The Clinton White House had a plan to take 50 tonnes of plutonium from each of the American and Russian stockpiles to blend into MOX fuel for PWRs. IIRC the Bush White House cancelled the plan.
In a 2000 agreement, the US said it would turn 75% of its plutonium stockpile into MOX which would be consumed in PWRs, but not reprocessed. The rest would be blended into reactor waste and vitrified, this was cancelled around 2000 and the MOX plant is still under construction.. The US also agreed to help the Russians build a MOX plant to dispose of 34 tonnes of their plutonium, but the cost rose from about $3 billion to more than $18 billion. So far the Russians have paid for their MOX plant from their own money.
In 2010, the US agreed an amendment to the 2000 agreement that will allow the Russians to turn their plutonium into fuel for two demonstration fast neutron reactors that are under construction with the stipulation it will not reprocess any spent fuel from the reactors before all 34 tonnes have been passed through the plants.
US plutonium is kept at Pantex near Amarillo in Texas and at Savannah River, Georgia. Not sure about the Russian plutonium.
France offered to sell a MOX plant to the US, but was refused. Both Britain and France have (mostly) operational MOX facilities largely designed for exporting fuel to Japan which was suspended, but which has now resumed, following the Fukushima meltdowns.
BTW. A large amount of the highly enriched uranium in the Megatons to Megawatts programme actually came from Soviet era nuclear submarine fuel which was often enriched to over 90% so that the core could be made smaller and quieter.