Depleted uranium stockpiles (tails)

Nuclear power is mainly produced in reactors fuelled with enriched uranium. In the enrichment process, for each kilogram of enriched uranium produced, an average of 8 kg of depleted uranium (range 5 to 10 kg) is produced. Consequently, more than three quarters of the total uranium devoted to fuelling reactors is now in the form of depleted uranium (or tails), and the accumulated stockpiles of tails represent a signifi-cant quantity of uranium. Whether the depleted uranium stockpiles represent a valuable energy source or a waste to be disposed of has been debated for three decades.

The answer to this question has evolved over time, and will most likely continue to change. In the 1970s and 1980s the answer was clearly that depleted uranium is potentially a valuable energy source for the future. At that time uranium prices were high, development of

fast breeder reactors was considered by many to be unavoidable within one or two decades and transforming fertile ²³⁸U into fissile Pu was considered as the appro-priate answer to the lack of uranium. Today the answer is more controversial and less certain. Low uranium prices and the economic burden of tails management have altered the equation so that depleted uranium is now more often considered to have no current use at present Western enrichment costs.

However, re-enrichment of tails to recover more fissile uranium is still being conducted, and this activity is likely to continue as long as low cost enrichment capacity is available and there remains a supply of tails with a sufficient ²³⁵U residual content. In addition, the Russian Federation is reportedly using tails to downgrade weapons grade HEU into commercial grade material.

Furthermore, when addressing supply issues 50 years into the future, potentially lower cost enrichment tech-nologies and new reactors such as fast breeder reactors could once again elevate depleted uranium tails from a waste to a potentially valuable energy source. This change in philosophy could be accelerated by a signifi-cant uranium price increase over time, which can be expected if nuclear power remains a significant option in the energy mix. Since depleted uranium storage does not represent a significant hazard when de-converted to a stable form such as U₃O₈, storage costs are likely to remain low, thus ensuring their availability for future needs. Appendix II provides an example of tails re-enrichment economics that helps put the remainder of this discussion in an economic context.

FIG. 11. Projection of uranium equivalent displaced by the MOX contribution to 2050.

0

2000 2005 2010 2015 2020 2025 2030 2035 2040 2045 2050

Year

t U

Stop MOX MOX

3.2.4.2. Current uses of depleted uranium

Depleted uranium is suitable for fuelling reactors, assuming re-enrichment or mixing with other fissile material (e.g. Pu for MOX, HEU for dilution). Other non-fuel uses involve only small amounts of depleted uranium, mainly for radiological shielding. Uses of depleted uranium for fuelling reactors include the following.

— Re-enrichment. From a purely economic point of view, depleted uranium can be reused as feed for a further enrichment step if the ratio between the enrichment unit cost and natural uranium prices allows such a recovery. To some extent this is currently the case for limited quantities.

— MOX matrix. The quantities involved are small but still constitute about 94% of MOX heavy metal content.

— HEU dilution. The quantities of depleted uranium tails presently being used for dilution of HEU are reported to be significant as a result of the Russian HEU deal. They are already counted in the HEU impact figures, and should be deducted from the tails stockpile totals.

— Core blankets. Pellets made with depleted uranium are quite often used peripheral to the reactor core as neutron shielding. This is a potentially important use assuming the development of fast breeder programmes, but its current use is very limited in LWRs and CANDUs (5 to 10 t/a).

3.2.4.3. Existing stockpiles of depleted uranium The total quantities of depleted uranium tails have been estimated at approximately 1.1 million tonnes at year end 1995 [14]. Assuming an average ²³⁵U content of 0.3%

and a re-enrichment tails assay of 0.15%, this total could provide 294 000 t of natural uranium equivalent (or more than one and one half times the resources of the McArthur River deposit). However, the true content is probably lower, because the Russians have been operating at low tails assays for a considerable period of time, and even the Western gaseous diffusion plants (GDPs) were operating at lower tails assays before the 1980s.

The estimate of the allocation among the various forms of tails is: 365 kt of uranium in depleted uranium hexa-fluoride (DUF₆) at an average of 0.32% ²³⁵U (Table X);

730 kt of U in DUF₆at an average of 0.25% ²³⁵U; 131 kt of uranium in other forms at an average of 0.25% ²³⁵U.

FIG. 12. Projection of uranium equivalent displaced by the RepU contribution to 2050 — current burnup and high burnup scenarios.

0 500 1000 1500 2000 2500 3000 3500 4000 4500

2000 2005 2010 2015 2020 2025 2030 2035 2040 2045 2050 Year

t U

High burnup Current burnup Current burnup scenario

High burnup scenario

Table XI shows that the readily available natural uranium equivalent content of tails stockpiles worldwide is limited compared to some published reports. In addi-tion, the effect of the yet to be announced decision regarding the fate of the USDOE and USEC tails could have a significant negative impact on the future avail-ability of the more than 40% of tails having a significant

235U residual content.

3.2.4.4. Scenario for depleted uranium use to 2050 A large part of the ongoing use of depleted uranium is already accounted for within other supply components, including HEU down blending and use as a MOX matrix.

Therefore the related quantities of depleted uranium must be deducted from the stockpiles potentially avail-able in the future. Because of these uses and the low residual tails assays currently in use in the Russian Federation, the ²³⁵U content of the world’s depleted uranium stockpiles has been significantly reduced. For the purposes of this discussion, it is assumed that only tails at or above 0.3% ²³⁵U could be commercially attrac-tive for re-enrichment. It is also assumed that MOX matrix and other uses will come from depleted uranium with a content of less than 0.3% ²³⁵U.

Strictly commercial re-enrichment of depleted uranium depends mainly upon the availability of very low cost (marginal) separative work unit (SWU) capac-ity. US and western European GDPs have relatively high marginal SWU costs and are unlikely candidates for tails re-enrichment. The capacities at western European centrifuge plants are currently committed to normal enrichment contracts. Future expansion of these plants could free up some capacity for tails re-enrichment, but by the time these expansions are completed the supply of economically attractive tails (>0.3%) is likely to be largely depleted. Therefore, until the market price of uranium reaches US $65/kg U, significant expansion of tails re-enrichment in the West seems very unlikely.

In the near term, re-enrichment of tails will probably be limited to Russian centrifuge plants, which reportedly have available marginal capacity and thus can offer fuel contracts on a marginal cost basis. In order to supply Soviet design reactors, the Russian Federation has to supply fuels with a content of 8300 t U equivalent assum-ing 0.3% tails assay and 4400 kSWU. These totals are expected to increase to 9900 t U equivalent and 5100 kSWU by 2010. [Note: the Russian Federation reportedly runs their enrichment plants at 0.15% tails. However, the effect of lower tails is not included in the tails usage scenario, in order to achieve a more global perspective.]

In addition to satisfying its traditional markets, down blending of HEU requires about 3500 kSWU (assuming feed tails of 0.3% and residual tails assay of 0.15%). The Russian Federation also currently exports 3600 kSWU/a, which could increase to 4000 kSWU by 2010. Table XII projects the allocation of Russian SWU and, assuming a stable enrichment capacity of 20 000 kSWU/a, the remaining capacity available for tails re-enrichment.

Based on the above assumptions, the available capacity for tails re-enrichment is projected to total 6500 kSWU/a in 2000, diminishing to 5100 kSWU/a by 2010. The base case for tails re-enrichment, which was used as a component of secondary supply in the demand cases, is constrained by: (1) the availability of low cost SWUs; and (2) safeguards related limitations on transferring large quantities of depleted uranium in the form of UF₆to Russian enrichment plants and leaving the secondary tails in the Russian Federation.

Therefore, as shown in Table XIII, tails re-enrichment is scheduled to end in 2011 after having contributed 43 400 t U equivalent.

The base case scenario will by no means utilize all depleted uranium tails with a content of 0.3% or greater.

The 365 000 t U as UF₆ listed in Table X represent 110 000 t U equivalent, although the near term availability of the USDOE–USEC tails is uncertain. In addition, if uranium prices remain at less than US $52/kg U, assum-TABLE X. DEPLETED URANIUM STOCKPILES AT YEAR END 1998

Enricher Total depleted U (t U) U/DUF₆(t U) Estimation of U/UF₆supply 0.3% (t U)

USDOE–USEC 47 000 47 600 120 000

Eurodif 168 000 37 000 25 000

Urenco 29 000 29 000 25 000

British Nuclear Fuels Limited 30 000 30 000 25 000

Russian Minatom 495 000 495 000 150 000

China 20 000 20 000 15 000

Other 8 000 8 000 5 000

Total 1 226 000 1 095 000 365 000

ing current SWU prices, new tails totalling 17 000 t U/a in the form of UF₆will be produced through enrichment of newly mined and processed uranium. To re-enrich these tails totally would yield 4545 t U equivalent/year, which would significantly extend the lifetime for tails as a secondary supply source. It would, however, also require enrichment capacity comparable to the Russian Federation’s capacity currently available for the re-enrich-ment of tails. The potential of these tails assumes that tails storage technology will allow their retrievability in the future. It also assumes significantly higher uranium prices, lower SWU costs or expansion of low marginal cost SWU capacity to realize full utilization of the re-enrichment of tails.