Editing Nuclear meltdown (section)

==Soviet Union–designed reactors==

=== 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]].

===MKER===
The [[MKER]] is a modern Russian-engineered channel type reactor that is a distant descendant of the RBMK, designed to optimize the benefits and fix the serious flaws of the original.

Several unique features of the MKER's design make it a credible and interesting option. The reactor remains online during refueling, ensuring outages only occasionally for maintenance, with uptime up to 97-99%. The moderator design allows the use of less-enriched fuels, with a high burnup rate. Neutronics characteristics have been optimized for civilian use, for superior fuel fertilization and recycling; and graphite moderation achieves better neutronics than is possible with light water moderation. The lower power density of the core greatly enhances thermal regulation.

An array of improvements make the MKER's safety comparable to Western Generation III reactors: improved quality of parts, advanced computer controls, comprehensive passive emergency core cooling system, and very strong containment structure, along with a negative void coefficient and a fast-acting rapid shutdown system. The passive emergency cooling system uses reliable natural phenomena to cool the core, rather than depending on motor-driven pumps. The containment structure is designed to withstand severe stress and pressure. In the event of a pipe break of a cooling-water channel, the channel can be isolated from the water supply, preventing a general failure.

The greatly enhanced safety and unique benefits of the MKER design enhance its competitiveness in countries considering full fuel-cycle options for nuclear development.

===VVER===
The [[VVER]] is a pressurized light-water reactor that is far more stable and safe than the RBMK. This is because it uses light water as a moderator (rather than graphite), has well-understood operating characteristics, and has a negative void coefficient of reactivity. In addition, some have been built with more than marginal containments, some have quality ECCS systems, and some have been upgraded to international standards of control and instrumentation. Present generations of VVERs (starting from the VVER-1000) are built to Western-equivalent levels of instrumentation, control, and containment systems.

Even with these positive developments, however, certain older VVER models raise a high level of concern, especially the VVER-440 V230.<ref>{{cite web |url=http://www.insc.anl.gov/neisb/neisb4/NEISB_1.1.html |title=INL VVER Sourcebook |publisher=Timetravel.mementoweb.org |date=31 August 2010 |access-date=2019-09-09 |archive-date=22 December 2008 |archive-url=https://web.archive.org/web/20081222005537/http://www.insc.anl.gov/neisb/neisb4/NEISB_1.1.html |url-status=dead }}</ref>

The VVER-440 V230 has no containment building, but only has a structure capable of confining steam surrounding the RPV. This is a volume of thin steel, perhaps {{convert|1|-|2|inch|cm}} in thickness, grossly insufficient by Western standards.
* Has no ECCS. Can survive at most one {{convert|4|in|cm|abbr=on}} pipe break (there are many pipes greater than that size within the design).
* Has six steam generator loops, adding unnecessary complexity.
** Apparently steam generator loops can be isolated, however, in the event that a break occurs in one of these loops. The plant can remain operating with one isolated loop—a feature found in few Western reactors.
The interior of the pressure vessel is plain alloy steel, exposed to water. This can lead to rust, if the reactor is exposed to water. One point of distinction in which the VVER surpasses the West is the reactor water cleanup facility—built, no doubt, to deal with the enormous volume of rust within the primary coolant loop—the product of the slow corrosion of the RPV.
This model is viewed as having inadequate process control systems.

Bulgaria had a number of VVER-440 V230 models, but they opted to shut them down upon joining the EU rather than backfit them, and are instead building new VVER-1000 models. Many non-EU states maintain V230 models, including Russia and the CIS. Many of these states, rather than abandon the reactors entirely, have opted to install an ECCS, develop standard procedures, and install proper instrumentation and control systems. Though confinements cannot be transformed into containments, the risk of a limiting fault resulting in core damage can be greatly reduced.

The VVER-440 V213 model was built to the first set of Soviet nuclear safety standards. It possesses a modest containment building, and the ECCS systems, though not completely to Western standards, are reasonably comprehensive. Many VVER-440 V213 models operated by former Soviet bloc countries have been upgraded to fully automated Western-style instrumentation and control systems, improving safety to Western levels for accident prevention—but not for accident containment, which is of a modest level compared to Western plants. These reactors are regarded as "safe enough" by Western standards to continue operation without major modifications, though most owners have performed major modifications to bring them up to generally equivalent levels of nuclear safety.

During the 1970s, Finland built two VVER-440 V213 models to Western standards with a large-volume full containment and world-class instrumentation, control standards and an ECCS with multiple redundant and diversified components. In addition, passive safety features such as 900-tonne ice condensers have been installed, making these two units safety-wise the most advanced VVER-440s in the world.

The VVER-1000 type has a definitely adequate Western-style containment, the ECCS is sufficient by Western standards, and instrumentation and control has been markedly improved to Western 1970s-era levels.