Dirty Bombs And Nuclear Weapons

For some reason, there is once again a flurry of concern that Iran will make DIRTY BOMBS, about which we are all supposed to be afraid. These DIRTY BOMBS create as much fear in some people as nuclear weapons, and they have used that name for them, further confusing themselves.

I did a thread about this on Bluesky, but obviously I need to do this again. I will assume that people know what atoms and subatomic particles are. Please look up those terms in Wikipedia if you are having a hard time with this explanation.

For the pedants who are reading this, I am deliberately simplifying some things and leaving others out, because my read is that those things confuse people. So please don’t add them all in again.

Radioactivity frightens people. But we are all bombarded all the time, with cosmic rays and radioactivity from soil and rocks. Radioactivity is the breakdown of atoms. It is not a miasma that floats around. It is connected to particular kinds of matter. There are several ways that matter can break down. It can release energy or particles.

Uranium isotopes are mildly radioactive. That means they are normally not much more of a danger than the soil around you. That also means that they are poor candidates for a dirty bomb. The purpose of a dirty bomb is to spread a dangerous level of radioactivity over an area.

The much more radioactive cobalt-60 and cesium-137 are what would be used in a dirty bomb. They have been found mainly in hospital and food irradiation machines, but they are being replaced by accelerators, whose radiation can be turned on and off. Occasionally a commercial source (that cobalt-60 or cesium-137) is improperly disposed of, and people open it to their injury, or it gets into steelmaking.

I’ll add, for dirty bombs, that it’s actually hard to disperse materials through explosions. And I’ll observe that nobody has bothered to make a dirty bomb, for all the worry about them.

Uranium isotopes have a few ways of breaking down, but they do it slowly, so they don’t produce a lot of radiation, unlike cobalt-60 and cesium-137. I’ll focus on one of their paths, spontaneous fission. That’s what clued people in to the idea of nuclear weapons.

Occasionally a uranium atom splits into two roughly equal parts and releases neutrons and energy. That is called spontaneous fission. Uranium atoms can also split into two roughly equal parts if they absorb a neutron. This happens with the 235 isotope (U-235) more readily than the 238 isotope (U-238).

To make a nuclear weapon (fission bomb), it’s necessary to separate the U-235 from the U-238. That’s what is done in centrifuges. What results from the centrifuges is uranium enriched in U-235 and uranium depleted in U-235. I mention both because those adjectives, enriched and depleted, are often used as if they denote something about danger to human health. They don’t. It’s just how much U-235 they contain.

Enriched uranium is what is needed for fission bombs. The uranium of concern in Iran is 60% enriched, meaning 60% of it is U-235. The natural abundance of U-235 is 0.7%. Iran also has some 20% enriched uranium, and some enriched to lower levels. A fission bomb can be made of 60% enriched uranium, but it’s easier to do with 90% enriched uranium.

In a fission bomb, uranium atoms split in rapid succession – milliseconds – releasing neutrons and energy. The neutrons split one atom after another in a chain reaction taking much, much less time than it takes to write this sentence. The fragments of the splitting are themselves radioactive. Cesium-137 is one of them.

That’s totally different than a dirty bomb, and the energy released can be thousands or millions of times as much. Plus it spreads radioactive materials.

Credit for top diagram: By MikeRun – Own work, CC BY-SA 4.0