chapter 7a - accident analysis oheads · 2015-01-11 · november 19, 2006 lecture 10 – accident...
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November 19, 2006Lecture 10 – Accident Analysis
cont'd.ppt Rev. 4 vgs 1
Accident Analysis – cont’d
Dr. V.G. Snell
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November 19, 2006Lecture 10 – Accident Analysis
cont'd.ppt Rev. 4 vgs 2
Where We Are
Probabilistic RequirementsDeterministic Requirements
Design BasisAccidents
Plant safetyas operated
Safety GoalsExperience
CredibleAccidents
Plant safetyas designed
Safety CultureGood Operating
Practice
MitigatingSystems
ProbabilisticSafety Analysis
SafetyAnalysis
Chapter 2
Chapter 3
Chapter 1
Chapter 2
Chapter 4
Chapter 5
Chapter 6
Chapters 7 & 8
Chapter 9
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November 19, 2006Lecture 10 – Accident Analysis
cont'd.ppt Rev. 4 vgs 3
Severe Accidents
n Severe accidentsn core geometry is preservedn fuel is damaged but remains inside intact
pressure tubes
n Severe core damage accidentsn fuel channels fail and collapse to the
bottom of the calandria
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November 19, 2006Lecture 10 – Accident Analysis
cont'd.ppt Rev. 4 vgs 4
Examplesn Loss of coolant + Loss of Emergency
Core Coolingn Loss of all secondary side heat sinks +
Loss of shutdown cooling system, moderator available
n Loss of coolant + Loss of Emergency Core Cooling + Loss of moderator heat removal
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November 19, 2006Lecture 10 – Accident Analysis
cont'd.ppt Rev. 4 vgs 5
System ContinuousHeat RemovalCapability (%full power)
Time to heat upand boil off, noheat removal
Moderator 4.4% > 5 hours
Shield Tank 0.4% 10 to 20 hours
Water Near Core
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November 19, 2006Lecture 10 – Accident Analysis
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Loss of Core Geometry - 1n E.g., Loss of all heat sinks + inability to
depressurize HTSn Pressure rises to relief valve setpointn Loss of water through relief valvesn Overheating of fuel and pressure-tubes
at high pressuresn Failure of a few pressure tubes (6-10
MPa) – depressurize HTS
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November 19, 2006Lecture 10 – Accident Analysis
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Loss of Core Geometry - 2n Remaining pressure
tubes strain to contact calandria tube
n Boil-off of moderatorn Sag & failure of
channels at lowpressure at ~1200C as moderator level falls
n Collapse of channels onto lower neighbours
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November 19, 2006Lecture 10 – Accident Analysis
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Characteristics of Debris Bedn Top channels collapse when moderator is half voided,
so they sag into a pool of watern Debris likely to be composed of coarse pieces of
ceramic materialsn Bed will not be molten until all the moderator water
is boiled off - will then dry out and heat up due to decay heat & remaining Zircaloy-steam reaction
n No energetic fuel-coolant interactionn Models for heat transfer from debris bed to calandria
walls developed by T. Rogers et al. for dry debris, and also debris with molten centre
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November 19, 2006Lecture 10 – Accident Analysis
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Debris Bed Modelsn Uniform porous mixture
of UO2, ZrO2 and/or Zircaloy
n Fuel decay heat + metal water reaction
n Thermal radiation to inner surface of calandria from top of the bed
n Conduction through bottom of calandria to shield tank water
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November 19, 2006Lecture 10 – Accident Analysis
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Debris Bed Heatupn Melting of
debris starts about 7 hours after the event
n Upper & lower surfaces of debris bed stay below melting temperature
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November 19, 2006Lecture 10 – Accident Analysis
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Calandria Wall Temperaturesn Outer surface
temperature below 140C
n Stainless steel wall
n Do not expect creep under applied stresses
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November 19, 2006Lecture 10 – Accident Analysis
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Heat Flux to Shield Tankn Heat flux to shield tank
15 times less than CHFn Calandria will remain
intact while shield tank water boils off
n Behaviour insensitive to porosity and timing of metal-water reaction
Critical heat flux200 W/cm2
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November 19, 2006Lecture 10 – Accident Analysis
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Summary of Time-scalesTime (hr) Event
0 Loss of heat sinks, reactor shutdown
0.75 Steam Generators boil dry, liquid relief valvesopen, fuel cooling degrades
0.83 A few pressure tubes fail and depressurize heattransport system
0.86 High pressure ECC initiated; medium pressureECC assumed to fail
1.1 Heat transport system empty
5 Moderator boiled off, channels sagged to bottomof calandria
25 Vault water boiled off to top of debris bed,calandria fails
Days Interaction of debris with vault floor &penetration to containment basement
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November 19, 2006Lecture 10 – Accident Analysis
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Uncertainties
n Mechanical and thermal behaviour of end-shields
n Capability of shield tank to relieve steamn Local effects in molten pool and hot-
spotsn Lack of experimental validation of debris
melting transientn Demonstration of core collapse mode
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November 19, 2006Lecture 10 – Accident Analysis
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Tests on Channel Collapsen 1/5 scale studyn Scaling retains full
size stress levels, ratio of bundle size to channel length and channel length to pitch height of assembly
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November 19, 2006Lecture 10 – Accident Analysis
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Test Rig
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November 19, 2006Lecture 10 – Accident Analysis
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Channel Failure Mode
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November 19, 2006Lecture 10 – Accident Analysis
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Channel Collapse – Early Results
n Significant sag occurs only above 800C.n Sag is creep-controlledn PT wall thins at the bundle junctions -
debris may be two to three bundles long
n The end-load is not sufficient to pull out the channel from the end-fitting
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November 19, 2006Lecture 10 – Accident Analysis
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Containmentn Containment heat removal (local air coolers)
may or may not be available depending on the accident
n If not available, pressure initially controlled by dousing sprays
n Will eventually rise above design pressuren Structure will remain intact due to leakage
through cracks and pressure relief
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November 19, 2006Lecture 10 – Accident Analysis
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Observationsn Severe core damage in CANDU is very different from LWRsn Low power density (16 MW/Mg of fuel at full power)n Long heatup times (hours)n Gradual collapse of the core into a coarse debris bedn Dispersion of the debris in the large calandria
n shallow molten pool about 1 metre deep
n Presence of two large sources of water in or near the coren Potential to stop or slow down the accident at two points:
n channel boundary (moderator)n calandria boundary (shield tank)
n Hydrogen control a necessity in short- and long-term
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November 19, 2006Lecture 10 – Accident Analysis
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Coherent vs. IncoherentLWRsn Fuel melts rapidly
(minutes) and “candles” down to the bottom of the vessel
n Vessel fails suddenly ejecting molten fuel
n Potential for steam explosion in vessel and in containment
CANDUn Fuel melts slowly
(hours) and slumps gradually down to the bottom of the vessel
n Vessel fails after a day; shield tank provides further barrier
n Continual steam release - explosion less likely
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November 19, 2006Lecture 10 – Accident Analysis
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Severe Accident Conclusionsn Severe accident mitigation requirements for new
reactors stress two design measures:n core debris spreading arean ability to add water to cool debris
n CANDU: calandria spreads the debris, and shield tank provides cooling water
n Long time scales allow for severe accident counter-measures and emergency planning
n Independent makeup to moderator and shield tankfor EC6 and ACR
n Future: backup or passive containment heat removal
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November 19, 2006Lecture 10 – Accident Analysis
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Uncertainty Analysisn Disadvantages of conservative
approach:n Overestimates / exaggerates riskn Misleadingly small marginsn Useless as operator guiden Distorts safety resource allocation
n UA useful only with best-estimate methods (BE+UA)
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November 19, 2006Lecture 10 – Accident Analysis
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Types of Uncertaintyn Physical models
n Model biasn Experimental scattern Example: WIMS predicts coolant void reactivity for
37-element fuel with a bias of –1.6 mk. and an experimental uncertainty of ±1mk.
n Plant idealizationn Example: How many spatial nodes for
convergence?n Plant data
n Example: Uncertainty in flow measurement
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November 19, 2006Lecture 10 – Accident Analysis
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Focus of Uncertainty Analysis
n UA stated only for key safety parameters output by the code & compared to acceptance criterian E.g., peak fuel temperaturen E.g., not internal parameter such as fission
gas pressure
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November 19, 2006Lecture 10 – Accident Analysis
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Issuen Assume there are < 8 contributors to a key
parametern E.g., key parameter is fuel temperature in LOCA
power pulsen Contributors are void reactivity, sheath-to-coolant
heat transfer, delayed neutron fraction etc.n Cannot afford to vary 8 simultaneously using
fundamental codesn Surrogate needed (curve fitting or functional
form)
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November 19, 2006Lecture 10 – Accident Analysis
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Simplified Methodologyn Generate cases for
variation of ncontributory parameters using fundamental code
n Fit surface with functional form
n Test goodness of fitn Sample surface based
on statistical distribution of each parameter
Probability distribution ofContributory parameter(2 examples)
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November 19, 2006Lecture 10 – Accident Analysis
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Graphical ExampleKey output parameter
n1, n2, n3
Key output parameter
n1
Code run
Functional or curve fit
Mean value &95% confidencelevels