Editing Nuclear meltdown (section)

=== RBMKs ===
Soviet-designed [[RBMK|RBMK reactors]] (''Reaktor Bolshoy Moshchnosti Kanalnyy)'', found only in Russia and other post-Soviet states and now shut down everywhere except Russia, do not have containment buildings, are naturally unstable (tending to dangerous power fluctuations), and have emergency cooling systems (ECCS) considered grossly inadequate by Western safety standards.

RBMK [[emergency core cooling system]]s only have one division and little redundancy within that division. Though the large core of the RBMK is less energy-dense than the smaller Western LWR core, it is harder to cool. The RBMK is moderated by [[graphite]]. In the presence of both steam and oxygen at high temperatures, graphite forms [[Syngas|synthesis gas]] and with the [[Water gas shift|water gas shift reaction]], the resultant hydrogen burns explosively. If oxygen contacts hot graphite, it will burn. Control rods used to be tipped with graphite, a material that slows neutrons and thus speeds up the chain reaction. Water is used as a coolant, but not a moderator. If the water boils away, cooling is lost, but moderation continues. This is termed a positive void coefficient of reactivity.

The RBMK tends towards dangerous power fluctuations. Control rods can become stuck if the reactor suddenly heats up and they are moving. Xenon-135, a neutron absorbent fission product, has a tendency to build up in the core and burn off unpredictably in the event of low power operation. This can lead to inaccurate neutronic and thermal power ratings.

The RBMK does not have any containment above the core. The only substantial solid barrier above the fuel is the upper part of the core, called the upper biological shield, which is a piece of concrete interpenetrated with control rods and with access holes for refueling while online. Other parts of the RBMK were shielded better than the core itself. Rapid shutdown ([[SCRAM]]) takes 10 to 15 seconds. Western reactors take 1 - 2.5 seconds.

Western aid has been given to provide certain real-time safety monitoring capacities to the operating staff. Whether this extends to automatic initiation of emergency cooling is not known. Training has been provided in safety assessment from Western sources, and Russian reactors have evolved in response to the weaknesses that were in the RBMK. Nonetheless, numerous RBMKs still operate.

Though it might be possible to stop a loss-of-coolant event prior to core damage occurring, any core damage incidents will probably allow massive release of radioactive materials.

Upon entering the EU in 2004, Lithuania was required to phase out its two RBMKs at [[Ignalina Nuclear Power Plant|Ignalina NPP]], deemed incompatible with European nuclear safety standards. The country planned to replace them with safer reactors at [[Visaginas Nuclear Power Plant]].