With the explosion of “nuclear devices” (aka “bombs”) in the past couple of weeks, India and Pakistan have joined the “nuclear club,” and also, alas. brought nuclear weapons into the forefront of the news for the first time in years. So perhaps it’s time for a little refresher course on just how a nuclear bomb works–and how (and why) it’s tested.
The original atomic bomb, exploded on July 16, 1945, near Alamagordo, New Mexico, by the United States, was a fission bomb. Nuclear fission was discovered in 1938 by the German physicists Otto Hahn and Fritz Strassmann, who split the nuclei of uranium atoms by bombarding them with neutrons (uncharged particles that make up atomic nuclei along with the positively charged protons).
Fission happens spontaneously in some large atomic nuclei. Uranium-238, for example, the most common form of uranium (mined in northern Saskatchewan), exhibits this spontaneous fission–but so slowly that it would take 8,000 trillion years for even half of it to spontaneously split. (Don’t wait up.)
However, as Hahn and Strassmann discovered, fission can also be induced by bombarding certain nuclei with neutrons. A rarer form of uranium, Uranium-235 (the number is the total number of protons and neutrons in the nucleus) can be split much more easily than Uranium-238. Not only that, but when it is split in two, it produces an average of 2.5 loose neutrons, which are then free to bombard other U-235 nuclei nearby, which will also split and send out neutrons to bombard other nuclei, and so on, and so on, and so on, producing a self-sustaining “chain reaction.” The total mass of the two medium-sized nuclei left after the fission is less than that mass of the large nuclei they sprang from. That missing mass is converted into energy–quite a lot of energy, as Einstein’s E=mc2 points out.
U-235, used in the bomb dropped on Hiroshima, is only one “fissile material.” An even better one is plutonium (used in the Nagasaki bomb and probably in the Indian and Pakistani bombs), which is produced from uranium with the help of a nuclear reactor. Plutonium makes a much more efficient bomb than U-235.
With either U-235 or plutonium the chain reaction fizzles out if the total mass of the fissile material isn’t large enough. That’s because too many neutrons, relative to the total mass, escape from the surface. For every fissile material and every given shape, there is a “critical mass,” however, which is large enough to keep enough neutrons internally to let the chain reaction reach its full potential. For U-235, critical mass is a sphere about the size of a baseball. If it can be held in that shape for just long enough, a large number of the uranium nuclei in the mass will split within about a millionth of a second, releasing enormous amounts of energy that superheat everything in the vicinity to tens of millions of degrees, resulting in an enormous explosion–equivalent to about 20,000 tons of TNT. The larger the mass of U-235 or plutonium you start with, the larger the resulting explosion.
There are two basic ways of achieving critical mass. The Hiroshima bomb used a gun-like design, in which explosives fired a slug of U-235 down a barrel to smash into and fuse with another mass of U-235. The more common approach, and the one probably used in the Indian and Pakistani bombs, uses a sphere of explosives to put the squeeze on a spherical mass of plutonium, increasing its density to the point where a chain reaction begins.
But fission bombs are nothing compared to so-called hydrogen bombs. Hydrogen bombs use fusion, not fission. In fusion, two small atomic nuclei are combined to form one larger nucleus. Oddly enough, when you’re dealing with very light elements, two small nuclei actually have more mass than a single nucleus of twice the size, which means that after fusion there is once again missing mass which has been turned into energy.
Fusion is harder to achieve than fission, because it takes a lot of energy to shove together two small nuclei which, having like charges, tend to repel each other like the matching poles of two magnets. The sun, which gets its energy from fusion, uses its vast size to cram everything together at a density 100 times that of water and heat it up to 15 or 20 million degrees. Here on Earth, there’s only one way to achieve something typical: use a fission bomb. Fission bombs can, with further testing, serve as the triggers for hydrogen bombs, which are hundreds of times more powerful. (The U.S. atomic bomb had an explosive force equal to 20,000 tons of TNT; the U.S.’s most powerful test of a hydrogen bomb, on March 1, 1954, yielded an explosion equivalent to 15 million tons of TNT, creating a fireball more than 4.8 kilometres in diameter. At least one of the devices exploded by India is thought to have been a hydrogen bomb.)
Becoming a nuclear power is not an easy task. It takes the resources of an entire country, many years, and many millions or even billions of dollars. Much of the cost and effort must be expended on creating the fissile material: U-235 is very hard to separate from U-238, and to create plutonium, you must first build or buy a nuclear reactor, then modify it. Creating the weapon itself is easier, but still not the sort of thing (contrary to popular belief) that’s going to be easily carried out in a basement workshop. And once you have a design, there’s only one way to be sure it will work, and then to improve it: through testing.
In the early years of the Atomic Age, tests were carried out above-ground: the U.S. carried out 193 atmospheric tests and the U.S.S.R. 142. Britain, France and China also carried out atmospheric tests. However, eventually, due mainly to increased concern about radioactive fallout, atmospheric tests gave way to underground tests. As of January 30, 1996, there had been 2.044 nuclear tests: 528 above ground, the rest underground. Now you can add 10 more.
Underground tests can take a variety of forms, but the most common involves digging a shaft anywhere from 200 to 700 metres deep and two to four metres wide and lowering the nuclear device into the hole. The hole is then filled with cement, sand or gravel, and the device is detonated. Sensors in the blast chamber and the shaft transmit data to the scientists monitoring the blast.
A less common form of underground test involves drilling a horizontal shaft instead of a vertical one. This can make it easier to install specialized test equipment.
The first true underground testing of a nuclear explosive was Plumbob Rainier, which exploded a 1.7 kiloton device September 19 at the end of a tunnel driven into the side of a mountain. In the years since, there have been thousands of underground tests all over the world, with the U.S. and the Soviet Union leading the way.
Underground tests take place so far beneath the surface that the only immediate apparent effect up above is a cloud of dust raised from the ground by the shock wave. But many minutes later, the ground collapses, forming a crater. That’s because the blast creates a huge, glass-walled cavity in the rock. The roof of the cavity tends to collapse into it, creating a new roof, which also collapses. Layer after layer of rock sheers away, and eventually the surface layer of rock and soil also drops.
There is usually no release of radioactivity from underground tests, but that’s hardly assured: occasionally radioactive dust and gases from the blast cavity find their way to the surface, and underground explosions can also contaminate underground streams which can eventually find their way into surface water.
In 1996, the U.S. China, Russia, France and Britain all signed the Comprehensive Test Ban Treaty. China conducted the last known underground test by any of the five original nuclear powers; at the time, there was hope that it would be the last explosion of a nuclear weapon ever, anywhere.
Alas, two years later, that hope has, literally, exploded.