The seven men were working on a top secret missile engine on a sea platform off the coast of northern Russia,
Five were killed by the blast on Thursday, with two more casualties dying from their injuries that night.
Russian nuclear agency Rosatom said experts were blown off the platform by the force of the blast.
They were reportedly testing a classified 'isotope power source' for an engine.
Radiation levels in the area spiked for 40 minutes after the explosion, but are said to have soon returned to usual background levels.
One more employee was injured in the accident.
The incident has led to speculation Russia is testing a new nuclear-powered missile in the Arctic, and even claims the weapon could be a 'dirty bomb.'
Schools were evacuated after the blast and residents ordered to stay indoors. Stocks of iodine, which can stop radioactive substances building up in the thyroid gland, were snapped up by worried locals.
However, it's believed the radiation spike was local and within safe limits, with no radiation increases detected in Europe.