The Dirty Nuke - What It Is and Isn't
On March 18, the Washington Post published an article by staff writer Jo Warrick, discussing the potential danger posed by old Soviet RTGs (Radiothermal Generators) falling into the hands of terrorists. Warrick's thesis is that strontium-90, which is used to power some of these RTGs, could be used in the construction of dirty bombs - conventional bombs that are wrapped in a layer of radioactive material, so when they explode, they spread a layer of radioactive contamination over a wide area.
In support of this thesis, Warrick cites the Institute for Energy and Environmental Research and the Federation of American Scientists. Both these groups are left-wing, anti-nuclear organizations whose agendas include eliminating anything nuclear from modern life. In support of their anti-nuclear agenda, these guys routinely distort the nature of radioactivity and its effects, and they exaggerate its consequences. They can dupe the public with their distortions and exaggerations because the organizational names they have chosen evoke public trust. Warrick's real focus in this article is to use the public's fear of terrorists obtaining material for dirty nukes as a way to eliminate further deployment of RTGs.
Within the context of this discussion, it will be useful to examine more closely what an RTG really is and what environmental threat it really poses.
In its simplest implementation, an RTG consists of a radioactive source that produces heat that activates a thermocouple to produce electricity. Most Western RTGs employ plutonium as the radioactive source, since plutonium produces the most concentrated heat from the smallest amount of source material, and so allows for very compact RTGs. Since the amount of Plutonium is far below that needed for a nuclear explosive device, the only danger is that related to contamination from the plutonium itself.
These compact devices are particularly useful as power sources in spacecraft that will be out of contact with the concentrated sunlight, and to power devices in remote terrestrial locations, such as underwater sensors, navigational beacons, transponders, etc. Lead shielding makes up most of the mass of RTGs. Plutonium powered units weigh only a few pounds and can mass even less when power requirements are very small. The typical Soviet style RTGs weigh in at one to three thousand pounds, because they have relatively high power requirements, and they are usually powered with strontium-90. One can assume that the Soviets used the less efficient strontium-90 in their RTGs, because they slated all their plutonium for nuclear weapons.
The total amount of nuclear material contained in a typical RTG is insufficient to cause a contamination problem over greater than a few square yards. As is explained below, the strontium-90 loaded Soviet RTGs are virtually useless as sources for "dirty nukes." Most of the remaining RTGs are miles below the ocean surface or millions of miles away in deep space, doing what they were designed to do.
So, what is the "best" nuclear material to use in a "dirty nuke"? This depends on your objective. If you want to irritate your enemy for a very long time, say 75 thousand to 4.5 billion years, you use a low-level long half-life material like uranium-234 or 238, or thorium-230, but if you want to deny your enemy use of the contaminated territory for 20 thousand years or so, use a plutonium mixture, and use radium-226 to keep him out for over 1.5 million years.
On the other hand, if you want to harm a bunch of people and close off an area for about a decade you use cobalt-60. But if you want to do some real damage to humans while regaining access to the area in a couple of weeks or so, use iodine-131.
The problem with using strontium-90 in a dirty bomb is that it really causes very little human damage. It gives out only beta particles (high speed electrons), which can be stopped by a few feet of air, a sheet of paper, or even your skin. External burns resulting from intimate contact with concentrated strontium-90 are indistinguishable from any other burns and heal in the same way. The only significant danger from strontium-90 is when it is ingested and taken up in the bones, where it affects the bone marrow and can eventually cause one of several cancers. Fortunately, the body throws off any strontium relatively quickly, so even when ingested, the danger eventually goes away. It does hang around for 30 or so years in the environment, but you can live with it. As bomb stuff, it's really not worth the effort.
The point is, if you go to the lengths necessary to detonate a dirty bomb over one of our major cities in the United States today, you're NOT going to use strontium-90. You're wasting your time.
Either you go for maximum short-term human impact, or you hang up a long-term keep out sign, but you don't piddle around with strontium.
Warrick and the Post are nothing more than mouthpieces for the anti-nuclear Left. Unfortunately, their scare tactics work, because the general readership understands only what it reads and hears, and these guys supply most of the words and sounds.