Nuclear Power Without the Meltdowns?
As the world increasingly tries to go beyond carbon fuel, some environmentalists view nuclear energy as a key component to a cleaner fuel future. Others (and not just those who live near nuclear power plants) are nervous about the prospect of increased use of nuclear energy. One word in particular is scary: meltdown. But what is a meltdown? Can one be avoided?
The reaction in a power plant begins when many different sources of neutrons – fuel pellets – are combined. The pellets bombard each other with neutrons and cause fission. Fission occurs when a heavy element, most often some form of uranium, is broken into smaller pieces, releasing energy. As each fuel atom splits, it releases more neutrons, which split even more atoms. This is a nuclear chain reaction. Water keeps the reactor cool and helps slow neutrons down to a rate at which they can actually cause fission (with conventional reactor fuel, fission does not occur on its own). To prevent the chain reaction from moving too fast, control mechanisms between the fuel tanks absorb additional neutrons. As an additional precautionary measure, nuclear power plants are shielded by containment: layers of steel and concrete that have been damaged by accidents.
Although not apocalyptic, meltdowns are still very bad.
A by-product of the fission is heat. (Another by-product can be gamma rays.) The heat boils (clean) water and creates steam that turns a turbine and generates electricity. If the cooling process stops for any reason or the chain reaction goes too fast, the fuel will overheat. The fuel pellets are typically sealed in zirconium jacketed tubes. With enough heat, zirconium and water react to generate additional heat and large amounts of highly flammable hydrogen. When this hydrogen is exposed to oxygen, it can cause a massive explosion (think Hindenburg). This is the main misconception about nuclear accidents: there can be massive, fatal explosions, but they are conventional, non-nuclear explosions.
If the solid fuel heats up even further, it literally melts: a core meltdown. In a core meltdown, a highly radioactive lump is loose in the core. The molten, toxic lump can melt through the core and into the soil, especially if containment is compromised by hydrogen explosions that contaminate groundwater and land. Explosions can blow up radioactive vapor and particles through eventual containment breaches with long-term health and environmental consequences.
While not apocalyptic, meltdowns are very bad. Another type of reactor, first introduced in the 1960s and now attracting renewed interest, uses liquid fuel fluoride Thorium and a salt. There are security benefits. On the one hand, the fuel is already liquid and therefore cannot melt. In a solid fuel reactor, the cooling water must be pressurized to prevent boiling (see the ideal gas law), to increase the pressure on the containment, and to make violations more likely. When liquid fuel overheats, it expands, reducing the density of neutrons and slowing down a reaction that has gotten out of hand (but can still leak). Proponents also insist that liquid fuel reactors make waste disposal easier. There is reason to be skeptical about any technology that claims to solve any problem, but it is well worth a look.
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From: Lars Högberg
Ambio, Vol. 42, No. 3 (APRIL 2013), pp. 267-284
Springer on behalf of the Royal Swedish Academy of Sciences
Creators: Robert Hargraves and Ralph Moir
American Scientist, Vol. 98, No. 4 (July-August 2010), pp. 304-313
Sigma Xi, Scientific Research Society