Using Nuclear Materials To Prevent Nuclear Proliferation.
G.P.Gilfoyle1,2,3* and J.A.Parmentola1.
1
Defense Threat Reduction Agency, Dulles, VA2
American Association for the Advancement of Science, Washington, D.C.
Abstract
Efforts to halt proliferation of nuclear weapons are threatened by vulnerability of weapons-usable material to smuggling especially in Russia. Mixing 232U into highly-enriched uranium (HEU) makes it readily observable and harder to steal. Adding a proportion of 233U associated with a specific storage site enables attribution to be performed on stolen HEU that has been recovered. Incorporating 244Pu into plutonium does the same for this material. US programs for radioactive surplus disposition could provide a source for tags. Current US-Russian efforts to dispose of surplus nuclear weapons open opportunities to incorporate tags into large amounts of weapons-usable material.
While the end of the Cold War greatly reduced the threat of an all-out nuclear war between the United States and the Russian Federation, it created a new set of challenging national security concerns. The political and economic volatility in Russia has resulted in growing fears about the lack of fissile material security there and raised the specter of this material falling into the hands of `rogue' nations, terrorists, and other opportunists (1-3). While a nuclear blast would have dire consequences even acquisition of weapons-grade, nuclear material by such groups would damage US interests. In this report we describe a way of significantly reducing these threats by making it easier to detect smuggled fissile material and creating a nuclear fingerprint to aid law enforcement in identifying the source of a `leak'. We also describe a unique confluence of opportunities that make this nuclear tagging feasible at this time.
We propose uniformly mixing specific substances into fissile materials to act as intrinsic tags. The tags work in two ways. First, they brighten the radioactive signature of highly-enriched uranium (HEU) making it easier to detect. HEU emits far less radiation than plutonium and is more difficult to detect. The HEU tag emits a high-energy, penetrating gamma ray that would set off passive, non-intrusive monitors located at nuclear facilities, border stations, and other choke points; making theft of the fissile material much more difficult. There is no need to intensify the plutonium signature because it is already bright. Second, the tags provide the opportunity to perform attribution on stolen plutonium or HEU. A storage-site-specific amount of the tag is added to the fissile material during chemical processing which associates the tagged material with the site; different amounts label different sites. If material is stolen and later recovered, analysis of it with readily available techniques would identify the source (the nuclear fingerprint) and enable law enforcement to cut off the leak at its source
We propose adding a small amount (about 1 ppb) of 232U, an isotope of uranium, to highly-enriched uranium (HEU) to make the material easier to detect. A sample of 232U produces a high-energy (2.6 MeV), penetrating, gamma ray that is actually emitted by one of the daughter nuclei produced in the series of radioactive decays that occur before a stable, final nucleus is reached. This gamma-ray signature can be detected by radiation monitors at Russian nuclear sites and exit points (border crossings, airports, etc.).
These tags are effective only if they cannot be easily defeated even by a technically skilled person with access to the weapons-usable material. This `insider' is considered the most likely candidate to steal weapons-usable material from a storage site (4). Here we consider several possible methods a would-be smuggler might try. In each case, the presence of the tags makes theft considerably more difficult. A smuggler could shield the HEU and block the gamma rays emitted by the detection tag to reach any radiation detectors at a monitoring site. This requires a lead shield with a thickness of about two inches. For the typical size of a piece of HEU (about 8 kg), a box to shield the detection tag would weigh about ninety pounds, would be cumbersome to move, and could show up quite easily on an x-ray monitor like the ones already in use at Russian border crossings.
A technically skilled smuggler could try to chemically remove the 232U detection tag from the HEU. We have intentionally chosen the tagging materials to be the same element as the tagged material (HEU or plutonium). Different isotopes of the same element are nearly identical under most chemical processes. Hence, it requires costly and expensive equipment (i.e. gas centrifuges) to separate isotopes of the same element. These techniques are beyond the capabilities of most smugglers. Note here that this feature is a general property of all the tags we propose. None of them can be easily removed with methods available to smugglers.
A smuggler could try to remove the daughter nucleus of 232U that emits the high-energy gamma ray. This gamma ray is actually emitted by the decay of one of the members of the 232U decay chain, 208Tl, not by the 232U itself. The 208Tl nucleus is not the same element as uranium so it is possible for a technically-skilled smuggler to chemically remove the 208Tl and defeat the tag. However, the smuggler must move quickly after removing the 208Tl because the continuing decay of the 232U (which is still in the material) will produce more of the gamma-ray-emitting 208Tl. We have chosen the amount of added 232U so the detection tag is observable again in about a month.
Finally, consider an attempt to mask the 233U attribution tag for HEU or the 244Pu attribution tag for plutonium by adding more of the tagging material. Each attribution tag is a unique material and only small quantities exist (neither element is found in nature). This means it is difficult to 'cover up' the tag because a smuggler simply can't get the 233U or the 244Pu to add to the stolen nuclear material. As a bonus, the uniqueness of the attribution tag material means there will be little natural background contamination to confuse investigators.
None of the tags significantly add to the radiation hazard of HEU or plutonium or disrupt possible future fuel use. The 232U detection tag for HEU is added in small amounts; half the maximum permitted to meet industry specifications. Similarly, the amount of the 233U attribution tag is kept small to avoid radiation hazard from alpha emission. The 244Pu attribution tag is stable and does not emit much radiation. All of the tags are added in very small quantities so they won't damage the quality of the materials if they are later incorporated into reactor fuel (a potential future use of the material).
A US-Russian program to enhance weapons-usable material security with such tags could take advantage of a timely confluence of opportunities which could enable the tagging of a significant amount of material. There are three essential steps in building a program: (1) creating the tags, (2) incorporating them into the weapons-usable material, and (3) building the network of monitors to detect smuggled, weapons-usable materials. In each case we have found existing or planned programs in the US and Russia that could be easily extended to include tagging at a small marginal cost.
The first step is to create the tags. Adequate amounts of the tagging materials (232U, 233U, and 244Pu) are already in US stockpiles. Batches of uranium isotopes at United States' Oak Ridge National Laboratory contain mixtures of 232U and 233U that could be used to tag HEU (5). An adequate supply of 244Pu is at United States' Savannah River Site. The amounts of the material in the US stockpiles and the requirements for tagging are shown in Table 1. The first column lists the tagging isotopes and the second column lists the US supply. The right-most column contains the amount of material needed to tag the Russian surplus that is the product of the tag proportion (column 3) and the amount of Russian weapons-usable material surplus (column 4). The cost of making the tags is a fraction of the funds necessary for final disposition or storage of the surplus.
The second step in developing a tagging program for weapons-usable material is to incorporate the tags into the HEU and plutonium. Existing US-Russian programs make this step feasible for a portion of the surplus Russian material (6). The US is committed to spending $20 billion by the year 2013 to buy 500 metric tons of Russian HEU as part of the HEU Purchase Agreement (7). This represents about 40% of the Russian stockpile (2). HEU is first transformed into an oxide and then processed further to make low-enriched uranium for reactor fuel (7). Ideally, the uranium oxide would be quickly processed into reactor fuel, but past experience reveals the HEU in oxide form is sometimes stored for a period from a few weeks to two years. To tag the HEU during the oxide-processing step one would uniformly mix a small (about 3 millimeter across) piece of material containing the detection and attribution tags. Since the tags are the same chemical element as the HEU this step adds little to the cost and complexity of the HEU processing. In another case, the US and Russia have a cooperative program costing about $1.3 billion for the dismantling of Russian nuclear weapons and storing them in the Fissile Material Storage Facility (FMSF) at Mayak in Siberia in unclassified shapes (6). The FMSF is expected to be certified for use by August 2002. The method for dismantling the weapons is under negotiation with two techniques under consideration: melting and recasting the plutonium or HEU into an unclassified shape or turning the weapons-usable material from the weapons into an oxide. In either case one would add a step during processing to uniformly mix the tags into the uranium or plutonium. This additional step would add little to the complexity and cost of the proposed processes.
This tagging proposal would also enhance other US-Russian nuclear security activities. These are programs totaling over $265 million for FY2000 with the explicit aim of enhancing physical security of weapons-usable material in Russia (e.g., DOE's Second Line of Defense program) (6). As part of these efforts radiation detectors are being installed at Russian storage sites, border crossing, and exit points (i.e., airports). These programs have had some success at detecting smuggled radioactive materials, but still face the difficult problem of detecting HEU because of its dim radioactive signature (8). The tagging method we've described here attacks the problem from a different direction and reduces the problem by making the tagged HEU visible to existing monitoring systems. It adds a capability that would be very costly otherwise.
It is worth considering other applications of the tags. Highly-enriched uranium that is not part of the HEU Purchase Agreement should be tagged. The 500 tons of HEU that will be purchased by the US under the HEU Purchase Agreement is less that half of the material in the Russian stockpile. The future disposition of the balance of the material is undecided and some of that additional HEU is vulnerable to theft (2,3). This material could be tagged during accelerated processing into oxide form and placed in secure storage. As time, funding, and facilities permit, it can then be downblended to low-enriched uranium that is not a proliferation risk and eventually burned as fuel. Similarly, surplus plutonium that has not been formally declared as excess should also be tagged. The 34 metric tons formally declared as excess for storage in the FMSF is only about one-quarter of the Russian stockpile (9). The remaining material should be processed in the same way as the current, declared excess, tagged, and placed in long term storage in an expanded facility at Mayak.
Another extension of the tagging concept is to apply it to the civilian plutonium industry. Reprocessing spent reactor fuel to extract plutonium poses a threat to US and international security because even 'reactor-grade' plutonium can be used to make a nuclear weapon. Britain, France and Russia all spend billions of dollars each year to process tons of plutonium. The worldwide inventory of civilian plutonium is about 170 tons; rivaling military plutonium in size (2,10). Adding the 244Pu attribution tag to civilian plutonium as the spent fuel is processed achieves the same goal of enabling investigators to identify the source of a plutonium leak after stolen material is recovered. An important caveat with this idea is that tagging the 34 tons of surplus, Russian, military plutonium uses much of the US stockpile of the 244Pu attribution tag. To deal with the shortfall Russian sources could be used if they have a stockpile of 244Pu or by employing an existing high-neutron-flux reactor to produce the additional quantities of the 244Pu attribution tag in a timely manner.
References and Notes
1
. P.Doty, Nature, 402 583 (1999).
2
. National Research Council, Protecting Nuclear Weapons Materials in Russia (National Academy Press, Washington, DC 1999).3
. M.Bunn, The Next Wave: Urgently Needed New Steps To Control Warheads and Fissile Material (Carnegie Endowment for International Peace and the Harvard Project on Managing the Atom, Cambridge, MA 2000).4
. S.A.Erickson, W.D.Smith, and L.Cantuti, "Understanding the Nuclear Smuggler", Nucl. Mater. Manage., 28 Proc. Issue-CD-ROM (1999).5
. C.W.Forsberg and L.C.Lewis, "Uses for Uranium-233: What Should Be Kept for Future Needs?", Oak Ridge National Laboratory Report ORNL-6952, (1999).6
. O.Bukharin and K.Luongo, "US-Russian Warhead Dismantlement Transparency: The Status, Problems, and Proposals", Princeton University Center for Energy and Environmental Studies Report 314 (1999).7.
A.J.Bieniawski and Y.N.Busurin, "Transparency Measures Associated with the US/Russian Intergovernmental HEU-to LEU Agreement", Nucl. Mater. Manage., 28 Proc Issue-CD-ROM (1999).8. T.Weiner, "Uzbeks Are Said to Seize Radioactive Cargo", New York Times, April 5, 2000.
9. United States - Russian Federation Plutonium Disposition Agreement-Fact Sheet, The White House, 04 June 2000.
10
. F.N.von Hippel, Nature, 394, 415 (1998).
Table 1 - Supply and Demand for Nuclear Tags.
|
Nuclide |
Tag Supply (kg) |
Tag Proportion |
Amount to be Tagged (metric tons) |
Tag Demand (kg) |
|
232 U |
0.3a |
1 ppb |
500 (HEU) |
0.0005 |
|
233 U |
351a |
20-100 ppm |
500 (HEU) |
10-50 |
|
244 Pu |
0.02b |
200 ppb |
34 (Pu) |
0.007 |
a
- Oak Ridge National Laboratory
Acknowledgements
We wish to acknowledge many helpful conversations with Dr. Frank Graham at the Savannah River Site and Dr. Charles Forsberg at the Oak Ridge National Laboratory.
* To whom correspondence should be addressed. E-mail: ggilfoyl@richmond.edu