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NRC Source Term Research Outstanding Issues and Future Directions

NRC Source Term Research Outstanding Issues and Future Directions. Farouk Eltawila, Director Division of Risk Assessment & Special Projects Office Nuclear Regulatory Research U.S. Nuclear Regulatory Commission. Accident Source Terms in the US regulatory process.

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NRC Source Term Research Outstanding Issues and Future Directions

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  1. NRC Source Term ResearchOutstanding Issues and Future Directions Farouk Eltawila, Director Division of Risk Assessment & Special Projects Office Nuclear Regulatory Research U.S. Nuclear Regulatory Commission

  2. Accident Source Terms in the US regulatory process • Releases of fission products to the containment: • Defense in depth • Regulatory evaluation of engineered safety features (ESFs) • Releases of fission products to the environment: • Consequences of reactor accidents • Accident management and emergency response

  3. HISTORY • Most current reactors licensed to the TID-14844 Source Term to the containment • Derived from heating irradiated fuel in a furnace • Releases • 100 % Noble gases • 50% iodine as a gas (half of this deposits) • 1% of all other radionuclides as particles • Instantly available in containment

  4. HISTORY continued • The accident at TMI changed perceptions • Severe accident possible • Source term different • Nuclear Regulatory Commission asked for a better, more realistic source term • NRC Research initiated a major initiative to develop a mechanistic source term • Tie to risk important accidents – not DBAs • Eventually about $500 million spent

  5. NRC Severe Accident Source Term Research • Massive undertaking to understand • Accident progression within RCS • In-pile tests (PBF, ACRR, DF, FLHT, etc.) • Exvessel phenomena • Melt-concrete interactions • Steam explosions • Hydrogen combustion • Fission product chemistry • Deposition in RCS • Aerosol physics • Containment integrity

  6. Culminated in • Alternate Source Term for licensing • NUREG-1465 • NUREG-1150 Level III risk analysis of five representative US nuclear power plants • 3 PWRs • 2 BWRs • Integrated, systems-level, accident analysis computer code to preserve understanding • Initially, Source Term Code Package • Later MELCOR

  7. Alternate Source Term • Timing based on four accident phases in BWRs and PWRs separately: • Gap release (clad ballooning and rupture) • In-vessel release (core degradation) • Ex-vessel release (melt/concrete etc.) • Late in-vessel release (revaporization) • Release magnitudes based on mechanistic analysis of important accident sequences for many plants • 8 chemical groups of fission products • Most aerosol • 5% of released iodine gaseous

  8. Alternate Source Term for Licensing PWRs

  9. Alternate Source Term • Very popular with licensees • Timing features allow safer • Diesel start times • Isolation valve closure times • Etc.

  10. A Research Climax but not a Conclusion • NUREG-1150 made clear many uncertainties remained • Estimated release fractions to environment could vary by factors of 10 to 1000 • Source terms adequate for regulatory needs of the time • As use of PRA and more advanced reactors developed, better source term understanding needed

  11. Examples of Radionuclide Release Uncertainties from NUREG-1150 RSS = Reactor Safety Study predictions circa 1975

  12. Collaborative Experimental Research • PHEBUS-FP: realistic FP chemistry in RCS and containment • ARTIST: mitigation of risk dominant accident for PWRs • RASPLAV/MASCA: feasibility of in-vessel retention and FP release in late stage degradation • MCCI: attenuation of ex-vessel source term with water PreTest Post Test

  13. Comparison of Alternative MELCOR Models of Cesium Release to Data from PHEBUS test Validation of the Alternate Source Term for reactor licensing

  14. Issues for Current LWRs • Iodine behavior in containment • Steady-state gaseous iodine in containment atmosphere • Interactions with paint • Utility of containment sump buffering • PHEBUS-EPICUR and AECL tests • Chemical form of cesium released to containment • Cesium molybdate versus cesium hydroxide • PHEBUS-CHIP tests

  15. Issues for Advanced LWR Certification • AP-1000 • Diffusiophoretic deposition of aerosol on containment walls • Effects of aerosol shape factors • ESBWR • Iodine behavior in the drywell and passive safety systems

  16. FUTURE Fuel Kernel • Next Generation Nuclear Plant • Source terms from gas-cooled graphite reactors • Triso fuel • Completed phenomena identification and importance ranking exercises graphite Silicon carbide Coated particle fuel for gas reactor

  17. FUTURE • Global Nuclear Energy Partnership • Source terms from sodium-cooled reactors • Defining regulatory approach

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