Accident Analysis

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Narora Fire accident

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Accident Analysis

  1. 1. By-DIVYA DEEPAK M.Engg-Mechanical Engg(University of Windsor)
  2. 2. <ul><li>NAPS consists of Pressurized Heavy Water Reactor. </li></ul><ul><ul><li>CANDU-220 MWe </li></ul></ul><ul><ul><li>Second Generation Advanced Candu Reactors </li></ul></ul><ul><li>FUEL USED : Natural Uranium. </li></ul><ul><li>PRIMARY COOLANT : Heavy Water </li></ul><ul><li>MODERATOR : Heavy Water </li></ul><ul><li>STEAM GENERATOR COOLANT: Light Water. </li></ul>
  3. 7. <ul><li>Failure of turbine blades. </li></ul><ul><li>Vibrations in turbine. </li></ul><ul><li>Hydrogen gas release. </li></ul><ul><li>Small amount of fire produced. </li></ul><ul><li>Oil spill from the severed lubricating & seal oil pipelines. </li></ul><ul><li>The oil fire burning of power cables. </li></ul><ul><li>Emergency transfer relays burnt, turbine& generator panels burnt </li></ul><ul><li>Complete loss of Class I& Class II power supply. </li></ul><ul><li>Emergency declared and corrective actions taken </li></ul>
  4. 8. TIME EVENT 0 Turbine trip 72 ms UT-1 breaker trip 105 ms Generator Tranformer-1 breaker trip 117ms Main generator-1 field breaker trip 140 ms 6.6kv bus section breaker trip 38.512s Reactor trip manually 38.592s PSS got actuated 3M 0.247s Power MG-2 breaker trip 4M 54.552s Control MG-1 breaker trip 5M 47.942s Manual crash cool down initiated 5M 47.957s SSS got actuated 6M 47.862s All PHT pumps tripped-complete loss Of class-IV supply 7M 4.347s Complete MG-3 breaker trip, Complete loss of control power supply
  5. 9. 7M.39.228 SEC Control MG3 breaker trip (complete loss of Control Power Supply) About 8 Min PLANT EMERGENCY DECLARED 10 Min Two diesel driven fire water pumps started 1 Hr. 30 Min Fire was extinguished in ground and mezzanine floors on TB 2 Hr. 10 Min Boron poison added through GRABS ABOUT 3 Hr. 50 Min Both diesel driven fire water pumps tripped 4Hr. 25 Min First entry to reactor building by the operating staff 4 Hr. 25 Min PHT pressure noted as 42 Kg/cm 2 from master gauge at E1.103M 4 Hr. 35 Min Fire water hose connected to the end shield system 5Hr. 35Min One fire water pump restarted at T=5hrs.35mins , cooling to end shields provided in addition to putting water to steam generators. 5Hr. 35Min D.G #3 was started 6Hr. 00Min Class Bus Q was charged 7Hr. 00Min End shield metal temp 70-80 deg C 7Hr. 30Min End shield metal temp. 70-80 deg C
  6. 10. 9Hr. 10Min PHT pressure noted to be 2 KG/cm2 9Hr. 30Min Active HPPW started 3335-MW-15 &16 (D2O accumulator outlet valves) found in nearly closed position 10Hr. 00Min Instrumented IRVs closed manually 13Hr. 10Min control room made habitable 13Hr. 30Min 5.5 Te of D2O from accumulator was taken into PHT system. (System pressure increased to 5 Kg/cm2) 15Hr. 50Min Environmental survey showed normal radiation fields 17Hr. 05Min Shutdown cooling pump #2 was started 17Hr . 45Min FM supply pump #2 was started and PHT system pressure increased to 31 Kg/cm2 19Hr. 15Min Plant Emergency Lifted 20Hr and beyond Moderator pump, end shield recirculation pump, PC ventilation fan, Purge dryer fan started
  7. 11. <ul><li>First cool down of primary PHT initiated by manual opening of small atmospheric steam discharge valves. </li></ul><ul><li>later crash cool down initiated by opening of large ASDV. </li></ul><ul><li>Thus, secondary shutdown system got actuated as per logic. </li></ul><ul><li>Complete loss of ClassI&II power supply. </li></ul><ul><li>Manual insertion of boron to prevent attainment of re-criticality. </li></ul><ul><li>Diesel driven generators employed to start supply of water to the secondary side of steam generator. </li></ul>
  8. 14. <ul><li>Root Cause analysis. </li></ul><ul><li>Defense in Depth. </li></ul><ul><li>Top-Down approach. </li></ul>
  9. 15. TIME LINE EVENT CAUSE REASON FOR OCCURANCE OF THE EVENT 0 m.s Rupture of turbine blades ROOT CAUSE Accumulated Stress 38 sec Rupturing of hydrogen Seals CONTRIBUTING CAUSE Vibrations in the Generator Unit 40 sec Rupturing of lube oil lines CONTRIBUTING CAUSE Vibrations in the Generator Unit 1M 20sec Fire spread through the generator bus duct in the Turbine Building (TB) and through cables into the Control Equipment Room (CER) DIRECT CAUSE Lubricating oil as a medium of transport 3M 24sec Power cables Burning ROOT CAUSE Lack of fire resistant insulation Negligence of the organization ROOT CAUSE …………………… ..
  10. 17. <ul><li>PREVENTION : </li></ul><ul><ul><li>Process Water </li></ul></ul><ul><ul><li>Backup Heat Sinks </li></ul></ul><ul><ul><li>Safety Valves. </li></ul></ul><ul><ul><li>Control systems. </li></ul></ul><ul><ul><li>Feedback Systems. </li></ul></ul><ul><ul><li>Primary heat transport system. </li></ul></ul><ul><ul><li>Trained and skilled personnel. </li></ul></ul><ul><li>PROTECTION: </li></ul><ul><ul><li>External Natural Disasters </li></ul></ul><ul><ul><li>Trip Systems. </li></ul></ul><ul><ul><li>Shut down systems. </li></ul></ul><ul><ul><li>ECCS. </li></ul></ul><ul><ul><li>Containment building. </li></ul></ul><ul><ul><li>Automatic system Responses </li></ul></ul><ul><li>MITIGATION </li></ul><ul><ul><li>Venting. </li></ul></ul><ul><ul><li>Dedicated heat removal Chain. </li></ul></ul><ul><ul><li>Large Fire Water For Emergency. </li></ul></ul><ul><ul><li>External Warnings. </li></ul></ul><ul><ul><li>Off Site Responses. </li></ul></ul><ul><li>ACCOMODATION </li></ul><ul><ul><li>Post Shutdown Heat Removal System. </li></ul></ul><ul><ul><li>Valves for injection of POISSONS. </li></ul></ul><ul><ul><li>External Fire Extinguishers </li></ul></ul>
  11. 19. <ul><li>STRENGTHS: </li></ul><ul><ul><li>The entire reactor was properly designed with all the safety equipments and the various control and shut down mechanisms. </li></ul></ul><ul><ul><li>Positioning of the valves for sudden shut down of the reactor was perfect. </li></ul></ul><ul><ul><li>The core and other systems were properly utilized according to the available resources. </li></ul></ul><ul><ul><li>The double containment had an advantage even at LEVEL 3 scenario. </li></ul></ul><ul><ul><li>Presence of back up systems </li></ul></ul>
  12. 20. <ul><li>WEAKNESSES: </li></ul><ul><ul><li>Lack of considerations on the turbine blade dynamics. </li></ul></ul><ul><ul><li>Lack of alternative power supply systems for the control room analysis. </li></ul></ul><ul><ul><li>Cables of backup power systems laid in the same duct without any seperation. </li></ul></ul><ul><ul><li>Lack of proper material to prevent fire in the cables used for signal transmission. </li></ul></ul><ul><ul><li>Position of the hydrogen coolant for the turbine. </li></ul></ul><ul><ul><li>Position of the oil lubricant for the turbine. </li></ul></ul><ul><ul><li>Poor design of indicator alarms in the fire room. </li></ul></ul><ul><ul><li>Missing water source to prevent sudden fire accident. </li></ul></ul>
  13. 21. <ul><li>Various on site responses observed. </li></ul><ul><li>Declarations of emergency. </li></ul><ul><li>Manual crash cooling. </li></ul><ul><li>Manual operation of the diesel generators. </li></ul><ul><li>Manual operation of injection of boron. </li></ul><ul><li>Observation of various parameters w.r.t time. </li></ul><ul><li>Calculation of radiation dose and allowance w.r.t permissible levels. </li></ul>
  14. 22. <ul><li>Redesign of turbine blades. </li></ul><ul><ul><ul><li>Increasing the thickness of the turbine blades and the convexity of the turbine blades. </li></ul></ul></ul><ul><ul><ul><li>Increased convexity avoids back-rake stress reduction. </li></ul></ul></ul><ul><ul><ul><li>Displacement sensor to be placed to measure eccentricity of shaft. </li></ul></ul></ul><ul><li>Increase coating of fire resistant material in the cables. </li></ul><ul><li>Increase the distance between oil tanks and the turbines. </li></ul><ul><li>Laying down cables in different ducts so that if one duct catches there is no damage to the power cables in other ducts. </li></ul>
  15. 23. <ul><li>Increase the insulation layer in turbine building flooring. </li></ul><ul><li>Increase the distance of control room from the turbine . </li></ul><ul><li>Adding additional fire control system before the control so that the control room is not damaged. </li></ul><ul><li>Smoke detector inclusion. </li></ul><ul><li>Inclusion of Adsorbers materials like charcoal to absorb smoke in case of accidents. </li></ul>
  16. 24. <ul><li>The electrical and mechanical problems coupled </li></ul><ul><li>in a turbo generator set should never be neglected. </li></ul><ul><li>If the mechanical system is weak, even minor disturbances in electrical systems can cause failures. </li></ul><ul><li>For the safety of power plant equipment, a good maintenance of the system is of utmost importance. Utmost attention should be given to even seemingly simple and unimportant issues. </li></ul><ul><li>The mechanical and electrical problems seem to </li></ul><ul><li>pretty harmless in the presence of a nuclear reactor </li></ul><ul><li>and usually extreme importance is given for the </li></ul><ul><li>maintenance of nuclear side of the plant. It should </li></ul><ul><li>be remembered that a chain is as strong as the </li></ul><ul><li>weakest link in it </li></ul>

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