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.

The public relations department of LNPP meanwhile has said the reactor is on the mend.

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.”

LNPP denies ‘indefinite’ shutdown

LNPP spokesman Mikhail Melikhin on Thursday strongly denied to the BaltInfo news agency that the shutdown of the reactor was indefinite, as reported by STUK’s head and other sources.

Melikhin said instead that leaving it off the grid for now was simply part of planned maintenance – which has simply been extended. The reactor’s operational license, he noted, is valid through 2021.

“There have at the moment been no decisions taken about [the reactor’s indefinite shutdown],” Melikhin said. “All is proceeding according to plan.”

Melikhin continued to say that the condition of all graphite moderators at Russia’s RBMKs were being inspected. He said if they had swelled to a point that begins to put pressure on the construction, “compensatory measures” would be taken ­– and insisted that was what is currently underway at LNPP’s reactor No 1.

Melikhin told the Fontanka news agency that “it remains to be decided whether the reactor will be put back online with or without generating power.”

According to Andrei Ozharovsky, another nuclear physicist with Bellona, putting a reactor online without using it to generate power is a necessary precursor to decommissioning the unit altogether.

Igor Kudrik, a Bellona expert on the Russian nuclear industry, said Melikhin’s denials did not ring true.

“It sounds like they are trying to paper over a very serious problem and make it all sounds okay and predictable,” he said. “But the denial is unconvincing and raises more questions than it answers.”

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.

Vladimir Slivyak, co-chair of Russia’s Ecodefense said that conditions such as those discovered at LNPP’s reactor No 1 were not surprising, given repeated decisions by Russian nuclear authorities to grant engineered life-span extensions as long as 15 years to their RMBK reactors, adding “this was not the intention of their designers.”

“It is clear this is a serious problem that would occur sooner or later,” said Slivyak in an email. “ The problem with the graphite moderators [of RMBKs] has long been known.”

Like Bøhmer, Slivyak said that Russia’s RBMKs should be shuttered.

“Rosatom must shut RBMKs down immediately in order to avoid large-scale accidents. Otherwise, there is a very high risk of new Chernobyl-Fukushima type of event,” wrote Slivyak.

Both LNPP, 80 kilometers west of St. Petersburg and Kursk NPP, located 541 kilometers south of Moscow, run four RBMK reactors. Smolensk NPP, 393 kilometers west of Moscow, operates the remaining three.

The very youngest of Russia’s RMBKs are found at Smolensk NPP. The first of the plant’s RMBKs was built in 1982, with the following two put on the grid in 1985 and 1990.

In a blog posted late last month, Yevgeny Romanov, head of Rosenergoatom, Russia’s nuclear power station operations subsidiary wrote that his agency was investigating LNPP’s reactor Nos 1 and 2.

He said that until an expert committee had determined the cause of the defects in the graphite moderator at reactor No 1, it would not be powered up again and added that, “we have said time and again, and I will repeat it now: The most important issue to us is safety. So long as we don’t understand, the reactor will not be launched.”

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