What is a dirty bomb and what do you do if one goes off?

In a series of telephone calls on Sunday, Russian Defence Minister Sergei Shoigu told his counterparts in Britain, the United States, Turkey, and France that Ukraine was planning to detonate a radioactive dirty bomb on its own soil as part of a plan to discredit Russia. Though quickly dismissed by NATO ministers as a convoluted attempt by Russia to drive a wedge between Ukraine and its supporters or as the set up or a false flag attack to justify another Russian offensive, the allegation raises the question, what is a dirty bomb and how dangerous is one?

Essentially, a dirty bomb is a way of spreading radioactive material over a wide area. Where a nuclear weapon, like an H-bomb, initiates a nuclear chain reaction to release enormous amounts of energy in an instant and radioactive fallout as a byproduct, the dirty bomb uses a conventional explosive to disperse a cloud of radioactive dust to contaminate a target area.

The idea isn't a new one. It was first made popular by Robert A. Heinlein in his 1941 science fiction novella Solution Unsatisfactory, where a US Army general develops a way to crop dust radioisotopes over Europe to end the Second World War. In the real world, after the war, the nuclear powers toyed with the idea of encasing a nuclear warhead in materials like cobalt and iodine to produce a more radioactive fallout. However, aside from inspiring the plot device in the 1964 thriller Goldfinger to irradiate the US gold reserves, this never went anywhere.

A dirty bomb is so simple to make that even the poorest of nations could build one. All one needs is some TNT, the appropriate detonating and fusing devices, and a canister filled with a suitably nasty radioisotope like caesium-137.

Radioisotopes are very common around the world. They're used in medicine, industry, science, food preservation, agriculture, and many other fields. This is one reason why governments place heavy restrictions on such isotopes and monitor them closely. Otherwise, there's the chance of such isotopes not only falling into the wrong hands, or horrible accidents like the one in Brazil in 1987 when a pair of metal scavengers broke into an abandoned clinic and stole a teletherapy source capsule containing powdered caesium-137. Unaware of what it was, their mishandling of it resulted in 249 people contaminated, 20 seriously ill, and five dead.

However, there have only been two attempts to use a dirty bomb. One was in 1995 and the other in 1998 by a Chechen separatist group, though neither bomb was actually detonated.

But what if a dirty bomb is set off? What would be the result? The details depend on a lot of factors, like the isotopes used, the target, the surrounding geography, and the number of people in the area. Though, in general, the effects would be surprising.

The US Department of Energy conducted a series of simulations of a dirty bomb attack and found that such device was largely useless as any sort of a weapon intended to achieve a military objective or even to simply cause death and damage.

These simulations found that the immediate deaths would be of, at most, a few people in the immediate vicinity and, even then, only from the explosion. After this, the spread of the radioactive dust is dependent on the wind or other air currents. Experiments with chemical and biological warfare and terrorist attacks involving nerve gas show that simply releasing something into the air is a very poor way of dispersing an agent. This means that, in the example of nerve gas, a chemical that could theoretically kill an entire city might only affect a handful of people.

For radioactive dust, dispersal is complicated because the dust particles are relatively heavy and, unlike fallout, they aren't thrown high into the air. This means that the dust falls out of the air in a very short time, resulting in a small, roundish pattern.

The most surprising result of the simulations was how the radioactive dust would affect people in the area. It turns out that even if people remained in the area of highest contamination for a full year without shelter, it would give a radiation exposure of about two rems, which is the equivalent of two full-body CT scans.

Exactly what this means is hard to assess because the effects of very low levels of radiation are still very much in question, with some scientists holding to the accepted idea that there is no safe lower limit to radiation exposure, while others point to more recent studies showing that the human body has some ability to repair radiation damage at low levels. However, even in the extreme scenario of the DOE simulation, the effect would be basically a small increased cancer risk.

This doesn't mean that a dirty bomb isn't dangerous. The target area would need decontamination, which could cost millions, if not billions of dollars, depending on where the bomb was detonated, and could take years. The cumulative economic effect could be enormous.

However, the biggest danger is from panic. The general ignorance and misconceptions about radiation have turned it into an almost demonic force in the minds of many. Even if the actual casualties from a dirty bomb were very small, the panic could cause terrible damage hundreds or even thousands of miles away. It's for this reason that dirty bombs are regarded as terror weapons, or "weapons of mass distraction."

What should you do if a dirty bomb does go off near you? According to the Center for Disease Control (CDC), if you are outside, the best thing to do is to cover your mouth and nose with a cloth to avoid breathing in the dust, avoid touching anything, and get inside a building with intact windows, doors, and walls. Once inside, remove your outer clothing and seal it in a plastic bag for later disposal. Then wash with soap and water, not forgetting your hair. Then tune in to local radio for instructions.

As to Ukraine, the Foreign Ministers of the US, UK, and France have rejected the Russian allegations as "transparently false" and a pretext for further aggression. A United States State Department spokesman has also said that if Russia does detonate a dirty bomb or any nuclear device in Ukraine, there would be consequences.

Hopefully, the whole episode will turn out to be a horrible bluff.