How does a nuclear bomb work?

When you mine uranium out of the ground, you get two kinds of uranium atoms. One is called U-235 and the other is called U-238. U-235 is the kind of uranium that you need for a nuclear power plant, and it is also possible to create a nuclear bomb with it.

Image courtesy USAF/Dept of Defense MGM-118A Peacekeeper nuclear missile

If you refine the uranium so that it is almost pure U-235, we call it highly enriched uranium. If you take about 150 pounds of highly enriched uranium and form it into a ball, it will explode with unbelievable power. Just a few pounds of enriched uranium contains the same amount of energy as a million gallons of gasoline. All of that energy gets released in an instant. Even the smallest nuclear bomb explodes with the force of several million pounds of TNT.

Here is what happens. When a U-235 atom gets hit by a neutron, it splits into two smaller atoms. It also shoots out three or four new neutrons of its own, and it creates a lot of heat. If the neutrons that it shoots out hit other U-235 atoms, they will split too, and they throw out new neutrons. And so on.

You can imagine that if you rolled the uranium out like a sheet of paper, a lot of neutrons would end up flying off into the air. In that case, nothing really happens. If you only have a little bit of U-235, the same thing happens. No matter how you shape it, most of the neutrons fly out into the air.

Radiation All Around You We have already talked about smoke detectors. The question is, are there other ways for you to be exposed to radiation in your house? The answer is yes. If you have a watch that has glow-in-the-dark hands, it might be radioactive. Some watches use normal phosphorous. For these watches to glow in the dark, you have to expose them to light first. Other watches use tritium to energize a phosphor. In these watches, the hands glow all night long. As we mentioned earlier, many homes contain a radioactive gas call radon. When uranium in the ground decays, it creates radon. This radon then leaks into the house. It is thought that one out of 15 houses in the U.S. have a radon problem. Radon is bad because it causes lung cancer.

But if you have about 150 pounds (or more) of U-235, and if you shape it into a ball, you have what is called a critical mass. Many of the neutrons in the ball will hit other U-235 atoms rather than flying into the air. In just a few nanoseconds, a gigantic number of U-235 atoms split. And you get a big explosion.

So, to create the simplest nuclear bomb, you take 150 pounds or so of U-235. You split it into two parts, neither of which has a critical mass. Then, to make the bomb explode, you bring the two masses together.

That is how the bomb that exploded over Hiroshima, Japan worked in WWII. When it exploded, it had the energy of about 30 million pounds of TNT. It was enough energy to level much of the city. You can do a lot of evil if you have a bomb that can level a city.

Nuclear Non-proliferation Nuclear bombs pose a big threat to humanity. There are enough nuclear bombs on the planet to kill humanity many times over. To try to stop the spread of nuclear weapons to more countries, a treaty called the Nuclear Non-Proliferation Treaty was created in 1968. Most of the countries in the world (188) have signed the treaty. The treaty has thee parts. The first part tries to discourage proliferation. Only a few countries are allowed to have nuclear bombs according to the treaty. The second part of the treaty tries to reduce the number of nuclear bombs. This part has worked to some degree - the United States used to have more than 30,000 nuclear bombs, but now has about 10,000. The third part of the treaty allows countries to build nuclear power plants if they want to. According to the treaty, countries agree to allow their power plants to be inspected, and this prevents them from building bombs.