Uranium Downblending

(last updated 2 Sep 2021)

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 research reactors.
Surplus HEU can, however, 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.

On Aug. 23, 2018, the U.S. Department of Energy's National Nuclear Security Administration (DOE/NNSA) announced plans for TVA to down-blend highly enriched uranium for use in commercial reactors to produce tritium for U.S. nuclear weapons, because there is no source of unobligated enriched uranium available through the open market today.

On Sep. 1, 2021, the U.S. Department of Energy's National Nuclear Security Administration (DOE/NNSA) announced Norway's plan to eliminate its HEU stockpile by downblending.

Blending process

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 UF₆ 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 UO₂, before it can be used in the production of commercial nuclear fuel.

HEU Feed

• 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

Mass and SWU balance of the Downblending Process

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)

• 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)

• 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)

• 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)

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

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