Two more reactors at Fukushima Daini have now achieved cold shutdown with full operation of cooling systems. Engineers are working for the same at the last unit.
The power plant's four boiling water reactors stopped automatically on last week's earthquake. At unit 3 the shutdown appears to have gone exactly as expected, with no systems damaged by the huge earthquake or tsunami. It went from power production to cold shutdown - where coolant water is at less than 100ºC - in about 34 hours.
All the reactors have remained safe, but damage to the emergency core cooling systems of units 1, 2 and 4 led to the announcement of emergency status. Those reactors used their a secondary system, the make up water condensate system, and this was used to maintain coolant levels above the reactor core. An additional emergency notice came from unit 1 concerning the temperature of a suppression chamber, which reached 100ºC after some time.
The plant then continued in a relatively steady state throughout 12 March, with coolant levels always maintained but a high level of attention from engineers. Units 1, 2 and 4 were prepared for potential pressure release, but this was never required.
In the last 48 hours, Tepco has carried out repairs to the emergency core coolant systems of units 1, 2 and 4 and one by one these have come back into action. Unit 1 announced cold shutdown at 1.24 am today and unit 2 followed at 3.52 am.
Repairs at unit 4 are now complete and Tepco said that gradual temperature reduction started at 3.42pm. An evacuation zone extends to ten kilometres around the plant, but this is expected to be rescinded when all four units are verified as stable in cold shutdown conditions.
Researched and written
by World Nuclear News