Determining the causes of the accident was not easy, because there was no experience of comparable events to refer to. Eyewitness reports, measurements carried out after the accident, and experimental reconstructions were necessary. The causes of the accident are still described as a fateful combination of human error and imperfect technology.
The test during which the accident happened was conducted under time pressure. Shortly after it started, on Friday 25 April 1986, the test run was interrupted for nine hours. Electricity still had to be supplied to the capital, Kiev. The test then took place at night. Today, several flaws in the technical design of the reactor type are thought to have been decisive.
These include the handling of the control rods. In a reactor, the power level is controlled by raising and lowering the control rods: the fewer control rods are positioned between the fuel elements, the greater the reactor power. In this type of reactor, however, the management of the "braking" process has a fatal flaw. If the control rods are raised and then, to "put on the brakes", lowered between the fuel elements, the initial effect is the exact opposite: reactor power is increased.
If, as was the case in the test at Chernobyl, too many control rods are raised at once and then reinserted simultaneously during an emergency shutdown, the power level rises so dramatically that the reactor is destroyed. A similar error, but with much less severe consequences, had already occurred in a reactor of the same type in Lithuania in 1983. This experience, however, was not passed on to the operating crew in Chernobyl.
The test during which the accident happened was conducted under time pressure. Shortly after it started, on Friday 25 April 1986, the test run was interrupted for nine hours. Electricity still had to be supplied to the capital, Kiev. The test then took place at night. Today, several flaws in the technical design of the reactor type are thought to have been decisive.
These include the handling of the control rods. In a reactor, the power level is controlled by raising and lowering the control rods: the fewer control rods are positioned between the fuel elements, the greater the reactor power. In this type of reactor, however, the management of the "braking" process has a fatal flaw. If the control rods are raised and then, to "put on the brakes", lowered between the fuel elements, the initial effect is the exact opposite: reactor power is increased.
If, as was the case in the test at Chernobyl, too many control rods are raised at once and then reinserted simultaneously during an emergency shutdown, the power level rises so dramatically that the reactor is destroyed. A similar error, but with much less severe consequences, had already occurred in a reactor of the same type in Lithuania in 1983. This experience, however, was not passed on to the operating crew in Chernobyl.