An explosion ripped through a smelter at a nuclear power plant outside the northern city of St. Petersburg, badly injuring three people, Russia's nuclear agency said Friday.

Rosenergoatom said radiation levels were not affected as the reactor in that part of the Leningrad nuclear plant was undergoing repairs and not in operation at the time. But Thursday's blast raised questions about safety at an onsite scrap metal operation.

The blast caused molten metal to burst out of the smelter, where a private company was reprocessing scrap metal, said a plant spokesman, Sergei Averyanov.

Averyanov blamed the explosion on violations of production rules.

The Leningrad nuclear plant is in the town of Sosnovy Bor, 400 miles northwest of Moscow and 50 miles west of St. Petersburg, the chief city in the region with 5 million people.

The plant has four units, or reactors, and the smelter is located in the complex containing the second one. The second unit was shut down for major repairs in July, the agency said.

"There were no violations of safety levels and operating conditions of the energy units of the Leningrad nuclear plant," Rosenergoatom said in a statement.

One of the reactors at the 30-year-old plant is the same type as the one at the Chernobyl nuclear power plant that exploded in Soviet Ukraine in 1986, in the world's worst nuclear accident.

The station is the main supplier of electricity to St. Petersburg, formerly known as Leningrad, and there are plans to transport some of its power to Finland.

Two of the injured had burns over 90 percent of their bodies, the Emergency Situations Ministry said.

In March 1992, an accident at the Sosnovy Bor nuclear plant leaked radioactive gases and iodine into the air, according to nuclear watchdog groups.

Sosnovy Bor, a center of nuclear technology, was founded 25 years ago and has 60,000 people.

The educational level and living standard there is higher than the average in Russia. Almost everyone in Sosnovy Bor is connected with nuclear technology, and most are not native to the region.