The heavy water reactor will use about .7 percent of the uraniums energy value and the light water ractor will use about .5 percent, they both do terribly. At normal pressures water will boil at 100º Celcius, this isn't nearly hot enough to generate electricity effectively. So water cooled reactors have to run at over 70 atmospheres of pressure. You have to build a water cooled reactor as a pressure vessel.
The No. 1 accident that people worry about is that pressure is lost and water that is being held at 300º Celcius flashes to steam and it's volume increases by a factor of 1000. If you do not get emergency coolant to the fuel in the reactor it can overheat and melt. This is what drives the design of this building.
So if this happens, all of the steam is captured in this building. The reactors we have today use uranium-oxide as a fuel. It's a ceramic material chemically stable, but not very good at transfering heat. If you lose pressure you lose your water and soon your fuel will melt down and release the radioactive fission products within it.
So there are a series of emergency systems designed to always keep the core covered with water; we saw the failure of this at Fukushima Daiichi in Japan.
Now let's talk about another type of Nuclear power,
The Liquid Flouride Thorium Reactor (LFTR). We can fully burn up the thorium in this reactor versus only burning up part of the uranium in a typical light water reactor. It isn't based on water cooling and it doesn't use solid fuel. You have to heat up the fuel to about 400º Celcius to get them to melt but that's actually perfect for trying to generate power in a nuclear reactor. Here's the real magic, they don't have to operate at high pressure, they dont have to use water for coolant, and there's nothing in the ractor that's going to make a big change in density.
Unlike the solid fuels that can meltdown if you stop cooling them, these liquid floruide fuels are already melted. In a LFTR there is something called a freeze plug made of salt, if there is an emergency and all power is lost to the nuclear power plant the blower that keeps the freeze plug frozen, the plug melts, and the liquid flouride fuel drains into a tank called the drain tank. In water cooled reactors you generally have to provide power to the plant to keep the water circulating and to prevent a meltdown. If you lose power to the LFTR it shuts itself down, all by itself, without human intervention; this is a staggeringly impressive level of safety, even if there is physical damage to the reactor.
Thorium is a naturally occuring nuclear fuel that is 4x more common in the earths crust than uranium It's so energy dense that you can hold a lifetime supply of thorium energy in the palm of your hand. We can use thorium about 200x more efficiently than we are using uranium now. This reduces the waste generated by using thorium by factors of hundreds and over fossil fuels by factors of millions.
One mine will pull up about 5000 tonnes of thorium per year - enough to power the entire planet. In 2007 we used 5000000000 tonnes of coal, 31000000000 barrels of oil, 5000000000000 cubic metres of natural gas, and 65 000 tonnes of Uranium.
