27-8 Neptunium, Plutonium,and Americium
The extraction of plutonium from uramum fuel elements involves (a) removal of the highly radioactive fission products that are produced simultaneously in compar- able amounts, (b) recovery of the uranium for reprocessmg, (c) remote control of all the chemical operations because of the radiation hazard. An additional hazard is the extreme toxicity of Pus 10-6 g of which is potentially lethal; a particle of 239 Pu02 only I micrometer diameter can give a very high dose of radiation, enough to be strongly carcinogemc.
The separation methods Of N", Pu, and Am from U are based on the following chemistry.
l. Stabilities of oxidation stares. The stabilities of the major ions involved aren > > Pu022+ > Ari > pu3+ N/ + U 4 It IS thus possible by choice of suitable oxidizing or reducing agents to obtun a solution containing the elements in different oxidation states; they can then be separated by precipitation or solvent-extraction. For example, Pu can be oxidized to Pu022+ while Am remains as Arn3+ The former can then be removed by solvent extraction or the latter by precipitation of AmF3.
2. Extractability inro organic solvents. The MO, 2 + ions are extracted from nitrate solutions Into ethers. The M ions are extracted Into tributyl phosphate m kerosene from 6M nitnc acid solutions; the M Ions are similarly extracted from 10—16M mtnc acid, and neughbonng actinides can be separated by a choice of conditions.
3. Precipitation reactions. Only M and M 4+ give insoluble fluorides or phosphates from acid solutions. The higher oxidation states glue either no pre- clpitate or can be prevented from precipitation by complex formation v.lth sulfate or other Ions.
4 Ion exchange methods These are used mainly small amounts Of material as in the separation of Am and the followmg elements, as discussed later.