Impact of isotopic enrichment on uranium supply: Tails assays adjustment impact and re-enrichment of depleted uranium (tails)

3.7.1. Background

Nuclear power is mainly produced in reactors fuelled with enriched uranium, and the trend is to extend this further to heavy water reactors which are currently fuelled with natural uranium. In the enrichment process, the fissile isotope ²³⁵U is preferentially extracted from natural uranium to produce enriched uranium. Depending on the extent to which the isotopic separation is achieved, uranium requirements can vary by more than 20% under already experienced cost conditions. The first and major impact of enrichment is on the range of that variation.

To give orders of magnitude, for each kilogram of enriched uranium produced, an average of 8 kg of depleted uranium (range: 5–10 kg) is produced from 9 kg of natural uranium feed. Consequently, about 80% of the total amount of uranium devoted to fuelling reactors is now stored in the form of depleted uranium (otherwise called enrichment tails). Significant quantities (about 80% of uranium deliveries to enrichment plants) are added annually to the stockpiles. These tails contain a large quantity of total uranium potentially available for future use. Some of them have a relatively high residual ²³⁵U content and are re-enriched, thus allowing a secondary separation of additional usable uranium.

3.7.2. Economic optimum enrichment tails assay:

Effect of lowering the tails assays and the resulting savings

Until this year, many reports, including an IAEA analysis of uranium supply to 2050 [1], were using an enrichment tails assay of 0.3% ²³⁵U, or close to this value. At the same time, for economic optimization reasons, a majority of utilities were asking for tails assays between 0.3 and 0.35%. However, for various reasons (e.g. excess marginal capacity and the availability of low cost uranium), enrichment plants were operating at a lower global average, resulting in an actual average close to the 0.3% assumption.

With the increase of uranium prices, and the relative SWU price stability, utilities notify lower tails assays and enrichers tend to adjust, thus pushing the average assay significantly below 0.3%.

In the long term, it is believed that the general trend would be a uranium price increase for all scenarios except a nuclear power decline. In the same time frame, the prices of isotopic enrichment services are likely to remain steady or even decrease due to improvements in technology. Hence, a declining trend for tails assays is foreseen. The resulting uranium savings are not strictly falling into the definition of secondary supplies, except in the case of economic conditions departing significantly from average market values. This is currently the case for enrichment in the Russian Federation, reportedly operating at tails assays below 0.15% ²³⁵U.

If needed, it is also possible to forecast uranium savings beyond assumed optimum tails assays for the long term in order to bridge a potential uranium gap through the use of additional enrichment capacity, which is easier to build than finding new uranium deposits.

Figure 7 and Table 6 illustrate the major impact on uranium demand resulting from a change in enrichment tails assays. These savings are up to 20%

in the case of uranium prices rising from 10 US $/lb U₃O₈ (22 US $/kg) to 40 US $/lb U₃O₈ (88 US $/kg). Obviously this kind of change will have an

FIG. 7. Market impact from adjustment of tails assays (nw: norm of waste).

impact only on reactor requirements on the basis of EUP based fuels (and will arithmetically lower the savings from MOX and RepU fuels). It is, however, important to emphasize that these savings imply a correlated increase of SWU consumption in a similar proportion.

Tables 6 and 7 show that, on the basis of current SWU and conversion market prices, the optimum tails assay for the current uranium price of US $80/kg U (US $30/lb U₃O₈ (US $66/kg)), is now tending to around 0.24%

235U (owing to the effect of long term uranium contracts, a multi-year lag is observed for full tails assays adjustment). However, because of the rather flat curve of the optimum, one can see that the economic incentive to depart from a 0.30% tails assays is limited (less than 2% of the EUP cost).

TABLE 6. URANIUM FEED REQUIREMENTS FOR 1 kg OF 4.2% ²³⁵U LEU AT VARIOUS TAILS ASSAYS

(optimum tails assays are assuming a fixed conversion cost of 10 US $/kg U and a fixed SWU cost of 100 US $/SWU)

0.338 0.303 0.240 0.193 0.122

Uranium

Tables 7 and 8 also show that use of a tails assay below 0.15% ²³⁵U is justified only in the case of very costly uranium (about 100 US $/lb U₃O₈ (220 US $/kg), i.e. more than three times the current level), or for the current price, in the case of very low cost SWU, typically below 50 US $/SWU, such as marginal excess centrifuge SWUs.

This paper must remain consistent with the updated IAEA study published in these proceedings ([10], Paper 1.1). In this study, uranium require-ments were calculated using a 0.3% tails assay for Western plants, and 0.1% for Russian supplied reactors. Therefore, additional uranium savings between 0.3% and future optimum tails assays required by incremental marginal resources cost will be calculated and evaluated as ‘secondary supplies’. We will do the same calculation to evaluate the savings already achieved through Russian enrichment operations.

TABLE 7. COST OF 1 kg OF 4.2% ²³⁵U LEU AT VARIOUS TAILS ASSAYS AND URANIUM COSTS ASSUMING A FIXED CONVERSION COST OF 10 US $/kg U AND A FIXED SWU COST OF 100 US $/SWU

(cost expressed in US $/kg U)

NatU cost (US $/kg U) 26 40 80 130 260

NatU cost (US $/lb U₃O₈) 10 15.4 30.8 50 100

At optimum tails assays 901 1040 1395 1795 2748

At fixed tails assays of 0.30% 907 1040 1420 1894 3128 At fixed tails assays of 0.25% 933 1053 1396 1824 2938 At fixed tails assays of 0.15% 1062 1163 1452 1813 2752

TABLE 8. COST OF 1 kg OF 4.2% ²³⁵U LEU AT VARIOUS TAILS ASSAYS AND SWU COSTS ASSUMING A FIXED CONVERSION COST OF 10 US $/kg U AND A FIXED NatU FEED COST OF 66 US $/kg U₃O₈

(cost expressed in US $/kg U)

SWU cost (US $/kg U) 100 80 60 40 30

Optimum tails assay 0.240 0.216 0.185 0.142 0.111

Uranium requirements 8.41 8.05 7.64 7.13 6.82

SWU requirements 6.38 6.74 7.27 8.23 9.15

EUP cost at optimum tails assays

1395 1263 1123 971 888

Since the last study, prices have risen from less than a level of 26 US $/kg U to around the 80 US $/kg U level, i.e. the lower limit of our high cost project. We believe this price spike is still disconnected from ‘the last marginal production cost’, and mostly reflects disequilibrium between supply and demand. When analysing the updated scenarios it appears that the current price level is probably a sound long term reference. As a result, considering tails assays in the range of 0.25–0.20% ²³⁵U for the period until 2025, and of 0.15–0.20% until 2050 is certainly a reasonable assumption for the reference case.

The formulas show that lowering the tails assay from 0.30% (the demand assumption) to 0.225% results in a savings of 140 t U per 1000 t U of demand, and that lowering it from 0.30 to 0.175% results in a savings of 210 t U per 1000 t U of demand. Therefore, in the reference case, the savings from the average tails assays reduction represent about 8000 t U annually. In addition, owing to the lower tails assays achieved at Russian enrichment facilities, a further 15% reduction in the requirements must be applied to the reactors served by the Russian enricher. In total, for the period from 2005 to 2025, the impact of the tails assays is likely to represent 105 000 t U. Similarly, for the period 2025–2050, the savings amount to 438 000 t U and for the entire period up to 2050 would total 543 000 t U (they have been evaluated as 590 000 t U in the Key Issue paper ([10], Paper 1.1).

3.7.3. The depleted uranium (tails) potential

3.7.3.1. The depleted uranium status debate

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 probably continue to change according to changes in national policies, the uranium market, isotopic enrichment costs and fission reactor technologies. 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 breeders was considered by many to be unavoidable within one or two decades and transformation of fertile ²³⁸U into fissile plutonium was considered the appropriate answer to the lack of uranium. During the 1990s, the answer was less certain and the issue of depleted uranium management started to be controversial. Low uranium prices and the economic burden of tails management have altered the equation so that depleted uranium was more often considered to be a waste. At present, recent market trends and the perspectives of a nuclear renaissance are likely to change the conclusion again, and there is a renewed interest in depleted uranium re-enrichment.

3.7.3.2. Current uses of depleted uranium

Depleted uranium can be used as a raw material for fuelling reactors, assuming re-enrichment in the case of sufficient ²³⁵U residual content, or mixing with other fissile material (e.g. Pu for MOX and HEU for dilution).

Non-fuel uses in the reactor cores involve small amounts of depleted uranium, mainly for radiological shielding. Uses of depleted uranium for fuelling reactors include:

(a) 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 the natural uranium price allows such a recovery. This is currently the case for limited but growing quantities.

(b) MOX matrices: The quantities involved are small but still constitute about 94% of MOX heavy metal content.

(c) HEU dilution: The quantities of depleted uranium tails at present being used for dilution of HEU are reported to be significant as a result of the Russian HEU agreement. They are already counted in the HEU impact figures, and should be deducted from the tails stockpile totals.

(d) Core blankets: Pellets made with depleted uranium are quite often used peripherally to the reactor core as neutron shielding. Currently this use is very limited in LWRs and CANDUs (5–10 t/a). However, assuming the development of fast breeder programmes, this use can become very significant, multiplying the duration of uranium resources use by a factor of at least 50. It should be noted that the majority of the so-called Gen-4 list of future reactors belong to the fast breeder type.

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. This is of particular interest for the tails already having a very low residual ²³⁵U content, whether it is the result of a low initial tails assay or of a secondary process (tails of tails).

3.7.4. Existing stockpiles and flows of depleted uranium

A detailed updated estimate of the quantities of depleted uranium tails is provided in Table 9, the total being at approximately 1.5 million tons of depleted uranium at the end of 2004 (more than an OECD Nuclear Energy Agency estimate [11] and to be compared with a cumulative world uranium production exceeding two million tonnes of uranium). Table 9 also provides an estimate of the chemical form of these tails (depleted uranium hexafluoride

(DUF₆) versus defluorinated oxide forms) and the quantities that are believed to have a ²³⁵U content in excess of 0.3% and are therefore considered as potential sources for our study.

Table 10 provides an estimate of the recoverable natural uranium equivalent content of tails having a ²³⁵U content in excess of 0.3%. For the calculation, an average of 0.35% is utilized; we considered this to be a high figure. A secondary tails assay of 0.15% is considered; we consider this to be an economically achievable figure in the period up to 2025. The result shows that the readily available natural uranium equivalent content of tails stockpiles worldwide is limited compared with some published reports [11].

In addition, there is some uncertainty about the fate of part of the USDOE and USEC tails, in principle set to be defluorinated and sent to a repository. Some of these are currently identified for re-enrichment at the USEC plant [12].

Under the above assumptions (considered to be optimistic), the total could provide about 110 000 t of natural uranium equivalent, comparable to the resources of a large deposit, but would imply a significant SWU consumption.

3.7.4.1. Likely trend for depleted uranium use (2050 and beyond)

With the exception of limited quantities such as its use as MOX matrices or axial blankets in certain types of reactor, the uses of depleted uranium must be considered from now as an ‘on-line’ process. During the last decade, TABLE 9. DEPLETED URANIUM STOCKPILES ESTIMATE AT THE END OF 2004 (updated from Ref. [1])

Enricher Total depleted

uranium (t U) U/DUF6 (t U)

Estimate of U/UF6

with U-235 content over 0.3%

USDOE–USEC 535 000 535 000 120 000

Eurodif 225 000 35 000 15 000

Urenco 61 000 61 000 40 000

BNFL 30 000 30 000 25 000

Rosatom 565 000 565 000 80 000

China 27 000 27 000 10 000

Other 15 000 15 000 10 000

Total 1 458 000 1 268 000 300 000

stockpiled depleted uranium has been utilized for HEU blending down purposes in the Russian Federation. Owing to the limited availability of quantities having a high residual ²³⁵U content, this use is declining and is being progressively replaced by the use of on-line tails and imported tails that are already fed to the enrichment cascades under normal commercial programmes.

Because of these practices, and of the low residual tails assays currently in use in the Russian Federation, the ²³⁵U content of the world’s depleted uranium stockpiles is permanently decreasing. In many places, it is also assumed that keeping these depleted uranium inventories as UF₆ is now worthless with regard to the schedule for their potential future use, and programmes are ongoing or planned to convert the inventories to an oxide form through defluorination, thus allowing safe and efficient long term storage.

In addition to the availability of ²³⁵U rich-enough tails, commercial re-enrichment of depleted uranium depends mainly upon the availability of relatively low cost unutilized (marginal) SWU capacity.

Figure 8 illustrates some aspects of the economics of tails re-enrichment.

At the level reached by the uranium market price in the middle of 2005, rich tails are clearly becoming attractive when enrichment capacity is available.

The US and West European gaseous diffusion plants have relatively high marginal SWU costs, but as there is free capacity at the US plant, limited re-enrichment operations are currently starting, owing to the changes in the TABLE 10. RECOVERABLE NATURAL URANIUM EQUIVALENT FROM ‘RICH’ TAILS

Total 300 000 108 000 89 225

* According to Ref. [13] only about 15 500 t U are likely to be recovered from US tails, but this was evaluated before the increase in price of natural uranium.

uranium market. The capacities at West European centrifuge plants are currently fully committed to normal enrichment contracts. Future expansion of these plants could release some capacity for tails re-enrichment but they will be primarily devoted to normal enrichment operations, probably at lower tails assays.

In the near-term, the largest contribution to the re-enrichment of tails is coming from Russian centrifuge plants, which reportedly have large available marginal capacities, and thus can offer fuel contracts on a marginal cost basis.

Total enrichment capacity exceeded 20 MSWU/a in 2005, and to supply Soviet design reactors, the Russian Federation has to supply fuels with a content of 4.4 MSWU if calculated for a 0.3% tails assay. Because the Russian enrichment plants are operated at tails assays below 0.15%, an additional SWU consumption of about 50% is needed, increasing the requirements to about 6.6 MSWU.

Table 11 projects the allocation of Russian enrichment capacity potentially available for tails re-enrichment.

FIG. 8. The economics of tails re-enrichment of 0.35% ²³⁵U tails to natural uranium for four secondary tails assays (nw2) versus SWU unit cost (assuming no feed related cost or credit).

4. APPRAISAL OF IMPACT OF SECONDARY SUPPLIES ON THE