Nuclear reconsidered


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Nuclear reconsidered

  1. 1. Should Nuclear be Reconsidered?Nuclear worries surface after Japan quakes
  2. 2. Lurid Headlines• “Partial meltdown at Japanese reactor likely”• “US Nuclear Experts Worry About Possible Japan Reactor Meltdown”• “Sailors at sea receive month’s dosage in one hour three miles from plant” – No mention this is far less that any space launch• “US Begins Airlift as Japan Battles Nuclear Reactor Leaks”• “Help Wanted: Save Japan From Nuclear Disaster”• “Japan’s Nuclear Crisis Stokes Fear In Europe”• “After Japan Earthquake, Groups Question Nuclear Power”• “Japan’s crisis: The world rethinks nuclear power”• “Helicopters drop water on Japan’s troubled reactor” – False• “Nuclear power in disarray”
  3. 3. What is the truth?• How does this type of reactor work? – And how is it designed?• What are its worse case scenarios?• How much radiation was released? – How much of a danger is that? • Close to the reactor? • In Japan? • Elsewhere on earth?• Was there any danger of “meltdown”?
  4. 4. Event timeline• Quake hits – Control rods inserted under full cooling conditions • Reactor powered down 94% and much lower temp – Loses coolant • Unclear how long. Over six hours can damage cladding in control rods• Saturday explosion – Damaged only most outer containment – But release of radioactive iodine and hydrogen • “could” suggest decay of zircaloy cladding in fuel rods (temp > 2000 C) • But normal small defects in rods could also produce this • But there is possibility of partial fuel meltdown – Much confusion calling the fourth level containment building “reactor containment”
  5. 5. Event Timeline 2• Seawater pumped in – Actually only pumped in between 2nd (reactor vessel) and third level (containment building) to bring overall temperature down • Core never exposed to sea water or vented to ocean – Some reports say that seawater was pumped even into core » Especially as plant was scheduled for 40 yr end of life in two weeks anyhow.• Coolant flow is reestablished
  6. 6. Japanese Reactor Type• Boiling Water Reactor (BWR) – Operating temperature 250 C – GE BWR reactor• Fuel uranium oxide or MOX in zircaloy rods – One or more fuel pellets per rod (1 here) – Rods melt at > 2300 C• Control rods mediate the reaction soaking up neutrons produced by the fuel. – With fuel rods in only secondary radiation is present (particularly iodine and cesium) – Control rods were inserted immediately when quake hit• Three levels of containment (four in this reactor)
  7. 7. Containment Levels• Level 1 – Cladding – Zirconium alloy sheath that surrounds fuel pellets keeping it in bet geometry for management and cooling and contains fission products• Level 2 – Reactor Vessel – Thick steel container housing reactor and its coolant – part of coolant loop• Level 3 – Containment building – Thick steel reinforced concrete to withstand very high pressure and heat• Level 4 – Dry-wall outer shell mainly only meant to protect from elements, not keep anything in.
  8. 8. BWR characteristics• BWR reactor generates steam in the reactor vessel.• No steam generator and pressurizer are needed.• Control rods are inserted from the vessel bottom• BWR has lower operating pressure than PWR does.• The coolant flow rate of a BWR is smaller than that of a PWR of equivalent power output.• The BWR reactor vessel is larger than PWR vessel.• Its primary coolant system connects to turbine units• Circulation pump assembly is needed.• The engineering of a BWR is more difficult than PWR.• Basic game plan of BWR, keep pouring water on. If pressure gets to high vent steam into containment building.
  9. 9. Reactor Vessel AssemblyCore Thermal Power: ~3,500 MWthVessel Diameter (I.D.): ~6 mVessel Height: ~22 mVessel Wall Thickness: ~14.5 cm to 16.4 cmCoolant: WaterCoolant Flow Rate: ~13,000 Kg/sPressure: ~1,000 psiaSteam Temperature: 551°FFeed Water Temperature: 420°F
  10. 10. Reactor Vessel Assembly• GE Fuel Rod – Fuel Pellet Diameter: 1.04 cm – Fuel Pellet Height: 1.04 cm – Fuel Material: UO2 – Cladding Diameter (O.D.): 1.23 cm – Cladding Thickness: 0.81 mm – Cladding Material: Zircaloy
  11. 11. Reactor Vessel Assembly ABB Fuel Assembly and Control Rod
  12. 12. Reactor Vessel Assembly• GE BWR Fuel Assembly – Fuel Assembly: 8 ´ 8 array – Number of Assemblies: 746 – Active length: 3.6 m – Total Number of Fuel Rods: 46,376 – Fuel Weight: 155,000 Kg – Number of Control Rods: 177 – Fuel Center Temperature: 3,330°F – Cladding Temperature: 579°F
  13. 13. Reactor ContainmentDry Well: – Seals the reactor from the rest of the buildingPressure Suppression Pool: – Store water for emergency useContainment Shell: – Steel shell, a sealed barrier against radioactive releaseReactor Building: – Concrete structure, last barrier.Negative pressure is maintained betweencontainment shell and building wall to preventradioactive release.
  14. 14. Bottom Line• There was no real danger of major meltdown• There was very very small local danger from radioactive iodine. No real immediate danger.• Actually given a 9.0 quake (very rare) – The reactors stood up very well even with both power systems down. No reason for reevaluation of nuclear power from this.• Much of the world goes into hysteria re nuclear power – Why? • Associating with nuclear weapons? • Creeped by radiation/radioactivity – And associated confusions? • Plot to instill fear of some types of energy? • General reactive clamor so prevalent in hu-mans? • Lack of understanding of science/technology and fear thereof?
  15. 15. Bottom Line 2• Spent fuel pool in reactor 4 – Much was made of this but.. • Spent fuel doesn’t get dangerously hot even after several days without coolant • It is sometimes stored for years with only normal airflow cooling in storage buildings • At most this was a minor problem not fixed with far less difficulty than claimed in much of the press.• Top dose to any worker was about 10 mrem – Not that high considering normal background dose over year is 250- 500 mrem• Overall not bad for a once in 300 years event with an older reactor design. – Two weeks from its scheduled 40 year decommission.