Page 11 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN001F-11 May 7, 2001 12:19

Actinide Elements 219

from leach solutions can be affected by ion exchange, sol- for metal production, and PuF 4 is most commonly used as
vent extraction, and chemical precipitation. Most leach the starting material. The crude plutonium metal may be
solutions are now treated by anion-exchange methods or refined by electrolysis in molten salts.
solvent extraction or both for purification prior to pre- Americium and curium can be obtained from the aque-
cipitation. The two principal methods of precipitation are ous waste of the Purex process. This americium is a mix-
now neutralization with ammonia or the precipitation of ture of 241 Am and 243 Am. Isotopically pure 241 Am, the
uranium peroxide, UO 4 · xH 2 O, with hydrogen peroxide. decay product of 241 Pu, can be obtained from aged plu-
The precipitates (“yellow cake”) are dried and ignited tonium. Solvent extraction and ion-exchange procedures
to U 3 O 8 or UO 3 , depending on temperature. To produce are used to recover americium from waste streams. Ameri-
nuclear-grade material, these raw products are normally cium metal is produced by lanthanum reduction of the ox-
further refined by solvent extraction or fluoride volatility ide, followed by vacuum distillation of the americium at
◦
processes. The purified uranium is converted to UO 3 , re- 1400 C.
duced with hydrogen to UO 2 , and converted to UF 4 with 243 Cm and 244 Cm are minor constituents of nuclear
hydrogen fluoride. The UF 4 can either be reduced to ura- waste. Gram quantities of 242 Cm and 244 Cm were pro-
nium metal or fluorinated to UF 6 for isotope enrichment duced by neutron irradiations of 241 Am and plutonium,
by gaseous diffusion. respectively. The Tramex process based on the extraction
The production of uranium metal usually involves the with tertiary amines and high-pressure ion-exchange sys-
◦
reduction of UF 4 with magnesium at 700 C. The metal tems was developed for the recovery of curium. Curium
may be refined by molten-salt electrolysis followed by metal is advantageously produced by thorium reduction
zone melting. Because of the low melting point of ura- of the oxide, followed by vacuum distillation of the metal
◦
nium, the van Arkel process is not as feasible as for tho- at 2000 C.
rium and protactinium. Weighable quantities of the transcurium elements
The principal source of neptunium ( 237 Np) is irradiated berkelium ( 249 Bk), californium ( 252 Cf), and einsteinium
nuclear reactor fuel based on 235 U. A slightly modified ( 253 Es) for use in research are produced in the high-
Purex (plutonium-uranium recovery by extraction) pro- flux nuclear reactors HFIR at Oak Ridge and SM-2 at
cess can be used to separate neptunium from uranium, Dimitrovgrad, Russia. 257 Fm in picogram quantities was
plutonium, and fission products during reprocessing of produced only at Oak Ridge. Targets containing pluto-
nuclear reactor fuel. Ion-exchange methods are used for nium, americium, and curium are irradiated in the high-
the final purification and concentration. Neptunium metal flux reactors and then processed. After target dissolution
is produced by reduction of NpF 4 with calcium metal us- followed by impurity, rare-earth, and curium removal,
ing iodine as a booster. Refining is accomplished by vac- the transcurium elements are separated by high-pressure
uum melting. Plutonium was the first synthetic actinide cation exchange using ammonium α-hydroxyisobutyrate
element to be produced on a large scale. It is produced in as the eluent. Berkelium metal in microgram to milligram
nuclear reactors by the so-called pile reactions [Eqs. (11) amounts is produced by reducing BkF 3 or BkF 4 with
and (4)]: lithium metal, followed by the removal of lithium fluo-
◦
ride at 1200 C from the less volatile berkelium metal.
238 1
0
92 U + n → fission products + 2.5n +∼200 MeV The more volatile californium, einsteinium, and fermium
(11)
metals can be prepared by reduction of the oxides with
−β − −β −
238 1 239 239 238 lanthanum metal, followed by a distillation of the ac-
92 U + n → 92 U −−→ 93 Np −−−→ 92 Pu (4)
0
23 min 2.3 days tinide metals. To prepare the metals free of a supporting
The most widely employed method for plutonium repro- material at least a few milligrams of metal have to be
cessing used today in almost all of the world’s reprocess- distilled.
ing plants is the Purex (plutonium-uranium reduction ex- Californium is the heaviest actinide for which data like
traction) process. Tributylphosphate (TBP) is used as the the enthalpy of sublimation have been determined directly
extraction agent for the separation of plutonium from ura- withbulkquantitiesofabout2mgofpuremetal.Duetothe
nium and fission products. In effecting a separation, ad- limited availability of the heaviest actinides down to the
vantage is taken of differences in the extractability of the “one-atom-at-a-time” scale, the preparation of the metals
various oxidation states and in the thermodynamics and becomes an integral part of an experiment for studying the
kinetics of oxidation reduction of uranium, plutonium, and metals. Unusual experimental approaches like the mea-
impurities. Various methods are in use for the conversion surement of partial pressures of the actinide under study
of plutonium nitrate solution, the final product from fuel over an alloy, studies of diffusion of actinide atoms in met-
reprocessing plants, to the metal. The reduction of pluto- als, and adsorption studies of actinide atoms onto metal
nium halides with calcium proved to be the best method surfaces by thermochromatography have been reported.