3.3.1 Precipitation Processes

The earliest separations were developed in support of the WW2 Manhattan Project and exploited the different solubilities of Pu(iv) and U(vi). The primary centre for these activities was the Hanford site in Washington State, USA.7 Most fission products are soluble in strongly acid conditions, whereas the fluorides and phosphates of the tri — and tetravalent actinides are not.

Clint A. Sharrad, Laurence M. Harwood and Francis R. Livens — Total for fuel

— Fission and activation products

Thus, these separations were based on dissolution of fuel in nitric acid, with control of the uranium oxidation state to vi and of the plutonium oxidation state to III or IV. On addition of Bi, followed by phosphoric acid, BiPO4 was precipitated, carrying plutonium and separating it from uranium and many fission products. The crude precipitate was then dissolved in strongly oxidising conditions, for example with BiO32 or MnO4 in the presence of Cr2O72 holding oxidant, which switched the plutonium to oxidation state VI. Further purification was achieved by lanthanum fluoride precipitation, which elimi­nated lanthanide fission products and isolated plutonium.

These processes were carried out on an industrial scale from the end of 1944 until the early 1950s and were used to manufacture tonne quantities of pluto­nium from hundreds of tonnes of uranium (the irradiated fuel contained about 250 mg Pu tonne 1U). In the early 1950s, precipitation separation was replaced by solvent extraction, leading to the development of the Purex process and a substantial increase in throughput. By the early 1960s, Hanford was separating between 1.5 and 2 tonnes of plutonium from 7000 tonnes of irradiated uranium each year, 80% of this from the Purex process.