科學與社會課件：Reactor Accident Concerns - II

1、Reactor AccidentConcerns - IICANDUGas-Cooled, Na-CooledPebble Bed, GEN IVCANDU 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 ReactorsMagnox Graphite Can Absorb a Lot of Heat C

2、ore Meltdown Not Credible Single-Channel Fuel Melting Possible Some Release of Radionuclides to EnvironmentAGR 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 Poten

3、tial Release of Radionuclides from a Meltdown (Should it Occur) is GreaterLMFBR 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 an

4、d Possibly Breach the Containment Structure Melting of Fuel will Terminate Reaction and Molten Material will Collect at Bottom of VesselPebble Bed Does Not Achieve Temperature Hot Enough to Melt Fuel Spheres LOCA Accident Causes Immediate Shutdown Helium Coolant is Inert and Fireproof No Phase Trans

5、itions in Coolant Multiple Layers in Fuel Sphere and in Containment Design Potential Problem is in Flammability of Graphite at High Temperature (Ongoing Debate)Generation IV Generation IV Reactors have Similar Safety Considerations and Concerns as their Previous Design Counterparts Additional Safety

6、 Measures are Included and Design Simplifications are Implemented in Order to Reduce Potential Reactor Accidents and Contain Released RadiationDefense inDepth Reinforced Structures Redundant Safety Systems Two or more ways to ensure safety measures can be maintained Highly Trained Operators Federall

7、y licensed every two years Train 1 week out of 5 Required emergency preparedness drills and exercisesNuclear Plant Event Classifications Notification of Unusual Event Minor Operational or Security Threat No Radiation Release Expected Alert Potential Reduction in Plants Safety Level Security Threat t

8、o 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 (

9、TMI)Plant Safety over the YearsNRC Significant Events(annual industry average per plant)NRC Information DigestFission 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

10、 Surrounding Area Greatly Dependent Upon Weather ConditionsEmergency 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 R

11、adius 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 exposureEmergency Planning Zone (EPZ) Upon Containment of the Accident, Environmental

12、 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

13、 the Event of an EmergencyEmergency Planning ZoneNuclear Energy Institute: Nuclear Power Plant Emergency Preparedness (Dec 2005)RegulatoryOversight The Nuclear Regulatory Commission is Responsible for Safety Oversight of Nuclear Power Plants Drill and Exercise Performance Percentage of Emergency Res

14、ponse 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, Safety in a Nuclear PlantU.S. Nuclear(Accidents per 200,000 worker-hours)U.S. ManufacturingU.S. Finance, Insurance, Real

15、 EstateOSHA Accident RatesU.S. Bureau of Labor Statistics, NEIIts safer to work in a nuclear plant than in an office!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 l

16、ink to Department of Homeland Security Threat Information & Assessment Established response procedures and contingency plans“Nuclear power plants are probably our best-defended targets. There is more security around nuclear power plants than anything else weve got. Its infrastructure, especially

17、 against these kinds of terrorist threats, is extremely good.”John Hamre, President Center for Strategic & International StudiesBisconti Research Inc. & Nuclear Energy InstitutePublic OpinionMore Public OpinionFavor 83% Not sure 1% Oppose 16% Residents within 10 miles of each nuclear plant:“

18、Do you favor or oppose the use of nuclear energy as one of the ways to provide electricity in the United States?”Several states are offering $ millions in incentives to build new nuclear energy plants adjacent to existing plants in their states Bisconti Research Inc. & Nuclear Energy InstituteEx

19、amples 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 need

20、ed for cooling the bed with water without causing dryoutExamples and Problems 6.2 By interpolation, the minimum particle size would by about 0.4 mm to prevent dryoutExamples and Problems 6.2 Other Problems Use different heat rates Use different times Additional Analysis What about non-uniform decay

21、beds of varying geometry or heat rate production? What would happen when dryout occurs in the debris bed?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 re