Conant, who knew of the experiments that had been going on, understood at once. He said: "Is that so? Were the natives friendly?" And Compton replied: "Everyone landed safe and happy." This was the first day of the atomic age. The reactor had been started, had been stopped--and had produced enough power to light one small flashlight bulb! 7 TINY ATOMS, BIG POWER Atomic power has grown quickly since that day in 1942. Atomic power plants now make electricity to light large cities in many parts of the world. Atomic power doesn't make electricity directly. It makes heat. The heat turns into steam. Then the steam turns turbines and the spinning turbines drive the generators which make electric current. Ordinary steam power plants depend on fossil fuels--coal, oil, or gas--to make heat. It has been figured out that if only coal were used for fuel, the world's supply would be used up in 350 years. Oil and gas would last for 40 years. But there are enough nuclear fuels to last for at least 8,500 years! There are several kinds of atomic power plants, but the best known is the Pressurized Water Reactor. This long name is usually abbreviated to PWR. The PWR isn't really very different from Fermi's pile in Chicago. There is the same big stack of atomic fuel--usually uranium--with control rods sticking out of holes in the fuel bars. Just like Fermi's pile, when the control rods are pushed in they soak up the flying neutrons and there is no reaction. When the control rods are pulled out, the chain reaction takes place. [Illustration] One of the curious things about a chain reaction is that it won't work if the neutrons are flying too fast. They hit the new atoms at such great speed that they just bounce off and keep going. In order for the neutrons to do their splitting job, they have to be slowed down. Fermi used graphite bricks for this purpose. The PWR uses water, which works very well. And the water also serves another purpose. It absorbs the great heat which is formed in the reactor. Now, everyone knows that when water is heated to a high temperature, it boils. But _this_ water must not boil. To prevent its boiling, the water is kept under very high pressure, and that is how the Pressurized Water Reactor got its name. The water is sealed in special tubes and reaches a temperature of about 600° F. The tubes then heat _other_ water which turns into steam. A simpler kind of atomic power plant is the Boiling Water Reactor, or BWR. The BWR is just a tank which holds a reactor and water. In this case, the water is _not_ under pressure and the heat released by the chain reaction makes it boil. The steam which comes from the boiling water goes directly to the turbine. Whether they use PWRs or BWRs, atomic power plants don't look very much like ordinary plants. There is no smoke, no dirt, and no fire. Everything is controlled by automatic switches and there may be no more than two or three men in sight. [Illustration]