Americium-241 ingrowth

Not all impure plutonium metal produced by multicycle direct oxide reduction needs to undergo metal chlorination. However, if the plutonium has sat in storage for many years, it will have undergone radioactive decay of plutonium-241. This isotope has a half-life of 13.2 years and decays by beta decay into americium-241. The americium-241 has a 433-year half-life and yields alpha particles and gamma rays when it decays. The gamma rays add significant worker exposure from aged plutonium and removal of americium reduces dose to personnel during subsequent processing. Americium also represents an unwanted impurity in weapons-grade plutonium.

Coalescence/ingot casting

After the americium is extracted from the impure metal, the plutonium metal must be cast into a cylindrical geometry that is compatible with the electro-refining cell. Pieces of plutonium from the metal chlorination process, along with pieces and turnings not requiring americium removal, are cast in a heated, stirred magnesia crucible, sometimes containing calcium chloride salt and a small amount of calcium metal. The resulting ingots are the correct size for use in the electrorefining cell.

Scraps of plutonium metal are melted into a single piece in the coalescence process. Impure plutonium metal is also cast into ingots (shown above) in preparation for electrorefining—the final step in the purification process.

Next: Electrorefining