Light Water Reactors: Chernobyl, Three Mile Island, Fukushima

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Researched by Industrial Info Resources East Asia (Kofu-shi, Japan)--A light water reactor (LWR) is a type of thermal reactor that uses seawater or fresh water as a coolant. LWRs are the most common type of reactor and have three subclasses: pressurized water reactors (PWRs), boiling water reactors (BWRs) and supercritical water reactors (SWRs). All LWRs produce heat via nuclear fission controlled by rods filled with neutron-capturing material. When control rods are inserted into the reactor, neutrons emitted by the core material, usually uranium, are blocked from the chain reaction of fission. Once the control rods are removed, the neutrons are free to participate in fission and produce heat.

BWRs are encased in steel, water-filled pressure systems called reactor vessels. The water used to create steam is also used to maintain safe temperatures within the reactor. PWR reactor vessels are separate from their secondary loops, which also serve as coolant systems. If pressure within the reactor vessel becomes too great in cases of control rod malfunction or other types of disruption, it becomes difficult for water to enter the coolant system, thus preventing temperatures from stabilizing.

In BWRS, heat generated by fission in the reactor turns the water within the reactor vessel into steam, which directly powers the turbine to generate electricity. Heat generated by PWRs is transferred to a secondary loop via heat exchangers, and steam is generated there. Steam produced in the secondary loop drives the turbine to generate electricity.

Chernobyl nuclear power station, which was located in Ukraine, was comprised of four reaktor bolshoy moshchnosti kanalniy (RMBK) reactors. Each reactor had a generation capacity of 1,000 megawatts (MW), accounting for about 10% of Ukraine's electricity at that time. RMBK reactors are a type of BWR that use graphite moderators within the control rods. Graphite makes reactor control rods comparatively more effective, however it is highly flammable in the presence of oxygen. In the Chernobyl disaster of 1986, graphite in the control rods of Unit 4 were exposed to air and ignited, causing the subsequent explosion.

The Three Mile Island accident of 1979 in Pennsylvania involved the partial meltdown of Unit 2, which stemmed from failures in the non-nuclear secondary loop. Electrical or mechanical failure caused certain pumps to shut down, resulting in a chain reaction of system failures that caused Unit 2's turbine to shut down. Despite the insertion of control rods, the reactor began generating excess heat, as steam was no longer being removed from the reactor vessel to the turbine. All three of Three Mile Island's reactors were PWRs.

Fukushima Daiichi and Daini nuclear power stations, both owned and operated by Tokyo Electric Power Company Incorporated (TYO:9501) (TEPCO) (Tokyo, Japan), generate electricity through the use of BWRs. Both power stations experienced a magnitude 6 shock wave originating from the magnitude 9.0 earthquake that occurred off the eastern coast of Miyagi, Japan, on Friday, March 11. The shockwave itself initiated automatic shutdown sequences, but the subsequent tsunami knocked out the coolant systems at both power stations.

The International Nuclear Events Scale, created by the International Atomic Energy Agency (IAEA), placed the Chernobyl accident at a Level 7, its highest level. Three Mile Island was considered to be a Level 5 incident, while events in Fukushima, Japan, are currently ranked as Level 4, the highest level to ever be experienced in the country.

For recent information on Fukushima Daiichi and Daini, see today's article - Updates for Fukushima Daiichi, Daini Nuclear Power Stations and continue checking the IIR website for more information.

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