As Reactor Four was to be shutdown for routine maintenance on the 25 April 1986 it was decided to take advantage of this to run a test. The test was to check whether, in the event of a shutdown enough electrical power to operate the emergency equipment and core cooling pumps until the diesel power supply came online.
As the reactor shutdown proceeded, the reactor operated at approximately half power when the electric load dispatcher refused to allow further shutdown. As part of the test the emergency core cooling system was switched off and the reactor carried on at half power. At about 23:00hrs on 25th April the grid controller agreed to a further reduction in power.
For the test the reactor was to be stabilised at 1,000MW prior to shutdown, however, due to operational error the power fell to about 30MW where the positive void coefficient became a problem. The operators tried to raise the power by freeing all the control rods manually and at 01:00hr on 26 April the reactor stabilised at 200MW.
Shortly afterwards an increase in coolant flow and a drop in steam pressure occurred requiring the operators to withdraw nearly all the rods. The reactor then became very unstable and the operators had to make adjustments every few seconds to maintain a constant power.
At this time the operators reduced the flow of feedwater to maintain the steam pressure. Also pumps that were powered by the slowing turbine provided less and less cooling to the reactor. This created additional steam in the cooling channels (positive void coefficient) and the operators could not control a power surge estimated to 100 time the nominal power.
The sudden increase in temperature caused part of the fuel to rupture, fuel particles then reacted with the water creating a steam explosion which destroyed the reactor core. A second explosion added to the destruction two minutes later.
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发表于 2010-9-15 15:48:40