It was the fire protection system which shut the reactor down automatically after the fire was discovered in an electrical cable outside the reactor block. Both the German company Siemens and the French company Framatom have delivered new fire protection systems to Kalinin Nuclear Power Plant, which operates two VVER-1000 reactors. The reactor that was shutdown on Monday had been operating since December 1986.
More than 120 incidents at Kalinin Nuclear Power Plant were reported in the period from 1990 to 1994. The Russian civilian nuclear inspection agency Gosatomnadzor stated in 1994 that the plant is only allowed to operate its two reactors at 90 percent capacity, due to safety concerns.