Reactor No 2 at the Leningrad Nuclear Power Plant II has achieved criticality and was brough to the minimum controlled power level late last month, officials with Rosenergoatom, Russia’s nuclear utility, have said.

The commissioning of the reactor – a VVER-1200 – is being carried out in stages, which began on July 19 when its first nuclear fuel assembly was loaded into the vessel, the company said. Commercial operation of the rector will begin in 2021.

“The minimum power level will allow us to safely conduct a series of tests and verify the physical characteristics of the reactor core for compliance with the design requirements, Alexander Belayev, Leningrad II’s chief engineer said in a statement.  “After these operations are completed and the calculations are received by Rostekhnadzor (Russia’s environmental agency), it remains for us to obtain a permit for power start-up and to begin a phased increase in capacity.

During the first week of August, engineers conducted more than 50 tests in accord with the physical launch schedule, World Nuclear News reported. Those reviews will help to specify the neutron-physical parameters of the nuclear reactor’s first fuel loading and confirm that all the nuclear safety systems are working in a reliable manner.

The reactor is the second VVER-1200 to go into operation at Leningrad II, which Russia is building to replace the original Leningrad Nuclear Power Plant. Both facilities are located in Sosnovy Bor about 50 kilometers west of St Petersburg on the Gulf of Finland.

Retirement of the older plant has already begun. Last year, 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.

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 scheduled to go into operation.

At the time, officials took pains to obscure the emergency reactor shutdown that led to the repairs, keeping the scram under wraps for several days. It was later reported that the generator issue was common to several other VVER-1200 that were then under construction and Rosatom, the state nuclear corporation, made upgrades.

Rosatom 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.

Two 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 two, the first of which was fueled last month. Turkey has ordered four VVER-1200 reactors, and both Bangladesh and Hungary are in line to build two each.