There are a number of different reactors in use, most commonly water-cooled pressurised or boiling water reactors - though other reactor types are kept cool with the help of liquid metal, gas or molten salt.
Iran, by comparison, is producing near-weapons-grade uranium, but it also has a heavy-water reactor being developed that could produce weapons-usable plutonium.
Iran is also building a heavy-water reactor that could produce plutonium, another bomb ingredient.
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North Korea insists it has built the bomb already, but now wants America to provide it with a modern light-water reactor as the price for disarming.
The light-water reactor, which is not capable of producing weapons-grade plutonium, was promised to North Korea in the early 1990s for the North giving up its nuclear weapons.
The prototype light-water reactor that North Korea is constructing at Yongbyon, and which requires enriched-uranium fuel, could add to the fissile material stockpile when it is completed, possibly by the end of the year.
The enrichment plants at Natanz, Qom and perhaps elsewhere give Iran an early route to HEU. A planned heavy-water reactor at Arak will produce large quantities of plutonium as a by-product, but will not be completed for some years.
But disputes, like the current argument about whether North Korea gets a light-water reactor as part of a deal for dismantling its weapons program, have happened before, and as of now the six-party negotiations scheduled to resume in November appear to be still on.
The 825-megawatt pressurized water reactor at Three Mile Island, about 10 miles outside Harrisburg, Pennsylvania, has been in service since 1974 and can power about 800, 000 homes.
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The operation came a day after workers plugged a highly radioactive water leak from a concrete pit connected to the number two reactor, using sodium silicate - known as ''water glass''.
Moreover, between the Russian-built, soon-to-be-opened nuclear reactor in Bushehr and the illicit heavy water reactor in Arak, Iran will have the capacity to build plutonium-based bombs within two years.
This outfit is part of the scheme hatched by America, South Korea and Japan five years ago to wean North Korea off producing plutonium, with which nuclear weapons can be made, by offering to replace its plutonium-rich nuclear reactor with a pair of less proliferation-prone western-designed light-water reactors.
At Purdue University in Indiana, scientists are working on a reactor-like device that uses water circulated by gravity to keep the core cool.
CNN: Today/Tomorrow - Helping nuclear plants keep their cool
Harrison also said the North demanded the completion of the light-water reactors as compensation for the dismantling of the Yongbyon nuclear reactor.
Finland is using a new breed of pressurized water reactor that is being built by a German-French consortium for around three billion euros.
The gravity-based system responded as it was designed to, circulating water and steam in a continuous loop to keep the scale-model reactor at a safe temperature automatically.
CNN: Today/Tomorrow - Helping nuclear plants keep their cool
The study identified future potential electric generation capacity of 158 gigawatts for advanced coal, 78 gigawatts for water and dry-cooled concentrated solar energy and 515 gigawatts for large nuclear reactor plants.
Meanwhile, engineers are continuing to pump some 11, 500 tonnes of low-level radioactive seawater into the sea so the more highly contaminated water from reactor No 2 can be stored in waste buildings.
Spain's oldest nuclear power station - the Santa Maria de Garona plant - was shut down on Sunday because the Ebro river water became too hot to cool its reactor.
In fact, the military is constantly testing green-power projects, such as a contraption that provides power and purifies water, or even a mobile nuclear reactor.
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