Graphite moderator, RBMK. reactors

FIG. 20.2. Russian graphite-moderated boiling light water cooled power reactor, RBMK. 

An RBMK is a graphite-moderated, water-cooled system. The Chernobyl reactor that exploded in 1986 was an RBMK. Fourteen RBMKs are in operation. Several problems are inherent in RBMKs. If several pressure tubes rupture simultaneously, the force will raise the reactor lid, releasing radioactive fission products. An RBMK is susceptible to power instabilities. Increased boiling can boost power levels, creating an uncontrolled reaction. The emergency core-cooling, fire protection, and electronic control-and-protection systems in the older RBMKs do not meet international standards. 

RBMK. A Russian reactor design which uses a graphite moderator and natural water coolant. 

RBMK light water cooled, graphite moderated, channel type reactor (Soviet design)... 

Accident Analysis for Nuclear Power Plants with Graphite Moderated Boiiing Water RBMK Reactors... 

ACCIDENT ANALYSIS FOR NUCLEAR POWER PLANTS WITH GRAPHITE MODERATED BOILING WATER RBMK REACTORS... 

Accident analysis for nuclear power plants with graphite moderated boiling water RBMK reactors. — Vienna International Atomic Energy Agency, 2005. 

Unlike the BWRs of Western design, in which boiling water is both moderator and coolant, the RBMK reactors use graphite as a moderator while the heat is removed from the reactor core by boiling water in separate cooling channels. Be-... 

However, steam voidage can also have another effect because, as well as slowing down neutrons, water also absorbs some neutrons. Hence, if the neutrons are already fuUy moderated (e.g., by the graphite in the RBMK reactor core) then increased steam voidage will just mean that fewer neutrons are absorbed in the water this means that increased steam voidage can lead to an increase in reactivity - exactly the effect that we do not want This is called a positive steam voidage coefficient. 

The Russian Graphite Moderated Channel Tube Reactor, NPC(R) 1275, Nuclear Power Company Ltd, March 1976.(This report does not appear to be available on-line. It gave an early Western view of the RBMK design and noted some of its major weaknesses.). 

Figure A1.36 Simplified schematic of 11-MWei EGP-6 (power heterogeneous loop reactor) NPP graphite-moderated, boihng hght water-cooled with natural circulation (also, LGR), pressure channel power reactor for production of electricity and heat, air-cooled condenser, prototype of RBMK and smallest operating reactor by power.

All four Chernobyl reactors shared the same Soviet design, the Reaktor Bolshoy Moshchnosty Kanalny (RBMK), high-power channel reactor (Anon., 2010). The RBMK was a uniquely soviet design that evolved from the early Russian graphite moderated reactors used for the Soviet Union s production of plutonium for then-weapons program. As a result, the RBMK design included several features that made it distinctly different from the commercial reactors developed in the West. One major... 

Unlike Western reactors that have negative feedback coefficients in all phases of operation, the RBMK reactors had a positive feedback coefficient under some circumstances. While the graphite provided the vast majority of neutron moderation, the cooling water also absorbed some neutrons. Under certain operating conditions, especially at lower power levels, an increase in power could turn some of the water in the core into steam. This increase in steam would then cause a reduction in neutron absorption because of the reduction in liquid water density. Thus, if power increased while at a low power state, the positive void feedback coefficient would drive the power higher, and potentially exacerbate an accident situation. 

The Soviet RBMK graphite moderated (Ghemobyl-style) reactor was a direct descendant of their first graphite core, a unit that reached criticality in December of 1946 and was very similar to Eermi s Ghicago Pile 1 (Josephson, 2000). Unlike western countries, the communist Soviet Union did not rely on economic forces to guide the development of new technologies. Instead, the Soviets counted on their scientists to advocate for the best available technologies. 

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