In its national statement at the 2016 Nuclear Security Summit, the United Kingdom announced that
The UK will transfer around 700kg of Highly Enriched Uranium (HEU) to the United States. In return, the United States will provide HEU in a different form for use in European reactors to produce medical isotopes used in the diagnosis and treatment of conditions including thyroid cancer.
The material in question appears to be part of stock of "unirradiated high enriched uranium fuels" described in the "Exotic Fuels and Nuclear Materials - Dounreay: Credible Options" report, issued by the U.K. Nuclear Decommissioning Authority in February 2012. The report describes this material, named "group 2 material" in the text, as follows (p. 11):
There is approximately 1 tonne of group 2 material stored on the site, in the form of oxide powders and pellets, and also some uranium metal and alloys. It is stored individually in several locations in small quantities [...].
The unirradiated high enriched uranium has a wide range of enrichment values, presenting operational and disposability difficulties. Some of the items such as swab ash, slag and graphite crucibles contaminated with small quantities of unirradiated high enriched uranium are irrecoverable, are considered to be uranium contaminated waste which has a lower security classification.
All this material contains uranium with enrichment above 20%, although exact U-235 contents is unknown. It is possible that 700 kg included in the U.K.-U.S. exchange is the amount of recoverable material. It is not clear if it refers to the mass of uranium in the material to be sent to the United States.
The 2012 NDA report identified several "credible" options for long-term handling of this material, one of which was to "send material overseas for reprocessing and utilise product" (p. 15). A subsequent report, released in February 2013, named "[transfer] to Sellafield for long term management" the preferred option for dealing with the material. As documented by CORE, a U.K. environmental group, the decision to send HEU to the United States was made without an appropriate consultation with the public.
According to a comment from NNSA, obtained by Tom Clements of the Savannah River Site Watch, U.K. HEU cannot be directly used in the medical isotope production process. It will be down-blended to LEU that could be then used in reactor fuel. NNSA would not name the where the material will be stored in the United States, but it stated that it will not be sent to the Savannah River Site. Most likely the down-blending will be done at Nuclear Fuel Services in Erwin, Tennessee.
The nuclear security value of the HEU exchange is highly questionable. While the removal of HEU from Dounreay will improve the security situation at the site, the overall security benefit is likely to be much smaller because of the risks associated with the transfer of the material.
More importantly, the terms of the exchange will effectively legitimize the continuing supply of U.S. highly-enriched uranium to Europe, where it is used for medical isotope production. The last transfer of this kind was authorized in February 2015.