The No 1 reactor at the Leningrad Nuclear Power Plant II that was taken offline due to an unknown system failure before the weekend was again operational Monday, according to a release from the plant.

The plant’s statement issued early Monday, reported that the reactor had again been connected to the electricity grid and was operating at 100 percent capacity, but failed to mention the reason why the unit’s emergency systems had tripped the shutdown in the first place.

The reactor, which came online last year, is one of the first of the VVER-1200 line, which Rosatom, Russia’s state nuclear corporation, has worked to establish as its flagship product despite a series of glitches that have beset its earlier releases.

It also the first reactor of the four-reactor Leningrad Nuclear Power Plant II, which Russia is building to replace the original Leningrad Nuclear Power Plant – both of which are located about 50 kilometers west of St Petersburg on the Gulf of Finland.

Retirement of the older plant has already begun. Last month, the first of its four Chernobyl-style RBMK-1000 reactors was taken out of service after 45 years of operation, with the remaining three slated to follow by 2021.

But the new line of VVER-1200 reactors that will be replacing the older workhorses of the Soviet nuclear industry have proved fussy in early operations.

And while there is no official information suggesting that Friday’s scram of the Leningrad station’s No 1 reactor posed any danger, Russian officials have, as a rule, been loathe to admit it when VVER-1200s – which constitute their prime offering to domestic and international markets ­– have run into problems in the past.

Russia’s first VVER-1200 reactor, which went into service in 2016 at the Novovoronezh Nuclear Power Plant, contained a flaw in its electric generator that kept it offline for several months after it was put into operation.

But Rosatom officials took pains to obscure the emergency reactor shutdown that led to the repairs – first by keeping the scram under wraps for several days, and then by backdating press releases on the subsequent repairs to hide the initial hush-up.

It was later reported that the generator issue was common to several other VVER-1200 that were under construction at the time, and Rosatom made upgrades.

Russia is currently planning to build as many as 16 of these reactors throughout the country, either as extensions to current nuclear power plants or to constitute new plants altogether.

Three more are slated for construction the Leningrad Nuclear Power Plant II, and three more are underway at the Novovoronezh plant south of Moscow, which likewise will replace an older plant by the same name. Should construction ever begin on the Kostroma and Nizhny Novgorod nuclear power plants, those would have VVER-1200 reactors as well.

Yet more VVER-1200 reactors are under construction abroad. Belarus is building one near its border with Lithuania, and Finland is planning to build one as well. Turkey has ordered four VVER-1200 reactors, and both Bangladesh and Hungary are in line to build two each.