Re: DU suitcase shipment (reply) - "radiation signature"

["Jaro" <jaro-10kbq@sympatico.ca>]

Barbara Hamrick wrote on Friday, September 12, 2003 9:32 PM

To: jaro-10kbq@sympatico.ca; radsafe@list.vanderbilt.edu

Subject: Re: DU suitcase shipment (reply)



<snip>

The problem I haven't had a chance to work through is the claim that the

"radiation signature" would have been the same for a more heavily shielded

package of HEU.



OTOH, even if it would, it begs the question, what are we doing to remotely

detect shipments of smallpox, anthrax, ebola, sarin, etc.?  We could bring

international commerce to a halt and thoroughly inspect every single

shipment through every single port, but I expect the cure would be worse

than the disease in that case.

===========================



Regarding your first sentence, I got curious about the "radiation signature"

too, and decided to see if I could figure something out on my own, with the

help of various references, including past Radsafe postings (thanks to all

the un-named sources !).

Here's what I found:



The difference between DU and natural uranium is quite small -- 0.2% U235

vs. 0.72% U235 (the half-lives of the isotopes (U238 & U235) are within an

order of magnitude of each other (4.51GY & 0.71GY respectively)).



By contrast, for HEU the difference is fairly drastic, since the enrichment

process sharply increases the very rare U234 fraction, which has a T½ of

only 247,000 Y (only about 10 x that of plutonium-239).

That's what should make it much easier to detect HEU than DU.



Specifically, the proportions are as follows:



                Natural Uranium        Depleted Uranium

234U           0.0057 %                  0.0005 % (that's 11.4 x less !)

235U           0.7204 %                  0.2500 %

238U         99.2739 %                99.7495 %



The 0.0057% by mass of U-234 in natural uranium is one of the progeny in the

decay chain of U-238, and in equilibrium therefore accounts for about the

same radioactivity as the 99.3% of U-238.

In other words, the radioactivity of pure natural uranium (ie. no

non-uranium daughters) is divided 50/50 between U238 and U234, despite their

huge (17,400 x) content proportion difference.



In the isotope separation process, much of the U-234 is removed with the

U-235, becoming the HEU product, while the radioactivity of the depleted

uranium by-product is decreased.

Overall, depleted uranium is about half as radioactive as natural uranium.

Measured DU radiation dose is usually ~ 200 to 250 mrad/hr beta and 5 to 10

mrem/hr gamma on contact.



Your typical 0.2 wt% U-235 depleted uranium gives the following percent

constituent activity:



U-234 = 17.53%

U-235 = 1.05%

U-238 = 81.42%



Note the high relative activity of U234 -- despite its tiny proportion in

both DU & NU -- but also the fact that its no longer in equilibrium, thus no

longer 50/50 between U238 and U234.



In HEU, most of the ~ 4% that's not U235 is U234, i.e. about 8000 times the

proportion in DU !! (obtained simply by dividing 4% by 0.0005%, which equals

8000 ---  this is one point I need to get more accurate figures on: exactly

how much of that 4% is U234 and how much is U238 ? ....hopefully this isn't

classified info.)



All the U isotopes in freshly-made HEU are alpha emitters, thus easily

shielded.

But during storage, following enrichment, you get a gradual build-up of the

"daughter" isotopes (to eventual equilibrium levels), which include beta and

gamma emitters.



In the case of U235 you get Th-231 with T½ = 25.5h and Ac-227 with T½ =

21.6y (this chain also includes Pa-231, a close analogue to Pu-239, with T½

= 32,000 y).



In the case of U234 you get the same decay series as U238 - including the

Pb-214, Bi-214 & Pb-210 gamma emitters - only the approach to equilibrium is

a lot quicker (or equivalent to a far larger mass of DU or natural U), and

the eventual specific activity of each isotope in the entire cascade is

about 18,300 times higher per gram of that material, than in the U238 series

(because that's the difference in specific activity between pure U234 and

pure U238, to begin with ....it being equal to the ratio of the half-lives

of U238 and U234).



But since Pu-239 decays through long-lived U-235 (and the rest of that

series), the approach to equilibrium is slow, so one would expect the gamma

activity (for smuggling detection purposes) to be significantly higher for

slightly aged HEU (with its high U-234 content) than for bomb components

made of weapons-grade plutonium.



Anyway, it seems that the "radiation signature" (gammas) for HEU would be

hundreds, if not thousands of times stronger than for the same amount of DU,

for anything but HEU straight out of the enrichment plant. Or, conversely,

it would be equivalent to thousands of pounds of DU, not the 15 pounds

smuggled by ABC (lots more shielding required to bring the signature down to

15 lbs DU level).

Anyone care to take guess at how many years your typical stockpiled HEU

might have been aging ? ....say some HEU batch stolen by terrorists from a

poorly-guarded FSU warehouse ?



 Jaro

^^^^^^^^^^^^^^^^^^^^^



PS. its also interesting that, according to ANL's "Human Health Fact Sheet"

on Depleted Uranium (Oct. 2001), the following calculations are suggested,

without any *explicit* regard for U-234 content :



<begin quote>

Specific Activity of Enriched and Depleted Uranium

(For additional information, see the Uranium fact sheet.)

The specific activity (SA) of a uranium compound depends on its isotopic

composition.

The SA of natural uranium (containing 0.72% uranium-235) is 6.77×10-7 curies

per gram (Ci/g).

The SA for other mixtures of uranium-238, uranium-235, and uranium-234 can

be estimated using the approach developed by the U.S. Nuclear Regulatory

Commission (see 10 CFR 20):

For depleted uranium (uranium-235 containing less than 0.72%):

SA = 3.6 × 10^-7 Ci/g



For enriched uranium (uranium-235 containing more than 0.72%):

SA = {0.4 + 0.38(enrichment) + 0.0034(enrichment)^2}×10^-7 Ci/g,

where enrichment is the percent uranium-235.

Thus, the SA of depleted uranium is approximately half that of natural

uranium. (To express SA in standard international units, multiply the value

in Ci/g by 3.7 × 10^10 becquerels [Bq]/Ci.)

<end quote>



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