What Went Wrong?
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Problems with RBMK Reactors
As discussed on the previous page, the Chernobyl nuclear reactors are RBMK type reactors, which suffer from instability at low power. This sometimes causes a rapid, uncontrollable power increase. The design of other types of reactors helps prevent instability from occurring by using water rather than steam because water is a better coolant than steam. Another reason RBMK reactors aren't as good as other types, is because the steam doesn't act as a moderator and a neutron absorber, slowing down the reaction, while water in other reactors does. Next, positive void coefficient, meaning that if the water in the reactor boils in some spot, a bubble of steam is produced. In other reactors, this causes reactivity, causing the nuclear reaction to slow down. In RBMK reactors, it speeds up. And lastly, RBMK reactors are built with out a containment shell, meaning in an accident, the radioactive materials spread, rather than being contained.
Not only was the design of the Chernobyl reactors weak, but the plant's operators made several bad decisions. Reactor Four was shut down on April 25, 1986 for maintenance, and the staff decided to take an advantage of this and run a test. The test they conducted was to see if in the event of shut down, whether or not enough electrical power to operate the emergency equipment and core cooling pumps until the diesel power supply came. In addition, they were testing with only 6-8 control rods, even though there was a standard minimum of 30 rods to control. Beyond that, the emergency cooling system was turned off. To top that, there was a communication breakdown, preventing communication between the group in charge of the test, and group responsible for the operation of the nuclear reactor.
Sequence of Events, before the Meltdown
Once Reactor four was shut down, the reactor operated at around half power, because the electric load dispatcher wouldn't allow complete shutdown. The test required the emergency core cooling system to be turned off and the reactor continued to be run at half power. Around 11:00 PM, on April 25, the reactor allowed even more reduction in power. The test also required the reactor to be stabilized at 1,000 MW before shutdown, but due to operational error, the power dropped down to around 30 MW, causing trouble. The operators tried to raise the power by freeing all of the control rods manually at about 1:00 AM on the 26th, stabilizing the reactor at 200 MW.
Shortly after, an increase in coolant flow and a drop in steam pressure occurred, requiring the operators to withdraw most of the rods. The reactor then became very unstable and the operators had to make adjustments every couple seconds just to maintain a constant power. By then, operators had reduced the flow of feedwater to maintain the steam pressure. Also, pumps that were powered by the slowing turbine provided less and less coolant to the reactor. The created additional steam in the cooling channels and the operators couldn't control a power surge, estimated to be 100 times more powerful than normal.
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 occurred less than two minutes later. A disaster had just begun.