Nuclear Forensics

Several months ago I was involved in an interesting conversation about improvised nuclear devices – specifically, the best way to tell the public what had happened. In particular I recall a several-minute discussion on whether or not the public should be told that an attack had taken place. I have to admit I was somewhat incredulous – at one point I asked if anyone really thought that a member of the public, seeing a mushroom cloud rising over downtown, would think that it was a friendly explosion. Of course a nuclear explosion in one of our cities would be an attack – the question is not “what happened” so much as “who did it?” This is where nuclear forensics comes into play.

Consider – a nuclear attack would be hugely destructive; far more so than any terrorist attack in history. Just as we waited to confirm al Qaeda’s role in the September 11 attacks before we attacked Afghanistan, so too would we need to be absolutely sure to retaliate correctly to a nuclear attack.

In the “good old days” of the Cold War there were fewer nuclear nations and no terrorist groups who wanted to use them against us. What we worried about were bombs that arrived at the tip of a missile or in the belly of a bomber – weapons that came with a return address as it were since we could (presumably) trace the trajectory back to the point of origin. Planning for an all-out nuclear attack ruled out a number of possibilities – massive numbers of nuclear weapons arrive through air or space, not by UPS. But things are different today – there are more nuclear-capable nations (a number of which lack both large numbers of weapons and missiles), there are terrorist groups who have expressed a determination to use nuclear weapons against us, and there are an increasing number of possibilities for these groups to try to get their hands on them. This means that trying to sort out who attacked and where they got their nuclear weapon will not be nearly as simple as we had once expected.

In Tom Clancy’s novel The Sum of All Fears there is a scene in which savvy nuclear forensic experts pinpoint the exact reactor in which the plutonium comprising a nuclear weapon originated. A neat scene – but is it plausible? Say, for example, a nuclear weapon goes off in one of our major cities and we can figure out that it contained plutonium that originated in Russia. Do we blame the Russians for the attack and retaliate, do we accuse the Russians of having sold a nuclear weapon to a terrorist group (or to a rogue nation), do we decide that a terrorist group (or a rogue state) managed to get their hands on Russian plutonium, or do we suspect that whoever attacked us just happened to produce plutonium that was identical to something produced by a Russian reactor?

In 2010 the National Academies of Science concluded that America was losing its touch in nuclear forensics, recommending that actions be taken to restore our Cold War capabilities. To help address this need the government has opened a new facility at the national laboratory in Oak Ridge – the place where some of the first weapons-grade uranium was produced – the National Uranium Materials Archives. As reported in the Knoxville press, this collection will help nuclear forensic specialists to pinpoint the source of nuclear materials that are used or intercepted.

The idea behind nuclear forensics is that every bit of enriched uranium or plutonium is unique, a product of the environment in which it was produced. Plutonium, for example, is produced in nuclear reactors, when a U-238 atom captures a neutron and begins a process that leads to Pu-239. But reactor fuel contains more than just U-238, U-235, and Pu-239 – consider these, for example:

  • Fission products from U-235 fission
  • Various nuclides of plutonium from neutron capture (Pu-239, Pu-240, Pu-241)
  • Fission products from the plutonium nuclides
  • Impurities from the original fuel and the nuclides formed when they capture neutrons

The concentrations of all of these radionuclides will vary in a way that, in theory, can identify the exact nuclear reactor in which a batch of plutonium was produced. And, with careful analysis, one can even make some very educated guesses as to where a batch of uranium was mined, processed, and enriched. Putting all of this information together with the sort of samples Oak Ridge is collecting can help us to pinpoint the origin of a batch of uranium or plutonium used in a nuclear attack. And with that information, should there be a nuclear attack against us or, for that matter, against any other nation, we just might be able to figure out who attacked us and where they got their weapon.

In the wake of the Cuban Missile Crisis President Kennedy foresaw “the possibility in the 1970s of the president of the United States having to face a world in which 15 or 20 or 25 nations may have these weapons.” This obviously didn’t happen, thanks in large part to the Treaty on the Non-Proliferation of Nuclear Weapons (usually abbreviated as the NPT). But we have seen the number of nuclear states increase in the last decade and there is the possibility that this number could grow even further in coming years and we increasingly face the possibility that such weapons will be developed or obtained by nations that may be tempted to sell or to give them to terrorist groups for use against us or against our allies.

Consider, for example, a North Korea that, in the aftermath of a successful 20 kiloton nuclear test, sold a nuclear weapon to a terrorist group who then used it to attack Tel Aviv, Riyadh, Paris, Washington DC, or another major city. Delivered in a shipping container or rented truck, the weapon would arrive without a return address – we would have only the claims of various terrorist groups to go by in trying to figure out who had attacked. But if nuclear forensics could fingerprint the weapon as containing uranium or plutonium from North Korea we would know who made the weapon and we could hold them accountable for their role in the attack. But we can only do this if we have the capability of identifying the weapon as having originated in that nation.

Absent solid knowledge of the precise composition of North Korean uranium and plutonium coupled with a robust nuclear forensics capability we might well be unable to figure this out or to be able to convincingly justify retaliation. This is why it is vitally important that we retain our skills in nuclear forensics and why we need to build our uranium materials archive – every nation that has or is developing nuclear weapons has to be made fully aware that, should one of their nuclear weapons be used in an attack anywhere in the world, their role in the attack will come to light and they will be punished.


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