In 2000, the United States and Russia signed an agreement in which each country would convert 34 metric tons of surplus weapons-grade plutonium into a form no longer useful for weaponry. They also would have to approve the other side’s method, and the International Atomic Energy Agency would serve as monitor.

For both countries, the idea was to mix the plutonium with uranium to make mixed-oxide (MOX) fuel for use in reactors that produce electrical power.

While Russia chose to use its MOX in what are known as fast reactors, the United States initially chose using MOX in light-water reactors. The overall U.S. approach required constructing a new MOX facility, another to disassemble the nuclear weapon triggers (or “pits”) and a facility to handle radioactive waste.

It also required finding a buyer for the reactor fuel.

In 2002, the Energy Department estimated the total cost of the MOX project over 20 years would be $3.8 billion. The department’s facility at Savannah River, S.C., was chosen as the site for the plutonium conversion.