Enrichment waste: Depleted uranium

By Peter Diehl


For the use of uranium as fuel in light water reactors, the percentage of the fissile uranium isotope uranium-235 has to be raised from its value of 0.71% in natural uranium to a reactor grade of 3.2% (for Boiling Water Reactors - BWRs) or 3.6% (for Pressurized Water Reactors - PWRs). The enrichment technologies commercially available at present are the gaseous diffusion process and the centrifuge process. Both of them require the prior conversion of the uranium to the gaseous form of uranium hexafluoride (UF6). The product stream of enriched UF6 obtained is then converted to the form of UO2 for further processing to nuclear fuel assemblies.

The enrichment process not only produces the enriched product, but aiso a waste stream of uranium hexafluoride depleted in uranium-235 ("depleted uranium"), typically to 0.3%. The degree of depletion of uranium-235 (the "tails assay") in this depleted uranium waste is a parameter that can be adjusted to economical needs, depending on the cost of fresh natural uranium and on the enrichment cost (expressed in $ per Separative Work Unit SWU).

In the example shown, the depleted uranium waste stream is seven times larger than the enriched uranium product stream.

Cylinder storage of depleted uranium hexafluoride
Most of the depleted uranium produced to date is being stored as UF6 in steel cylinders in the open air in so-called cylinder yards located adjacent to the enrichment plants. The cylinders contain up to 12.7 tonnes of UF6. In the US alone, 560,000 metric tonnes of depleted UF6 have accumulated until 1993; they are currently stored in 46,422 cylinders. Meanwhile, their number has grown by another 8,000 new cylinders.

At ambient temperature, UF6 is a crystalline solid, but at a temperature of 56.4°C, it sublimates Ibecomes a gas). Chemically, UF6 is very reactive: with water (atmospheric humidity!) it forms the extremely corrosive hydrofluoric acid and the highly toxic uranyl fluoride (UO2F2). The hydrofluoric acid causes skin burns, and, after inhalation, damages the lungs. Further health hazards result from the chemical toxicity of the uranium to the kidneys, and from the radiation of the uranium (an alpha emitter).

In the storage yards, the cylinders are subject to corrosion. The integrity of the cylinders must therefore be monitored and the painting must be refreshed from time to time. This maintenance work requires moving of the cylinders, causing further hazards from breaching of corroded cylinders, and from handling errors.

As a worst-case scenario, the crash of an airplane into a cylinder yard must be assumed. If cylinders are involved in longlasting fires, large amounts of UF6 can be released within a short time. If the whole contents of a cylinder is released during a fire, lethal air concentrations of toxic substances can occur within distances of 500 to 1,000 meters.

Civilian uses of depleted uranium
Historically, uranium has been used as a colouring matter in pottery. More recent civilian uses include the use of uranium as a steel-alloying constituent, and the use of several uranium compounds in chemical processes, for example as a catalyst. For its high density of 18.9 g/cm3 (67% higher than that of lead and slightly lower than that of tungsten), uranium can be used in dense metal applications such as counterweights or flywheels. For example, the first 550 Boeing 747 aircrafts built utilized depleted uranium weights for mass balance of outboard elevator and upper rudder assemblies. But this use of depleted uranium in the form of uranium metal also included drawbacks: over 20% of these weights were corroded at each major aircraft overhaul and had to be reprocessed, although nickel and cadmium plated. In more recent aircraft designs, however, the use of counterweights has been minimized due to advanced design technology.

During the production process of uranium metal applications, the pyrophoric behaviour of small uranium metal particles constitutes a problem. These particles, such as finely divided metallic saw turnings and chips, sawdust, and abrasive saw sludge are capable of spontaneous ignition, and have caused many incidents. Inhalation of dust from fires involving uranium metal can cause high radiation doses.

Another possible use of depleted uranium based on its high density is the use in radiation shields: though an alpha-radioactive material itself, it is suitable for shielding penetrating gamma-radiation better than lead.
For all of the uses mentioned, it doesn't matter other than for use as nuclear fuel, that the uranium is depleted in uranium-235.

To date, none of the civilian uses of depleted uranium has brought an appreciable decrease of the existing stockpiles of this material. In the US, however, the Department of Energy (DOE), urged by the increasing maintenance problems of its cylinder yards, is now performing the first steps towards a large-scale civilian use of depleted uranium. The DOE's preferred alternative for the management of its 560,000-metric-tonne stockpile is to use the entire inventory of material in the form of metal or oxide, mainly for radiation shielding in casks for spent fuel and high-level waste, but also for other industrial uses to be developed. The depleted uranium, now contained at a few sites, would then be dispersed over a wide range of products. The DOE now plans to build two plants to convert the UF6 to more stable forms that could be manufactured to marketable products or used for disposal, at costs of nearly $200 million each.

Long-term storage or disposal
The portion of the depleted uranium for which no use can be identified must be disposed of, or must be safely stored in the long term for possible future uses. According to the nuclear industry, changes in the market or new enrichment technologies might allow for an economical recovery of the residual uranium-235 still contained in the depleted uranium in the future.
For long-term storage or disposal, the depleted UF6 must be converted to a less reactive chemical form: candidates are UF4, U3O8, and UO2. UF4 has the advantage of being easily reconvertible to UF6, while U3O8 is the most stable form, also existing as a natural mineral.

The depleted uranium long-term storage project at Bessines (France)

France's nuclear fuel company Cogema is going to store 199,900 metric tonnes of depleted uranium at the site of the former uranium mill of Bessines-sur-Gartempe (Haute Vienne) near Limoges. The project was licensed on December 20,1995.

This license was revoked by the Administrative Tribunal of Limoges on July 9, 1998, mainly for the reason that the depleted uranium had to be regarded as a waste under current conditions, though an extraction of the residual uranium-235 might be viable in the future.

On Nov. 5, 1998 however, a Bordeaux appeals court ruled that the material is no waste, but a "directly usable raw material that is effectively used for multiple uses". Following the court decision, Cogema sent the first depleted uranium shipment to Bessines on Nov. 12,1998.

Originally, Cogema had applied for the storage of 265,000 tonnes, but during the hearings held on the project, it became obvious that Cogema had "forgotten" to consider some radionuclides (artificial uranium-236, among others) in its calculation of the total activity inventory: the specific activity of the depleted uranium is 21,100 Bq/g instead of 15,902 Bq/g. The project would therefore have exceeded the 100,000 Curie (3.7 10'5 Bq) limit, requiring a different type of license (Installation Nucleaire de Base) involving wider public participation. Cogema was not able to provide a reasonable explanation for the presence of the uranium-236.

The depleted uranium is a residue of the Eurodif Tricastin gaseous diffusion enrichment plant in the Rhone valley. Its residual contents of uranium-235 is 0.2 to 0.3% and it has the chemical form of uranium hexafluoride (UF6). Cogema doesn't declare it a waste, but wants to store it for possible future use. Cogema hopes that the stored depleted uranium can be useful, if future enrichment techniques would allow for economic extraction of the residual uranium-235, or if uranium prices would rise significantly.

For storage, the UF6 is converted to the chemically more stable form of U308 at Cogema's Pierrelatte facility. Then it is transported by rail to the Bessines site and stored as a powder in iron containers. The containers (8.5 or 11 tonnes each) are to be stored in special storage buildings. Each building can store 2,500 containers. The maximum dose that an individual would be exposed to at the fence of the facility is calculated at 0.7 mSv (70 mrem) per year, far below the (extremely high) French limit of 5 mSv (500 mrem) for the public. The total investment is planned at 60 million French Francs (approximately US$ 10 million) over a period of 15 years.

Surprisingly, the recovery of the residual uranium-235 contained in the depleted uranium no longer is a matter of the future: it has been practised for several years now.

Depleted uranium from European uranium enricher Urenco (with plants operating in the United Kingdom, The Netherlands, and Germany) and others is now being enriched in Russia. The centrifuge enrichment plant of Minatom's Ural Electrochemical Integrated Plant (UEChK, formerly Sverdlovsk-44) at Novouralsk near Ekaterinburg is enriching tails for Urenco. Minatom, while further depleting ("stripping") Urenco's depleted uranium, produces uranium of natural contents (0.71 %) in uranium-235. It thus re-enriches or upgrades the tails to natural uranium-235 grade. This product is then delivered back to Urenco for further enrichment to reactor grade. In 1996 alone, more than 6,000 metric tonnes of tails were upgraded. [Nuclear Fuel, October 6, 1997]

In the case of Figure 2 (underneath):

• 9.5% of Urenco's initial natural uranium feed is recovered, thus lowering the need to mine fresh uranium,
• the recovery rate of natural uranium is 1.57 kg U per Separative Work Unit (SWU) spent at Minatom, and
• the amount of the depleted uranium tails decreases by 11 %, not exactly an impressing figure.

The economics of tails re-enrichment
Assuming 1997 world market prices for uranium and enrichment services, the break-even point for tails upgrading according to the assumptions made above would be reached at a recovery rate of "natural" uranium of 2.63 kg U/SWU at Minatom. The obtained recovery rate of 1.57 kg U/SWU only reaches 60% of this value. So additional factors must be taken into consideration to understand the economics of re-enrichment.

1) Minatom possibly does not charge the full enrichment cost
Minatom has an estimated 9 million SWU/year of enrichment capacity in excess of Russia's needs [Nuclear Fuel Oct. 19, 1998]. It is therefore possible that Minatom does not charge the full enrichment cost, but its operating cost only. The US DOE's Engineering Analysis Report for the Long-Term Management of Depleted Uranium Hexafluoride of May 1997 estimates operating costs of $20-$30/SWU for centrifuge enrichment plants (there are no such plants in the US though). With US$30/SWU, for example, the break-even point would be reached at 0.88 kg U/SWU. The obtained recovery rate is 78% higher for the case shown above. The highest absolute cost savings would be obtained at a tails assay of 0.21% at Minatom, in this case.

2) Minatom possiby strips the tails further than contracted
In case of Minatom not quoting the full enrichment cost, also another consideration can be made: According to George White, a consultant with Uranium Exchange Co., it is likely the Russians have contracted with Urenco to strip tails from 0.3% to 0.25% U-235. But the Russians are then probably stripping the tails further to 0.12% U-235 to produce uranium for their own account, White has suggested [Nuclear Fuel, Oct. 1 9, 1 998l.

If Russia used all of its excess 9 million SWU/ear to strip Urenco's tails in the described way from 0.3% to 0.12% U-235, then 7,290 tonnes/year of uranium of natural isotope composition would be recovered, 4,680 tonnes of which would be on Russia's own account.

In this case,

• 26.7% of Urenco's initial natural uranium feed would be recovered,

• the recovery of "natural" uranium were 0.81 kg U per SWU spent at Minatom, only slightly below the break-even point, assumed above for enrichment costs of 30 US$/SWU, and

• the mass of the depleted uranium tails would decrease by 30.5%.

This procedure also would be an explanation why Russia's uranium stockpile doesn't expire...

3) Urenco's avoided disposal cost
The new tails produced during the upgrading process remain in Russia, according to the answer of the German government to a parliamentary question in 1997. This, together with the fact that the upgrading process results only in a minor reduction of the amount of tails, gives reason to have a look at Urenco's avoided disposal cost.

Assuming market conditions, the tails upgrading does not make an economic sense, if the recovery of the uranium were its only purpose: the recovered uranium would be 68% more expensive than fresh uranium.

The re-enrichment does, however, make sense, if the avoided disposal cost for the tails are taken into consideration. For the German branch of Urenco, for example, disposal in the proposed Gorleben HLW deposit must be assumed, since the German LLW deposits don't allow for storage of such amounts of uranium. The excess upgrading cost over the market value of the uranium recovered would be about 10 % only of the storage cost at Gorleben. *1

Urenco's main purpose of the deal, therefore, seems to be to "solve" its waste management problem by transferring the depleted uranium to Russia.

The German Federal Government, however, stresses the results of an investigation it has conducted together with the governments of the United Kingdom and The Netherlands. The study has approved that the re-enrichment in Russia is not connected to a management of residues violating international rules, standards, or obligations.

Re-enrichment would also be an option for the management of the depleted uranium stockpile of the US DOE - in particular, since roughly 30% of the DOE inventory has a rather high tails assay in the 0.3 - 0.4% range. But, since there exist no low-cost enrichment plants such as centrifuge plants in the US, this option is not seen viable at present.

*1- These figures are based on 1997 market prices for uranium (11 US$/lb U3O~ and 34.2 US$/kg U as UF6), and enrichment services (90 US$/SWU), a product assay of 3.6% (PWR grade) and a tails assay of 0.3% at Urenco, and an assumed tails assay of 0.25% at Minatom. The storage cost for a 200-liter barrel at the proposed Gorleben HLW deposit is estimated at 15,000 DM; the volume needed for disposal of the tails as UO2 after cementation in barrels is estimated at 550 litre/t UO2.

Contact: Peter Diehl, Am Schwedenteich 4, 01477Arnsdorf, Germany.
Tel/Fax: + 49-35200-20737 E-mail: p.diehl@sik.de



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