Shortly before 3 pm on the afternoon of Friday, March 11, a massive earthquake of approximately 9.0 magnitude struck offshore of Japan.
At the Fukushima Daichi plant, Units 1, 2 and 3 were in operation. The other three units were in various conditions of maintenance shutdown. As designed these three reactors shutdown automatically in response to the earthquake loads. Although, this earthquake exceeded by a large margin, the earthquake accelerations for which the reactors were designed, it appears that no adverse affect resulted.
1 Core with fuel rods
3 Equipment pool
5 Fuel storage pool; spent fuel area
8 Reactor pressure vessel
10 Secondary concrete shield wall
11 Free standing steel drywell
16 Vent header
17 Downcomer pipe
18 Water (wetwell)
20 Basement
21 Reactor building
22 Refuelling platform
24 Pressure supression chamber (runs in a torus around the reactor)
25 Vent (81 inch diameter)
26 Crane
27 Spent Fuel
29 Feedwater pipe
30 Steam pipe (to turbine-generator)
31 Control rod drives
39 Control rods
40 Steam separators (water normally goes to this level)
41 Steam dryer
The earthquake also caused the loss of electrical power from the national electrical grid. As a result, the redundant emergency diesel generators were activated on loss of external power which were provided to power all required emergency loads. Note, two full capacity diesel generator units are required at nuclear plants as required by the Single Failure Criterion design requirement. This requires that all emergency functions be available assuming a single failure of any given component.
Simultaneously, the emergency steam turbine driven feedwater pump was activated, providing feedwater cooling to the reactor vessels of units 1, 2 & 3.
Flowing through the reactor core fuel elements, this water was turned to steam by the residual heat remaining in the fuel rods. This steam, in turn, was directed downward into the torus water pool, where it was condensed.
Some 15 minutes later, The tsunami unleashed by the earthquake struck the Fukushima facility flooding out the emergency diesels, the electrical switchgear, and all the required electrical driven emergency pump motors.
With the loss of power from the grid and the damage to the diesel generators, the plant was now totally without power in which to drive emergency equipment. The only remaining emergency function remaining was the steam-driven pump which would only be functional for a few hours when the steam pressure had been reduced below working pressure.
At this time, emergency cooling water pumps should have been activated, but, had no electrical power available to power them, however, this would have been to no avail, as they had been damaged by the flooding waters.
Subsequently, without electric power, without cooling, the pressure within the nuclear units began to rise. This pressure buildup is the result of residual decay heat causing the coolant, which is not being circulated, to evaporate
The central problem behind almost every hurdle faced by the workers at the Fukushima nuclear power plant has been – and remains – a lack of power supply. Since electricity was knocked out by the Tsunami it has been impossible to run the pumps that cool the reactor cores and circulate water around storage pools used to keep spent fuel rods cooled.
