UC Berkeley

Cristhian Galvez, Nicolas Zweibaum, Per PetersonThermal Hydraulics Laboratory

Department of Nuclear EngineeringUniversity of California, Berkeley

2010 RELAP 5 International Users Seminar

Design and Analysis of the PB-AHTR using RELAP5

UC Berkeley

Outline

• Introduction

• Overview of Plant Design

• Modeling needs

• Plant system->process modeling breakdown

• Solution methodology

• Results

• Conclusion + Future Work

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Introduction

• The Pebble Bed Advanced High Temperature Reactor (PB-AHTR) is a pebble fueled, fluoride-salt cooled, 900-MWt reactor under development at UC Berkeley.

• Design features large thermal margins to fuel damage. Thermal limits are imposed by metallic primary loop structures. Peak core outlet temperature is the parameter of interest

1600°C

Fuel failure fraction vs. temperature

max.PB-AHTR

temp

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Overview of Plant Design: Diagram

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Overview of Plant Design: 3D render

Reactor

Primary Pumps

Recuperator

Turbines

Compressors

Generators

Intercoolers

Precoolers

Helium heaters

Intermediate pumps

Intermediate heat exchangers

Intermediate drain tank

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Coolant Flow diagram

• Primary heat removal system composed of 4 Intermediate Heat Exchangers (IHX).

• Passive decay heat removal mechanism accomplished through 8 Direct Reactor Auxiliary Cooling System (DRACS). Heat is absorbed by the Direct Heat Exchanger (DHX), which is similar in design to the IHX and rejects heat to the environment through air-cooled Natural Draft Heat Exchangers (NDHX)

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Annular-type Core

Annular core and components diagram Lateral cross section

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Annular Pebble Bed core design

• Radially-zoned injection of buoyant pebbles• Alternative injection of seed and blanket pebbles (axial zoning)• Pebble Recirculation Experiment (PREX-2), 42% actual core size, high

density polyethylene spheres, dry

Radially and axially zoned pebble bed core PREX-2 filled with 129,840 pebbles

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Channel-type core

Pebble channel assemblycore and components

Elevation view andlateral cross section

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Channel Pebble Bed core design

Baseline design for lower half of PCA showing configuration of pebble

channels

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Fuel and Coolant

Flibe Primary Coolant (Li2BeF2)

• Excellent heat transfer

• Transparent, clean fluoride salt

• Boiling point ~1400ºC

• Reacts very slowly in air

• No energy source to pressurize containment

RELAP5-3D pebble fuel model description from pebble center (left) to pebble surface

(right) for the annular pebble design

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Active cooling system: Intermediate Heat Exchanger (IHX) and pumps

C/L

Shell inside Shell Outside

Y

X

Baffles

=> Cross- flow

IHX

• Tube and shell, disk and doughnut baffled heat exchanger

• Primary coolant (Flibe) on tube side, Intermediate coolant (Flinabe) on shell side

• Forced convection on external and internal side driven by active centrifugal conventional pumps

• Derived from MSBR heat exchanger design

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Passive cooling system (DRACS): DHX, NDHX, Fluidic diode

NDHX

• Tube and shell helical heat exchanger

• Natural circulation coolant (Flinabe) on tube side, Natural draft coolant (Air) on shell side

• Radiation heat transfer important

Fluidic Diode

• Low resistance during forward flow, high resistance during reverse flow

• Passive operation

DHX

• Tube and shell heat exchanger

• Forced primary coolant (Flibe) on shell side, Natural circulation coolant (Flinabe) on tube side

• Radiation heat transfer possibly important

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Reactivity control: Feedback mechanisms

Fuel and Moderator Temperature Feedback

•Monte Carlo studies performed to determine fuel and moderator temperature reactivity feedback coefficients

•Study was done for various fuel-burn up levels, however RELAP5 analysis assumes average burn-up

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keff vs. Rod Position, Rod Geometry

0.8

0.85

0.9

0.95

1

1.05

0 10 20 30 40 50 60 70 80 90 100

% Rod Insertion

kef

f

Cruciform Rods Cylindrical Rods

Reactivity control: Shutdown rodShutdown-rod design

• Neutrally buoyant rod remains above the core during normal operation at typical coolant temperatures, but looses buoyancy and sinks into rod channel during above-normal coolant temperatures during transients

• Analytical and experimental work to determine rod insertion speed and rod worth

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Analysis Objectives

• Steady state: Mass, Pressure and Temperature distribution

• Transient: Peak core outlet temperature

• Safety system performance

• Decay heat removal system performance

• Experiment design analysis

Design and analysis of the PB-AHTR requires investigation employing analytical, computational and experimental tools

In order to obtain variables of interest and capture important phenomena, a methodology to breakdown the system and model it is used

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System-process breakdown: Core

Core

Coolant ReflectorFuel

1-ϕ liquid pebble bed void volumeSolid spherical Solid cylindrical

COEnergy COMass COMom COEnergyCOEnergy

-Energy generation-Conduction

-Continuity -Convection -Form loss -Friction loss

-Convection-Conduction

22 2

1p

d dT dTkr q C

r d r dr dt

1

p

d dT dTkr C

r dr dr dt

0.6 0.332 1.1Re Prh

kh

D

3003.5

Ref

( )

I I

I

t tI

t oI I

P P e e

Not available in current version of RELAP

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Active cooling

Secondary PumpIntermediate Heat Exchanger IHX

1-ϕ liquid tube side volume

Solid cylindrical

1-ϕ liquid pump volume

COEnergy COMass COMom COMomCOEnergy

-Conduction -Continuity -Convection -Form loss -Friction loss

-Momentum addition

1-ϕ liquid shell side volume

Primary Pump

1-ϕ liquid pump volume

COMom

-Momentum addition

p H g

P T

gQH

T

0.618 0.333 0.14

0.6 0.33 0.14

0.346Re Pr ( )

0.128Re Pr ( )

crossh w

parallelh w

lam

tubeside turh

NatCirc

kh

D

kh

D

Nuk

h NuD

Nu

1p

d dT dTkr C

r dr dr dt

10

0.6

0.6

64

Re

1 / 2.512log

3.70 Re

cross restric

paral restric baffle cut

tubeside

f N

f N F

f lam

fD

turbf f

System-process breakdown: Active Cooling

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Passive cooling

Direct Heat Exchanger DHX

1-ϕ liquid tube side volume

COMass COMomCOEnergy

-Continuity -Convection -Form loss -Friction loss

1-ϕ liquid shell side volume

Natural Draft Heat Exchanger NDHX

1-ϕ gas shell side volume

1-ϕ liquid diode volume

COMom

0.618 0.333 0.14

0.6 0.33 0.14

0.346Re Pr ( )

0.128Re Pr ( )

crossh w

parallelh w

lam

tubeside turh

NatCirc

kh

D

kh

D

Nuk

h NuD

Nu

10

0.6

0.6

64

Re

1 / 2.512log

3.70 Re

cross restric

paral restric baffle cut

tubeside

f N

f N F

f lam

fD

turbf f

unbaffled

curved pipe

f

f

curved pipeh

System-process breakdown: Passive Cooling

Fluidic diode

-Form loss-Friction loss

1-ϕ liquid tube side volume

forward

reverse

f

f

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RELAP5-3D Model

Evolutionary steps taken to deal with modeling ‘gaps’

• 1st: Input heat and flow loss coefficients manually– Only valid for steady state calculations

• 2nd: Input heat and flow loss coefficients manually as a function of time with self-consistent heat / flow loss coeficients and mass flow history

– Approximation for transient

• 3rd: Manipulate existing LWR options in RELAP5-3D to add user-input factors to replicate correlation using multipliers (fouling factor for h and internal junction form loss for f)

– Better approximation but still incomplete since power exponents of Re and Pr do not exactly match with available correlations coded in RELAP5-3D (Shah & ESDU cross flow)

• 4th: Implement pebble bed correlations into source code

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Annular Core RELAP5-3D Model

• 1/8 symmetric core modeled• 3 multi-dimensional axial zones: inlet, mid-section and outlet• Active mesh: inlet: 47, mid-section:81, outlet:32• Fixed T,P at coolant sources and fixed P at coolant sinks• Power distribution resulting from coupling studies with MCNP5

Geometrical Configuration of the Core and the RELAP5-3D Model

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Annular core flow distrubition

1.2 1.4 1.6 1.8 2 2.2 2.4

0.5

1

1.5

2

2.5

Velocity vector Field ofMid Core at time t=500

Cross sectional view

Axis

dis

tances in M

ete

rs

1.2 1.4 1.6 1.8 2 2.2

0.5

1

1.5

2

2.5

3

Velocity vector Field ofInsertion at time t=500

Cross sectional view

Axis

dis

tances in M

ete

rs

1 1.5 2

0.5

1

1.5

2

2.5

Velocity vector Field ofDe Fueling at time t=500

Cross sectional view

Axis

dis

tances in M

ete

rs

Core Diagram

COMSOL FEM Multiphysics Model

RELAP5-3D Model

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Outlet Temperature Parametric Analysis

Inlets and Outlets Distributions in the Bottom, Mid-Section and Upper Core

(a) (c)(b) (d)

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Outlet Temperature Distributions

Temperature distributions of the outlets in different model variations

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Best Model Variation

• ΔT=97K, optimal difference

Best model variation sketch and simulation result

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Channel Core RELAP5-3D Model

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Transient Description

• Several transients are analyzed, but focus of this study is Loss of Forced Circulation (LOFC) and Loss of Heat Sink (LOHS)

• LOFC involves the trip of the primary pumps, LOHS involves the trip of the intermediate pumps

• Both transients are evaluated under a different assumed safety system response

1. Normal scram immediately after primary or intermediate pumps. Shutdown rod bank inserted.

2. Failure to actively scram reactor with shutdown rods. Passive, buoyancy driven shutdown rod insertion occurs. Scram accomplished after a delay

3. Failure to scram reactor with either system. Power reactivity coefficient is the only mechanism present to shutdown the reactor

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Transient Results: Loss of Forced Circulation

1000 1500 2000 2500 3000 3500

620

640

660

680

700

720

740

760

Core Inlet and Outlet Temperature of LOFC transient[C]for Active Shutdown (AS) and Passive Shutdown (PS)

Time [s]

Tem

pera

ture

[C

]

AS Inlet

AS Onlet

PS Inlet

PS Outlet

1000 1500 2000 2500 3000 3500700

800

900

1000

1100

Average Fuel temperature of LOFC transient[C]for Active Shutdown (AS) and Passive Shutdown (PS)

Time [s]

Tem

pera

ture

[C

]

AS

PS

• Fast loss of primary flow at t = 1000 s. Passive shutdown rod insert ~32 s after transient initiation. Average fuel and core outlet coolant temperatures rise to acceptable levels

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Transient Results: Loss of Forced Circulation

• Fast loss of primary flow at t = 1000 s. Flow within the Direct Heat Exchanger passively inverts shortly after the transient initiation. Steady state natural circulation for decay heat removal is rapidly obtained. Temperatures in metallic heat exchanger remain acceptable during severe transient

1000 1500 2000 2500 3000 3500-40

-20

0

20

40

60

80

DHX mass flow rate during LOFC transientfor Active Shutdown (AS) and Passive Shutdown (PS)

Time [s]

Mas

s flo

w r

ate

[kg/

s]

PS

AS

1000 1500 2000 2500 3000 3500550

600

650

700

Core Inlet and Outlet Temperature of LOFC transient[C]for Active Shutdown (AS) and Passive Shutdown (PS)

Time [s]

Tem

pera

ture

[C

] PS Inlet

PS Oulet

AS Inlet

AS Outlet

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Transient Results: Loss of Heat Sink

• Fast loss of intermediate flow at t = 1000 s. Passive shutdown rod insert ~32 s after transient initiation. Coolant temperatures rise to acceptable levels

800 1000 1200 1400 1600 1800 2000

650

700

750

Core Inlet and Outlet Temperature of LOHS transient[C]for Active Shutdown (AS) and Passive Shutdown (PS)

Time [s]

Tem

pera

ture

[C

]

AS Outlet

AS Inlet

PS Inlet

PS Outlet

800 1000 1200 1400 1600 1800 2000

650

700

750

800

850

900

950

Average Fuel Temperature of LOHS transient[C]for Active Shutdown (AS) and Passive Shutdown (PS)

Time [s]

Tem

pera

ture

[C

]

AS

PS

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Transient Results: Loss of Heat Sink

• Fast loss of intermediate flow at t = 1000 s. Intermediate coolant flow is quickly reduced to negligible amounts. Thermal reactivity feedback shuts down the reactor quicker in the case of LOHS transients vs. LOFC transients.

1000 1100 1200 1300 1400-10

-8

-6

-4

-2

0

Core reactivity insertionfor LOHS and LOFC transient with passive scram

Time [s]

Rea

ctiv

ity [

$]

LOFC

LOHS

800 1000 1200 1400 1600 1800 20000

200

400

600

800

LOHS transientIntermediate cooling system mass flow rate

Time [s]

Mas

s flo

w r

ate

[kg/

s]

mass flow rate

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Conclusions

• RELAP5-3D model matches well with its analytical results, confidence in model exists for steady and transient conditions

• Passive and inherent reactor control mechanism perform well under postulated transients and maintain temperatures well below thermal damage limits for fuel (~1600 oC) and metallic structures (~765 oC) for Hastelloy 800 H

• Model provides preliminary insights on passive safety performance of the PB-AHTR. Additional work is necessary in order to consider other limiting cases such as 1) partial loss of flow 2) partial core flow blockage 3) partial heat exchanger flow blockage

• Need to configure the annular core model for transient simulations with optimized coolant outlet geometric distribution