Perform a fault tree analysis of the last 1 minute of the accident.  (10 marks)

• The main trick to this fault tree analysis is to understand that at each stage the positive void coefficient is magnifying the accident.  The steam increase would not cause a power surge without the positive void coefficient.  Removing the control rods (which increases reactivity) would not have increased steam without the p.v. coefficient.  The other aspect that is important is that at 1:23, the reactor is already in an unsafe state since much of the cooling water has been converted to steam already. 

• In marking there should be 5 clear levels of events connected by AND connections.  Insight is required to determine the correlated conditions.  4 marks for the four of these.  1 mark for a reasonable tree structure connecting between the levels.

 Reactor explodes

 Reactor power surges very high  AND plant only designed to contain a certain power level

 Reactor steam increase AND reactor design with positive void coefficient/cooling water is steam

 Removal of automatic control rods AND reactor design with positive void coefficient/cooling water is steam

 Cut flow to the turbines AND cooling water largely converted to steam