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Commentary on Uranium Forensics 

By Lucas Royland

In May 2004 the United States Government reported, based on the analysis of the isotope Uranium 234 (U234) that Libya had obtained uranium from North Korea.

While the ratio of U235 and U238 is the same for virtually all natural uranium, the abundance of U234 varies among uranium mines, allowing the origin of the uranium to be determined, in principle. The following analysis shows, however, that the concentration of U234 can be varied enough to obscure the origin of the uranium. Thus, the Libyan uranium might be from North Korea or from some source that doctored the uranium to cover its provenance. We considered three different approaches, each with contingent degrees of feasibility.A more technical version can be found here.

  Simple Mixing of Samples from Different Mines

The concentration of U234 in a natural deposit is dependent on many factors. The age of the rock, amount of ground water, water permeability of rocks, and assorted geological factors all play a part in altering this concentration. Since the processes dependent on these factors and the half life of U234 (2.445×105 years) affect concentration over geologic time periods, unique and relatively unchanging amounts tend to exist in specific deposits. It should also be noted that even deposits in the same mine can vary in the U234 amount [1].

There are a number of uranium mines around the world. Uranium taken from all mines has a world-wide average U234 concentration of about 53.8 parts per million (ppm) and varies over a range from 48.4 to 62.1 ppm among different mines [1].

Given two samples of natural Uranium, one with a concentration of U234 equal to cA and one with a concentration of U234 equal to cB, it is possible to combine amounts of these two samples to yield a resultant sample with any concentration in between cA and cB. This means that a composite sample can be produced, from natural sources, inside the range 48.4 to 62.1 ppm if adequate amounts of ore from different source mines are involved.


Mixing from Isotope Separation Procedures

We must also consider the possibility of combining samples that have undergone some form of isotope separation. The general purpose of isotope separation is to enrich the U235 (about .7% in natural uranium) relative to the U238 (99.3% in natural uranium). Isotope separation of natural uranium generally neglects the other uranium isotopes.

The process starts with an amount of Uranium Hexaflouride (HF6 ) being fed into the largest set of centrifuges in the cascade. Each of these centrifuges produces a product material with a higher concentration of U235 and a waste material with a lower concentration of U235. The product material is sent to another set of centrifuges to be further enriched and the waste is sent to another set to be “stripped.” After an amount of Uranium has passed through such a cascade, there will be a final waste amount (also called “Tails”) and a final product amount. The typical U235 concentration of the waste is .2-.4%. The final product can have a U235 concentration of .7%-20% (Low Enriched Uranium or LEU ), 20%+ (Highly Enriched Uranium or HEU ), or 90%+ (weapons grade uranium).

In addition to changing the concentrations of U235 and U238, these processes also alter the concentration of U234 in the products and wastes (and the relative concentrations U234/U235). Below are the final concentrations of U234 for various cascades in products and wastes. We assume that the natural Uranium will have a U235 concentration of .711%, a uniform U235 waste concentration of .2%, an initial U234 concentration of 60 ppm, and specified product concentrations of U235 from 1%-90%.


Table 1. Product concentrations of U235 (C235) and U234 (C234). [2]
C235 C234(ppm)
.01 88.67
.02 190.01
.03 292.46
.04 395.27
.05 498.27
.2 2047.15
.9 9281.56

Table 2. Waste (Tails) concentrations of U235 (C235) and U234 (C234) [2].
C235 C234(ppm)
.002 9.31
.002 8.46
.002 8.11
.002 7.91
.002 7.78
.002 7.35
.002 7.23

If one were to specifically take waste concentrations and product concentrations from different processing schemes (say a the product from a 5% enrichment and the waste from a 1% enrichment) and mix them in amounts such that the U235 concentration was equal to .711% and U238 concentration equal to 99.289%, the resulting sample would have a different U234 concentration than the initial feed samples but still equal the U235 and U238 levels of natural uranium.

In addition to being excessively work-intensive, this method can only shift the concentration of U234 by small amounts (less than 10 ppm). However, it could provide the potential to decrease the U234 concentration; an effect unobtainable by other methods but still not significant enough to warrant the process.


U234 from Plutonium 238

Another possible source of U234 is from the alpha decay of Plutonium 238. Pu238 is generally formed in nuclear reactors as a product of Neptunium 237 irradiation. The half-life of Pu238 is 87.7 years, so older supplies of plutonium will have significant concentrations of U234. Pu238 is not used in nuclear weapons. It is, moreover, the primary isotope present in fuel for NASA’s Radioisotope Thermoelectric Generators (RTG) [3] and there are considerable quantities of it in the world .[4]. In table 2 we start with an initial kilogram of Pu238 and show what amount will decay to become U238 after a certain number of years.


Table 3. U234 and Pu238 amounts, over time, of initial 1kg Pu238.
Years of Decay U234(kg) Pu238(kg)
0 0.00 1.00
1 0.01 0.99
5 0.04 0.96
10 0.08 0.92
15 0.11 0.89
20 0.15 0.85
30 0.21 0.79
50 0.33 0.67
87.7 0.50 0.50

If U234, separated from Pu238 in an ion exchange column, is added to natural uranium, the U234 content would be increased. Thus, if we start with natural uranium that has a high concentration of U234 this approach does not allow us to lower it. If, however, we start with natural uranium with a low concentration of U234, this approach allows us to raise it to any value within the natural range.

The primary concern in producing U234 by way of Pu238 is that there are not pure samples of Pu238. Most plutonium samples contain a number of other plutonium isotopes each of which can decay to an isotope of uranium. A sample containing too much of these decay products would not resemble natural uranium. For samples with a very high concentration of Pu238, however, this method could be viable.

Further Remarks

(1) U234 concentrations in the range of 48.4 to 62.1 ppm can be created by the combination of natural uranium from different mines.

(2) Enriched/depleted uranium can be combined so that the resultant sample equals the U235 and U238 concentrations of natural uranium but has a shifted U234 concentration. The effect is, however, small and probably not practical.

(3) Large existing stocks of Pu238 will produce significant amounts of U234 and could be added to natural Uranium to produce a higher U234 concentration. But, the presence of uranium daughter products from other plutonium isotopes could keep samples from resembling natural uranium.

Given the potential for these methods to alter U234 concentration combined with other types of enrichment, utilizing this concentration to identify the source of a given quantity of uranium is questionable. It follows that assertions made based on this concentration are potentially invalid.


References:
[1] G.S. Solov’ev, A.V. Saprygin, I.S. Izrailevich, and A.A. Makarov. Determination of 234U Content In Uranium From Different Deposits. Atomic Energy, Vol. 92, No.4, 2002.

[2] G.S. Solov’ev, A.V. Saprygin, V.V. Komarov, and A.I. Izrailevich. 234U Content in Enriched Uranium As A Function Of The 234U Concentration In The Initial Material. Atomic Energy, Vol. 95, No. 1, 2003.

[3] Final Environmental Impact Statement for the New Horizons Mission. National Aeronautics and Space Administration, Vol. 1, Executive Summary (chpts. 1-8), July 2005.

[4] Plutonium: The First 50 Years. Washington, D.C.: DOE, Feb. 1996.

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