Nuclear Power Technology
The basic principle behind a nuclear reactor is simple: the heat produced by a controlled nuclear fission chain reaction is used to create steam pressure that drives a power-generating turbine.
But the technology required to implement this principle efficiently and safely is enormously complex. The fission reaction must be maintained at the correct rate and quickly adjusted or stopped when necessary. Water temperature and pressure must be carefully controlled. Elaborate, redundant cooling systems are needed to guard against the possibility that the nuclear fuel will overheat and start to melt. Different reactor designs approach these requirements in different ways.
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Current Reactor Types
All of the commercial nuclear reactors currently operating in the United States are either boiling water reactors (BWRs) or pressurized water reactors (PWRs).
Both BWRs and PWRs use ordinary ("light") water to transfer heat from the nuclear fuel to drive the turbines that generate power, so they are sometimes referred to collectively as light water reactors (LWRs). The most important difference between the two concerns the way in which water is used to transfer heat.
Boiling Water Reactors
A BWR uses a single coolant loop: the water that surrounds the fuel inside the reactor pressure vessel (RPV) is heated to boiling, and the resulting steam drives the turbine.
Boiling water reactor (NRC illustration).
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Pressurized Water Reactors
In a PWR, by contrast, there are two separate loops. The water in the primary loop, surrounding the reactor vessel, is heated to high temperatures but is kept under high pressure so that it does not boil; this hot, pressurized water passes through a steam generator—an elaborate heat exchanger—and boils water in the secondary loop, which drives the turbine.
Pressurized water reactor (NRC illustration).
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About two-thirds of operating U.S. reactors are of the PWR type; the rest are BWRs. Each type has advantages and disadvantages. The reactors at Three Mile Island, site of the most significant accident to date at a commercial U.S. nuclear plant, are PWRs, while the reactors at the Fukushima Dai-ichi plant in Japan, site of the March 2011 accident, are BWRs.
Alternative Reactor Designs
Small Modular Reactors
Several alternative reactor designs have been proposed. The designs that have received the most attention in recent years are collectively known as small modular reactors (SMRs). SMRs generate up to about a third as much power as typical current reactors. Because of their smaller size, they can be manufactured offsite and transported to the power plant, rather than being constructed from scratch at the plant site like older designs.
Proponents suggest that SMRs will be both safer and more cost-effective than older designs, allowing them to be distributed more widely and sited in more densely populated areas. UCS analysis, however, questions these claims, and cautions against relaxing safety and security standards for SMRs.
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Thorium Reactors
Liquid fluoride thorium reactors are a type of molten salt reactor (MSR) that was first developed decades ago at Oak Ridge National Laboratory. In recent years some people have advocated thorium reactors as a potentially safer alternative to current designs.
As with SMRs, however, thorium reactors have significant issues, including safety problems, proliferation risks, and waste disposal concerns.
