"Arktika" and the entire "Arktika"-class icebreaker fleet are outfitted with two OK-900A reactors, which deliver 171 MW each. Each reactor is contained in its own closed compartment and weighs 160 tonnes. They are shielded by water, steel, and high density concrete, and ambient radiation is monitored throughout the ship by 86 sensors. The reactors were originally fueled by a 90% enriched, zirconium-clad, uranium fuel. Those reactors still in operation today now use a 20%-90% enriched with 60% average enrichment uranium dispersed in an aluminum matrix. The chain reaction can be stopped in 0.6 seconds by the full insertion of safety rods.