Russian officials say the explosion damaged a metal smelter on the grounds of the Leningrad Nuclear Power plant near the city of St. Petersburg.
The state nuclear agency Ros-energo-atom released a statement saying the blast did not affect the nuclear reactors at the plant, and did not cause the release of any radiation. The smelter reprocesses scrap metal.
A spokesman at the plant says two of the injured workers suffered serious burns.
The accident occurred on Thursday morning, but word of the blast only came out early Friday.
The nuclear watchdog organization, Bellona, has won a court case against the company operating the metal plant, arguing that such a facility should not operate near a power plant. Bellona officials say this is the second time an accident has occurred at the smelter, even though it is a new facility.
The overall state of nuclear safety in Russia has long been of concern, and the Norway-based Bellona Foundation has sought to focus international attention on the issue.
Igor Kudrik works with Bellona in its Oslo office. He says a recent study by the foundation finds that not enough is being done to improve safety. "Our conclusion is that the nuclear industry is not reforming," he said. "It is spending money on sustaining old Soviet-era nuclear facilities, and not investing into safety."
Mr. Kudrik says he is hopeful that this may change, with the recent appointment of former Russian Prime Minister Sergei Kiriyenko as head of the Russian atomic energy agency.
Mr. Kudrik says the previous director of the agency had links with the industry, and did little to invest in modernizing aging power plants.
The Leningrad plant is more than 30 years old.