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Photos placed in horizontal position with even amount of white space

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Emerging issues in Source Term Estimation

following the Fukushima Daiichi Accident

Presented at IAEA Technical Meeting on Source Term

Evaluation for Severe Accidents

21 to 23 October 2013

Vienna International Centre, Austria

Randall Gauntt

Sandia National Laboratories

Used by permission from TEPCO Used by permission from TEPCO

Post Fukushima reactionsHydrogen control

Containment venting (filtered?)

Multi-unit accidents

Small Modular Reactors

Accident Management

Consequence Mitigation

2001

Aircraft

Vulnerability

Study

MELCOR Severe Accident Phenomena

Small Scale Release Tests

VERCORS and ORNL VI � Small scale tests use only a few pellets

� Conditions are uniform compared to integral tests like FPT-1

� Cladding often fully oxidized prior to release measurements

� Temperature raised in steps with long plateaus

Validation of Fission Product Release Models

VERCORS 4

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pera

ture

(K

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data

CORSOR-M

Temperature

VERCORS 4

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Rele

ase F

racti

on

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pera

ture

ORNL Booth

CORSOR M

I data

COR-TFU.102

VERCORS 4

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mp

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ture

(K

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ORNL Booth

CORSOR M

Te data

Temperature

VERCORS 4

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ture

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CORSOR M

Mo data

Temperature

Cs Release I Release

Te Release Mo Release

The Phebus Experiment Facility� Irradiated fuel heated in

test package by Phebusdriver core

� Fuel heatup

� Zr oxidation, H2

� Fission product release

� Circuit (700 C) transports FP through steam generator tube

� Deposits in circuit and SG

� Containment receives FP gas and aerosol

� Settling

� Iodine chemistry

FPT-1 Deposition: Prediction versus Experiment

� Distribution of transported fission products

� Predictions versus experiment

� Performance reasonable for application

Cs Distribution as Fraction of Bundle Inventory: ORNL-Booth (m odified)

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5000 7500 10000 12500 15000 17500 20000

tim e [sec]

fra

cti

on

of

inv

en

tory

bundle

plenum

hot leg

steam generator

cold leg

containment

Total

- Plenum

- Hot Leg

- Stm Gen

- Containm ent

- total

FPT-1

Final

Dist

FUKUSHIMA DAIICHI UNIT 1

ANALYSIS WITH MELCOR

….setting the stage for discussions….

SNL Fukushima MELCOR Reactor Models

• BWR Mk-I model from the NRC’s State-of-the-Art Consequence Analysis (SOARCA) project used as a template

– 20+ years of BWR model R&D

– Current state-of-the-art/best practices

• Incorporated reactor-specific information into the template to create Fukushima reactor models

• Developed surrogate information for unavailable Fukushima information

• Analyses performed using MELCOR 2.1

Mark-I Containment

Browns Ferry from Wikipedia

NRC Training Manual

RPV Pressure During First Hour� IC heat removal

nominally 42 MW

per IC

� 2 IC’s operated

initially (exceeded

tech spec cooldown

rate)

� Later only 1 IC was

cycled

� After 1 hour, IC’s

were assumed off

following SBO

Sequential Degradation of Control Blades

and Fuel Rod Geometry

� Core damage starts at ~ 4 hours – Control Blade fails first

� Progressive fuel damage after 6 hours

� Core exit gas temperatures very high

0

500

1000

1500

2000

2500

3000

0 5 10 15 20

Tem

pe

ratu

re [

K]

time [hr]5 hr 7 hr 8 hr

C/B Fails

Fuel Fails

High Gas Temperature Weakens

Steam Line � Zr oxidation produces

hydrogen and high

gas temperature

� Cycling SRV vents hot

gas through steam

line and into

suppression pool

� High RPV pressure

ruptures weakened

steam line

� TEPCO prefers gasket

blowout theory

� SRV failure by seizing

open is completely

plausible alternative

� Subsequent SA

progression not

strongly affected

CV 200

CV 220FL012

FL020

CV 220

FL021

FL903

( DW head

flange failure )

CV210CV210

CV 205

FL014Personnel

access

FL016CRD hyd .

pipe chase

FL015

CRD removal

FL904(DW liner

melt-through)

FL017(DW nominal leakage)

FL022(RB-WW vacuum breaker to NE torus corner room)

FL023(RB-WW vacuum breaker to SE torus corner room)

CV 201

CV 202

FL

20

2

FL200

FL910(Wetwell Hard-Pipe ventto atmosphere)

SRV Seizure Versus MSL Rupture

CV 200

CV 220FL012

FL020

CV 220

FL021

FL903

( DW head

flange failure )

CV210CV210

CV 205

FL014Personnel

access

FL016CRD hyd .

pipe chase

FL015

CRD removal

FL904(DW liner

melt-through)

FL017(DW nominal leakage)

FL022(RB-WW vacuum breaker to NE torus corner room)

FL023(RB-WW vacuum breaker to SE torus corner room)

CV 201

CV 202

FL

20

2

FL200

FL910(Wetwell Hard-Pipe ventto atmosphere)

Main Steam Line Rupture vents

Fission products to drywell

Release to environment via head

Flange failure or drywell liner melt through

SRV Seizure vents fission products

Into wetwell

Wetwell scrubbing prevents release

To the environment

MSL Rupture

SRV cycles long time

MSIV

SRV sticks open

Containment Pressure

� 1F1 suffered loss of

instrumentation

� Unaware of too high

pressure

� Unable to vent

anyway

� Containment

vented itself

� Uncontrolled

release

� Manual venting of

suppression pool

vapor space could

have reduced

releases to

environment

� Emerging Issue:

Venting strategies

� When ?

� Where ?

� Filtered ?

� Active management

A

P

“n” bolts witharea “a” andlength “L”

L

δL

Drywell Head Pressure Response

Cs Release 1F1

Radiation monitors at front gate jump at ~13 hr

Cs Envrelease about 400,000 Ci

0.0

0.1

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0.7

42 48 54 60 66 72 78 84 90 96

Pre

ssu

re [

MP

a]

time [hr]

simulation-1

simulation-2

simulation-3

simulation-4

TEPCO drywell pressure

TEPCO wetwell pressure

RPV depressurizes

core damage and in-vessel oxidation

S/C penetration leak

in simulations 3&4

actual time of 1F3

reactor building

explosion (68.2 hr).

S/C vent 1 hr before

Unit 3 Containment Pressure, Venting

and Water Injection� Containment pressure

trends captured in analysis

� Two vents (4 and 5) were

assumed to match

observed trends

� Operator action or

containment venting

itself ?

� Self-venting could explain

H2 path to R/B

� Water injection now

thought too large

� Over-cooling core instead

of generating steam

� Sensitivity cases show

improved pressure trends

ExplosionExplosion

Vent 1Vent 1

Vent 2Vent 2

Vent 3Vent 3

Vent 4Vent 4

Vent 5Vent 5

Containment Venting Strategies

� Containments traditionally designed for “Design Basis

Accidents”

� Not generally expected to withstand “Severe Accidents” (beyond

design basis)

� Intentional venting an “emerging” element of accident

management

� BWR suppression chamber (SC) venting is a “filtered” release

� Containment sprays reduce airborne but also reduce time window for

SC venting (SC will over-fill with water)

� “Drywell venting” could benefit from filters – expensive backfit

� SBO “hardening” of venting capability is of concern

Long Term RCIC Operation

� RCIC pump is driven by “Terry

Turbine”

� Prior PRA assumptions and SAMG’s

hold that steam line flooding would

kill RCIC

� 1F2 experience shows RCIC ran

uncontrolled in self-regulating

mode for 3 days

� Should this be realistically modeled

in safety analyses ?

� Of course it should !

RPV pressure drop caused by large 2-phase enthalpy flow through robust Terry turbine

SAND2012-6173

Spent Fuel Pool Risks

� Fukushima accident caused great

concern over SFP safety

� Frequency of major loss of coolant very

low

� Consequence of pool fire very high

BWR Spent Fuel Pool

� Air-oxidation more energetic

� Air-oxidation increases volatility of Ru

� Sandia tests provided data for air-

oxidation

� Pool-fire is bad, but loading pattern can

reduce risks

Spent Fuel Pool Emerging Issues

� Air ingression changes things

� Increased energetics and probable increased Ru release

� Recent studies at SNL characterize ignition conditions for pool

fire

� Recent work focuses on fully drained pool response

� Partial draindown likely similar to boildown dynamics

� Accident management is the next question

� Water spray is a likely management response

� What spray characteristics are needed to prevent ignition conditions

from occurring ? And…

� What spray characteristics are needed to put out a fire once ignited

SC Saturated

Containment vent

H2 Explosion

Unit 1level loss

RCIC - CST

Possible Fuel damageContainment vent

Noise heard ?

RCIC from suppression poolUnit 2

RPV Depressurization

Unit 4 (SFP)Explosion in Unit 4

RCIC operating

Containment vents

H2 Explosion

HPCI operating

RPV Depressurization

low pressure emergency injection

Unit 3

Friday 11 Saturday 12 Sunday 13 Monday 14 Tuesday 15 Wednesday 16

Earthquake at 14:46: LOSP

Tsunami at 15:41: SBO

Timeline of Major Fukushima Damage Events

Fuel Damage

Level loss

Fuel damage

low pressure emergency injection

low pressure emergency injection

more damage possible ?

SC Saturated

SC Saturated

Level loss

sea waterfresh water

night day

• Loss of isolation condenser cooling following tsunami and station blackout produced “hands off” damage progression

• Loss of isolation condenser cooling following tsunami and station blackout produced “hands off” damage progression

• Low pressure water injection was well aligned with operator depressurization

• Water injection believed to minimize core damage initially

• Loss of injection later believed to lead to significant core damage

• Low pressure water injection was well aligned with operator depressurization

• Water injection believed to minimize core damage initially

• Loss of injection later believed to lead to significant core damage

• Unit 2 operated RCIC/HPCI pumps well beyond expected duration

• Unit 2 operated RCIC/HPCI pumps well beyond expected duration

Unit 1

Unit 3

Unit 2

Summary of Emerging Issues

� ST predictive technology on reasonably sound footing

� Important to account for natural attenuation processes and engineered safety

features

� Fallout and deposition

� Water pool scrubbing

� Spray scrubbing

� Managing releases

� Containments generally not designed to withstand severe accidents

� Judicious use of containment sprays and venting to optimize management

� Real-world response of safety related components under severe accident

conditions

� RCIC operation

� Safety relief valves

� Spent Fuel Pools

� Zr-fire has high consequence but low frequency

� Water sprays to prevent or terminate fire and mitigate release

� Multi-unit disasters are not over in 24 hours