Source Term modeling for CANDU reactors

Source Term modeling for CANDU reactors IAEA Technical Meeting on Source term Evaluation for Severe Accidents October 21-23, 2013

Objectives of presentation To provide overview of the current state in modeling of fission product release (Source Term) in Canada 2

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 3

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 4

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 5

CANDU reactor 6

CANDU Reactor Reactor Assembly 7

Channel 8

Fuel bundle 9

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 10

SOURCE code release from fuel Phenomena modelled: Diffusion Grain growth Fuel cracking Gap transport UO 2+x, UO 2-x formation UO 2 Zircaloy interaction Fission product volatilization Fuel melting Fission product leaching 11

Severe Accident phenomena in SOURCE Some CANDU Design Basis Accidents involve phenomena common with severe accidents UO 2 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 12

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 (UO 2 /UO 2+x /U 4 O 9 ) to orthorhombic (U 3 O 8 ) for oxidation at temperatures < ~1550 C Forms fine U 3 O 8 powder at T<~650 C Release of FP grain-boundary inventory (GBI) SOURCE allows modeling of FP release in such a scenario 13

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 14

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 15

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 16

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 17

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) 18

MAAP Containment FP transport 19

5 Example of MAAP4-CANDU ST Calculations Mass of CsI + RbI (kg) 4 3 2 1 Loop disassembly and core Mitigating Steaming/Flashing actions such of water as re-establishing can collapse increase FP the Calandria liberate large Vessel fractions Cooling of System FP aerosols and vapours (SAG-2) can assist (Calandria in terminating Vessel failure accident leading progression, to including corium-water containment interaction and and environment steam releases FPR begins with of explosions FP in the Shield Tank) Potential releases to fuel damage the environment if containment fails Mass in Containment (Unmitigated) Mass to Environment (Unmitigated) Mass in Containment (SAMG Action) Mass to Environment (SAMG Action) 0 0 24 48 72 96 120 144 Time (hours)

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 21

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 hightemperature transient AECL HCE6 experiment series will test fuel subjected to hightemperature transients and oxidation by steam 22

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 23

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 24

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 25

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 26

Questions? nuclearsafety.gc.ca