Russia has reneged on an agreement to deliver a total of 10 kilograms of plutonium-238 to the United States in 2010 and 2011 and is insisting on a new deal for the costly material vital to NASA’s deep space exploration plans. The move follows the U.S. Congress’ denial of President Barack Obama’s request for $30 million in 2010 to permit the Department of Energy to begin the painstaking process of restarting domestic production of plutonium-238. Bringing U.S. nuclear laboratories back on line to produce the isotope is expected to cost at least $150 million and take six years to seven...