By Charles Digges | Bellona

The reactor is of the fatally flawed Soviet era RMBK-1000 series – one of which exploded at Chernobyl in 1986 ­– and is one of 11 such reactors still in service in Russia. RBMKs are moderated by graphite rather than water, which is used in more contemporary designs. RBMKs from the outset, then, pose a far more severe risk during operation as graphite is flammable.

According to Reponen, who maintains close relations with Russia’s nuclear authorities, LNPP reactor No 1 has been in cold shutdown since May 6, when it was switched off for routine maintenance. It was then that the cracking and swelling of the graphite moderator was discovered, said Reponen, although reports about it in the Russian media began to appear only this week.

Reponen also said it was unclear if the reactor would be allowed to go back online after investigations into the malformations of the graphite moderator were complete.

“If it does go back online it could be only for a short period of time” said Reponen, citing the 38-year-old age of the reactor. RBMK’s were initially designed to operate for only 30 years. He said the reactor has been shut down “indefinitely,” a decision he welcomed in Finnish press reports on Thursday.

Bellona general manager and nuclear physicist Nils Bøhmer said that: “The LNPP’s reactor No 1 should never go back online again.” Russia’s state nuclear corporation Rosatom would not comment on its further use.

What are the risks?

The chief dangers discovered at LNPP reactor No 1 are “deformations of the whole form of the core,” said Reponen, which, in conditions of its ongoing operation, could lead to coolant cut offs and uncontrolled chain reactions.

Reponen said that reactor cooling could be lost if fuel rods come into contact with the graphite. The other concern, he said, is the impossibility of introducing control rods into the contorted graphite, which could lead to a chain reaction.

Reponen emphasized that the reactor does not pose any radiological threat while in cold shutdown.

Alarm for Russia’s RBMK fleet

But the condition of the reactor is a loud warning bell for the entire RBMK fleet that Russia runs – all of which are approaching or have well surpassed their engineered operational lifespans, said Bøhmer.

”The discoveries and LNPP show that the clock is ticking very fast for RBMKs in Russia,” he said. “There is an urgent need for Russia to investigate its oldest graphite moderated reactor cores and they should be shut down as soon as possible.”

Reponen indicated during his telephone interview that deterioration within RMBKs is happening much faster than initially predicted because plutonium, a by-product of burning uranium fuel, changes the way that graphite behaves as a moderator.

Reponen cited that Russian nuclear authorities should be looking into the condition of the graphite moderator at reactor No 2 at LNPP, which is only two years younger than unit one. Another especially concerning unit on the Russian nuclear landscape is reactor No 1 at the Kursk NPP, which runs 4 RBMKs. That reactor is 40 years old – a decade over its intended engineered lifespan.

Reponen said he trusted that Russian nuclear officials were taking these reactors into account as they continue inspections of Russia’s RMBK fleet.

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