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Three Mile Island (TMI-2)

Three Mile Island (TMI-2). Three Mile Island reactor number 2 (TMI-2) in Pennsylvania had a core meltdown in March 28, 1979. The reactor was a PWR design. Main feedwater pump failed Backup system opens emergence feedwater system Valves were closed

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Three Mile Island (TMI-2)

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  1. Three Mile Island (TMI-2) Three Mile Island reactor number 2 (TMI-2) in Pennsylvania had a core meltdown in March 28, 1979. The reactor was a PWR design.

  2. Main feedwater pump failed • Backup system opens emergence feedwater system • Valves were closed • No one noticed the Red Warning Light in the control room.

  3. Water in the core overheated – control rods inserted • PORV opened to release pressure • Normal pressure restored PORV closed • PORV didn’t close completely - control light said it had

  4. Fission stopped by control rods but beta-decay continues • Beta-decay lasts for about 1 hour, each decay releases 15 MeV • All water evaporates through PORV and core melts • Molten core stays in bottom of pressure vessel

  5. Chernobyl Water cooled – Graphite moderator • Safety test planned so turned power down to 50% • Realised station needed for grid so kept in low power for a day • Xenon build up in core due to fewer neutrons • Removed control rods completely out of core to maintain power output • At 1:23:04 on 26 April, 1986 safety test began! • Turned off emergence cooling system • Turned off one of the turbines • Cut electricity to remaining turbine to see if inertia in rotating blades was sufficient to keep turbines spinning, and therefore power to cooling pumps, for a few seconds until emergency power started. • This lead to a slight drop in water flow into the steam generators • Less heat now extracted from core coolant water which began to boil

  6. Positive Void Coefficient • Water was coolant in Chernobyl • Water also absorbs neutrons and so slows reaction • Water can act as a moderator but moderation dominated by graphite in Chernobyl • When bubbles form fewer neutrons absorbed therefore reaction rate increased • More heat more bubbles – this feedback called a positive void coefficient • Not true for AGR/PWR - physics prevents Chernobyl accident in AGR/PWR!

  7. Inserted the Control Rods • 1:23:04 on 26 April, 1986 safety test began • 1:23:40 to control the positive void coefficient inserted control rods • Bottom of control rods were made of graphite so first thing that happened when control rods were inserted was an increase in reaction rate • 1:24:00 reactor went critical

  8. Summary • Chernobyl wouldn’t have exploded if it didn’t have a positive void coefficient – poor design • Chernobyl wouldn’t have exploded if the control rods didn’t have moderators on the bottom –poor design • Chernobyl wouldn’t have exploded if managers had waited for normal operating conditions before starting the test – poor management and safety culture • Estimates vary enormously for total expected fatalities – we don’t really know the full cost. Estmates of projected fatalities vary from a 1000 to 200,000. You must decide for yourself which of these is true: • Chernobyl proves that nuclear accidents are easily avoided by proper design • Chernobyl proves that nuclear power is madness

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