The actinoid elements in this case consist primarily of the unconsumed remains of the original fuel (typically
U-235,
U-238, and/or
Pu-239).
Chemical process
The fuel is first dissolved in
nitric acid at a concentration around 7
M. Solids are removed by filtration to avoid the formation of
emulsions, referred to as
third phases in the solvent extraction community.
The
organic solvent consists of 30%
tributyl phosphate (TBP) in a
hydrocarbon such as
kerosene. Uranyl(VI) UO2+ 2 ions are extracted in the organic phase as UO2(NO3)2·2TBP complexes; plutonium is extracted as similar
complexes. The heavier actinides, primarily
americium and
curium, and the fission products remain in the aqueous phase. The nature of uranyl nitrate complexes with trialkyl phosphates has been characterized.[10]
Plutonium is separated from uranium by treating the TBP-kerosene solution with reducing agents to convert the plutonium to its +3 oxidation state, which will pass into the aqueous phase. Typical reducing agents include N,N-diethyl-
hydroxylamine,
ferroussulphamate, and
hydrazine. Uranium is then stripped from the kerosene solution by back-extraction into nitric acid at a concentration around 0.2 M.[11]
PUREX raffinate
The term PUREX
raffinate describes the mixture of metals in
nitric acid which are left behind when the
uranium and
plutonium have been removed by the PUREX process from a
nuclear fuel dissolution liquor. This mixture is often known as high level
nuclear waste.
Two PUREX raffinates exist. The most highly active
raffinate from the first cycle is the one which is most commonly known as PUREX raffinate. The other is from the medium-active cycle in which the uranium and plutonium are refined by a second
extraction with
tributyl phosphate.
Deep blue is the bulk ions, light blue is the
fission products (group I is Rb/Cs) (group II is Sr/Ba) (group III is Y and the
lanthanides), orange is the
corrosion products (from stainless steel pipework), green are the major actinides, violet are the
minor actinides and magenta is the
neutron poison)
The PUREX plant at the
Hanford Site was responsible for producing 'copious volumes of liquid wastes', resulting in the radioactive contamination of groundwater.[12]
Greenpeace measurements in
La Hague and
Sellafield indicated that radioactive pollutants are steadily released into the sea, and the air. Therefore, people living near these processing plants are exposed to higher radiation levels than the naturally occurring
background radiation. According to
Greenpeace, this additional radiation is small but not negligible.[13]
^Paiva, A. P.; Malik, P. (2004). "Recent advances on the chemistry of solvent extraction applied to the reprocessing of spent nuclear fuels and radioactive wastes". Journal of Radioanalytical and Nuclear Chemistry. 261 (2): 485–496.
doi:
10.1023/B:JRNC.0000034890.23325.b5.
S2CID94173845.
^Burns, J. H.; Brown, G. M.; Ryan, R. R. (1985). "Structure of dinitratodioxobis(triisobutyl phosphate)uranium(VI) at 139 K". Acta Crystallographica Section C Crystal Structure Communications. 41 (10): 1446–1448.
Bibcode:
1985AcCrC..41.1446B.
doi:
10.1107/S0108270185008125.
^J.H. Burns (1983). "Solvent-extraction complexes of the uranyl ion. 2. Crystal and molecular structures of catena-bis(.mu.-di-n-butyl phosphato-O,O')dioxouranium(VI) and bis(.mu.-di-n-butyl phosphato-O,O')bis[(nitrato)(tri-n-butylphosphine oxide)dioxouranium(VI)]". Inorganic Chemistry. 22 (8): 1174–1178.
doi:
10.1021/ic00150a006.