I wonder why they are pursuing sodium, as opposed to something like this:
http://thoriumenergy.blogspot.com/2006/04/introducing-liquid-fluoride-reactor.html France, Japan and the United States agreed Friday to cooperate in making prototypes of so-called "4th generation" nuclear reactors, according to statements released by each country's energy ministries.
These sodium-cooled reactors, which would not come on line until mid-century, produce more energy per unit of fuel than nuclear reactors currently in operation.
But early prototypes have been plagued with problems. The liquid sodium cooling agent is highly volatile, bursting into flames if it comes into contact with air, and exploding if it comes into contact with water.
http://www.energy-daily.com/reports/France_Japan_US_cooperate_on_nuclear_reactors_999.html