U.S. Nuclear Regulatory Commission released a Report to Congress on the Current Disposition of Highly Enriched Uranium Exports Used as Fuel or Targets in Nuclear Research or Test Reactors (PDF file). The report, that was prepared in response to the National Defense Authorization Act for Fiscal Year 2013, deals with HEU exported by the United States.
According to the report, the records indicate that as of 31 December 2012,
Since 1957, ... the United States has exported a total of approximately 22,600 kilograms (kg) of HEU for use as fuel or targets in RTRs to a total of 35 countries ... . The records indicate that approximately 6,100 kg of that U.S.-supplied HEU presently remains in 20 countries, with 95 percent of that material located in five of those countries. Many of the 20 countries have converted their RTRs from HEU to low enriched uranium (LEU) fuel or have committed to doing so in the future.
... approximately 7,700 kg of HEU has been returned to the United States primarily as irradiated fuel.
Available information indicates that more than 4,300 kg of the U.S. HEU has been eliminated by down-blending to LEU; approximately 500 kg of HEU has been eliminated in highly-dilute processing waste; and at least 2,400 kg of HEU has been destroyed (burned up) through irradiation in RTRs. As of the issuance of this report, the NRC staff was not able to definitively reconcile records for approximately 1,600 kg of HEU that was exported by the United States.