Reactor Accident Concerns - II

1. Reactor AccidentConcerns - II CANDU Gas-Cooled, Na-Cooled Pebble Bed, GEN IV

2. CANDU Significant Fuel Melting Would Not Likely Occur Ruptured Pressure Tubes Could Incur a Steam Explosion to Rupture Containment and Cause Fuel Melting Such as in Light Water Reactors

3. Magnox Graphite Can Absorb a Lot of Heat Core Meltdown Not Credible Single-Channel Fuel Melting Possible Some Release of Radionuclides to Environment

4. AGR Graphite Can Absorba Lot of Heat Core Meltdown Not Credible Higher Temperatures of Operation Possible Due to Fuel Type Used LOCA Accident Slower to Occur than in a Light Water Reactor Potential Release of Radionuclides from a Meltdown (Should it Occur) is Greater

5. LMFBR Pump Failure Causes Boiling of Sodium and Increased Reactivity and Heat Production in the Reactor Meltdown in a Fuel Channel Would then Occur within Seconds Two Possible Outcomes Fuel Blows Apart to Terminate Reaction and Possibly Breach the Containment Structure Melting of Fuel will Terminate Reaction and Molten Material will Collect at Bottom of Vessel

6. Pebble Bed Does Not Achieve Temperature Hot Enough to Melt Fuel Spheres LOCA Accident Causes Immediate Shutdown Helium Coolant is Inert and Fireproof No Phase Transitions in Coolant Multiple Layers in Fuel Sphere and in Containment Design Potential Problem is in Flammability of Graphite at High Temperature (Ongoing Debate)

7. Generation IV Generation IV Reactors have Similar Safety Considerations and Concerns as their Previous Design Counterparts Additional Safety Measures are Included and Design Simplifications are Implemented in Order to Reduce Potential Reactor Accidents and Contain Released Radiation

8. Defense inDepth Reinforced Structures Redundant Safety Systems Two or more ways to ensure safety measures can be maintained Highly Trained Operators Federally licensed every two years Train 1 week out of 5 Required emergency preparedness drills and exercises

9. Nuclear Plant Event Classifications Notification of Unusual Event Minor Operational or Security Threat No Radiation Release Expected Alert Potential Reduction in Plant�s Safety Level Security Threat to Personnel or Plant No Radiation Release Expected Site Area Emergency More Serious Event Major Failure in Safety Equipment Potential for Minor Radiation Release that would Not Exceed EPA Standards General Emergency Serious Event Radiation May Leak Outside the Plant and Beyond the Plant Boundaries (TMI)

10. Plant Safety over the Years

11. Fission Product Dispersion Assumed All Gaseous Fission Products Released (Benign Noble Gases) 10% of Cesium and Iodine Radionuclides Released 1% of Other Radionuclides Released Dispersion to Surrounding Area Greatly Dependent Upon Weather Conditions

12. Emergency Planning Zone If a Serious Reactor Accident Occurs Evacuation and Emergency Responders Focus within a 2-mile Radius Surrounding the Reactor Building and 5 miles Downwind (Keyhole Approach) Emergency Planning Zone within a 10-mile Radius Surrounding the Reactor Building has Emergency Plans in Place KI Available to Reduce Thyroid Cancer Risk (Shelters or Evacuation Procedures are Set Up) Population beyond 10-mile radius not at risk from direct exposure

13. Emergency Planning Zone (EPZ) Upon Containment of the Accident, Environmental Assays are Performed within a 50-mile Radius of the Reactor Building Food-Chain Exposure Possible Emergency Responders Focus on Individuals Most at Risk from Direct Exposure Each Year, Nuclear Power Plants Provide Information to the Public within the 10-mile Radius Concerning Protective Measures in the Event of an Emergency

14. Emergency Planning Zone

15. RegulatoryOversight The Nuclear Regulatory Commission is Responsible for Safety Oversight of Nuclear Power Plants Drill and Exercise Performance Percentage of Emergency Response Participating Testing and Maintenance of Alarms and Sirens Every Reactor Receives at Least 2,500 hours each year of NRC Inspection Inspection Findings on the Web, www.nrc.gov

16. Safety in a Nuclear Plant

17. Nuclear Security Nuclear power plants have the highest security in American industry Well-armed, trained security forces New, strong physical security barriers, post 9-11 Continuous link to Department of Homeland Security Threat Information & Assessment Established response procedures and contingency plans

18. Public Opinion

19. More Public Opinion

21. Examples and Problems 6.2 Formation and Cooling of Debris Beds After a LOCA event, the reactor core partially melts and forms a 0.75 m deep particle bed with a porosity of 0.4. The decay heat from the bed is 1000 kW/m3 at 3 h Use the data shown to determine the minimum particle size that would be needed for cooling the bed with water without causing dryout

22. Examples and Problems 6.2 By interpolation, the minimum particle size would by about 0.4 mm to prevent dryout

23. Examples and Problems 6.2 Other Problems Use different heat rates Use different times Additional Analysis What about non-uniform decay beds of varying geometry or heat rate production? What would happen when dryout occurs in the debris bed?

24. Examples and Problems 6.3 Steam Explosions A severe accident in a PWR drops 50 metric tons of molten core to the bottom of the reactor vessel with a temperature of 3000 K A steam explosion occurs, releasing 3% of the thermal energy and transmitting the rest to a 10-ton slug of water that rises up the 500 ton vessel How high will the vessel rise from the impact assuming the thermal energy of the fuel is 1.5 GJ per ton?

25. Examples and Problems 6.3 Total energy released by 50 tons (3%) is 2.25 GJ KEslug = �mv2; vslug = 670.8 m/s Conservation momentum Vvessel = mslug*vslug/mvessel = 13.42 m/s KEvessel = 45.0 MJ Convert kinetic energy to potential energy h = KEvessel/(mvessel*g) = 9.17 m

26. Examples and Problems 6.3 Other Problems Account for energy losses from friction Increase the thermal load Additional Analysis Account for energy needed to rupture the vessel containment What would happen if a flaw in the vessel caused the explosion to be directed sideways?