Physics Design of Indian Thorium fuelled Pebble Bed Reactor with FLiBe Coolant

Anurag Gupta, Indrajeet Singh

D.K. Dwivedi, P.D. Krishnani

Reactor Physics Design Division Bhabha Atomic Research Centre

CMSNT-2013, Mumbai, January 11, 2013

High Temperature Reactors

Process temperature Up to 700°C Up to 900°C Up to 950°C

Electricity production

Rankine (steam) cycle Brayton (direct) cycle

Utility applications

Desalination H2 via steam reforming of methane

Thermochemical H2 production

Oil and chemical industry

Tar/oil sands and heavy oil recovery, Syncrude, Refinery and petrochemical

Syngas for ammonia and methanol

Thermochemical H2 production

Requirement: Process heat applications

lhttp://www.world-nuclear.org/info/inf116_processheat.html

High Temperature Reactors

Applications

l Hydrogen Production Future fuel

l Electricity generation Better Efficiency

l Desalination Lower temperature use

Desirables

l Thorium Proliferation Resistance

l Safety -ve reactivity coefficients

l Higher Burnup

Moderator Graphite or BeO

Inert Coolant He, Pb-Bi eutectic, Molten Salt (2FLi-BeF2)

Fuel TRISO coated particle Form

Indian High Temperature Reactor Program

Compact High Temperature Reactor (CHTR) A technology demonstration facility

Nuclear Power Pack (NPP)

To supply electricity in remote areas not connected to grid

Innovative High Temperature Reactor (IHTR-H)

For hydrogen generation

Compact High Temperature Reactor (CHTR)

Basic design guidelines:

Ø Reactor Power 100kWth

Ø Time interval between refueling: 15 EFPYs

Ø Coolant outlet temperature: 1000 oC

Ø Use of thorium based fuels

Ø Passive core heat removal by natural circulation

Ø Passive rejection of entire heat to the atmosphere under accidental conditions

CHTR Basic Characteristics

Reactor Power 100 kWth

Core Life 15 years

Fuel TRISO particles of (U233Th)C2 or HEUC2

Fuel Mass Total 8.0 kg of heavy metal

Coolant Lead-Bismuth Eutectic

No. of Fuel Tubes 19

ID/OD of Fuel Tubes 35/75 mm

Hexagonal Pitch 135 mm

Moderator BeO

Reflector BeO and Graphite

Power Regulation

Primary Shutdown System

(Ta with W) rods in 12 outer coolant channels

(Ta with W) rods in inner 6 coolant channels

Secondary SDS (SDS-2) 12 holes in BeO reflector blocks filled with Indium

Core height / diameter 100 cm / 127 cm

CHTR Core Schematic

Triso coated Fuel particle CHTR Fuel Assembly Fuel Compact

Cor

e cr

oss-

sect

iona

l vi

ew o

f CH

TR

CH

TR la

ttice

co

mpo

nent

Fuel Tubes

Graphite Reflector

BCR

Downcomers

BeO Moderator

SDS-2

Th-U233 fuel Ø 2.9 kg of U (with 93% U233) + 5.1 kg of

Th232

Ø Burnup compensation rods are used to control of initial excess reactivity along with Gd as burnable poison.

Ø beff = 4.2 mk.

Ø Fuel temperature coefficient (FTC) = – 5.65 × 10–6 /°C.

Ø The average radial and axial power peaking factor with Th-U233 are 1.18 and 1.57 respectively.

Variation of keff with burnup in CHTR in hot operating condition with Th-U233

CHTR Neutronics

0 1000 2000 3000 4000 5000 60000.98

1.00

1.02

1.04

1.06

1.08

1.10

1.12

1.14

1.16

ff

Burnup (FPDs)

0 gm Gd with all BCR OUT 29.75 gm Gd with all BCR OUT 0 gm Gd with all BCR IN 29.75 gm Gd with all BCR IN

0 1000 2000 3000 4000 5000 60000.98

1.00

1.02

1.04

1.06

1.08

1.10

1.12

1.14

Keff

Burnup(FPDs)

no burnable poison and BCRs are OUT no burnable poison and BCRs are IN

U235 as fissile in UC2 with 50% enrichment

HEU fuel Ø CHTR Fuel inventory: 8.0 kg of 50%

enriched Uranium for 15 EFPYs.

Ø Burnup compensation rods are used to control of initial excess reactivity.

Ø beff = 8.3 mk.

Ø Fuel temperature coefficient (FTC) = –8.6 × 10–6 /°C.

Ø The average radial and axial power peaking factor with HEUC2 fuel are 1.34 and 1.30 respectively.

Variation of keff with burnup in CHTR in hot operating condition with HEU.

CHTR Neutronics

CHTR has two independent and capable shutdown systems in addition to a separate control system:

Control: 12 control rods (CRs) made from a mix of Tantalum and Tungsten in the coolant channels of the twelve outer fuel assemblies.

Primary Shutdown System: 6 shutoff rods (PSRs) like control rods falling in six coolant channels in the second hexagon ring from the centre.

Secondary Shutdown System: 12 holes in BeO reflector blocks filled with Liquid Indium.

Burnup compensation: 6 burnup compensation rods (BCRs), in BeO reflector blocks.

CHTR: Control and Shutdown Systems

CHTR: Worth of Shutdown systems

At the beginning of core life:

Reactor state keff

Hot condition Reactor Startup

All CRs, BCRs , PSRs OUT & SDS-2 not activated 1.0880 1.1470

All 6 BCRs IN 1.0217 1.0781

Only 5 PSRs IN 0.8823 0.9365

Only 11 holes in BeO blocks filled with Indium

0. 8965 0.9591

Primary

Secondary

Point reactor analysis shows that in case of inadvertent withdrawal of single control rod of maximum worth in 5 seconds, the power rises to 6.5 times & stabilizes at 2.8 times the initial power and fuel temperature rises to 1238 °C & stabilizes at 1184 °C.

0 200 400 600 800 1000

1

2

3

4

5

6

7

Pmax/Po=6.54Pcon/Po=2.84

Rel

ativ

e Po

wer

Time(seconds)

0 200 400 600 800 1000

950

1000

1050

1100

1150

1200

1250

Tc(stablize)=1042 0CTc(max)=1069 0C

Tf(stablize)=1184 0CTf(max)=1238 0C

Tem

pera

ture

(o C)

Time(seconds)

Preliminary safety analysis of CHTR Inadvertent withdrawal accident of single control rod in critical condition

(U23

3 -Th

)C2

HEU

C2

For HEU fuel, Point kinetics analysis in case of fast transient shows that in the case of inadvertent withdrawal of single control rod in hot critical condition, power initially rises and stabilizes at about 3.0 times the initial power and fuel temperature stabilizes at 1220 oC.

0 100 200 300 400 500 600 700 800 9000

1

2

3

4

5

6

7

8

9

Pstable/Po = 3.0

Pmax/Po= 8.0R

elat

ive

Pow

er

Time (seconds)0 100 200 300 400 500 600 700 800 900

950

1000

1050

1100

1150

1200

1250

1300

Tc(stable) = 1057 oCTc(max) = 1097 oC

Tf(stable) = 1220 oC

Tem

pera

ture

Time (seconds)

Tf(max) = 1290 oC

Ø The core is small having large neutron leakage.

Ø Simulation of TRISO particles, consideration of double heterogeneity: Reactivity equivalent Physical Transformation (RPT) method

Ø The fuel temperature coefficient is less negative: Study for use of Erbium as burnable poison.

Ø The design of a control system with reduced maximum worth of a control rod at criticality is quite challenging.

Ø Core being small, the design of 2 independent shut down systems is a challenging job.

Ø Implementation of liquid Indium as secondary SDS is challenging task: sticky Indium gives high negative reactivity

Ø The cross sections for the some of the non-standard materials (e.g. Be, Ta, Bi etc.) are very different in different cross section libraries. For some, materials (e.g. Bi), the cross sections were not available.

Physics design challenges in CHTR

Innovative High Temperature Reactor (IHTR-H)

For hydrogen generation

IHTR-H: Selection of Coolant

Difficulty with He coolant

q High Pressure reactor operation

q Low power density

q Higher fuel temperature and Lower outlet temperature Liquid coolant

l Better heat removal

l Natural circulation

Liquid Metal Lead-Bismuth Eutectic

Liquid salt LiF-BeF2

Neutronic efficiency for comparison Materials and Coolant Candidates

Isotopes Cross sections (barns) at 0.0253ev

Be 6.2 (scattering) F 3.7 (scattering) 6Li (7.5%) 941.1 (absorption) 7Li (92.5%) 1.0 (scattering)

candidate coolants Material

Total neutron capture (per unit volume) relative to graphite

Moderating ratio (avg.over 0.1–10 eV)

Thermal Conductivity (W/m.0C)

Melting /Boiling (0C)

ρCp (kJ/m3.0C)

Light water 75 246 0.56 0/100 4040 Graphite 1 863 25-470 3652/4200 3230 Sodium 47 2 62 97/883 1040 LiF-BeF2 8 60 1.0 459/1430 4540 He 0 - 0.29 -- 20 LBE - <1 16 123/1670 1700

Reactivity coefficients for pebbles containing 12 g of uranium with 10% enrichment. All coolants at ambient pressure except Helium.

Fluoride Salt

K-inf Complete voiding reactivity ($)

Uniform temperature reactivity coefficient (pcm/ K)

Li- Be 1.39 -2.30 -7.68 Na- Be 1.11 21.5 -2.53 Li- Na- K 0.71 87.9 8.14 Na- Zr 1.10 23.0 -0.465 Na- Zr- K 0.81 65.1 5.42 Li- Na- Zr 1.15 17.7 -1.53 Na- NaB 0.86 56.2 8.32 Helium 1.36 -0.11 8.58

Source : PHYSOR-2006 Conference, Vancouver, Canada S.J. de Zwaan, J.L.Kloosterman,D.Lathouwers& B.BoerFaculty of Applied Sciences (TNW) Department of Radiation, Radionuclides & Reactors (R3)Section Physics of Nuclear Reactors (PNR)September 10-14, 2006

Liquid Salt Coolant Candidates

World Wide Work on Similar Reactor Concepts

q Oak Ridge National Laboratories design: AHTR / LS-VHTR

q TU Delft design: Liquid Salt-cooled Pebble Bed reactor (LSPBR)

q BARC: Innovative High Temperature Reactor(IHTR)

AHTR/LS-VHTR LSPBR IHTR Offline refueling

Online refueling Online refueling

Wide range in volume fractions

Fixed coolant volume fraction

Fixed coolant volume fraction

Pebble Channel Assemblies

Pebble bed fuel geometry

Pebble bed fuel geometry

6cm Pebble 6cm pebble 10cm pebble

LiF-BeF2 coolant LiF-BeF2 coolant LiF-BeF2 coolant

Reactor Power

600 MWth for following deliverables: Hydrogen: 80,000 Nm3/hr Electricity: 18 MWe Drinking water: 375 m3/hr

Fuel (233U-Th)O2 Based TRISO Coated Particles

Moderator material Graphite

Reflector material Graphite (inner & Outer)

Coolant Molten Pb-Bi or FLiBe Molten Salt (2LiF-BeF2)

Control B4C control rods/balls in outer and inner reflector

Burnup 900 Full Power Days (~78,000 MWD/T)

Basic Design Guidelines

600 MWth Pebble Bed IHTR-H

600 MWth Pebble Bed IHTR-H

TRISO Particles/ Fuel Pebbles:

OD of TRISO Particle 900 microns

ID/OD of Fuel Pebble 9/10cm

Packing Fraction of TRISO Particles in Pebble

8.6 %

U-233 content 7.6 %

Number of TRISO particles per Pebble 86,000

(U+Th)C2 Kernel (250 mm)

Pyrolitic Graphite (90 mm)

Inner Dense Carbon (30 mm)

Silicon Carbide (30 mm)

Outer Dense Carbon (50 mm)

Graphite Layer

Fuel Zone

Regular and Annular Pebbles

Pebble Bed Core:

OD of Inner Reflector 2.0 m

ID/OD of Outer Reflector 3.5m / 4.0m

Active Core Height 800-1100 cm

Total Number of Pebble in Annular Core 1,48,681

Packing Fraction of Pebbles in Core 60%

U-233/HM per Pebble 3.5g/46.5g

Total U-233 requirement 521 Kg

600 MWth Pebble Bed IHTR-H

Central Reflector

De-Fuelling Chute

Side Reflector

Bottom Reflector

Core Barrel Support

Fuelling pipe

Coolant Outlet

Pebble Retaining MeshPebbles and Coolant

Coolant Inlet

Reactor Vessel

Coolant

Central Reflector

De-Fuelling Chute

Side Reflector

Bottom Reflector

Core Barrel Support

Fuelling pipe

Coolant Outlet

Pebble Retaining MeshPebbles and Coolant

Coolant Inlet

Reactor Vessel

Coolant

Activities:

600 MWth Pebble Bed IHTR-H

v The collision probability based code ITRAN is used in lattice calculation to generate 12-group lattice parameters.

v 3D diffusion codes (Tri-HTR, ARCH) in hexagonal geometry for core calculation using ITRAN generated 12-group cross section.

Computational Tools Used:

v Estimation of TRISO particle packing fraction and U-233 content optimization study has been done for 10 cm size pebbles.

v Studies to obtain optimum value of moderation, Packing fraction of TRISO particles in a pebble and U-233 content.

v Location of the fueled/unfueled zones interface in fuel pebble to have optimum moderation.

v Double Heterogeneity treatment.

Ø The fuel is lumped into tiny TRISO particles and a large number of TRISO particles are dispersed in a graphite matrix .

Ø Volume weighted homogenization of a fuel zone with TRISO particles results in significant reduction in the resonance self-shielding effect.

Ø This effect is known as the Double Heterogeneity and should be included in burnup computations.

Ø Codes have been developed to treat this effect and compared.

Double Heterogeneity of Fuel Pebble of IHTR-H

Reactivity with Burnup K-eff vs Burn-up of IHTR-600MWth at 200C without Xenon Load

0.90

1.00

1.10

1.20

1.30

1.40

1.50

0 50 100 150 200 250 300 350 400 450 500Burn-Up (FPD)

K-e

ff

RPT MODEL OF PEBBLE

SIMPLE VOLUME WEIGHTEDHOMOGENISED MODEL OF PEBBLE

600 MWth Pebble Bed IHTR-H

Comparison of Pb-Bi With FLiBe

Pb-Bi Results in • Low Reactivity (Annular/Regular Pebble)

• Lower Burn-up

• Almost no moderation

• Non Negative CVRC

FLiBe Shows • Better neutronic behaviour • CVRC can be made –ve at appropriate

loading of heavy metal

FLiBe Coolant Void Reactivity for (Th-U233)O2

U233, 10cm

U233, 6cm

U233 fuel pebble

To get under-moderation-

• Large capture in coolant result in less negative CVRC for under-moderated core

• PF should be more than 18% in 10cm

pebble.

• PF should be more than 22% in 6cm pebble

• U235 is found to be better fuel than U233

to have under moderated core for a lower PF and smaller kernel radius.

--- No Void --- 8% void

--- No Void --- 8% void

v It is challenging to model IHTR with ~150000 pebbles in core with 48 rods(CSD) in outer reflector and 24 rods (SSD) in central reflector. (B4C rod/pebbles)

v Due to Large excess reactivity, initial core has to be designed with low enrichment and use of Dummy pebbles in two ratios (F/M= 1:1 and 1:2).

v A core height of 10 meter is marginally safe & 11 meter height seems to give better worth of CSD.

Control and Shutdown systems for IHTR-H

Reactor core (Top view)

v For (Th-233U)O2, 18%PF, CVRC is –ve in the beginning and becomes less –ve with burn-up.

v Below 18% PF, CVRC is +ve.

v Enriched UO2 fuel with only 10%PF results in –ve CVRC and remains –ve for all values of Burn-up up to at least 500 FPD.

U233, 10cm

U235, 10cm

CVRC with burn-up for (Th-233U)O2 and (UO2 ) in 10 cm pebbles

E ø(

E)

Ø Spectrum of the IHTR core fuelled with enriched UO2 and cooled by FLIBE.

Ø Two spectra represent the Un-voided and voided conditions of the coolant.

Ø Blue curve is U-238 cross-sections.

Ø After voiding capture rate in U-238 increases and results in –ve CVRC. (10%PF)

ü Spectrum of the IHTR core fuelled with (Th-233U)O2 and cooled by FLIBE.

ü Two spectrum represents the Un-voided and voided conditions of the coolant.

ü Blue curve is of Th-232 cross-sections.

ü Capture rate in Thorium increases but not dominating over coolant capture and results in +ve CVRC at same PF.

IHTR-H Spectrum for (Th-233U)O2 and (UO2 ) in 10 cm pebbles

E ø(

E)

Thank You

l

Pb-Bi Coolant l

Graphite l

TRISO Fuel l

BeO

Fuel Assembly

l

(U+Th)C2 Kernel (250 mm) l

Pyrolitic Graphite (90 mm) l

Inner Dense Carbon (30 mm) l

Silicon Carbide (30 mm) l

Outer Dense Carbon (50 mm)

35 mm

Fuel Compact: Triso fuel particles embedded in graphite matrix

Fuel Compact and Triso Particle

lf 25mm

f 19mm

f 17mm

lTa

lW

lf 43mm

lf 31mm

lf 29mm

lTa

lW

In-Fuel CR

In-Reflector CR

Control Rods