Sunday, July 6, 2014

Will ISIS use Depleted Uranium in Iraq?

Greetings.   I've decided to expand the purpose of this blog beyond scientific analysis of the nuclear issues associated with the Fukushima meltdowns to a general discussion of specific issues of public interest relating to nuclear physics.

My inspiration for today's post is the book I am reading "The Making of the Atomic Bomb" by Richard Rhodes...and excellent and very readable book.  I just read how nuclear scientists like Leo Szilard tried desperately to make the US government aware of the importance of the discovery of nuclear fission by Lise Meitner & colleagues.    A key factor is the difference in the way the two main isotopes of  Uranium react: the rare U235, and the heavier (and more common) U238.

This got me thinking about U238. It comprises ~99% of Natural Uranium, and to make bombs & reactors, natural Uranium is enriched until the ~1% of it that is U235 becomes 20% or even 99% (highly enriched, or weapons grade).  Nuclear bomb makers thus remove most of the less reactive U238.....but that doesn't mean they can't use the leftovers for weapons.  They can and they do.

This leftover "Depleted Uranium" is shaped into high density and very hard bullets and anti-tank shells,  which were used extensively both US-Iraq wars, and has, apparently caused serious health problems, including reproductive harm.   While U238 is not as radioactive is the U235 made for bombs, its radioactivity lasts a very long time.   Some 400 tons of DU was dropped on Iraq, of which most has not been cleaned up.

My goal here is to determine, via  a back of the envelope calculation if this reservoir of DU could be used by ISIS, to create a weapon that poses a general threat to human health.   ISIS  currently occupy what looks like about 1/3 of Iraq  (and a portion of Syria which is not of interest here).  So, with some rough assumptions, I calculate:

A.)  Of 400 tones of DU dropped on Iraq, 1/3 was in ISIS controlled territories.   (I'll decrease this to 1/4 since much DU was concentrated in South Iraq...but a lot was used in Falluja as well)

B.) Perhaps only 10% of DU is recoverable (eg. using Geiger counters).  The rest would be vaporized, too broken up to trace, or otherwise already cleaned up.   The figure could be lower than this.

This leaves about 40 tones, or 40,000 kilograms of pure Uranium 238 possibly under ISIS control.  What could they do w/ that amount?  Fortunately, one answer is NOT "immediately build a nuclear weapon".   While only 50 kg of U235 (a ball 4 inches in radius) is needed for a nuclear weapon, Depleted Uranium is pure U238 and has no critical mass, so cannot be "enriched" into U235.

But it can be transmuted into Plutonium 239 by means of what's called a breeder reactor.   That Plutonium 239 can be made into nuclear weapons, like the one that destroyed Nagasaki.  Basically a neutron source is required to add an extra neutron to U238 making Pu238 which Beta decays into Pu239.   Building a full fledged breeder reactor seems implausible to me given the current state of affairs in Iraq.

However, there are many types of neutron source, with different neutron fluxes, ranging from the old fashioned Radium+Beryllium sources used by fission pioneers such as Otto Hahn (Nobel Prize winner, not to be confused with David Hahn, the "Radioactive Boy Scout" who also used this same type of neutron source) to home made & commercially available "Fusors"......It is difficult to calculate what neutron flux is needed to transmute appreciable U into the required 10 kg of Pu, using neutron reflectors and neutron moderators (eg. Heavy Water) to improve the process.

One report makes such transmutations sound easy, given the right facilities; but according to this website its well nigh impossible (but it does not consider the prospect of a large quantity of free DU).     I'll have to leave that calculation until later...if you have any thoughts feel free to chime in in the comments.   How could we tell if ISIS were doing this:   look out for large purchases of Geiger counters, Heavy Water, smoke detectors, etc.

Ostensibly it is unlikely that any viable attempt at nuclear weapons will made by non-state entities, so there probably is nothing to worry about.  ...After all, isn't it safe to assume that if the US military invades a country they will have thought through any of the adverse consequences like this?