Lets make a dirty bomb by Matt Giwer, © 2002 [June]
Making a dirty bomb is like making rabbit stew.
As the government has informed us that a high school dropout street punk can make a dirty bomb, lets do a thought experiment. Lets make one. The government descriptions are of radioactive material being dispersed by an explosion. Essentially all candidate materials are metals. The requirement is to disperse the metal. Clearly if it is in one piece we might as well just throw it. No explosion is going to turn it into more than a few pieces. Those pieces can be found quite easily by Geiger counters. To kill people we have to get them to eat it or inhale it. As people are not likely to eat pieces of metal they find on the street we are going to have to get people to inhale it or unnoticeable on their food. Therefore we must reduce the metal to a very fine powder. With metals that would require a file. The finer the file the finer the pieces. If you have ever done this or handled iron filings you know if you drop them they fall like stones. They do not float away in the wind and no one is going to inhale them and they will be spit out if chewed. We could drop them off of a high building in a strong breeze but that doesn't do much for dispersion. Even if as fine as talcum powder (no easy task) the particles are a hundred times heavier than talcum powder and fall rather than blow away in the wind. So we have to find a way to get the material fine enough to be dispersed by the wind. To do that the best way is to burn it. That will produce relatively light metal oxide particles. This can disperse through the air in a stiff breeze and can be inhaled or fall on food. The best candidate metal is uranium isotope number 235, the one used to make one type of atom bomb. This is slightly heavier than lead. Back when we had lead in gasoline the burned lead came out of the exhaust. That was so heavy it didn't get much beyond the sidewalks so we will still need that tall building. Uranium burns quite well once started so mixing the powder with thermite or magnesium will cause a nice burn and on top of a tall building about the best dispersion we are going to get. It is not "the taller the better" and "the stronger the breeze the better" as we don't want it falling beyond the target area or, depending on location, blowing out to sea. It would take some study to determine building height to wind velocity to maximize the results. Despite the government's implication we cannot drop into the local university and get a few ounces of U235. But if we could get hold of a few ounces it would come in a lead container weighing a half ton or more. And after having it and developing a filing machine and collection apparatus the machine would have to work in a lead enclosure in the ten ton range. The mixing with magnesium would have to be done in the room. This increases our requirement to a human operator having control of the equipment from outside the room drastically increasing the cost. As we can make the assumption this is a suicide mission the designated bomber can rush in to pick up a briefcase and head for the tall building and ignite it on the roof. We have now been successful suicide terrorists. Because of the necessary contamination of the assembly area that person will most certainly die of radiation poisoning due to massive exposure. If not that then certainly because whatever he carries it in will not have the half ton of lead shielding needed to prevent his death by radiation poisoning. But our success is limited. Exploded or burned outside on top a tall building with a stiff breeze either dust or burned Uranium can cover and contaminate a wide area -- until the first decent rain washes it down the sewers. What doesn't wash away will certainly collect in pools of rain water making cleanup efforts much easier. We also do not want it too dispersed else it will be hard to detect above natural background radiation and the media will not become as hysterical. We have also made a trade-off between burned Uranium which people have a chance of inhaling against a fine dust which is essentially cannot be inhaled. Dust has to fall on food unnoticed and eaten leaving the risk only to food exposed and being eaten while it is settling. This would of course leave the customers of open air food markets at risk but little would be eaten and the loss limited to one days' exposed food. With this success we have created massive hysteria in the media and in the people stirred up by the media. But with neither dust nor burning is there enough radiation in one place nor ingested or inhaled by one person to cause radiation poisoning. What we have achieved will only be measurable statistically. Starting no sooner than a year later there will be a small statistical increase in radiation related forms of cancer. Actual deaths will not start until a year or more after that. This statistical increase may be noticeable for another twenty years until things are back to normal. We will not have hospitals filled with thousands of victims of radiation poisoning. In the process of designing this weapon we have discovered there are very few useful radioactive materials. Those commonly available as in smoke detectors are in much too small a quantity and do not put out significant dangerous radioactivity. Another relatively safe isotope of Uranium, 238, is called depleted Uranium. It was used extensively in anti-tank shells during the Gulf War. That is all over Iraq along the 'highway of death' and thus freely available. There is no good source of U235 outside of a highly guarded weapons plant. Some universities do have it for research purposes but it is never in large quantities and most always in the form of salts or oxides. It is not in large quantities as the more in one container the more radioactivity it produces. The larger the sample the more dangerous it is to handle. Fortunately research other than for bombs requires only very small quantities. It is in the form of salts, such as chlorides so that it is soluble in water for some experiments or as an oxide exactly so that it cannot burn with the attendant risks. So any oxide stolen can be used only for dispersion not burning. Put it all together and a theft from a university source might yield a few thousandths of an ounce, a few milligrams. As it is in a form which can only be used for explosive dispersal it has that tendency to fall like a rock so it won't disperse very far and not likely to be inhaled. The real danger of its use would be the hysteria in the news media and the panic the media would cause and then cover. In the process of designing this weapon we have learned there is natural background radioactivity and the amount and type of radioactivity to harm people. We have learned the only people who would die in the first few years after it is set off would be those caught in the initial fire or explosion. We have also learned the safety requirements for preparing such a weapon and that the government can discover us by our purchases of lead and industrial robotic equipment. And if we don't do everything right we will be dead before half started. Above all we have learned high school dropout street thugs are not of use except as the designated suicide. Addendum The one other alternative are the spent core rods from nuclear reactors. If we could get some of this it starts as a new fuel rod at 3% instead of the over 90% concentration for weapons grade material. So the actual amount of radiation is drastically lower to begin with. When it is spent, ready for reprocessing, they have a wide range of highly radioactive elements which are quite suitable for dirty bombs. The problems of obtaining this are greater than stealing U235 from a research facility. Whereas a university would have a night watchman or two, nuclear power plants have 24/7 armed guards, security fences and intrusion detection systems. If these can be overcome we will find spent fuel rods in huge tanks of water. If we can remove them we have to put them into the multi-ton lead containers we brought with us. During the hours this would take with our huge truck and cranes the people working there have called the police. We could try hijacking a truck carrying the rods to the reprocessing plant. We discover, unlike the movies, the trucks do not travel back roads in the country but on the Interstate highways with police escorts. We would have about as much chance of getting away as a speeder on police video with ten times the resources after us. Addendum 12 July 2007 I presume everyone has read by now the Congressional "sting" on the Nuclear Regulatory Commission to buy radioactive material. If you followed it you read the NRC's answer which was, it would have been such a low level that the worst would have been a panic in the press but otherwise essentially harmless. That is what I said. Doing this can only panic the media. It is not a real threat. I make the presumption based on the logs indicate the searches which lead to this article have soared from about 3 a week before the story broke. I do not have a new estimate of searches yet. Also for the record the GAO was assuming explosive dispersion for this. That would be the only way to make it a measurable increase. It would be thousands of small particles. However they could not travel very far. Because of their low mass they would barely travel beyond the blast range of the explosion. Using the burning method I suggest it would travel much farther but would be of such low level as be ignorable except by the media looking for ratings. The NRC does not casually hand out licenses for material that is really dangerous.
First you catch one rabbit.
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