World Nuclear Fuel Facilities
a) Nominal capacity
MTU = metric tonnes of uranium
Uranium Downblending
(last updated 20 Aug 2005)
Uranium used in nuclear weapons is enriched to approx. 93% U-235, while uranium used as fuel in commercial nuclear power plants typically is enriched to 3 - 5% U-235.
Uranium enriched to more than 20% U-235 is called Highly Enriched Uranium (HEU) and can only be used in
nuclear weapons and in r
esearch reactors.
Surplus HEU can be downblended to Low Enriched Uranium (LEU) to make it suitable for use in commercial nuclear fuel.
Note: This downblending only concerns uranium: there is no plutonium involved, in contrast to the production of mixed oxide fuel (MOX).
In 1993, the U.S. and Russia concluded the
US-Russia HEU Agreement, under which
Russia was to supply the downblended uranium derived from 500 metric tonnes of HEU to the USA over a period of about 20 years. While the deliveries under this agreement are still ongoing, the U.S. now have begun
downblending some of their own surplus HEU.
Blending process
In a first step, the HEU and the blendstock have to be converted to the chemical form required for the selected blending process, if not already in the appropriate form.
For the
downblending process, there exist the following methods:
- Mixing of liquids:
- uranium in the form of uranyl nitrate hexahydrate (UNH): UO2(NO3)2·6H2O, or
- molten uranium metal
- Mixing of gases:
- uranium in the form of uranium hexafluoride (UF6)
The existing commercial
downblending facilities in the U.S. (BWXT in Lynchburg, Virginia, and NFS in Erwin, Tennessee) are using the UNH process, while the Russian facilities (in Novouralsk, Seversk, and Zelenogorsk) are using the UF
6 process.
Historically, downblending has also been performed at the following
DOE nuclear weapons facilities in the U.S.: the Y-12 Plant in Oak Ridge, Tennessee (UNH and molten metal processes), and the Savannah River Site (SRS) in Aiken, South Carolina (UNH process).
After the blending, the material has to be converted to UO
2, before it can be used in the production of commercial nuclear fuel.
HEU Feed
The HEU material can have various forms, such as metal, oxide, or alloys with aluminium, for example. The material may contain several impurities, among them some unwanted uranium isotopes:
- U-234 is a minor isotope contained in natural uranium; during the enrichment process, its concentration increases even more than that of U-235. High concentrations of U-234 may cause excessive worker radiation exposures during fuel fabrication.
- U-236 is a byproduct from irradiation in a reactor and may be contained in the HEU, depending on its manufacturing history. HEU reprocessed from nuclear weapons material production reactors (with an U-235 assay of approx. 50%) may contain U-236 concentrations as high as 25%, resulting in concentrations of approx. 1.5% in the blended LEU product. U-236 is a neutron poison; therefore the actual U-235 concentration in the LEU product must be raised accordingly to compensate for the presence of U-236.
Blendstock
The blendstock can be
natural uranium, or
depleted uranium. Often however, slightly enriched uranium at typically 1.5 wt-% U-235 is used as a blendstock to dilute unwanted byproducts, such as U-234 and U-236 contained in the HEU feed. Often, concentrations of these isotopes in the LEU product would exceed ASTM specifications for nuclear fuel, if natural, or depleted uranium were used as a blendstock (see
Uranium Downblending Calculator).
So, the HEU downblending generally cannot contribute to the
waste management problem posed by the existing large stockpiles of depleted uranium (DU).
There is one exception, however: The blendstock used for HEU downblending in Russia is made from re-enrichment of depleted uranium. For this re-enrichment, or tails upgrading process, surplus centrifuge enrichment capacities are used. (see also
Uranium Enrichment Tails Upgrading)
Mass and SWU balance of the Downblending Process
In the following, the mass and SWU (
Separative Work Unit) balance of the downblending process is shown for two cases (natural blendstock / 1.5% enriched blendstock). For each case, the reference case of straight enrichment of natural uranium is shown for comparison.Due to the decreasing efficiency of the enrichment process with higher product assays, only some fraction (max. 80% in the cases regarded) of the separative work spent for the past production of the HEU can be recovered through the downblending process. The overall SWU recovery rate is particularly poor (only 16%) in the case of depleted uranium being used as a raw material for the production of the 1.5% enriched blendstock.
Case 1: Mass balance of HEU downblending with natural uranium (per tonne of HEU)
Assumptions:
- 93% HEU assay
- 4.0% LEU assay
- 0.71% Blendstock assay
- 0.3% Tails assay
- no account for conversion losses, no account for any U-236 in HEU
|
Feed
226 t U
0.71% U-235 | ------> | Past High Grade Enrichment
200,000 SWU | ------> | HEU
1 t U
93% U-235 | | |
| | |
V | | |
V | | |
| | Tails
225 t U
0.3% U-235 | | Downblending | ------> | LEU
28 t U
4.0% U-235 |
| | | | ^
| | | |
| | | | Blendstock
27 t U
0.71% U-235 | | |
Reference Case 1:
|
Feed
253 t U
0.71% U-235 | ------> | Low Grade Enrichment
148,000 SWU | ------> | LEU
28 t U
4.0% U-235 | | |
| | |
V | | | | |
| | Tails
225 t U
0.3% U-235 | | | | |
|
Observations Case 1:
- the sum total amount of natural uranium feed and blendstock is equivalent to the reference case feed
- the amount of tails is equivalent to the reference case
- 74% of the SWU spent for the HEU production can be recovered
Cases 2 a/b: Mass balance of HEU downblending with uranium enriched to 1.5% (per tonne of HEU)
Assumptions:
- 93% HEU assay
- 4.0% LEU assay
- 1.5% Final Blendstock assay
- 0.71% (a) / 0.3% (b) Raw Blendstock assay
- 0.3% Tails assay (High grade enrichment)
- 0.3% (a) / 0.12% (b) Tails assay (Blendstock enrichment)
- no account for conversion losses, no account for any U-236 in HEU
|
Feed
226 t U
0.71% U-235 | ------> | Past High Grade Enrichment
200,000 SWU | ------> | HEU
1 t U
93% U-235 | | |
| | |
V | | |
V | | |
| | Tails
225 t U
0.3% U-235 | | Downblending | ------> | LEU
36.6 t U
4.0% U-235 |
| | | | ^
| | | |
Raw Blendstock
(a) 104 t U 0.71% U-235
(b) 273 t U 0.3% U-235 | ------> | Blendstock enrichment
(a) 32,900 SWU
(b) 161,000 SWU | ------> | Final Blendstock
35.6 t U
1.5% U-235 | | |
| | |
V | | | | |
| | Tails
(a) 68.4 t U 0.3% U-235
(b) 237.4 t U 0.12% U-235 | | | | |
Reference Case 2:
|
Feed
330 t U
0.71% U-235 | ------> | Low Grade Enrichment
193,000 SWU | ------> | LEU
36.6 t U
4.0% U-235 | | |
| | |
V | | | | |
| | Tails
293.4 t U
0.3% U-235 | | | | |
|
Observations Case 2a (1.5%-enriched blendstock obtained from enrichment of natural uranium):
- the sum total amount of natural uranium feed and raw blendstock is equivalent to the reference case feed
- the total amount of tails is equivalent to the reference case
- 80% of the SWU spent for the HEU production can be recovered (taking into account the malus for blendstock enrichment)
Observations Case 2b (1.5%-enriched blendstock obtained from enrichment of depleted uranium with 0.3% U-235):
- the natural uranium feed amount is 32% lower than in the reference case
- the blendstock enrichment consumes 21% more tails (at 0.3% U-235) than left over from the related HEU enrichment; the total tails balance of 189.4 t U is 36% lower than in the reference case
- only 16% (!) of the SWU spent for the past HEU production can be recovered (taking into account the malus for blendstock enrichment)
A more detailed analysis of the blendstock enrichment and downblending actually performed in Russia suggests that the
blendstock enrichment even consumes 20% more SWU than can be recovered. This means that no SWU recovery takes place at all, and the whole process is a SWU sink:
> Download report
Re-enrichment of West European Depleted Uranium Tails in Russia (313k PDF)
For other situations, use the
Uranium Downblending Calculator and the
Uranium Enrichment Calculator.
> See also:
Management of High Enriched Uranium for Peaceful Purposes: Status and Trends 
, IAEA TECDOC Series No. 1452, June 2005 (1M PDF)
Economy of the Downblending Process
See the
Recycled Nuclear Fuel Cost Calculator
