Chapter 7a - Accident Analysis Oheads · 2015-01-11 · November 19, 2006 Lecture 10 – Accident...

November 19, 2006Lecture 10 – Accident Analysis

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Accident Analysis – cont’d

Dr. V.G. Snell

November 19, 2006Lecture 10 – Accident Analysis

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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

November 19, 2006Lecture 10 – Accident Analysis

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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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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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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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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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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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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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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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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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Calandria Wall Temperaturesn Outer surface

temperature below 140C

n Stainless steel wall

n Do not expect creep under applied stresses

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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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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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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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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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Test Rig

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Channel Failure Mode

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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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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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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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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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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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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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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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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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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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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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Graphical ExampleKey output parameter

n1, n2, n3

Key output parameter

n1

Code run

Functional or curve fit

Mean value &95% confidencelevels