Part 5

The men who had gathered in the secret room were some of the finest scientists in the world. The leader of the group was Enrico Fermi (En-REE-ko FER-mee), an Italian scientist who had come to the United States.

For some weeks the men had been quietly at work, carefully stacking a huge pile of pure graphite bricks. Here and there among the bricks they placed pieces of uranium. Fermi believed that when the pile reached a certain size, a chain reaction would start. By December 2, the size seemed to be right.

Inside the pile were three control rods. They were made of cadmium, an element which soaks up flying neutrons like a sponge. With the rods in place, no reaction could take place. When the rods were withdrawn, the reaction would begin.

To make sure that the pile would not get out of hand, the three control rods were operated in three different ways. The first one was controlled by an electrical switch and was completely automatic. The second, called ZIP, was tied to a rope in the balcony. In case of emergency, there was a man ready with an axe. He had only to chop the rope and ZIP would go crashing back into the pile. The third rod was moved by hand.

It was time to begin. Fermi gave the signal for the automatic rod to be withdrawn. Immediately, the counters which measured radioactivity began to tick.

Then Fermi gave the command, "ZIP out!" ZIP was drawn up on its balcony rope and the ticking of the counters at once became faster.

Then Fermi turned to the man who controlled the last rod. This rod was marked in feet and inches, and Fermi said: "Pull it out to thirteen feet."

All eyes were on the instruments. Not yet. A little more. Pull it out another foot. Not yet. The men grew more and more tense as the careful work went on.

Finally, at about 3:25 in the afternoon, Fermi made a last check of his instruments and his calculations. Then he said: "Pull it out another foot. This is going to do it!"

No one dared breathe. The ticks of the counters became so rapid they sounded like a steady hum. The pointers on the instruments swung all the way over--and stayed there. The first atomic chain reaction had been achieved!

The pile was allowed to run for 28 minutes. Then the control rods were put back. Suddenly, all was quiet. There were no ticks from the counters.

Not only had these men started a chain reaction, they had also been able to stop it. At last man could control the energy of the atom.

One of the men present, Arthur H. Compton, ran to the phone to call James B. Conant, chairman of the U. S. National Defense Research Committee. But since our country was at war in 1942, it wasn't safe to talk about this important secret over the telephone. And so, on the spur of the moment, a quick-witted and historic conversation took place.

Compton said: "Jim, you'll be interested to know that the Italian navigator has just landed in the new world."

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!

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.

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.