emmanuelle Marquis, Andrew Morley, Steve Roberts, David Saxey, Paul Styman, Ceri Williams (Oxford)
• Colin English, Jonathan Hyde (NNL) • Peter Flewitt (Magnox / Bristol / Oxford) • Brian Connolly (Birmingham University) • Liz Rowsell, Allin Pratt+ (Johnson Matthey) • Michael Preuss, Andrew Sherry (Manchester) • Keith Wilford, Tim Williams , Dave Ellis (Rolls Royce) • Michael Miller (Oak Ridge Natl. Laboratory, USA) • G. Robert Odette (Univ. California , Santa Barbara, USA) • Staff of Institute of Nuclear Safety System, Fukui, Japan.
From: Energy Materials: Strategic Research Agenda, Materials UK Report 2007
Some Background Reading • Energy Materials Reports (1) Strategic Research Agenda, (3)
Nuclear Energy Materials, (Materials UK 2007) • The Mapping of Materials Supply Chains in the UK’s Power
Generation Sector, (Materials UK 2008) • Generic Design Assessment Consultation Documents for EPR
and AP 1000 Nuclear Power Plants (Environment Agency 2010) • Materials R & D for Nuclear Applications: The UK’s Emerging
Opportunities (Materials UK 2010) • Japanese Earthquake and Tsunami: Implications for the UK
Nuclear Industry, M. Weightman, (ONR, September 2011) • Nuclear Research and Development Capabilities. Report of
the House of Lords Select Committee on Science and Technology, published 22nd November 2011 [HL Paper 221]
The Global Nuclear Energy Scene 432 operable civil nuclear reactors worldwide 63 reactors under construction (27 in China, 6 India) 152 further reactors planned (51 in China, 17 India) 350 more reactors proposed (120 in China, 40 India) 14,680 reactor years of operation so far (30 countries) 14% global electricity produced from nuclear power (2010). Source: World Nuclear Association, October 2011
Major Nuclear Incidents and Alerts • 1957: Fire at Windscale
• 1979: Near-meltdown at Three Mile Island
• 1986: Explosion at Chernobyl
• 1999: Criticality accident at Tokaimura Plant
• 2002: Severe corrosion of pressure vessel at Davis-Bess Reactor,
Ohio, USA (a very “near miss”)
• 2004: 36-inch steam pipe failure at Mihama, Japan
• 2011: Fukushima: Earthquake, Tsunami, Meltdowns
Some Lessons from Major Incidents • One major incident per 2000 reactor operating
years is (rightly) unacceptable to our society • Failure types are exceptionally diverse:
– Design flaws
– Materials degradation
– Engineering component failures
– More complex system failures
– Operator errors or misdeeds
– Neglect of maintenance
– Unforeseen / unpredicted behaviour of system
– External environmental disaster
How to Optimise Nuclear Safety • Total system approach needed – from initial design
through construction to operation and maintenance, and eventual decommissioning
• Good stewardship of plant (and waste) required • Multi-level passive safety systems essential • Adequate supply of trained manpower is critical • Very long projected operating lifetime for new plant
(60+ years) creates major new challenges • Plant condition monitoring will be a key factor • Materials issues will be increasingly important
Materials Issues in PWR Engineering (excluding the fuel cycle)
• Embrittlement of pressure vessels and pipes due to thermal aging and / or irradiation
• Environmentally-induced cracking of pipes and welds • Corrosion and erosion of cooling systems • Creep-fatigue-irradiation interactions • Thermal fatigue of large structures • Behaviour of joints between dissimilar materials
NB: Reliable prediction of long-term stability of nuclear reactor materials requires a deep
mechanistic understanding of these phenomena
Field Ion Microscope (FIM) • Specimen in form of needle, 100nm end radius • Voltage applied to specimen generates high field • Gas ionised at apex generates image on screen
Channel plate and phosphor screen
Needle-shaped specimen (cooled)
High voltage (d.c.) Field ionised gas atoms
Vacuum chamber
FIM image of a Low-Alloy Steel
Pulsed Field Evaporation
Potential energy!Kinetic energy !
Identifying Single Atoms • Flight time t of ions removed from specimen is measured
over flight length d with (sub-)nanosecond resolution. • By equating potential energy for ion at specimen and
kinetic energy after field evaporation, can calculate mass-to-charge ratio: 1
2mv2 = neV
mn
= 2eV 1v! " # $
2
= 2eV td! "
# $
2
The 3-Dimensional Atom Probe • Single atoms removed from specimen and
identified • Position sensing gives original position to sub-nm • Continued removal gives 3-D atomic-scale map
Position- sensitive detector
Flight time signal
Specimen (cooled)
High voltage (d.c. + pulse)
Field evaporated
ions
Atom Probe Tomography (APT)
• At any point, 3DAP gives analysis of the surface elemental distribution
• Field evaporation leads to atomic layer ‘slicing’ through the material
• Data allows reconstruction of original 3–D distribution of elements
(x,y)
z=0 z=1 z=2 z=3 z=4 z=5
More About PressureVessels (RPVs)
• What happens in the matrix? – Size, number density and volume fraction of Cu-rich precipitates – Ni, Mn and Si at the precipitate-matrix interface – Fe content of precipitates
• What happens at grain boundaries/dislocations – Fast diffusion paths & heterogeneous nucleation sites
• New materials are generally lower in Cu – Suggestions of problems with Ni-Mn-Si precipitation
Cu Ni Mn Si P C Mo Cr Other Fe
0.44 1.66 1.38 0.75 0.018 0.19 0.24 0.054 0.0482 Bal
Nominal Composi,on of test steel (at.%)
PWR RPVs operate at ~293 C and up to 200 atmospheres pressure. Tests on low alloy steels, thermally aged at 330°C and 365°C for times up to 90,000 hrs (~10 years) show precipitation of impurity Cu a major concern. Other alloy additions (esp. Ni and Mn) enhance pptn. – why?
Fe2+
58Fe/58Ni
Mn
Mo3+
Cu Co
Ni Cr
C
Sensitivity ~ 0.002at.% (mass dependent)
APT Mass Spectrum of Model RPV Steel
Matrix Precipitation – 330°C & 365°C
19
90,000 hrs Mn Cu Si Ni
90,000 hrs Si Ni Mn Cu
Individual Cu ppts. in a low-alloy steel
• Cu • Ni • Mn
5nm
405˚C for 100 hours
330˚C for 18620 hours
Fe-0.5at.%Cu- 1.5at.% Ni-1.5at.%Mn-0.75at%Si aged at:
Interface segregation to Cu precipitates Aged for 18620 hours at 330 ˚C Aged for 100 hours at 405 ˚C
Grain Boundaries – Solute Segregation Solute segrega,on has been observed at grain boundaries in material aged to 50,000 hrs at 365°C.
C
Cr
Mo
P Ni
Mn
Si
The interfacial excesses observed:
Element Excess (x1017 atoms m-2)
Equivalent Monolayers
C 25 ± 0.3 0.14 Mo 12 ± 0.2 0.07 Mn 29 ± 2 0.17 Ni 19 ± 4 0.11 P 7 ± 0.1 0.04 Cr 2 ± 0.1 0.01 Si 3 ± 2 0.02
Grain Boundaries – Precipitation Copper Nickel Manganese Silicon
Carbon Phosphorus Molybdenum
Specimen aged for 50,000 hrs at
365°C
A
B
Matrix
30 nm
Cluster A
Cluster B
Ni-Mn-Si Precipitates
24
Grain Boundary
1 nm 1 nm 1 nm 1 nm 1 nm
Cu P Ni Mn Si
2 nm 2 nm 2 nm 2 nm 2 nm
Cu P Ni Mn Si
Disloca4on
Cu P Ni Mn Si
Pressure Vessel Summary • Good News: We now have a reasonable understanding of matrix Cu
precipitation during thermal aging, and of the effects of Ni, Mn and Si on this process.
• Bad News: Thermal precipitation processes complex, especially at dislocations and interfaces. At long times, evidence of intermetallic precipitation in the absence of Cu. Irradiation adds much further complexity to the pptn. process, moving it far from equilibrium.
• Worse News: We cannot say with full confidence what is going to happen to these steels after 60+ years exposure. There may be “late blooming phases” that could cause significant problems.
Primary Cooling System: Cast Duplex Stainless Steels
Pump castings and large-diameter cast pipes are generally fabricated from duplex (austenitic + ferritic) stainless steels.
Typical composition Fe-21.5%Cr -8%Ni(at%)
Ferrite phase composition: Fe-26%Cr-5%Ni The ferrite phase is thermodynamically unstable at reactor
operating temperatures at this composition Long-term thermal aging can lead to extensive hardening and
embrittlement (spinodal reaction)
Thermal Aging of Binary Fe-Cr Alloys
Fe-45at.%Cr aged at 500 C
5 nm
24 hours 100 hours 500 hours
• Isosurfaces drawn at approximately 40at.% Cr show interconnected structures
• Amplitude and scale of structure increases with time Reproduced by courtesy of Acta Materialia
Spinodal in Duplex Stainless Steel
• A10 stainless steel aged at 400 ˚C for 30,000 hours
• Isosurface reconstruction shows G-phase particles (red) forming in the spinodally decomposed ferrite phase – Red surface:
Ni + Si + … > 23 at.% – Blue surface:
Cr > 30 at.%
5 nm Data courtesy F. Danoix, Université de Rouen
Cast Stainless Steels Summary • Good news: We can now quantify the extent of spinodal
decomposition in Fe-Cr, study the evolution of microstructure, and extrapolate behaviour to longer times with confidence.
• Bad news: The ferrite phase of commercial duplex steels decomposes up to 1000 times faster than pure binary Fe-Cr, and at significantly lower temperatures (300 C!)
• Worse news: We do not understand why the reaction kinetics depend so sensitively on the exact composition of the steel, particularly the nickel content, and on heat treatment. Every candidate steel still needs to be individually tested.
Nuclear Fusion ((+ Gen IV Fission))
+Energy (17.6MeV) • 14 MeV neutrons
• 3.5 MeV alpha par,cle
Plasma facing materials will be exposed to:
• High heat flux of energe,c par,cles (0.1–20MW/m2)
• High temperatures (500–3200 ◦C) • Electromagne4c radia4on, spuZering
erosion, • Neutron-‐irradia4on (3–30 dpa/year),
• Instabili,es in the plasma edge
Fe-14% Cr ‘Low Activation’ Steel
Courtesy Rong Hu
• EBSD mapping:
200 µm
Σ 3 (72°)
Σ 39 (50°)
Random boundary (23°)
Σ 1 (12°)
Heavy-ion (Fe) irradiation carried out at University of Surrey EBSD used to select individual grain boundaries for study
FIB methods used to produce samples for APT analysis
Fe-14% Cr Grain Boundary Chemistry After Heavy Ion Irradiation
50nm 50nm
Implanted
Un-implanted
Region
The same grain boundary is examined in the ion irradiated and un-irradiated condition.
Cr
C • Atom maps for random boundary with 23º misorientation
Fe+ ions
1µm 2µm
Grain Boundary
Plane
Implanted Region Analysis
cylinder
100nm
600nm
2µm
Fe-14% Cr Irradiation: Segregation Profiles
Cr C
100nm
600nm
2µm
Cr
C
Implanted
Un-implanted
• 1D concentration profiles of Cr and C for random boundary with 23º misorientation
Courtesy Ceri Williams
Fe-14wt%Cr-2%W-0.3%Ti-0.3%Yttria. Mechanically alloyed powder Consolidated material (left); original alloyed powder (right)
Oxide nanoparticles provided high temperature strength, plus sink for helium. Nanoscale characterisation essential to alloy development.
Oxide Dispersion Strengthened Steels
Critical Timescales for UK Nuclear Power
0
2000
4000
6000
8000
10000
12000
14000 20
03
2005
2007
2009
2011
2013
2015
2017
2019
2021
2023
2025
2027
2029
2031
2033
2035
MWe
Existing stations Potential AGR life extension New Build
Historic Position over UK Materials Skills • UK experience over the last 40 years
has shown that understanding the evolution of materials properties is essential – UK has operated fleet of gas cooled
reactors (and more recently one PWR)
– No world wide database to call-on – Development of new designs (fast
reactor/fusion) – Materials involved in
decommissioning and waste management
• The volume of available expertise has been declining in the UK for many years
Individual Materials Experts in RPV, EAC, Zr (UK)
0
50
100
150
200
250
0 5 10 15 20
Years from 2005
Tota
l Man
Yea
rs o
f Ex
peri
ence
Nuclear R and D Investment in U.K.
Source: Nuclear Research and Development Capabilities. Report of
the House of Lords Select Committee on Science and Technology, published 22nd November 2011 [HL Paper 221]
Opal: UK National 3DAP Facility LEAP 3000X SI
50-200nm field of view Largest volume of analysis (107 nm3) Laser pulsing for resistive materials
Permits analysis of microtips
LAR 3DAP 30-100nm field of view
Highest available mass resolution Laser pulsing for resistive
materials Built-in reaction cell for surface
studies
UK Policy Issues: Safeguarding the Future
• Need to re-build UK nuclear R & D and nuclear skills capability from its present very low base
• Investment must be government-led, sustained and long-term: sector strategically vital to UK and potentially wealth-creating
• Clear vision needed of where we are going • Must quantify manpower and resource requirements • Joined-up thinking required: rebalance fission and fusion
programmes, re-think roles and responsibilities - and budgets - of NDA, NNL, RCUK, NII, HSE, DECC, Environment Agency
• Knowledge capture exercise is urgent priority • Harvesting of material from decommissioning programme
essential for forensic studies
A Couple of (Political) Dangers to Avoid • Words vs. Deeds:
“Wanted: A senior and distinguished figure to simulate research activity in a key strategic area” (University job advert placed during the run-up to a Research Assessment Exercise)
• Continually Rearranging the Deckchairs: “…every time we were beginning to form up into teams, we would be reorganised…..a wonderful method it can be for creating the illusion of progress while producing confusion, inefficiency and demoralisation” (Erroneously attributed to Gaius Petronius Arbiter, circa AD 60, actually taken from an article by Charles Ogburn, “Merrill’s Marauders”, Harpers Magazine January 1957)
Q: How Safe Is That Nuclear Reactor?
A: As Safe As The People Who Run It