The notorious RBMK graphite moderated , water-cooled reactors of Chernobyl Power Plant disaster were designed with a positive void coefficient with boron control rods on electromagnetic grapples for reaction speed control . To the degree that the control systems were reliable , this '' design '' did have a corresponding degree of '' active '' inherent safety . The reactor was unsafe at low power levels because erroneous control rod movement would have a counter-intuitively magnified effect . Chernobyl Reactor 4 was built instead with manual crane driven boron control rods that were tipped with the moderator substance , graphite , a neutron reflector . It was designed with an Emergency Core Cooling System ( ECCS ) that depended on either grid power or the backup Diesel generator to be operating . The ECCS safety component was decidedly not passive . The design featured a partial containment consisting of a concrete slab above and below the reactor – with pipes and rods penetrating , an inert gas filled metal vessel to keep oxygen away from the water-cooled hot graphite , a fire-proof roof , and the pipes below the vessel sealed in secondary water filled boxes . The roof , metal vessel , concrete slabs and water boxes are examples of passive safety components . The roof in the Chernobyl Power Plant complex was made of bitumen – against design – rendering it ignitable . Unlike the Three Mile Island accident , neither the concrete slabs nor the metal vessel could contain a steam , graphite and oxygen driven hydrogen explosion . The water boxes could not sustain high pressure failure of the pipes . The passive safety components as designed were inadequate to fulfill the safety requirements of the system
Three Mile Island accident
Three Mile Island accident
Passive nuclear safety