Source Term modeling for CANDU reactors

IAEA Technical Meeting on

Source term Evaluation for Severe Accidents

October 21-23, 2013

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Objectives of presentation

• To provide overview of the current state in modeling of fission

product release (Source Term) in Canada

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Outline

• CANDU reactors

• Source Term (ST) modeling for design basis accidents

– SOURCE, SMART (and other) codes

• ST modeling for severe accidents

– MAAP-CANDU models

• Current research priorities for ST

• Application of ST

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Canadian Nuclear Safety Commission

• Regulates the use of nuclear energy

and materials in order to prevent

unreasonable risk to the environment

and to the health and safety of

persons

• Disseminates objective scientific,

technical and regulatory information

concerning the effects of the use of

nuclear energy

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CNSC Regulates All Nuclear-Related Facilities and Activities

– Uranium mines and mills

– Uranium fuel fabricators and processing

– Nuclear power plants

– Waste management facilities

– Nuclear substance processing

– Industrial and medical applications

– Nuclear research and educational

– Export/import control

…From Cradle To Grave

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CANDU reactor

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CANDU Reactor

• Reactor Assembly

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Channel

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Fuel bundle

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ST modeling for design basis accidents

• SOURCE, SMART codes for fission product transport

• Supporting codes

– ORIGEN – fuel radionuclide inventory

– ELOCA – transient fuel element behaviour (temperatures, strain)

– SOPHAEROS – retention in PHTS

– LIRIC /IMOD-2 – iodine model

– GOTHIC – containment conditions

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SOURCE code – release from fuel

• Phenomena modelled:

– Diffusion

– Grain growth

– Fuel cracking

– Gap transport

– UO2+x, UO2-x formation

– UO2 – Zircaloy interaction

– Fission product volatilization

– Fuel melting

– Fission product leaching

– …

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Severe Accident phenomena in SOURCE

• Some CANDU Design Basis Accidents involve phenomena common with

severe accidents

– UO2 – Zircaloy interaction

– Fission product volatilization

– Fuel melting, etc..

• CANDU design traditionally considered events with localized fuel melting

such as

– LOCA + LOECI

– Flow blockage in a single channel

– Fuel ejection from a channel

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DB Accident - fuel ejection from a channel

• Fuel is ejected into containment when end fitting detaches from channel

• Fuel bundle breaks up into fuel element clusters

• Some fuel elements break, exposing fuel directly to air

– Tests on un-irradiated bundles at Stern Laboratories, Hamilton, and irradiated bundles at AECL Whiteshell Laboratories

• Fuel fragments oxidize in air to higher oxidation states than in steam

– Phase change from fluorite (UO2/UO2+x/U4O9) to orthorhombic (U3O8) for oxidation at temperatures < ~1550°C

– Forms fine U3O8 powder at T<~650°C

– Release of FP grain-boundary inventory (GBI)

• SOURCE allows modeling of FP release in such a scenario

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SMART code – transport in containment (1)

• Radionuclide (aerosol) removal processes:

1. Gravitational deposition of aerosols

2. Impingement of jet aerosols

3. Turbulent inertial deposition of aerosols

4. Turbulent diffusional deposition of aerosols

5. Diffusiophoretic deposition of aerosols

6. Thermophoretic deposition of aerosols

7. Moderator washout of aerosols

8. Radioactive buildup and decay

9. Iodine washout by dousing spray

10. Iodine washout by break spray

11. Filtration

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SMART code – transport in containment (2)

• Aerosol agglomeration mechanisms:

1. Brownian agglomeration of aerosols

2. Gravitational agglomeration of aerosols

3. Turbulent inertial agglomeration of aerosols

4. Turbulent diffusional agglomeration of aerosols

• Radioiodine processes

1. Chemical transformations between non-volatile and volatile iodine species in the aqueous phase

2. Partitioning of volatile iodine species among the gas, aqueous and adsorbed phases

• SMART could be used in some Severe Accident simulations, subject to validation conditions

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ST modeling for severe accidents

• MAAP4-CANDU (M4C) code

– Integrated code to predict severe accident progression at CANDU

– Developed for CANDU industry by FAI

– MAAP5-CANDU version is in development

• Source term prediction is just one of outputs of M4C

• MELCOR code is available but not customized for CANDU

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Severe Accident Source Term

• MAAP4-CANDU models

– 25 “fission products” allocated in 12 groups based on their volatility /

chemical properties

– Release from uncovered fuel while core geometry is maintained (A)

– Release from core debris (B)

• Two temperature-based release correlations, NUREG-0772 and NUREG-0956,

correspondingly for (A) and (B)

– Complex model for FP release due to MCCI

– FP release removes decay heat from fuel/debris

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MAAP Containment FP transport

• Convective transport

• Internal state transitions

1.vapour - aerosol (equilibrium evaporation)

2.vapour - uncovered surface (equilibrium evaporation, mass transfer rate)

3.aerosol - water (sedimentation, diffusiophoresis, thermophoresis)

4.aerosol - uncovered horizontal surface (sedimentation, thermophoresis)

5.aerosol - uncovered vertical surface (impaction, thermophoresis)

6.water - covered horizontal surface (dissolution/precipitation)

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MAAP Containment FP transport

0

1

2

3

4

5

0 24 48 72 96 120 144

Ma

ss

of

Cs

I +

Rb

I (k

g)

Time (hours)

Mass in Containment(Unmitigated)

Mass to Environment(Unmitigated)

Mass in Containment(SAMG Action)

Mass to Environment(SAMG Action)

Potential releases to

the environment if

containment fails

FPR begins with

fuel damage

Mitigating actions such as re-establishing the

Calandria Vessel Cooling System (SAG-2) can

assist in terminating accident progression,

including containment and environment releases

of FP

Example of MAAP4-CANDU ST Calculations

Steaming/Flashing of water can

liberate large fractions of FP

(Calandria Vessel failure leading to

corium-water interaction and steam

explosions in the Shield Tank)

Loop disassembly and core

collapse increase FP

aerosols and vapours

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Current research priorities for ST

• Releases into water (leaching from corium)

• Impact from hydrogen burns on FP volatility

• Iodine interaction with paints

• Ru oxidation and volatility

• Spent fuel pool, Multi-unit modelling

• FP removal processes

– Better understanding to help reduce release into environment

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ST through Leaching

• Release correlations accounting for leaching temperature, duration and salinity

• Are there notable differences in leaching releases in pH 10 water (CANDU ECC coolant) compared to fresh or sea water?

• Leaching releases from fuel that has been through a high-temperature transient

• AECL HCE6 experiment series will test fuel subjected to high-temperature transients and oxidation by steam

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Hydrogen burn impact on ST

• Recall MAAP approach to containment transport

• Convective transport

• Internal state transitions

1. vapour - aerosol (equilibrium evaporation)

2. vapour - uncovered surface (equilibrium evaporation, mass transfer rate)

3. aerosol - water (sedimentation, diffusiophoresis, thermophoresis)

4. aerosol - uncovered horizontal surface (sedimentation, thermophoresis)

5. aerosol - uncovered vertical surface (impaction, thermophoresis)

6. water - covered horizontal surface (dissolution/precipitation)

• Energetic event as a hydrogen burn will affect both convective

transport and state transitions

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Spent fuel pool / multi-unit effects

• Spent fuel pool - different geometry/materials/heat loads

– Presumably not a great challenge to adjust existing models for SFP

• Canadian reactors have shared containment systems – transport

of FP in containment in accidents involving several units is

affected

– Parallel processing of MAAP4-CANDU runs

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Application of Source Term

• Reactor Design

– In particular, design of mitigating systems

• SAMG

– Validation of effectiveness of the operator interventions

• Emergency response validation

• Environmental impact assessment

• Input in liability considerations

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Summary – key messages

• Models for predicting Source Term for both Design Basis and

Severe Accidents are available

– Uncertainties may be significant for specific phenomena or chemical

species, but state of the knowledge is generally adequate for the

purpose

• Fukushima presented a set of new questions and led to certain

revival of attention to ST in severe accidents

– Leveraging through international cooperation important

nuclearsafety.gc.ca

Questions?