USS Clueless - Nuclear power and nuclear weapons

Stardate 20040728.1502

(On Screen): Yesterday, Randy sent me this email:

I'm rather new to the BlogWorld, but after reading a bit of USS Clueless I thought you might answer an engineering question for me. I heard on the radio today that the nuclear power plant in Iraq that the Israelis blew up some years ago (sorry, I don't recall the name of the plant), was not capable of producing material that could be used in an atomic/nuclear bomb.

What do you think, is that true? Assuming it's true, what do you suppose would have been the Israeli rationale for destroying the power plant?

Today I learned that it was my old friend Regis Le Sommier who made that claim on the radio. I might have known...

I answered Randy by email, but I thought I'd post an answer, too, now that I know the source of that claim.

In one sense Le Sommier is correct: the French-built nuclear plant in Iraq is not capable of producing material which could be used in nuclear weapons.

That's because the Israelis destroyed it with their air strike before it went into operation.

I don't think that was what he meant. I think he was trying to contend that even if the plant had been finished and had gone into operation, it would not have aided Saddam's quest to develop nuclear weapons. That would exonerate France of any charge that it was helping Iraq develop nuclear weapons, and it would permit Le Sommier to argue that Israel's air strike wasn't justified.

If Le Sommier claimed that the French-built reactor in Iraq could not have produced anything that could be used in atomic bombs, either he was lying or he is dreadfully misinformed.

Theoretically speaking, there are a lot of ways of building power plants based on nuclear fission, and a lot of potential fuels which can be used. But right now, all civilian power plants use the same fuel, low-enriched uranium (LEU).

Naturally occurring uranium contains 0.72% of isotope 235 and negligible amounts of isotope 234. All the rest is isotope 238. U235 is fissionable, but U238 is not. Low-enriched uranium is uranium which has been processed to remove U238 so that the concentration of U235 is greater than 0.7% but less than 20%. Usually in fuel rods for power reactors it's between 3% and 5%.

You can't make a working atomic bomb out of low-enriched uranium. The threshold of 20% was adopted in the definition of LEU precisely because uranium consisting of 20% or less of U235 cannot form a critical mass. (A large mass of LEU would get extremely hot very rapidly, and would melt. If it was exposed to oxygen, it would also burn. The resulting mess would make Chernobyl look minor by comparison. However, it is physically impossible for it to detonate in a nuclear explosion.)

In a power reactor, energy is released because atoms of U235 undergo fission. Their nuclei break into pieces, and it's impossible to predict what the pieces will be, or how many there will be. Usually there are two big pieces, and nearly always one or more neutrons will also be released.

Some of those neutrons escape from the reactor core entirely and have to be absorbed by shielding. Some of those neutrons strike other U235 nuclei, and can cause them in their turn to fission almost immediately. But a lot of those neutrons strike other atoms in the reactor core. Some of them strike U238 nuclei and many of those are absorbed. If the fuel rods are based on LEU, it is impossible to prevent this from happening.

If U238 absorbs a neutron, it becomes U239. U239 β- decays with a half-life of 23.5 minutes, yielding Np239. That, in turn, also β- decays with a half-life of 2.35 days, yielding Pu239.

The fuel rods have to be replaced once a significant amount of the U235 in them has been used up. That usually takes many months, and sometimes takes years. By that point, a non-negligible amount of U238 will have been converted to Plutonium.

It won't all be Pu239. Plutonium is much better at capturing neutrons than U238, and it turns out that Pu240, Pu241, and beyond will make up a considerable percentage.

But that doesn't matter. What does matter is this: plutonium is chemically different from any of the other components of the fuel rod, and if the spent fuel rods are dissolved with acid, plutonium salts can easily be separated out using chemical means. Purified plutonium salts can then easily be reduced to plutonium metal. (Understand that "easy" is a relative term. It's dreadfully difficult and hazardous to do any of this, but it's extremely easy by co

Captured by MemoWeb from on 9/16/2004