Editor's Note: This story was originally printed in the December 2005 issue of Scientific American magazine.

Despite long-standing public concern about the safety of nuclear energy, more and more people are realizing that it may be the most environmentally friendly way to generate large amounts of electricity. Several nations, including Brazil, China, Egypt, Finland, India, Japan, Pakistan, Russia, South Korea and Vietnam, are building or planning nuclear plants. But this global trend has not as yet extended to the U.S., where work on the last such facility began some 30 years ago.

If developed sensibly, nuclear power could be truly sustainable and essentially inexhaustible and could operate without contributing to climate change. In particular, a relatively new form of nuclear technology could overcome the principal drawbacks of current methods—namely, worries about reactor accidents, the potential for diversion of nuclear fuel into highly destructive weapons, the management of dangerous, long-lived radioactive waste, and the depletion of global reserves of economically available uranium. This nuclear fuel cycle would combine two innovations: pyrometallurgical processing (a high-temperature method of recycling reactor waste into fuel) and advanced fast-neutron reactors capable of burning that fuel. With this approach, the radioactivity from the generated waste could drop to safe levels in a few hundred years, thereby eliminating the need to segregate waste for tens of thousands of years.

For neutrons to cause nuclear fission efficiently, they must be traveling either slowly or very quickly. Most existing nuclear power plants contain what are called thermal reactors, which are driven by neutrons of relatively low speed (or energy) ricocheting within their cores. Although thermal reactors generate heat and thus electricity quite efficiently, they cannot minimize the output of radioactive waste.

All reactors produce energy by splitting the nuclei of heavymetal (high-atomic-weight) atoms, mainly uranium or elements derived from uranium. In nature, uranium occurs as a mixture of two isotopes, the easily fissionable uranium 235 (which is said to be “fissile”) and the much more stable uranium 238. The uranium fire in an atomic reactor is both ignited and sustained by neutrons. When the nucleus of a fissile atom is hit by a neutron, especially a slow-moving one, it will most likely cleave (fission), releasing substantial amounts of energy and several other neutrons. Some of these emitted neutrons then strike other nearby fissile atoms, causing them to break apart, thus propagating a nuclear chain reaction. The resulting heat is conveyed out of the reactor, where it turns water into steam that is used to run a turbine that drives an electric generator.

Uranium 238 is not fissile; it is called “fissionable” because it sometimes splits when hit by a fast neutron. It is also said to be “fertile,” because when a uranium 238 atom absorbs a neutron without splitting, it transmutes into plutonium 239, which, like uranium 235, is fissile and can sustain a chain reaction. After about three years of service, when technicians typically remove used fuel from one of today’s reactors because of radiation-related degradation and the depletion of the uranium 235, plutonium is contributing more than half the power the plant generates.

In a thermal reactor, the neutrons, which are born fast, are slowed (or moderated) by interactions with nearby low-atomicweight atoms, such as the hydrogen in the water that flows through reactor cores. All but two of the 440 or so commercial nuclear reactors operating are thermal, and most of them—including the 103 U.S. power reactors— employ water both to slow neutrons and to carry fission-created heat to the associated electric generators. Most of these thermal systems are what engineers call light-water reactors.

7 Comments

1. 1. John Reynolds 07:53 AM 1/26/09

   Perhaps the "smartest" thing that we could do is to stop calling it nuclear "waste".
   
   As this article makes clear, the byproducts of nuclear fission are potentially valuable in and of themselves. Instead of elaborate schemes to bury this stuff for thousands of years, we should "mine" its possible uses.
   
   Treat it like garbage and it's a problem - treat it like gold and it won't end up dumped in a stream.

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2. 2. Giant Pistol 09:59 AM 1/26/09

   In retrospect it’s too bad our environmental friends gave the “man made global warming” treatment to nuclear power in the 60’s by using superstition and scare tactics to intimidate people with bad information. I’m sure they thought they we’re justified in their views at the time but now we realize the extreme damage of their ignorance. If we had gone nuclear 40 years ago we could have averted spewing gigatons of tons of carbon into our atmosphere and averted the “tipping” point we find our climate in today. Not to mention we could have spent the last 40 years making nuclear power safer and more efficient and the United States less reliant on fossil fuels. This is just one example of how environmentalist can do incalculable damage to our nation and to our climate when they start screaming before they know what they’re talking about.

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3. 3. Keller 10:21 PM 1/26/09

   Nuclear power's problems were and are primarily financial; the plants are simply not cost effective relative to the alternatives. We can build a 750 mW(e) combined-cycle plant for around $750 million or a 1200 mW(e) nuclear unit for 6 or 7 billion dollars.
   
   The fast reactor concept, while technically intriguing, is likely to be even more of an economic problem.

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4. 4. bhoglund 08:03 PM 1/28/09

   http://en.wikipedia.org/wiki/Generation_IV_reactors
   

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5. 5. bhoglund 08:04 PM 1/28/09

   Liquid Metal Fast Reactors (LMFRs), of which the authors are touting, have a terrible operational record. The USA has built 3, of which 2 have had unintentional core melts. In 1996, the Japanese Monju LMFR leaked 5 tons of its highly radioactive liquid sodium coolant which caught fire, and has been shut down for 10 years. The French Super Phoenix had power oscillations which caused them to shut it down too.
   
   In 1972, President Nixon fired ORNL's Director, Dr. Weinberg for advocating the meltdown proof Molten Salt Reactor (MSR) because the GOP had selected the LMFRs instead. MSRs can and have operated on all 3 fissiles (U235, U233, & Pu239) and can best utilize thorium. Why not restart this Generation IV reactor instead?
   
   REFs: http://en.wikipedia.org/wiki/Alvin_M._Weinberg
   http://en.wikipedia.org/wiki/Molten_salt_reactor
   http://en.wikipedia.org/wiki/Generation_IV_reactors

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6. 6. Chap in reply to bhoglund 05:28 PM 1/30/09

   The leak at Monju was not radioactive.

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7. 7. bhoglund in reply to Chap 12:17 PM 2/4/09

   Chap, you are correct, the sodium leak was not radioactive as it was in Monju's Secondary Coolant circuit. I apologize for the mistake I posted above.
   
   However, had the leak been in the Primary Coolant circuit while the reactor was operating, the leak would have been highly radioactive due to the 15 hr halflife of Na-24, which emits energetic 1.4 MeV & 2.8 MeV gamma rays. Large amounts of Na-24 are created by neutron absorption within the LMFR's core.
   
   Furthermore, sodium fires, even without complicating radioactivity, are difficult to contain because hot sodium reacts with air, water, carbon dioxide (CO2), and even concrete! It's ash is sodium oxide (Na2O), which will combine with any water to make highly caustic sodium hydroxide (NaOH), which is lye, or Draino (drain cleaner)!
   
   Molten Salt Reactors (MSRs), coolant and fuel is melted LiF-BeF2 into which sufficient fissile (U-235, U233, &/or Pu-239) are dissolved. Molten salts do not react with air or water and freeze below 500 C, thereby encapsulating the radioactive materials (e.g., Fission Products). Furthermore, they do not require pressure vessels as they operate at atmospheric pressure. The USA has built and successfully operated 2 MSRs at Oak Ridge National Laboratory: ARE (1954) & the MSRE (1960s).
   
   REFs:
   http://www.gen-4.org/Technology/systems/msr.htm
   http://nuclear.inl.gov/deliverables/docs/msr_deliverable_doe-global_07_paper.pdf
   http://www.ornl.gov/~webworks/cppr/y2001/pres/119930.pdf

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