Technicians at the original Leningrad nuclear power plant near St Petersburg have taken a major step in the decommissioning one of their oldest Soviet-built reactors by completely removing the uranium fuel from its core.

The fuel withdrawn from the No 1 reactor, a Chernobyl-type RBMK, will be reused in the two other RBMK units that are still operating at the plant – a process that that is typical for this graphite-moderated reactor model, but is impossible in more contemporary boiling water units.

The No 1 reactor was taken out of operation after 45 years of service in December 2018 as part of a gradual phase-down of the Leningrad nuclear power plant’s four aged reactors. Their power production is being replaced by the Leningrad Nuclear Power Plant II, where two VVER-1200 type reactors are now operating alongside the old plant’s site in the town of Sosnovy Bor, 70 kilometers west of St Petersburg on the Gulf of Finland.

The utility plans to re-use some of the fuel it extracted from the reactor. Fuel assemblies with more than half of their designed burn-up remaining will be transferred to reactors 3 and 4. Plant manager Vladimir Pereguda said this avoids the purchase of around 500 fresh fuel assemblies.

Launched in 1973, the reactor became the first unit of the RBMK-1000 type to be built in the Soviet Union. A reactor of the same model exploded at Chernobyl in April 1986 in the world’s worst nuclear accident, and Russian nuclear officials have been at pains to stress that Leningrad’s No 1 reactor had operated “reliably and safely” throughout its career.

The No 2 RBMK reactor at the Leningrad site, which was also built in the 1970s, was taken out of service in November of last year. Rosatom, Russia’s state nuclear corporation intends to take the remaining two out of service in the next several years.

Another seven RBMKs operate throughout Russia, and two similar RBMK-1500 style reactors were constructed at the now-decommissioned Ignalina nuclear plant in Lithuania. The remaining three RMBKs at Chernobyl continued operations after the No 4 reactor there exploded and were not shut down until 2000.

The hurdle to decommissioning RBMKs

While each of these Soviet RBMKs underwent comprehensive upgrades in the years following the Chernobyl catastrophe, no amount of remodeling has been able to finesse the reactors’ most deviling obstacle where safe decommissioning and dismantlement are concerned – their graphite stacks.

A graphite stack is essentially bulky cylinder about 7 meters high and 11 meters across made of graphite bricks and weighing about 2000 tons. Fuel is fed into the reactor via channels cut in the masonry, and the graphite acts as the moderator.

The concept originated in the late 1940s when the Soviet Union and the United States began building reactors to produce weapons-grade plutonium – a time when considerations about how to dismantle nuclear installations were not a priority. While most reactors in commercial operation around the globe are of the boiling water type, the Soviet Union’s first steps in civilian nuclear power were based on this unwieldy graphite moderated design.

Now, Russia’s RBMK reactors are reaching retirement age. But as Rosatom’s plans for dismantling the Leningrad nuclear plant’s reactors come into focus, it’s clear that the question of how to safely dismantle RBMKs remains largely unanswered.

According to Pereguda, the No 1 reactor will be treated essentially as an operating reactor, both financially and technically, for the foreseeable future – a phase called “operation without generation.”

This amounts to removing the reactor’s fuel and decontaminating what remains of the reactor’s structure aside from its graphite stack. After that, it’s essentially a process of waiting until nuclear science catches up with advances geared to handle the graphite stacks.

Presently, there are two Russian scientific studies underway – one in Seversk, underwritten by the UN’s International Atomic Energy Agency, and the other in Sosnovy Bor – aimed at solving the problem.

An expensive wait

In the meantime, dealing with the graphite stacks is a costly process. Without the profits from the sale of electricity generated by Leningrad’s No 1 reactor, the Russian business newswire RBK daily calculated that Rosenergoatom, Russia’s nuclear utility, will lose $203 million over the duration of the reactor’s decommissioning.

When that decommissioning will be completed, however, remains unclear. Experts who spoke with RBK estimated that the reactor might not be fully decommissioned for another 50 years because the technology for dealing with the irradiated graphite stacks simply doesn’t exist.

This presents a problem for Rosatom. The company intends to put the remaining two RBMKs operating at the Leningrad plant out of service by 2025. Those closures will be followed by the four RBMK 1000 reactors at the Kursk Nuclear Power Plant. Another three will be shuttered at the Smolensk Nuclear Power plant sometime within the next decade That leaves a lot of irradiated graphite hardware on Rosatom’s hands.

At the same time, there is little to suggest any breakthroughs in removing and storing the reactors’ graphite stacks. At Lithuania’s Ignalina – as well as at other graphite moderated weapons reactors both in Russia and the United States – decommissioning has involved simply mothballing the graphite stacks and waiting until better times, when more is known about how to safely deal with them.

That’s not to say that breakthroughs won’t come. The studies in Seversk and Sosnovy Bor may yet yield answers. It will be interesting to watch what materializes – especially for the good of those who must live among these reactors’ remains.