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Nuclear Safety

Nuclear Safety. BNEN 2012-2013 Intro William D’haeseleer. Well-known nuclear accidents. TMI Harrisburg PA 1979. Well-known nuclear accidents. Chernobyl Ukraine 1986. Well-known nuclear accidents. Dai-ichi Fukushima Japan March 11 2011. Serious nuclear accidents.

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Nuclear Safety

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  1. Nuclear Safety BNEN 2012-2013 Intro William D’haeseleer

  2. Well-known nuclear accidents TMI Harrisburg PA 1979

  3. Well-known nuclear accidents Chernobyl Ukraine 1986

  4. Well-known nuclear accidents Dai-ichi Fukushima Japan March 11 2011

  5. Serious nuclear accidents U* H3* Pu* Mo* Sr* Cs* Co* Xe* I*

  6. wash out Cloud radiation fallout Intake Serious nuclear accidents . Inhalation

  7. Reactor accident vs nuclear explosion • Because limited enrichment in U-235 • Because of confinement of fuel Impossible that nuclear reactor would explode like a nuclear bomb

  8. Reactor accident vs nuclear explosion Bomb • Basically pure U-235 or Pu 239 (>90%) • Establish super-critical mass via chemical explosion • During “implosion”, a fast build up of an exponential chain reaction • Inertia “helps” nuclear chain reaction • Correct timing very important

  9. Nuclear Bomb - principle

  10. Reactor accident vs nuclear explosion Bomb • Then suddenly release of massive amount of energy ~ 1012 Joule in short time • Huge power pulse leads to • Enormous temp rise (> 106 °C) • “vaporization” & ionization of matter • Huge fire ball

  11. Reactor accident vs nuclear explosion Bomb • Then suddenly release of massive amount of energy ~ 1012 Joule in short time • Huge power pulse leads to … • Enormous pressure wave (over several km) • Creation very strong EM pulse/waves leads to fires, fire burns, etc • Release big neutron flux • Release of fission products & actinides

  12. Reactor accident vs nuclear explosion Bomb • Immediate victims due to • Pressure shock wave • Vaporization & fires • Direct / Acute irradiation

  13. Reactor accident vs nuclear explosion Reactivity accident in reactor • Energy release much smaller, • even in most sensitive cases, in case of super-prompt-criticality • About ~ factor 103 à 106 less release of energy over “longer” period of time  Much much smaller “power” release

  14. Reactor accident vs nuclear explosion Reactivity accident in reactor • Chain reaction cannot continue to grow exponentially • Feedback mechanisms cfr Doppler • “flying apart” of structural material

  15. Reactor accident vs nuclear explosion Reactivity accident in reactor No possibility for • Fire ball, vaporization & E&M pulse • Pressure wave,…

  16. Reactor accident vs nuclear explosion Reactivity accident in reactor: Chernobyl • original “power excursion” of nuclear origin (over-criticality) • But the chemical explosion / steam explosion • Then establishment of graphite fires in the moderator • Large release radio-isotopes • Totally different sort of consequences!

  17. Safety philosophy in NPPs

  18. Safety philosophy in NPPs Fundamental safety principle: At the time of the design, during construction & start up, and during operation, There should “never” be a release of a large amount of radioactivity that may harm the public

  19. Safety philosophy in NPPs “Defense in depth” philosophy = Use of multiple successive barriers to prevent release of radioactivity to environment

  20. Russian doll principle Three subsequent physical barriers 1 2 3

  21. Safety philosophy in NPPs Three safety levels to avoid that none of the barriers is compromised as result of abnormal occurrences such as equipment failure, human error or natural phenomena

  22. Safety philosophy in NPPs • Three safety levels 1) “Design for maximum safety in normal operation and maximum tolerance for system malfunction. Use design features inherently favorable to safe operation; emphasize quality, redundancy, inspectability, and testability prior to acceptance for sustained commercial operation and over the plants lifetime”

  23. Safety philosophy in NPPs Examples: • Negative reactivity coefficients • Use only radiation-resistant materials

  24. Safety philosophy in NPPs • Three safety levels 2) “Assume that incidents & mishaps will occur in spite of careful design, construction and operation. Provide safety systems to protect operators and the public or minimize damage when such incidents occur”

  25. Safety philosophy in NPPs Examples: • ECCS to cope with LOCAs • Electric emergency power supply (diesels, batteries)

  26. Safety philosophy in NPPs • Three safety levels 3) “Provide additional safety systems as appropriate, based on evaluation of effects of hypothetical accidents, where some protective systems are assumed to fail simultaneously with the accident they are intended to control”  For unforeseen events or events with very small probability: DBA

  27. Safety philosophy in NPPs Example: • Assume ECCS fail: • Core melt… • Release radioactive material in reactor building • Containment must be leak free • Sprinkler system in containment building • Reduce p & T • Condense volatile isotopes (e.g., 131I) • Also ventilation & filtration in intermediate region

  28. Safety philosophy in NPPs Which accidents for DBA? “How safe is safe enough?” Rule of thumb USAEC (1973) “An event with afrequency >once/1000 yearsneed not be taken into account in the design”

  29. Safety philosophy in NPPs Which accidents for DBA? “How safe is safe enough?” By year 2000, estimated that 1000 reactors operational in USA With extra safety factor of 10 Probability limit 10-7/reactor-year

  30. Safety philosophy in NPPs Which accidents for DBA? “How safe is safe enough?” If P>10-7/reactor-year:  Necessity to perform safety analysis with the most pessimistic safety assumptions, to maximize the consequences

  31. Additional safety concepts • After TMI accident (lessons learned) • Now incorporated in all new “passive” or “inherently safe” designs • Fail safe • Full proof • Walk away safe • Forgiving

  32. Additional safety concepts • Fail safe • If, after shortcoming of an important component, the installation can be brought back into a safe state • Fool-proof • If it remains safe w.r.t. whatever human intervention (even with bad intentions) – safety locks –

  33. Additional safety concepts • Walk away safe • If the installation can be left alone for a “reasonable” time, after having been brought to a safe state in the beginning of the accident • Forgiving • If reactor “tolerates” a late or an erroneous human action, without giving rise to an accident.

  34. Typical accident classes • Reactivity accidentstransient behavior • Lack of coolingtransient behavior • Fuel manipulation (reloading, maintenance) • Site-related accidents (earth quakes, air plane crash)

  35. Typical accident classes

  36. Example: LOCA • After shut down reactor still produces heat ~ 7-10% of thermal heat at t=0 • Heat due to fission products • Heat decays exponentially • LOCA = breach in primary coolant circuit • May lead to dry cooking of reactor core  melt down

  37. LOCA – safety systems

  38. PRA • Risk = Probability x Consequences • Via ‘event trees’ and ‘fault trees’, probability of occurrence can be evaluated

  39. WASH 1400

  40. WASH 1400

  41. Actual & Predicted accidents

  42. Actual & Predicted accidents

  43. “Accepted accidents” • Deadly annual fatalities car accidents • About 1000+ in Belgium • Roughly 30 000 à 50 000 in USA • About 1.2x106/a worldwide (Ref WHO)

  44. “Accepted accidents” • Deadly annual fatalities car accidents • About 1000+ in Belgium • Roughly 30 000 à 50 000 in USA • About 1.2x106/a worldwide (Ref WHO)

  45. “Accepted accidents” • Deadly annual fatalities car accidents • About 1000+ in Belgium • Roughly 30 000 à 50 000 in USA • About 1.2x106/a worldwide (Ref WHO) • Deadly victims due to airplane crashes worldwide ~ 1000 à 2000/a • Industrial accidents…construction sector… • Largest accident in energy sector: failure of Hydro dam system in China 1975: 171,000 victimsRef http://en.wikipedia.org/wiki/Banqiao_Dam. See also Savacool (later)

  46. “Accepted accidents” • Deadly victims due to CO poisoning in B ~ 100/a Deadly victims developed world ~10-5/a  104 victims/a • Ghislenghien 2004 28 deaths • …

  47. TMI accident 1979

  48. TMI • PWR plant - LOCA type • Origin: failure feedwater pump secondary • LOCA through PRV’s pressurizer • Operators turned off ECCS (focused on level presurizer) • Health consequences: • Average absorbed dose ~ 0.01 mSv • Max off site dose ~ 0.8 mSv

  49. Chernobyl accident - RBMK

  50. Chernobyl accident • RBMK reactor, graphite moderated, water cooled • BWR in pressure tubes • RBMK has a positive void coefficient

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