Could Chernobyl Happen Again?

Ukrainian school teachers walk inside a nuclear bomb shelter inside the school in the city of Slavutich, Friday, April 21, 2006. Some 50,000 Pripyat residents were evacuated after the disaster, taking only a few belongings. They never returned, and workers and their families now live in the new town of Slavutich, 60 km (37,5 miles) from the plant. The city of Slavutich was built following the evacuation of Pripyat, which was just 1.5 km (about one mile) away from the Chernobyl plant. On April 26, the world will mark the 20th anniversary of the explosion at the Chernobyl nuclear power plant, which sent radioactive cloud across Europe.(AP Photo/Oded Balilty) (Oded Balilty - AP)

MOSCOW -- The Chernobyl plant sits idle 20 years after the world's worst nuclear accident, its last reactor taken out of service some six years ago.

But a dozen other reactors of the same design remain in operation and some could be in service for another 30 years. Could another one blow up?

The explosion of April 26, 1986, is attributed by experts to a fatal combination of design flaws and poor staff training. The design problems have been addressed, but doubt remains about the human factor.

The accident was a terrible irony, coming during a routine drill to test how long the electricity-generating turbines would spin and supply power during a shutdown.

But reactors of the RBMK type used at Chernobyl have a "positive void coefficient" in which excess steam, which absorbs neutrons less effectively than water, leads to an increase in reactor power. RBMKs are considered unstable at low power.

Automatic shutdown systems had been switched off for the test, and workers couldn't insert control rods in time once they started losing control of the reactor, according to generally accepted accounts of the blast.

Since then, the RBMK reactors in Russia and Lithuania have undergone modifications recommended by the International Atomic Energy Agency including speeding up the control-rod insertion time by about a third, to 12 seconds, and using uranium of a slightly higher enrichment in the core, which essentially means the reactor doesn't have to be driven as hard to spin the turbines.

Nuclear experts say the changes have substantially reduced the technical likelihood of a repeat of the Chernobyl blast.

"Very significant changes have been made in the technology," IAEA deputy director Tomihiro Taniguchi told The Associated Press. "The IAEA is firmly committed that such an accident not happen again."

"People are fairly relaxed about the RBMKs," said Ian Hore-Lacey, a spokesman for the World Nuclear Organization, which promotes peaceful uses of nuclear energy.

John Ahearne, a former U.S. Nuclear Regulatory Commission chief who now teaches at Duke University, agreed with the technical assessment, but was less sure about staff at plants using RBMKs.

"How well are they trained, how well are they paid _ that's harder to assess," he said.