The blast occurred on Thursday at the Leningrad nuclear power plant in the closed nuclear town of Sosnovy Bor, outside the northern city of St Petersburg and about 600km (400 miles) northwest of Moscow.
The smelter is operated by Ekomet-S, a company that reprocesses scrap metal, mostly from equipment at the plant - which can carry radioactive contamination.
The explosion highlighted what environmentalists called uncontrolled operations by such companies on sensitive sites.
Dmitry Artamonov, head of the St Petersburg branch of Greenpeace, said: "The enterprise ... functions illegally because there was no mandatory (state) environmental impact assessment on its construction."
He said Greenpeace had appealed against Ekomet-S to the Sosnovy Bor prosecutors' office but it took no action.
Rosenergoatom, Russia's nuclear agency, said that the smelter was in the grounds of the plant's second unit, and Sergei Averyanov, the plant spokesman, said it was about a kilometre from the reactor. The plant has four units in all.
But Oleg Bodrov, a physicist who heads the Green World ecological group in Sosnovy Bor, said the reactor was about 700m from the smelter, which is also about 50m from a radioactive waste pond.
The Emergency Situations Ministry said that two of the injured had burns over 90% of their bodies. A 33-year-old worker died of his injuries on Friday morning.