por admin » Mar Mar 15, 2011 9:53 am
BUSINESS TECHNOLOGYMARCH 15, 2011
How Nuclear Reactors Work ... And the Dangers When They Don't
By BEN CASSELMAN And REBECCA SMITH
How does a nuclear reactor work?
The core of a nuclear reactor contains both water and fuel rods made of zirconium and pellets of nuclear fuel, such as uranium, that set off a controlled nuclear reaction. The reaction, heats the water, creating 550-degree Fahrenheit steam, which powers a turbine, generating electricity.
What is a meltdown?
If the core gets too hot, the fuel rods can crack and release radioactive gases. In the worst case, the fuel pellets themselves can melt and fall to the reactor floor, where the hot, radioactive material may be able to eat through protective barriers and ultimately reach the surrounding environment.
In a partial meltdown, only some of the fuel or the reactor core melts, reducing the likelihood of breaching the containment structure.
What went wrong in Japan?
Nuclear reactors in Japan are designed to turn off automatically anytime a disaster knocks out the electric grid. That system worked properly in this case, shutting down the nuclear reaction.
Even with the plant shutdown, though, the nuclear fuel still held tremendous heat. Diesel-powered backup generators are meant to pump water into the plant to cool the fuel, but those systems failed in the tsunami that followed the quake. Emergency batteries provided some power, but not enough to run the water pumps.
What happens when the water pumps fail?
Without power to fuel the pumps, plant operators couldn't circulate water through the reactors to cool them down. The fuel rods began to boil off the remaining water, allowing water levels to drop and leaving the fuel at least partially exposed. That allowed temperatures to rise dangerously.
What caused the explosions at Reactors 1 and 3?
Details are unclear, but as the fuel rods begin to break down they can release gases that react with surrounding steam, generating hydrogen and allowing pressure inside the core to rise dangerously.
To prevent more serious damage, the plant's operators decided to release some of the pressure from the core by venting the built-up gas and steam. The escaping hydrogen reacted with oxygen in the atmosphere, causing an explosion that damaged nearby structures at Reactors 1 and 3. It is unclear what other systems were damaged, but officials say the main containment structures around the reactor cores—a key safety barrier—remained intact.
What is the situation at the different reactors now?
Plant operators are trying to pump seawater into all three reactors to cool the fuel. That will cause irreversible damage, but, it's hoped, will stabilize the plants. Workers are struggling to pump in water, however, for reasons that remain unclear.
At units 1 and 3, fuel rods were left exposed for long enough to allow at least some melting, experts believe. But workers have managed to restore water levels for the time being, and temperatures appear to be falling.
Unit 2 initially appeared to be in better shape than the other two reactors. But on Monday, water levels dropped, leaving fuel rods almost entirely exposed. Officials now consider Reactor 2 to be the most vulnerable of the three reactors to further damage, but no explosion had taken place as of early Tuesday morning.
What is the risk to the surrounding area?
Workers have released built-up gases to ease pressure inside the plant. That has released at least some radioactive material into the atmosphere, but officials say it isn't enough to be dangerous to anyone outside the immediate vicinity of the plant.
The worst-case scenario is a full meltdown, in which radioactive material eats through the various protective barriers and reaches the outside. Many experts consider that unlikely as long as workers can pump seawater into the plant and so long as containment structures remain intact.