[dennisw]

The Japanese were told years ago that they could have a meltdown if they lost power to their reactor and the auxiliary generators were not available. The tsunami took out the power to the plant AND flooded the auxiliary generators.

There are designs today that can stop the reactor and the nuclear fission in a passive way where zero power or very little is required to shut it all down. Can someone give a better description of this?

[fred2008]

Sure:

http://www.technewsdaily.com/nuclear-power-thorium-future-2400/

[fred2008]

A video presentation:

http://nucleargreen.blogspot.com/2011/04/kirk-sorensen-explains-lftr_02.html

[meatloaf]

Google Westinghouse’s AP1000.

[Spirochete]

There are designs today that can stop the reactor and the nuclear fission in a passive way where zero power or very little is required to shut it all down. Can someone give a better description of this?

Failsafe designs, e.g. they fail into a safe mode that causes no harm, quite the opposite of current designs.

For instance, the heat output of a pebble bed reactor falls to a safe idle level when all power and coolant is shut off. In this idle state, it loses as much heat through the vessel walls as the fuel generates.

[donmeaker]

The reactors were shut down. they normally work at 1.4 gigawatts (1,400 Mega watts) and when shut down, were still giving off 1.4 megawatts, because of the residual radioactivity of reaction byproducts in the fuel rods. That is what has been boiling the water. The radioactive Uranium reaction products themselves react and are consumed, with the power level dropping by about a factor of 2 every 7 days. The reaction workers are doing a good job of keeping the reactors under control as they wait for the reaction products to themselves react.