NRC Source Term Research Outstanding Issues and Future Directions

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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

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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

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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

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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

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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

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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

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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

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Alternate Source Term for Licensing PWRs
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Alternate Source Term

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

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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

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Examples of Radionuclide Release Uncertainties
from NUREG-1150
RSS Reactor Safety Study predictions circa 1975
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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
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Comparison of Alternative MELCOR Models of
Cesium Release to Data from PHEBUS test
Validation of the Alternate Source Term for
reactor licensing
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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

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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

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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
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FUTURE

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