Spin,
turbine, spin
Like
fossil-fuel powered generators, nuclear power plants use thermal energy
to turn turbines to generate electricity. The energy comes from a fission
reaction: A neutron emitted by a uranium nucleus strikes another uranium
nucleus, which emits more neutrons and heat as it fissions (breaks apart).
If the new neutrons strike other nuclei, a chain reaction is taking place.
The atoms get split in the reactor vessel. The primary loop (red) moves heat from the core to the steam generator. The secondary loop (blue) drives the turbine to make electricity. Courtesy Nuclear Regulatory Commission
Chain reactions are the source of nuclear energy, whether in hydrogen bombs or reactors that power toaster ovens or electric Barcaloungers.
In addition to making scads of heat, nuclear fission reactors make radioactive byproducts, including plutonium, a precious element if you're in the nuclear-weapons biz. (If spent fuel were reprocessed -- a controversial suggestion we don't have room to explore -- plutonium could also be used to power reactors.)
Nuclear reactors convert lightly-radioactive uranium fuel into isotopes that decay rapidly and release lots of radiation. You could almost hold reactor fuel in your hand, but you'd be nuts to do that after a fuel rod has been through a nuclear plant.
Fission reactions make a prodigious amount of heat from tiny amounts of fuel. Normally, this heat makes steam to drive the generating turbine. If the heat is not removed fast enough, the reactor fuel can get hot and melt down, destroying the vessel around it and possibly releasing radiation to the environment.
A
chain reaction occurs when neutrons from a fission strike another uranium
nucleus and create another fission. Courtesy University of Missouri-Rolla Student Chapter of the American Nuclear Society
A meltdown leaves a pile of highly radioactive sludge that's tougher to tidy than a teenager's room.
Coulda
been worse
The
meltdown at Three Mile Island (TMI) in Pennsylvania in 1979 released relatively
little radiation. According to the U.S. Nuclear
Regulatory Commission, only about 15 curies of radioactive iodine
131 -- a primary culprit at the later disaster at Chernobyl -- was released
-- although millions of curies were in the plant beforehand.
The accident was caused by design errors that were compounded "drastically" -- as the NRC put it -- by human error. After a valve released cooling water from the core, pumps began automatically replacing it -- until the operators shut the pumps down. Eventually, although about half the fuel melted, it did not destroy the containment, averting a "China syndrome" accident.
Far worse was the fire and explosion that scourged Chernobyl and its surroundings, killing 31 immediately and an unknown number over the long term.
Western reactor designs, we are told, make a Chernobyl rerun almost impossible. And nuclear plants have redundant safety measures, including emergency core cooling plumbing that pumps water into the core in an emergency.
And while multiple safety measures seem reassuring, the plants' very complexity opens the door to accidents like the one at TMI, which follow unforeseeable paths.
Leaving
meltdown town
Why
not design a reactor that could keep its cool without emergency core-cooling
equipment, thus sidestepping many key objections to nuclear power? Such
"passively safe" designs, which rely on reliable forces like gravity,
are a focus as the nuclear industry struggles back to its feet.
The three passive-safe reactor designs that have won NRC approval all heed the "simple is smart" credo. For example, the 600 million-watt AP 600, built by Westinghouse, omits lots of plumbing. James Winters, the project's engineering manager, wrote that the design incorporates "60 percent fewer valves, 75 percent less piping, 80 percent less control cable, 35 percent fewer pumps..." (see "The AP 600... " in the bibliography).
The plant is designed so gravity and convection, rather than electric pumps, will supply cooling water during an accident.
Most valves in the safety system, he wrote, "fail safe," requiring power to stay in their normal, closed position. When the light go out, they "open into their safety alignment."
Indeed, Winters writes, the reactor could cool itself, using on-site water, without any electricity and with the operator playing blackjack in Vegas.
Make
it simple!
Nuclear
engineers are angling for even greater simplicity. One example is the
proposed multi-application light water reactor now being developed at
the federal Idaho National Engineering and Environmental Laboratory (INEEL).
The idea is to eliminate unnecessary junk, tuck as many necessary parts
as possible into the containment vessel, and never add fuel.
Call it the disposable reactor.
Simplicity is bliss, says Jose Reyes, a professor of nuclear engineering at Oregon State University, who has worked on the reactor. "We got rid of quite a few components that might be susceptible to failure."
Instead of a complicated emergency core cooling system, the reactor would sit under a pond of water that would absorb excess heat and prevent a meltdown in an emergency, Reyes says.
Another troublesome component that's absent is a steam generator, which normally uses heat from the core to make steam for the turbine. Brittle steam generators have caused hyper-expensive repair programs in existing reactors.
Each reactor is a module that fits on a railcar, allowing factory construction rather than site-building. Two modules would be ganged to generate 100 million watts of electricity. That's minuscule compared to the large reactors built 30 years ago, which produce about 1,000 megawatts. But smaller units can go on line much faster.
A test model, scheduled to be built this summer, will be heated by electricity rather than nuclear fuel. If it works, the plan is to build a real reactor -- assuming that anybody wants to buy one, that is.
Pebbles! Some big money says you need a PBMR. A huh?
Bibliography | Credits | Feedback | Search