Fission-Product Release and Transport in the RCS

Lawrence Dickson, Fuel Modeling Section LeaderFuel and Fuel Channel Safety (F&FCS) Branch

Presented to US Nuclear Regulatory CommissionWashington DC

May 6-7, 2003

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Outline

• ACR Fuel and RCS Design• Fission-Product Release and Transport in Limited and

Severe Core Damage Accidents• Experimental Database• Computer Codes

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ACR Fuel Channel Details

Annulus Gas Fuel

Heavy Water Moderator

Light Water Coolant Annulus Spacer

Calandria Tube Pressure Tube

Fuel is uranium oxide clad with Zircaloy-4

Moderator is unpressurized and below 100oC

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ACR Fuel Channel

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ACR Shield Plug

• The ACR shield plug is based on a flow through design

• The shield plug is attached in the bore of the end fitting to locate the fuel bundlesand to provide shielding

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ACR Reactor Coolant System Layout

Similar to LWR above the headers

Below headers, feeders and horizontal fuel channels instead of a pressure vessel

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Outline

• ACR Fuel and RCS Design• Fission-Product Release and Transport in Limited and

Severe Core Damage Accidents• Experimental Database• Computer Codes

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Fission Product Release and Transport

• Fission product release from fuel and transport in the reactor coolant system are assessed to determine FP release into containment under accident conditions

• FP release and transport calculations are part of the source term analysis methodology

• FP release and transport simulations are used in estimating doses to the public, station staff and plant equipment

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Fission-Product Release Behavior

• Diffusion in fuel grains− Fuel oxidation increases diffusion rate

• Accumulation and venting from grain boundaries• Grain-boundary sweeping• Accumulation on fuel surface and in fuel-clad gap• Redox conditions (hydrogen vs. steam) of fuel

environment after cladding failure affect volatility− Noble gases (Kr, Xe) and volatile elements (I, Cs, Te, etc.) are

released from the fuel in significant amounts at high temps.− Other elements (e.g., Ru) may also be released if the fuel is

exposed to oxidizing conditions for extended periods

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FP Release Phenomena (1)

• Athermal Release (knockout, recoil and fission-spike) • Diffusion (from fuel grains to grain boundaries)• Grain-Boundary Sweeping / Grain Growth• Grain-Boundary Bubble Coalescence / Tunnel

Interlinkage• Vapor Transport / Columnar Grains• Fuel Cracking (thermal)• Gap Transport (failed elements)• Gap Retention

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FP Release Phenomena (2)

• Uranium Oxidation State − UO2-x ↔↔↔↔ UO2 ↔↔↔↔ UO2+x ↔↔↔↔ U4O9 ↔↔↔↔ U3O8

• UO2 – Zircaloy Interaction• UO2 Dissolution in Molten Zircaloy• Fuel Melting• Fission Product Vaporization / Volatilization• Matrix Stripping• Temperature Transients• Grain-Boundary Separation• Fission-Product Leaching

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Fission-Product Transport Behavior

• Noble gases transported to the break• Retention of other fission products can occur in the

reactor coolant system between the fuel and the break location

• Aerosol deposition, especially in− Complex geometries (e.g., end fittings)− Condensing steam (e.g., in feeder pipes)− Water-filled components (e.g., headers and steam generators)

• Fission-product vapor condensation• Fission-product vapor reactions with piping surfaces

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RCS FP Transport Phenomena (1)

• Fuel Particulate Suspension• Vapor Deposition and Revaporization of Deposits• Vapor / Structure Interaction• Aerosol Nucleation• Aerosol Agglomeration

− Gravitational, Brownian motion (diffusional), turbulent, laminar, and electrostatic mechanisms

• Aerosol Growth / Revaporization

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RCS FP Transport Phenomena (2)

• Aerosol Deposition− Thermophoresis, diffusiophoresis (Stefan flow), gravitational

deposition, Brownian motion deposition, turbulent deposition, laminar deposition, electrophoresis, inertial deposition and photophoresis

• Aerosol Resuspension• Pool Scrubbing• Transport of Deposits by Water• Chemical Speciation• Transport of Structural Materials

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ACR Shield Plug

Fission products will be deposited in complex flow paths such as through the shield plug.

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Outline

• ACR Fuel and RCS Design• Fission-Product Release and Transport in Limited and

Severe Core Damage Accidents• Experimental Database• Computer Codes

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FPR&T Experimental Database• Laboratory separate-effects tests

− UO2 oxidation-volatilization studies− Fission-product thermochemistry− Aerosol deposition in CANDU fuel channel end fitting

• Hot-cell fission-product release tests− FP release and transport from clad and unclad fuel samples

under accident conditions (Canadian, ORNL VI, Vercors)− Grain boundary inventory measurements− Direct-electric-heating tests

• In-reactor tests under accident conditions− Canadian severe-fuel-damage tests (BTF)− International severe accident tests (ACRR ST, Phebus FP)

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Knudsen Cell – Mass Spectrometer

• Used to measure vapor pressures and other thermodynamic properties of fuel and fission-product compounds at high temperatures

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Knudsen Cell - Mass Spectrometer

Cold Cathode GaugeCooled Shield

Turbo Pump

TurboPump

Ionization Gauge

Quadrupole Mass Spectrometer10-5 Pa

Bottom ApertureTop Aperture

TantalumKnudsen Cell

ShutterSapphire Window

Ion Source

Mass Spectrometer Shutter

Shutter

Faraday Cage

to Induction Heater

10-5 to 10-6 Pa Pyrometer

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Systems Investigated with KC-MS

• Iodine volatility over CsI / UO2 / MoOx and CsI / U3O8• Lanthanum volatility over lanthanum oxide / uranium

dioxide solid solutions• FP simulant volatility over SIMFUEL• Vapor pressure and stability of cesium molybdate

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End Fitting Aerosol Deposition Rig

612

34

5

8 9

10

11 12

13141516

36 18A19 20

22 21

steam temp.wall temp.outside wall temp.

32 condenser - inlet steam temp.33 condensate temp. 57 superheater -steam temp. at inlet58 superheater -steam temp. at CsI injection point59 superheater -surface temp

pressure sensorP

P

P

67

2329

30

31

2526 24

60 superheater -surface temp at outlet 61 superheater -steam temp at outlet

34

3735

EF11-7F.DRW

27A

45C

41C42C 43C

44C

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Hot-Cell FP Release Tests

• FP release and transport from clad and unclad fuel samples under accident conditions− > 300 tests performed− Maximum temperatures: 670 to 2300 K− Environments: H2, Ar/H2, steam/H2, steam, air− Heating rates: 0.2 to 50 K/s− On-line gamma-spectroscopy for FP release measurements− Post-test gamma-scanning for FP deposition measurements

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HCE4 Experiment Objectives

• Hot-Cell Experiment #4 (HCE4) was performed to provide data on fission-product releases (FPR) from CANDU fuel at 1650°C for validation of CANDU reactor safety codes

• Three sets of tests were performed to assess the effects of the following parameters on FPR:− Gaseous environment (Ar/2%H2, steam/0.5%H2 and air)− Fuel sample length (20 and 100 mm)− Heating rate (0.2, 1-2 and 6 K/s)

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Schematic of HCE4 Hot-Cell ApparatusAr

/2%

H2

argo

n

air

gas-monitoringγ-spectrometer

direct-viewing γ-spectrometer

water supply pump

sample

movable furnace1

23

4

5

emergencysuction system

off-gasscrubber

1: flow meters and valves2: steam generator3: inlet oxygen sensor4: outlet oxygen sensor5: condenser

pushrod tube

magnet

aerosol collector

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Fission-Product Release in J03 Test

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0 5000 10000 15000 20000 25000Time (s)

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Cs-134 Kr-85 Ru-106

Inert Steam Inert

Temperature 1600°C

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% FP Release (±1σ Uncertainties)

84 ± 286 ± 174 ± 20 ± 6137Cs

82 ± 284 ± 175 ± 12 ± 5134Cs

100 ± 1079 ± 3100 ± 1011 ± 4131I

0 ± 31 ± 313 ± 30 ± 4106Ru

73 ± 874 ± 10100 ± 118 ± 185Kr

6462636067125733Time /s

1650164016451620Temp /°C

Steam/H2Steam/H2AirAr/2%H2Environ.

J04J03J02J01Test

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Conclusions from Hot-Cell FPR Tests

• FP releases increase with increasing temperature• FP releases depend on oxygen potential of the

environment (hydrogen, steam, air)− Noble gases and volatile FP (I, Cs, Te, etc.) are released

rapidly under oxidizing conditions− Releases of other FP (Ru, Ba, Nb, Sr, etc.) depend on the

volatility of the stable condensed-phase species formed in the environment, e.g., Ru released under oxidizing conditions

− Non-volatile FP and actinides released by vaporization of the UO2 matrix

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Grain Boundary Inventory Tests

• Motivation− Data for validation of fuel performance codes

• normal operating conditions− GBI is released more rapidly under accident conditions

• safety analysis

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GBI Measurement Technique

• Heat to 500°C in Ar/H2, add air flow − Kr-85 GBI plus Kr-85 and Xe-133 500°C grain releases

• Heat to 1100°C in air− Release remaining Kr-85 and Xe-133 in grain

gas-monitoring γ-spectrometer

gas outlet

module

sample

furnace

flow meters

charcoal filterAr/H2

air

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GBI Measurement Technique

As-received

1100°C air500°C airStart of 500°C air

Trace-reirradiated

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Central and Mid-Radial GBI

0%

20%

40%

60%

80%

100%

20 30 40 50 60Peak Linear Power (kW/m)

GBI

Central Mid-radius

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GBI Conclusions

• GBI and total inventory show radial variation• Peripheral GBI < 5%• Central GBI starts to rise at 38 kW/m

− maximum of 90% for 58 kW/m high-burnup fuel• High-GBI region expands with increasing power• Absolute GBI saturates at center of fuel above 41 kW/m

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Direct-Electric-Heating Experiments

• Fuel heated using direct electrical current to obtain conditions expected in reactor accident scenarios:− Rapid heating rates− Large radial temperature gradients− Original Zircaloy cladding

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Direct-Electric-Heating ApparatusOptical Pyrometer

Pre-Heater

Alumina Tube

Tungsten Electrode

O-RingSeals

Quartz TubeAlumina Spacer

Alumina Cone

Natural UO2

Irradiated Fuel

Zircaloy Cladding

Helium

Stainless Steel Transition

Water-CooledBrass

Axial LoadSpring

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Observations in DEH Tests

• Electric current flows mainly along fuel pellet axis− Exaggerates radial fuel temperature gradients

• Switched DC current (0.5 Hz) used to minimize electrolysis

• Some noble gas FP releases observed• Volatile FP (e.g., Cs) redistribute from pellet center to

periphery but are not actually released

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Blowdown Test Facility

Research Program Goals− Verify our understanding of fuel behavior and FP release and

transport under high temperature conditions representative of severe-fuel-damage accident scenarios

− Provide data from integral in-reactor experiments for use in the validation of computer codes used for safety analyses and licensing of CANDU reactors

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NRU Reactor and BTF

Recirculating Side-Stream Sample

Fuel Element

NR

U R

eact

or C

ore

Lower Shield

Aerosol

Samplin

g

14 “U-T

ube”

Grab Sa

mples

Blowdown Tank

γ Spectrometers γ Spectrometers

Filter

Blowdow

n Line

NW2 NW3

NW4

NW6

NW7

“Trench”

BF AF

BT

TSBDL

Blowdown Valve

Isolation Valves

To/From U1 Loop

Loop Closure

GB / MS

Re-entrant Test Section

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BTF-105B Objective

• Measure fission product release under high temperature conditions− fuel-averaged temperature target of 1800-2000°C− try to preserve element geometry to measure retained fission

products and fuel performance− compromise resulted in a target fuel-averaged temperature

about 1800°C for 15 minutes

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BTF-105B Noble Gas Release Kinetics5

106

107

108

200

400

600

800

1000

1200

2700 3600 4500 5400 6300

88 Kr85m Kr87 Kr133 Xe135m Xe135 Xe

TFS03

Bq/

cm °C

Seconds1997 Nov. 1721h00

Dat

a G

ap

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BTF-105B I and Cs Gamma Activities10

510

610

710

8

2700 3600 4500 5400 6300

133 I135 I134 I132 I131 I138 Cs

Bq/

cm

Seconds1997 Nov. 1721h00

Dat

a G

ap D

ata

Gap

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131I, 137Cs Along Fuel Element-0

.50

0.5

11.

52

2.5

0 100 200 300 400 500

131 I137 Cs

Nor

mal

ized

Act

ivity

mm Up From Bottom of Fuel1997 Dec.,1998 Jul. 10

Thermocouple ClampsThermocouple Clamps

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BTF-105B PIE, Elevation 105 mm

10 m

m

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BTF-105B Elev. 105 mm Density Scan

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0.1

0.2

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X — mm

Y — mm

-Ln(Gamma Transmission)

Thermocouple

(proportional to density)at 105 mm

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BTF-105B Elev. 105 mm - 95Nb Activity

82

8486

8890

9294

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Y — mm

Nb-95 Activityat 105 mm

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BTF-105B Elev. 105 mm - 137Cs Activity

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Cs-137 Activityat 105 mm

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Summary of BTF Test Conditions

~ 2400< 60~ 1500~ 20Time at high temperature after fuel failure (s)

~ 4200~ 2900~ 2100~ 70Transient duration (s)

~ 2100 (volume-average)

~ 2100 (volume-average)

~ 2100 (volume-average)

≥ 2770 (peak)Maximum fuel temperature (K)

Saturated steamSaturated steamSaturated steamPressurized water

Pre-transient cooling

1 pre-irradiated1 fresh1 pre-irradiated1 pre-irradiated,2 fresh

Fuel elements

BTF-105B TestBTF-105A TestBTF-104 TestBTF-107 TestParameter

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Integral % FPR in the BTF Experiments

1.1 ± 0.3-2.5 ± 0.720.8 ± 1.3132Te

34 ± 7-59 ± 556 ± 3137Cs

21 ± 8< 2.020 ± 568 ± 34133I

21 ± 7< 2.033 ± 556 ± 2131I

11 ± 33 ± 27 ± 237 ± 288Kr

24 ± 7-47 ± 6-85Kr

25 ± 62 ± 110 ± 437 ± 385mKr

BTF-105BBTF-105ABTF-104BTF-107Isotope

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BTF Program Conclusions

• Data obtained for validation of CANDU fission-product behavior codes under severe-fuel-damage accident conditions

• Post-test simulations performed using CANDU safety analysis computer codes (CATHENA, ELOCA, SOURCE and SOPHAEROS)

• No new phenomena or phenomena interactions identified

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Outline

• ACR Fuel and RCS Design• Fission-Product Release and Transport in Limited and

Severe Core Damage Accidents• Experimental Database• Computer Codes

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SOURCE IST 2.0

• SOURCE IST 2.0 is the Canadian Industry Standard Toolset (IST) code for calculating fission-product release from fuel

• SOURCE IST 2.0 simulates all of the primary phenomena affecting FP release from CANDU fuel under accident conditions

• Release fraction is the key output of SOURCE

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SOURCE IST 2.0• Basis Unit: A geometric subdivision of a fuel bundle.

The smallest unit that the user has chosen to model. It could be a fuel element, an axial segment, or an annulus within a fuel element or axial segment. In the case of fragment tests, it could be the entire fragment.

• Bins (inventory partitions) (subdivisions of a basis unit):− Grain Matrix− Grain Boundary− Fuel Surface− Gap− Released

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SOURCE IST 2.0

• Validation in progress:− Canadian hot-cell FP release tests

• Steam: UCE12 TF01, HCE2 BM5, HCE2 BM4, HCE4 J03, HCE3 H03 & MCE2 TM19

• Air: GBI3 DL5, HCE3 H02 & MCE1 T4• Inert (Ar/H2): UCE12 TU09, HCE1 M12 & MCE2 TM03

− International hot-cell FP release tests• Vercors 04, Vercors 05 & ORNL VI-5

− Integral in-reactor tests• BTF-104, BTF-105B & PHEBUS FPT1

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SOURCE IST 2.0 Validation

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age

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1800

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pera

ture

(°C

)

Cs-134 meas. Cs-134 calc. Temperature

Ar/H2 SteamAr/H2

Comparison of Measured and Calculated Cesium Release (10 Annulus BaseCase) as Functions of Time for HCE3 Test H03 (Steam, 1840°C).

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SOPHAEROS-IST 2.0

• SOPHAEROS initially developed by IRSN (France) to simulate fission-product transport and retention in the RCS under LWR severe accident conditions

• SOPHAEROS-IST 2.0 adopted as Canadian Industry Standard Toolset code for calculating fission-product transport and retention in the RCS

• When development is complete, SOPHAEROS will simulate all of the primary phenomena affecting FP transport and retention in CANDU RCS under accident conditions

• Fractional retention is the key output of SOPHAEROS

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SOPHAEROS-IST 2.0

• Validation in progress:− Canadian laboratory FP transport tests

• Mulpuru, End-fitting aerosol retention− Canadian hot-cell FP release and transport tests

• HCE3 H01 & H03, HCE4 J01 & J03− International FP transport tests

• LACE LA3B, Falcon ISP1 & ISP2, Marviken 2b & 7, DEVAP 23, 25 & 26, STORM ISP, TUBA-D

− International hot-cell FP release and transport tests• VERCORS 04 & HT1, ORNL VI-2 & VI-5

− Integral in-reactor tests• BTF-104, BTF-105B, PHEBUS FPT0 & FPT1

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PHEBUS FP SOPHAEROS Validation Experiments

• PHEBUS FP in-reactor tests of fuel and FP behavior under LWR severe accident conditions

• Focusing on fuel relocation (molten pool formation), FP release, FP transport in RCS (including steam generator tube), and FP behavior in containment

• Two tests used for SOPHAEROS validation− FPT0 – bundle of 20 fresh fuel rods, one Ag/In/Cd control rod,

retention in steam generator tube simulated− FPT1 – bundle of 18 previously irradiated fuel rods (23

MWd/kgU), 2 fresh fuel rods, one Ag/In/Cd control rod, retention in whole circuit simulated

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SOPHAEROS-IST 2.0 Validation

0.00E+00

1.00E-03

2.00E-03

3.00E-03

4.00E-03

5.00E-03

6.00E-03

7.00E-03

8.00E-03

9.00E-03

Upper Plenum Horizontal Line SGT Hot Leg SGT Cold Leg Cold LineLocation

Mas

s D

epos

ited

(kg)

Experimental DataRun 0Run 5 (low Tg-Tw)Run 13 (high Tg-Tw)

Deposition of Cs as a Function of Position in PHEBUS FPT1 Circuit

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Summary

• Good technology base for understanding of fission-product release and transport behavior in CANDU reactor accidents− Phenomena− Experimental database− Computer codes

• Extension to ACR is straightforward

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