Three Mile Island reactor number 2 (TMI-2) in Pennsylvania had a core

meltdown in March 28, 1979. The reactor was a PWR design.

Three Mile Island (TMI-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.

• 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

• 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

Chernobyl

• 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

Water cooled – Graphite moderator

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!

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

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:1. Chernobyl proves that nuclear accidents are easily

avoided by proper design 2. Chernobyl proves that nuclear power is madness