The cause of a surprise reactor shutdown at Russia’s Kola Nuclear Power Plant’s No 4 reactor on Tuesday was finally explained by the station’s press service as “deterioration of the insulation of a power cable in the course of conducting scheduled tests on an auxiliary systems’ pump.

The initial emergency shutdown of the reactor on Tuesday morning at about 9:37 am Moscow time was originally reported without an explanation – something nuclear experts on Russia said is exceedingly rare, and cause for concern.

The reactor’s age and clearances to run above nominal generating capacity near the city of Murmansk added to worries during the day-long silence on why the No 4 unit had been pulled from the grid.

The Kola nuclear plant reported yesterday that the reactor was up and running again by 10:48 am Wednesday, after what the press service indicated was “an elimination of the malfunction” and “checks on the operational capabilities of equipment.”

The Kola plant release continued to say that “switching the reactor off the grid happened in accordance with engineered algorithms: No limits or conditions of safe use were violated.”

Inside Kola NPP's reactor No 1. (Source: Wikimedia Commons)

It added that a preliminary evaluation on the seven-point International Nuclear Event scale (INES) assigned the shutdown a “zero,” which indicates an event that, according to the scale, has “no safety significance” for the plant. The station also reported that background radiation levels at the plant and in surrounding areas remain unchanged since before the shutdown.

To what extent the burnt-out cable represents any threats to safety, however, will only be clear after an investigation by a committee to be appointed by Rostekhnadzor, Russia’s Federal Service for Environmental, Technological and Nuclear Oversight.

According to Andrei Ozharovsky, a Moscow-based nuclear adviser with Bellona, such investigations can be protracted for months, and their results aren’t generally made public.

Worries arose about the sudden, and initially unexplained shutdown because of two aspects in the reactor’s operational history.

First, the reactor, which is a VVER- 440 unit, is running on a 25-year engineering lifespan extension, meaning it will not be taken out of service until 2039, when it’s 60 years old.

The unit went online in 1984, and received its current service extension in 2014. Ozharovksy said this extension was granted in the absence of a state environmental impact assessment.

Second, the reactor since 2012 has been a part of an experiment to run at various intervals at 107 percent its nominal production capacity. The reactor had also been run at expanded capacities in 1986 and 1987 under Soviet rule, but the Chernobyl disaster of 1986 mothballed that for almost a decade and a half.

Alexander Nikitin, chairman of the Environmental Rights Center Bellona said on Thursday by email that, “it’s not important here how long the reactor has worked or will work – what’s important is that they allowed [such extensions] to begin with.”

The Kola Nuclear Power Plant. (Photo: Courtesy of b-port.com)

As to the experiments in boosting No 4’s power output, Nikitin said, “Difficulties can arise when a reactor is operated at any power, even at the minimum controllable level – but the official decision to stretch the power output and all other corresponding parameters to above normal, of course, adds to risks.”

Nikitin has in the past indicated that many nations with nuclear power plants routinely experiment with boosting reactor power output levels, not just Russia.

But Ozharovsky said the reactor is lacking in several contemporary safety components, such as appropriate containment and a core catcher to prevent leakages of molten core material in the event of a meltdown. This safety feature has been de rigueur in French reactors since the 1980s.

Kola’s No 4 reactor isn’t the plant’s only reactor that’s operation on borrowed time. Operations for reactors No 1 and 2 were extended to 2018 and 2019, respectively.

But the Kola plant drew up an investment project for operational extensions amounting to a total of 60 years for both these reactors. This means that they could both operate another 15 years on top of the 15-year extensions they’ve already got.

The Kola Nuclear Plant’s No 3 reactor in 2011 received 5-year operational extension.

The plant is likewise expecting to get the nod to run the No 3 reactor at boosted power outputs of 104 to 107 percent.

The boosted power regimens are not specific to Russia’s Kola nuclear station. Another six of Russia’s 31 reactors are operating above nominal capacity.

VVER-1000 reactors operating at Russia’s Balakovo, Kaliningrad and Rostov nuclear stations also undergo power boosting experiment to 104 percent over nominal output. Another reactor at the Rostov station may soon be taking part in these experiments as well, as soon as an environmental impact study is approved.

Some years ago, Russian state nuclear corporation Rosatom proposed running Chernobyl-type RBMK reactors at above nominal power output as well. In the wake of the Fukushima disaster in 2011, however, the plan was scrapped.