The Universe of Discourse

Tue, 16 Jan 2018

If I were in charge of keeping plutonium out of the wrong hands, I would still worry about [people extracting it from plutonium-fueled pacemakers].

This turns out to be no worry at all. The isotope in the pacemaker batteries is Pu-238, which is entirely unsuitable for making weapons. Pu-238 is very dangerous, being both radioactive and highly poisonous, but it is not fissile. In a fission chain reaction, an already-unstable atomic nucleus is hit by a high-energy neutron, which causes it to fragment into two lighter nuclei. This releases a large amount of nuclear binding energy, and more neutrons which continue the reaction. The only nuclei that are unstable enough for this to work have an odd number of neutrons (for reasons I do not understand), and Pu-238 does not fit the bill (Z=94, N=144). Plutonium fission weapons are made from Pu-241 (N=147), and this must be carefully separated from the Pu-238, which tends to impede the chain reaction. Similarly, uranium weapons are made from U-235, and this must be painstakingly extracted from the vastly more common U-238 with high-powered centrifuges.

But I did not know this when I spent part of the weekend thinking about the difficulties of collecting plutonium from pacemakers, and discussing it with a correspondent. It was an interesting exercise, so I will publish it anyway.

While mulling it over I tried to identify the biggest real risks, and what would be the most effective defenses against them. An exercise one does when considering security problems is to switch hats: if I were the bad guy, what would I try? What problems would I have to overcome, and what measures would most effectively frustrate me? So I put on my Black Hat and tried to think about it from the viewpoint of someone, let's call him George, who wants to build a nuclear weapon from pacemaker batteries.

I calculated (I hope correctly) that a pacemaker had around 0.165 mg of plutonium, and learned online that one needs 4–6 kg to make a plutonium bomb. With skill and experience one can supposedly get this down to 2 kg, but let's take 25,000 pacemakers as the number George would need. How could he get this much plutonium?

(Please bear in mind that the following discussion is entirely theoretical, and takes place in an imaginary world in which plutonium-powered pacemakers are common. In the real world, they were never common, and the last ones were manufactured in 1974. And this imaginary world exists in an imaginary universe in which plutonium-238 can sustain a chain reaction.)

Obviously, George's top target would be the factory where the pacemakers are made. Best of all is to steal the plutonium before it is encapsulated, say just after it has been delivered to the factory. But equally obviously, this is where the security will be the most concentrated. The factory is not as juicy a target as it might seem at first. Plutonium is radioactive and toxic, so they do not want to have to store a lot of it on-site. They will have it delivered as late as possible, in amounts as small as possible, and use it up as quickly as possible. The chances of George getting a big haul of plutonium by hitting the factory seem poor.

Second-best is for George to steal the capsules in bulk before they are turned into pacemakers. Third-best is for him to steal cartons of pacemakers from the factory or from the hospitals they are delivered to. But bulk theft is not something George can pull off over and over. The authorities will quickly realize that someone is going after pacemakers. And after George's first heist, everyone will be looking for him.

If the project gets to the point of retrieving pacemakers after they are implanted, George's problems multiply enormously. It is impractical to remove a pacemaker from a living subject. George would need to steal them from funeral homes or crematoria. These places are required to collect the capsules for return to Oak Ridge, and conceivably might sometimes have more than one on hand at a time, but probably not more than a few. It's going to be a long slog, and it beggars belief that George would be able to get enough pacemakers this way without someone noticing that something was up.

The last resort is for George to locate people with pacemakers, murder, and dissect them. Even if George somehow knows whom to kill, he'd have to be Merlin to arrange the murder of 25,000 people without getting caught. Merlin doesn't need plutonium; he can create nuclear fireballs just by waving his magic wand.

If George does manage to collect the 25,000 capsules, his problems get even worse. He has to open the titanium capsules, already difficult because they are carefully made to be hard to open — you wouldn't want the plutonium getting out, would you? He has to open them without spilling the plutonium, or inhaling it, or making any sort of mess while extracting it. He has to do this 25,000 times without messing up, and without ingesting the tiniest speck of plutonium, or he is dead.

He has to find a way to safely store the plutonium while he is accumulating it. He has to keep it hidden not only from people actively looking for him — and they will be, with great yearning — but also from every Joe Blow who happens to be checking background radiation levels in the vicinity.

And George cannot afford to take his time and be cautious. He is racing against the clock, because every 464 days, 1% of his accumulated stock, however much that is, will turn into U-234 and be useless. The more he accumulates, the harder it is to keep up. If George has 25,000 pacemakers in a warehouse, ready for processing, one pacemaker-worth of Pu-238 will be going bad every two days.

In connection with this, my correspondent brought up the famous case of the Radioactive Boy Scout, which I had had in mind. (The RBS gathered a recklessly large amount of americium-241 from common household smoke detectors.) Ignoring again the unsuitability of americium for fission weapons (an even number of neutrons again), the project is obviously much easier. At the very least, you can try calling up a manufacturer of smoke alarms, telling them you are building an apartment complex in Seoul, and that you need to bulk-order 2,000 units or whatever. You can rob the warehouse at Home Depot. You can even buy them online.

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