Pyrochemical salt processing

Pyrochemical processes involving molten calcium, potassium, and sodium chlorides are used at TA-55 to prepare and purify plutonium metal. Two advantages of these methods are the compactness of their equipment and their rapid reaction kinetics.

Multicycle direct oxide reduction

The plutonium dioxide from nitric acid or hydrochloric acid processing is converted to plutonium metal in a pyrochemical process called multicycle direct oxide reduction (MCDOR). In the MCDOR process, plutonium dioxide (PuO₂) is reduced with calcium metal to produce plutonium metal and calcium oxide (CaO). The reaction takes place in a molten calcium chloride solvent (CaCl₂), which dissolves the resulting CaO and allows the plutonium metal to coalesce in the bottom of the magnesium oxide (M_gO) crucible to form a metal button.

The reaction occurs at about 820 °C with a temperature spike to 875 °C, and takes about 15 minutes. After completion of the reaction, the melt is allowed to settle and cool while the plutonium forms a metal button at the bottom of the crucible, and the calcium chloride forms a salt cake that solidifies above the metal.

The calcium oxide is then converted back to calcium chloride in situ by bubbling chlorine gas through the molten salt. Once the salt is regenerated, more plutonium dioxide and calcium metal can be added to the crucible to produce more plutonium metal. The MCDOR process can be run several times before the plutonium metal button is recovered and the salt and crucible discarded as TRU waste.

Metal chlorination

The plutonium metal produced by MCDOR is not very pure, however, because calcium chloride always contains trace amounts of impurities that are readily absorbed by the plutonium. The method of purification depends to some extent on the nature of the impurities present. If the plutonium feed has been in storage for many years, it will have a high content of americium-241 that will need to be removed in a process known as metal chlorination (MC), formerly referred to as molten salt extraction (MSE). Americium-241 spontaneously grows into plutonium as a result of plutonium-241 beta decay. If the americium content is low, the metal can be purified by electrorefining (ER).

For americium removal by metal chlorination, the plutonium is heated above the melting point in a magnesium oxide (M_gO) crucible and reacted with chlorine gas. This produces plutonium trichloride (PuCl₃), in situ, which then reacts with the americium in the metal to produce americium trichloride (AmCl₃). The americium trichloride and some plutonium trichloride form an easily removed salt crust on top of the purified plutonium metal.

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.

Next: Americium-241 Ingrowth