BACKGROUND: Reactor design contributed to Chernobyl accident

Apr 25, 2011, 12:37 GMT

Kiev - The Soviet-era reactor used at the Chernobyl nuclear power plant contained several design flaws that contributed directly to the 1986 accident, studies have shown.

Unlike modern reactors, the one employed at Chernobyl, known as a high-power channel-type reactor (RBMK), was not designed to contain unexpected radiation leaks.

The reactor itself was set in a thin steel vessel surrounded by concrete, while the reactor building was standard industrial housing.

While the vessel and its housing were sufficient to contain radiation during normal operation, neither was designed to contain steam or hydrogen explosions caused by the heat of an out-of-control reactor.

Explosions at Chernobyl demolished the reactor core and much of the reactor building roof, leaving exposed highly radioactive debris.

The tools used by RBMK reactor operators to control the intensity of the nuclear reaction - 6-metre-long control rods - were another contributing cause to the accident.

Made primarily of boron carbide, which acts as an excellent damper to nuclear reaction, the RMBK control rods are pushed into the reactor core to slow the reaction down. If enough control rods are pushed in deeply enough, the nuclear chain reaction creating reactor heat comes to a halt.

But the RMBK's control rods were tipped with graphite, the same material as the reactor core. Graphite burns at 730 degrees Celsius - a relatively low and possibly dangerous temperature, given how much heat a runaway nuclear reaction might generate.

Also, the RMBK reactor's design allows the control rods to move into and out of the reactor core at a measured and relatively slow speed. In case of a power spike, the RMBK's control rods might reach a proper dampening position too late to prevent a rapidly accelerating nuclear reaction.

At Chernobyl, a very rapid increase in reactor heat caused the graphite tips of the control rods to melt. When engineers triggered emergency stop equipment, the control rods managed to move only one- third of their length into the core before becoming stuck in their channels. This proved insufficient to prevent the nuclear reaction from accelerating.

The graphite then caught fire and produced highly radioactive smoke, which made its way into the atmosphere via the roofless reactor building. Worse, graphite fires cannot be easily extinguished.

Another inherent design flaw in RMBK reactors is an unpredictable acceleration of nuclear reaction when operated at lower power levels.

Engineers at Lithuania's Ignalina plant, who also operated RMBK reactors, had uncovered this shortcoming before the Chernobyl accident.

The Soviet government made the Ignalina findings secret, and Chernobyl engineers were not aware of it when they conducted a low- power reactor test in the early morning hours of April 26, 1986.

Most RMBK reactors were taken out of commission after Chernobyl. Russia continues to operate six, which have received upgrades in control systems, housing and safety equipment.

Sources: International Atomic Energy Agency, Ukraine's Emergency Situations and Chernobyl Ministry.