Phenomenological Advances 

Accident Progressio

R.O. Gauntt, Reactor Analysis and M

Department 

Sandia National Laboratories 

Albuquerque, New Mexico 

Dept 6762 rogaunt@sandia.go
SOARCA Objectiv

• Perform a state-of-the-art, realisti

evaluation of severe accident progression, radiological releases offsite consequences for importanlikelihood accident sequences

• State of Art in

– Understanding and modeling of physics and 

phenomena

– Fidelity and realism in system representatio
Timeline of Nuclear 

Safety Technology Evolution

Timeline of Nuclear Safety Technology Evolution

OptimisticGuardedPessimistic

Risk Inform

- Mod

License Am

- MOX- Plan

Emergency Advanced R

- AP10

NGNP - HTGNEP - Fas

NRC

Chicago Critical Pile

Atomic Energy Act of 1954

WASH 1400

AEC

1982 Sandia

Siting Study TID 14844

source term

NU

al

so

Emerging

 Issues.....

WASH 740

NUREG-0772

NUREG-0956

MELCOR Development 

Core Heatup and FissioRelease

Aa– c- a

Fission product 

specie/volatility 

modified (Cs2 MoO4 ) – 

Phebus Tests – 

affects RCS 

deposition

Fuel failure criteria 

expanded via control 

function – Phebus 

tests – affects 

hydrogen generation 

and melt progression

BWR failure criteria 

expanded

RN Package expanded to 

allow analysis of FP 

release from mixed 

MOX/LEU core 

(French VERCORS and RT 

tests)
MELCOR Developmen

Late Phase Melt Progr

Cd(2

Coforpasid

Moadhea

Molten pool stratification into 

light and heavy layers in core 

and lower head 

(RASPLAV,MASCA)

Natural convection heat 

transfer in circulating melt 

pools

Fission product partitioning 

between melt phases 

(RASPLAV,MASCA)

Insights from OECD LH 

program in creep-rupture 

models for lower head
Phebus Program Be

with MELCOR
Fuel Heatup and D

400900

1400190024002900

0 5000 10000 15000 20000

time [sec]

Temperature [K]

TCW6TCW8TCS 31TC33

TU2R70TC2R70TSH170TSH270TCW6TCW8TCS31TC33

Oxidationtransient
Cladding Oxidatio

Hydrogen Genera

• Ste

moby therescalhydge
Fission Product Relea

Source Term

• ORNL Booth model captures kinetic• CORSOR-M over-predicts early rele

FPT-1 Class 2 Release - Cs

0

0.10.20.30.40.50.60.70.80.9

1

5000 7500 10000 12500 15000 17500 

time (sec)

Release Fraction
MELCOR Aeros

Mechanics - MA

• MAEROS section

– 10 sections [.1 – Condensed FP 

smallest section

• Particles grow in s

– Agglomeration– Water condens

• Particle fallout by • Particle deposition 

– Thermophores– Diffusiophores– Brownian moti

• PWR Ag aeroso

rods modeled

– Significant ae– Affects agglom

fallout

• Cs chemisorptio

– Iodine from C

reheated

0.1 1 10 100

aerosol diameter [

μm]

section mass 

0.1 1 10 100

aerosol diameter [

μm]

section mass 

Aerosol size distribution evolves in time, depending on sources, agglomeration and removal processes 

Time 1

Time 2
VANAM M3 Con

Aerosol Behavio

Aerosol Depletion in Dome

Time [h]

18 20 22 24 26 28 30

Aerosol Concentration [kg/m3]

10-710-610-510-410-310-2

184 Original Nodaliz

a

184 Modified Nodaliz

a

Experiment185 Modified Nodaliz

a
TMI-2 Assessm

• Accid

assesTMI-2 interncomp

• MELC

melt psignifbased exper
Realism in Modeling

• Early work required considerable 

abstraction in representation of pla

– Computer limitations on problem size– Problem “dumbed down” to permit an

• Current emphasis is on realism

– Spatial resolution (node size for resolv

physics)

– Accurate representation of plant

• Rooms and compartments• Heat structures• Transport time and deposition
Reactor Severe Accident Modeling 

Evolution of the State-of-the-Art 

Circa 1985 Circa 1990 Circa 1995 Circa 1998 

Timeline for Evolution of MELCOR Modeling P

• NUREG-1150• Basis for 

NUREG-1465 revised source term

• AP-600 design 

certification

• ESBWR design 

certification

• AP-1000 design 

certification

• Begin SGTR • Finish SGTR • MO

te

• Hi

so

Example Regulatory Applications

Modeling Techniques

STCP

CV210

SG Tubes

(Up)

CV200

Hot Leg

CV215

SG Tubes

(Down)

CV220

Pump

Suction CV240 Cold Leg To Vessel (downcomer)

From Vessel

(upper plenum)

 CV450

FL440

FL401

CV440

CV437

CV435

Accum FL441

CV460

 CV417

 CV416 FL431

FL450

CV465

FL454

FL471

Aux Feed

FL462

PORV/ADV

FL463

CV470

CV480

CV281

(steam line/turbine)

CV001

(environment)

FL485

Reactor Vessel

FL442

FL453

FL483

FL486

FL400

CV401

FL405

FL442 CV440 FL443

CV420

 CV413

 CV414

CV421

CV402

CV411 CV415

FL402

FL492

FL410 FL493

FL491 FL496

CV409

CV412

FL416

FL415

FL495

FL465

FL433

FL494

CV425

CV426

FL418

FL428

FL427

FL417

FL452

SRV

Steam Line

MSIV

See

Pressurizer

Detail

 CV450

FL440

FL401

CV440

CV437

CV435

Accum FL441

CV460

 CV417

 CV416 FL431

FL450

CV465

FL454

FL471

Aux Feed

FL462

PORV/ADV

FL463

CV470

CV480

CV281

(steam line/turbine)

CV001

(environment)

FL485

Reactor Vessel

FL442

FL453

FL483

FL486

FL400

CV401

FL405

FL442 CV440 FL443

CV420

 CV413

 CV414

CV421

CV402

CV411 CV415

FL402

FL492

FL410 FL493

FL491 FL496

CV409

CV412

FL416

FL415

FL495

FL465

FL433

FL494

CV425

CV426

FL418

FL428

FL42

FL

7

417

FL452

SRV

Steam Line

MSIV

See

Pressurizer

Detail

CV160

Upper Head

CV140

Core Region

CV110

Lower Plenum

CV130

Core

Bypass

CV110

Downcomer

CV150

Upper Plenum

CV160

Upper Head

CV140

Core Region

CV110

Lower Plenum

CV130

Core

Bypass

CV110

Downcomer

CV150

Upper Plenum

160

116

126

136

146154

716

724

736 746 756

701 101

110

Upper

Head

114 124

117 127 137 147 157

100

Downcomer

134 144

156

114

112 122 132 142

153152

124 134 144 154

199

156

172 182

193192

Bypass

718

726

738 748

Lower

Plenum

758

768 778 788 798

714

722

766 776 786 796762 772 782 792

712 752

760 770 780 790

720 750

732

740742

734

710 720 730 740 750 701

Ring 1 Ring 2 Ring 3 Ring 4 Ring 5

110

164 174 184 194162

116 126 136

146

152

706

710

728

730

744 754

764 774 784 794

153

112 122 132 142

160

116

126

716

724

736 

110

Upper

Head

114 124

117 127 

134 

114

112 122 132 

124 134 

172 182

718

726

738 

Lower

Plenum

768 778 

714

722

766 776 762 772 

712 

760 770 

720 

732734

710 720 730 

Ring 1 Ring 2 Ring 3 

164 174 18162

116 126 13

710

728

730

764 774 

112 122 132 

SO
Modeling Detail 

Minimizing Abstr

Old Nodalization

CV055

Dome

CV020

SG Cubicles

CV041

Pressurizer

Cubicle

CV050

Lower Dome

CV005

Basement

CV010

Cavity

Enhanced N

CV's

20,30,40

C

C

CV5 

C

R

V

Steam Generator

Grade

CV850, Auxiliary Building

CV800

LHSI

Spray Pumphouse
Auxiliary Building

Safeguards Building

Containment

Typically model entire plant, 

including all potential radiological release paths 

(and therefore potential fission 

product retention mechanisms)

Modeling of Plant Buildings
160

116

126

716

724

736 

110

Upper

Head

114 124

117 127 

134 

114

112 122 132 

124 13

172 

718

726

738 

Lower

Plenum

768 778 

714

722

766 776 

762 772 

712 

760 770 

720 

732734

710 720 730 

Ring 1 Ring 2 Ring 3 

164 174 

162

116 126 

710

728

730

764 774 

112 122 13

Old Nodalization Enhanced N

CV160

Upper Head

CV140

Core Region

CV110

Lower Plenum

CV130

Core

Bypass

CV110

Downcomer

CV150

Upper Plenum

Model Upgrades - Vessel Nodalization 
Natural Circulation 
Westinghouse PWR C

• Weight of core material • Engineering stress form

Roark)

• Failure based on excee

temperature

• Sequential failure of mu

structures treated

CL

Lower core

plate

Flow mixer plate

Core plate 

Support Columns

1

Ring 2 3 

Lower core plate

Flow diffuser plate

Core plate 

support columns

Core support forging
Impact of Modeling Advances

• Accident progression generally 

significantly more slowly developin

• Significantly more fission product 

in RCS and containment

• More time for mitigation of acciden• Significantly reduced source terms• Even SGTR bypass source term l

traditionally thought owing to RCS in containment

Summary

• Releases much delayed compare

– Mostly many hours to 10’s compared to ~1– Due to detail modeling of reactor systems a

mitigations

• Releases to environment much le

SST-1

– Detailed transport treatment and deposition – Delayed release enhances natural depositio

• Plant models account for 

– Improvements in modern plant design– Important operator actions

• Peer review and uncertainty quan

work next