Presentation on theme: "Three Mile Island (TMI-2)"— Presentation transcript:
1 Three Mile Island (TMI-2) Three Mile Island reactor number 2 (TMI-2) in Pennsylvania had a core meltdown in March 28, The reactor was a PWR design.
2 Main feedwater pump failed Backup system opens emergence feedwater systemValves were closedNo one noticed the Red Warning Light in the control room.
3 Water in the core overheated – control rods inserted PORV opened to release pressureNormal pressure restored PORV closedPORV 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 MeVAll water evaporates through PORV and core meltsMolten 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 dayXenon build up in core due to fewer neutronsRemoved control rods completely out of core to maintain power outputAt 1:23:04 on 26 April, 1986 safety test began!Turned off emergence cooling systemTurned off one of the turbinesCut 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 generatorsLess heat now extracted from core coolant water which began to boil
6 Positive Void Coefficient Water was coolant in ChernobylWater also absorbs neutrons and so slows reactionWater can act as a moderator but moderation dominated by graphite in ChernobylWhen bubbles form fewer neutrons absorbed therefore reaction rate increasedMore heat more bubbles – this feedback called a positive void coefficientNot 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 began1:23:40 to control the positive void coefficient inserted control rodsBottom of control rods were made of graphite so first thing that happened when control rods were inserted was an increase in reaction rate1:24:00 reactor went critical
8 SummaryChernobyl wouldn’t have exploded if it didn’t have a positive void coefficient – poor designChernobyl wouldn’t have exploded if the control rods didn’t have moderators on the bottom –poor designChernobyl wouldn’t have exploded if managers had waited for normal operating conditions before starting the test – poor management and safety cultureEstimates 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 designChernobyl proves that nuclear power is madness