Thursday, December 01, 2011

Micro Nukes: A new hope for nuclear energy?

Nuclear power fell into a long funk after the partial core meltdown at the Three Mile Island reactor in Pennsylvania in 1979.Many nuclear plant construction came to a halt, and before the industry could recover, the 1986 reactor breach at the Chernobyl nuclear plant in Ukraine seemed to seal the fate of nuclear power.
Now the technology is back again—this time in a good way—because it produces virtually no carbon emissions and it get us away from the turbulent politics and economics of oil.
Now the new idea reviving this energy industry is called micro nukes or micro nuclear plants. Miniaturized nuclear plants are small enough to be mass-produced, driving down costs, and they can be shipped just about anywhere by truck or boat, even to locations that are off the electric net. Also, micro nukes can be designed to run a long time without maintenance or refueling. They could be sealed like a big battery and buried underground for as long as three decades, so terrorists could not get into them and nuclear waste could not get out. A spent micro nuke could simply be plucked out of the ground and shipped whole to a waste-processing or recycling facility anywhere in the world; the old one could be swapped out for a new one, cartridge-style.
Like mainstream reactors, it is a “light water” design: The reactor is pressurized and filled with plain water that flows past the core, where the radioactive decay of uranium-235 generates intense heat. The heat boils a separate tank of water and turns it to steam, which in turn drives turbines that produce electricity. But there are differences. A conventional plant requires a vast, complex array of pumps, pipes, and valves to move enormous quantities of water between the reactor vessel, a separate steam-generating chamber, and a cooling tank. Micronukes keeps things simpler with a tall, thin, single-vessel design. Water heated by the core ascends in a chimneylike metal structure inside the reactor, then spills over the top of the chimney and sinks back down along the inside walls of the reactor to repeat the journey. High pressure inside the reactor prevents the superheated water from boiling. As the water climbs over the top of the chimney in the reactor, it passes over a long coil of pipe, transferring much of its heat to water inside the coil. Lower pressure in the coil allows the water to boil, and the resulting steam travels up the pipe to power a turbine.
However, sticking with proven light-water technology has some downsides. To keep the water from boiling and losing its heat-transferring properties, light-water reactors cannot run at the high temperatures that are most efficient for producing power. And even at lower temperatures, preventing boiling requires high pressure. In the unlikely event that an overheating core causes a reactor breach, the pressure could potentially cause an explosive venting of radioactive gases into the environment.

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