A-Bomb v. H-Bomb

    Since the 1950s people have said, "The bomb used against Hiroshima was horrible, but nuclear weapons today are thousands of times more powerful than that bomb was." This notion, that H-bombs are much more destructive than the atomic bomb that destroyed Hiroshima, is a commonplace. It is another example of loose and inaccurate thinking about nuclear weapons.

   It is true that you can build very large fusion bombs – there is, in fact, no theoretical limit to their size. In the early 1960s the Soviets tested a bomb that was quite large - roughly 50 megatons - which was 4,000 times greater in terms of yield than the Hiroshima bomb. But as a practical matter the bombs in most arsenals are not that large. Typically, the largest are 1 megaton bombs, which is only about 80 times greater than the Hiroshima bomb. In the US arsenal today the average is roughly 300 kilotons. This is only about 24 times more powerful than the bomb that destroyed Hiroshima.

    But there is another, more important difficulty. We lay people tend to imagine that there is a one-to-one correlation between the force of an explosion and the amount of destruction that it causes. If a bomb is 80 times more powerful than the Hiroshima bomb, we automatically assume that it will be 80 times more destructive. But that is not true. 

    To demonstrate this, let's look at the 5 psi circle. The 5 psi circle is the area in which the pressure created by a bomb's explosion is 5 pounds per square inch (or greater). At Hiroshima this area extended out .8 miles from the center of the blast. You might expect if an H-bomb's yield were 80 times more powerful, that the circle of destruction (in this case measured by its 5 psi circle) would be 80 times greater. This would mean that the 5 psi circle would extend out 64 miles from the center of the blast. The full diameter of this circle would be 128 miles. With a bomb this size, you could drop one bomb halfway between New York and Philadelphia and destroy both cities (and everything in between).

    But in fact the destruction is not 80 times greater. Most of the force of an explosion is concentrated at the center, not distributed evenly across its entire area. 

  The actual radius of 5 psi for a one megaton bomb is 4.4 miles, which is only 6 times greater than the area of the 5 psi circle at Hiroshima. Of course, all these figures are very rough. The 5 psi circle changes depending on the height at which you explode the bomb. The amount of destruction is also affected by the kind of terrain, the weather at the time of the explosion, and other factors. But the point is worth remembering: although it is true that hydrogen bombs are more powerful than the bomb that destroyed Hiroshima, and although it is true that the yeild of hydrogen bombs can be "thousands" of times more powerful than atomic bombs, as a practical matter the destructive power of the bombs that are in most arsenals today are not "thousands" of times greater than the Hiroshima bomb. They are 6 or 7 or 10 times more destructive, which is a significant difference. This mistake in size is typical of the tendency to imbue nuclear weapons with titanic power. We are so often tempted to see them as the apocalypse and not look closely at the reality.

    Today's sprawling cities also affect any calculation. A one megaton bomb used against Phoenix, Arizona would cover only 10% of the city with its 5 psi circle. A similar attack on Los Angeles would cover only 6% of the city with its 5 psi circle. In the 1980s Congress' Office of Technology Assessment imagined a hypothetical scenario of an attack against Philadelphia with two one-megaton bombs. They assumed that 10% of the population had already evacuated. The results were roughly equivalent to the damage at Hiroshima - one third of the population killed, two thirds of the city destroyed.

    Nuclear weapons are perilous weapons - vastly destructive. It would be foolish to take them lightly. But the only way to control them is to understand them clearly, without either fear or awe. It is a mistake to presume without thinking that if a nuclear weapon were used against a city today that the destruction would be far worse than Hiroshima, even "thousands" of times worse.

    The moral, I suppose, is that we are often misled by numbers. The numbers used to describe nuclear weapons' yields are misleading when used to think about the damage an attack might cause.