Nuclear Weapons

Trinitrotoluene, better known as TNT, is a chemical compound whose explosive power is a standard measure used to compare bombs and other explosive devices. When ignited, a kilogram of TNT releases approximately 4 million joules of energy, the equivalent of about 4 sticks of dynamite. Its explosive power comes from chemical potential energy, which—as you may recall from chapter 3—is due to the electromagnetic force. Nuclear chain reactions, on the other hand, are powered by the strong force. For this reason, some of the smallest nuclear weapons are far more powerful than the largest bombs fueled by chemical explosives.

The most powerful conventional (chemically-powered) bomb ever built, nicknamed the “Father of All Bombs,” was created in 2007 by the Russian military. It weighs 7.1 metric tons (7,100 kg) and employs a chemical explosive even stronger than TNT. The power of this monstrous weapon is equivalent to approximately 44 metric tons of TNT (or about 176,000 sticks of dynamite). That’s the largest conventional bomb ever made. Yet one of the smallest nuclear weapons ever built was far more powerful than that! The W-54 nuclear warhead, created and tested by the US military for use on air-to-air missiles, weighed only 23 kg. It was small enough and light enough to carry in a briefcase, but had an explosive power equivalent to 250 tons of TNT.See here for a list of all nuclear weapons built by the United States; see this Wikipedia entry for more on the W-54 warhead. In other words, this “baby nuke” was nearly six times more powerful than the Father of All (conventional) Bombs!

Although thousands of nuclear weapons have been detonated for testing purposes, only two have ever been used in warfare. In 1945, the United States dropped a uranium fission bomb on the Japanese city of Hiroshima. Its explosive power was equivalent to 12 kilotons of TNT. (That’s 12 thousand tons, or 12 million kilograms of TNT; roughly 48 million sticks of dynamite.) Three days later, a plutonium fission bomb was detonated over Nagasaki. Its explosive power was 23 kilotons, nearly twice as powerful as the previous bomb. Tragically, an estimated 200,000 people were killed by those two explosions.

Both of those bombs were fueled by nuclear fission reactions. However, the most powerful nuclear weapons use nuclear fusion as their primary source of energy. As mentioned on the previous page, fusion reactions require extreme temperatures: more than twice the temperature of the sun’s surface to fuse hydrogen atoms, and even greater temperatures to fuse other elements. So, how do you make hydrogen hot enough to start a nuclear fusion reaction? Easy! Just use a nuclear fission bomb (fueled by uranium or plutonium) to heat up the hydrogen! That’s how thermonuclear weapons (also called hydrogen bombs) work. A nuclear fission bomb is detonated inside or beside a tank of hydrogen fuel. The extreme heat from the fission bomb ignites a fusion reaction in the hydrogen, which releases far more energy than the initial fission bomb alone.

The fuel used in a thermonuclear weapon isn’t just ordinary hydrogen. The vast majority of naturally-occurring hydrogen is protium (hydrogen-1). But deuterium (hydrogen-2) and tritium (hydrogen-3) fuse more readily and energetically than protium, so thermonuclear weapons use those heavier isotopes as fuel for the fusion reaction.

Moreover, most thermonuclear weapons are fueled by lithium hydride (LiH) rather than pure hydrogen. Although the fusion reaction involves only hydrogen atoms, lithium hydride provides two advantages over pure hydrogen fuel. First, pure hydrogen is a gas at room temperature, so if you wanted to pack a lot of it into a small container, you’d have to put it under extreme pressure or make it so cold that it becomes liquid. Lithium hydride, on the other hand, is solid at room temperature. By using solid LiH as fuel, a lot of hydrogen atoms can be packed into a relatively small bomb without needing pressurization or cooling mechanisms.

A second advantage of using LiH for fusion fuel is that the lithium atoms undergo nuclear fission during the explosion. The fission of lithium produces tritium (hydrogen-3), which is an especially potent fuel for the fusion reaction.

The most powerful thermonuclear weapon ever tested by the United States, code named “Castle Bravo,” was detonated in 1954 at Bikini Atoll in the northern Pacific Ocean. Its explosive power was equivalent to roughly 15 million tons of TNT! The bomb was not intended to be that powerful, but one of the isotopes present in the nuclear fuel played an unexpected role in the reaction. (See the “fine print” section below for more details.) Consequently, the bomb exploded with more than twice the predicted power.

Castle Bravo was expected to produce the explosive power of 5 or 6 million tons of TNT. Unfortunately, the test did not go as planned. This was the first thermonuclear weapon to use lithium hydride as fusion fuel, and the bomb’s designers misjudged the role that lithium would play in the nuclear reaction.

As mentioned above, the fission of lithium produces tritium (hydrogen-3), and this extra hydrogen magnifies the power of the fusion reaction. The Castle Bravo engineers expected this to happen with lithium-6, but they did not anticipate that lithium-7 would likewise undergo fission and produce tritium. Naturally occurring lithium is about 92.5% lithium-7 and only 7.5% lithium-6. The lithium used in the bomb was enriched to about 40% lithium-6, but the remaining 60% of the lithium was not expected to undergo fission. Well, it did. As a result, much more lithium was converted to tritium, making the subsequent fusion reaction far more powerful than anyone had foreseen.See here for further details.
photo of the Castle Bravo fireball
the Castle Bravo fireballPhoto source: US Department of Energy (public domain). Original file here.

The unexpectedly extreme power of the Castle Bravo test had disastrous consequences. Nuclear fallout—radioactive debris thrown into the atmosphere by the blast—rained down on residents of the Marshall Islands hundreds of miles away. The islands of Rongelap (100 miles from ground zero) and Utrik (300 miles from ground zero) had to be evacuated, and the islanders were not allowed to return for three years. Even then, many who returned suffered from the effects of radioactive fallout that remained on the islands. In 1985—more than two decades after the Castle Bravo test—Rongelap was evacuated again, as persistent health problems indicated that the island was still contaminated.For more information, see this timeline of events related to the Castle Bravo test.

That was the largest nuclear test ever conducted by the United States, but the Soviet Union tested an even more powerful device called the “Tsar Bomba,” with an explosive yield equivalent to more than 50 million tons of TNT. That’s more than 14,000 times the combined power of the bombs that destroyed Hiroshima and Nagasaki. Its mushroom cloud reached over 40 miles high. Shock waves from the blast shattered windows 560 miles awaySee here for more details. and circled the earth three times.See this BBC article for more information. And that was merely the largest bomb ever tested. The soviets claimed to have built a bomb twice as powerful, with an expected yield equivalent to 100 million tons of TNT. Fortunately, it was never detonated.

These are unsettling facts, but our own nuclear arsenal is equally terrifying. The US presently has 450 armed Minuteman-III intercontinental ballistic missiles (ICBMs) ready to launch from missile silos in Montana, North Dakota, and Wyoming.More details are given by the National Park Service website, here. One missile carries up to three nuclear warheads, each with an explosive power up to 450 kilotons. (That’s 13 times the combined power of the bombs that destroyed Hiroshima and Nagasaki.) We also have numerous submarine-launched ballistic missiles (SLBMs) armed with thermonuclear warheads, along with plenty of nuclear bombs that can be dropped from aircraft.

Curious to know what would become of your hometown if a nuclear weapon were detonated nearby? Check out this website for an interactive map that allows you to visualize the destructive potential of nuclear weapons.