Apparently, some journalists for ABC News managed to smuggle depleted uranium into the Port of Los Angeles today in a shipping container from Jakarta, Indonesia.

The ABCNEWS project involved a shipment to Los Angeles of just under 15 pounds of depleted uranium, a harmless substance that is legal to import into the United States. The uranium, in a steel pipe with a lead lining, was placed in a suitcase for the shipment.

"If they can't detect that, then they can't detect the real thing," explained Tom Cochran, a nuclear physicist at the Natural Resources Defense Council, which lent the material to ABCNEWS for the project.

Cochran said the highly enriched uranium used for nuclear weapons, would, with slightly thicker shielding, give off a signature similar to depleted uranium in the screening devices currently being used by homeland security officials at American ports.

Is Tom Cochran's assertion correct? The best comparison I can find online between enriched uranium and depleted uranium is on the Canadian National Defence website.
In nature, uranium exists as a mixture of three isotopes --- U-238, U-235, and U-234. These isotopes of uranium have different weights, but each has 92 protons in its nucleus. Uranium isotopes differ in weight because they have different numbers of neutrons in their nuclei. The isotopes U-238, U-235, and U-234 are present naturally in the proportions of 99.28 percent, 0.72 percent, and 0.0055 percent respectively. ...

The Nuclear Regulatory Commission (NRC) defines depleted uranium as uranium in which the percentage of the U-235 isotope by weight is less than 0.711 percent. The military specifications designate that the DU used by the American Department of Defense contain less than 0.3 percent U-235. In actuality, DoD uses only DU that contains approximately 0.2 percent U-235. ...

The enriched uranium used in nuclear reactors contains about a 3 percent concentration of this isotope. Enriched uranium is also used in nuclear weapons. ...

Because of the high percentage of U-238 and its slow decay rate, naturally occurring uranium is, in fact, one of the least radioactive substances among unstable isotopes on the planet. DU can be up to 50 percent less radioactive than naturally occurring uranium depending on the degree of depletion. The material generally used by the U.S. Department of Defense (DoD) is 40 percent less radioactive than natural uranium.

Based on this information, I calculate that 3% enriched uranium is approximately 275% more radioactive than 0.7% natural uranium, and 687% more radioactive than 0.2% depleted uranium. Using arbitrary units:

99.28x +0.72y == 100 (natural uranium)
99.8x + 0.2y == 60 (depleted uranium)

Solve for x and y, the radioactivity of U238 and U235 respectively, and you get:

x == 0.446
y == 77.446

The radioactivity of enriched uranium is:

97*0.446 + 3*77.446 == 275.6

So, enriched uranium is about 7 times more radioactive than depleted uranium. But, uranium isotopes primarily emit alpha particles, and alpha particles can be shielded with a sheet of paper.

All uranium isotopes are primarily alpha particle emitters. These alpha particles travel only about 30 micrometers in soft tissue and, therefore, are unable to penetrate paper, glass, or even the dead superficial layer of skin. The gamma emissions of uranium are rather low.
This fact makes all forms of uranium rather difficult to detect from a distance, considering that background radiation is always present. Uranium doesn't emit a great deal of gamma radiation above what is present in the background, either.
In the Bulletin of Atomic Scientists the authors tried to estimate the possible external gamma-radiation levels on the battlefield by assuming that 100 tons of depleted uranium had been distributed uniformly over a one-kilometer-wide strip along 100 kilometers of the "Highway of Death" between Kuwait City and Basra, a city in southern Iraq. The average dose for someone who lived in the area for a year would be about one mrem - or about 10 percent of the dose from uranium and its decay products already naturally occurring in the soil. The dose rate immediately around a destroyed vehicle could be about 30 times higher. But even that figure would only add about 10 percent to the natural background radiation.
So, is Tom Cochran correct? It looks like it. Although enriched uranium is up to 7 times as radioactive as depleted uranium, it only takes a tiny bit of lead to drastically attenuate radiation exposure. A mere 0.5mm of lead can reduce radiation exposure by a factor of 1000, depending on the energy level of the particles in question. Considering all the work that goes into finding uranium deposits, I don't have trouble believing that we couldn't detect this shipment of depleted uranium -- and that we'd be hard-pressed to detect enriched uranium under similar circumstances.

Clayton Cramer disagrees, based on U235's greater emission of gamma radiation. That's fine, as far as it goes, but as Tom Cochran claims that shielded U235 would "look" very similar to depleted uranium to the various radiation detection devices. I am not a physicist, but my dad is and he confirms that lead-shielded enriched uranium would look pretty much the same as lead-shielded depleted uranium -- that is, it would look like lead.

Update 2:
I oringally wrote "U238" in the first update in places where I meant U235, as Clayton Cramer points out in his update (and in the comments here). That mistake/typo/whatever did severely confuse my point, and make me look stupid. Sigh.

I'm aware that U235 emits gamma radiation, and the point of this post was to say that, from what I've read and from what my dad has told me, it would not take a significant amount of lead shielding to attenuate said radiation.

I linked to a site in the post that talks about attenuating gamma radiation, and indicates that less than a millimeter would be needed to reduce exposure by a factor of 50 to 1000. I don't know whether that level of attenuation would be sufficient to disguise the presence of U235, and Mr. Cramer indicates that it would not.

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