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 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