The fear lingers: Could it happen again?

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, was attributed to a 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.

How it happened

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 said. "The IAEA is firmly committed that such an accident not happen again."

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.

A study by Greenpeace and the Russian Academy of Sciences found many nuclear workers in Russia showing up for work drunk or on drugs, said Vladimir Chuprov, head of energy issues at the Russian branch of Greenpeace. Yuri Sarayev, a nuclear expert at the Russian Academy of Natural Sciences, said pay hasn't kept up with the booming economy, so "specialists with solid training and 10 to 15 years' experience are leaving, and being replaced by less-prepared people."

Russian officials insist the RBMK reactors' future is bright and their service life will be extended from 30 to 45 years, with the last to close in 2036.

That confidence isn't universally shared. Lithuania, a former Soviet republic, has already mothballed one RBMK at its Ignalina plant and is to shut the other in 2009.

Can a Chernobyl-type disaster happen again?

Nikolai Tarakanov, a scientist and retired general who heads the Center for Social Support of Chernobyl's Invalids, replies: "No one can give you a guarantee that it will not happen tomorrow."