Reprocessing ofspent nuclear fuel by the PUREX method, first developed in the 1940s to produce plutonium for nuclear weapons,[1] was demonstrated commercially in Belgium to partially re-fuel a LWR in the 1960s.[2] This aqueous chemical process continues to be used commercially to separatereactor grade plutonium (RGPu) for reuse as MOX fuel. It remains controversial, as plutonium can be used to make nuclear weapons.[3][4]The most developed, though commercially unfielded, alternative reprocessing method, isPyroprocessing,[5] suggested as part of the depicted metallic-fueled,Integral fast reactor (IFR) asodium fast reactor concept of the 1990s. After the spent fuel is dissolved in molten salt, all of the recyclableactinides, consisting largely of plutonium and uranium though with important minor constituents, are extracted using electrorefining/electrowinning. The resulting mixture keeps the plutonium at all times in an unseparatedgamma and alpha emitting actinide form, that is also mildly self-protecting in theft scenarios.[6]
Structure of uranyl nitrate complex that is extracted in PUREX.[10]
The fuel is first dissolved innitric acid at a concentration around 7M. Solids are removed by filtration to avoid the formation ofemulsions, referred to asthird phases in the solvent extraction community.
Theorganic solvent consists of 30%tributyl phosphate (TBP) in ahydrocarbon such askerosene. Uranyl(VI)UO2+ 2 ions are extracted in the organic phase as UO2(NO3)2·2TBP complexes; plutonium is extracted as similarcomplexes. The heavier actinides, primarilyamericium andcurium, and the fission products remain in the aqueous phase. The nature of uranyl nitrate complexes with trialkyl phosphates has been characterized.[11]
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, andhydrazine. Uranium is then stripped from the kerosene solution by back-extraction into nitric acid at a concentration around 0.2 M.[12]
The termPUREXraffinate describes the mixture of metals innitric acid which are left behind when theuranium andplutonium have been removed by the PUREX process from anuclear fuel dissolution liquor. This mixture is often known as high levelnuclear waste.
Two PUREX raffinates exist. The most highly activeraffinate 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 secondextraction withtributyl phosphate.
Deep blue is the bulk ions, light blue is thefission products (group I is Rb/Cs) (group II is Sr/Ba) (group III is Y and thelanthanides), orange is thecorrosion products (from stainless steel pipework), green are the major actinides, violet are theminor actinides and magenta is theneutron poison)
Waste from the commercial PUREX process include acidic aqueous raffinate with high levels of radioactive materials (HLLW) as well as high volumes of lower-level waste streams and degraded organic solvents.[13] The PUREX plant at theHanford Site was responsible for producing 'copious volumes of liquid wastes', resulting in the radioactive contamination of groundwater.[14]
Greenpeace measurements inLa Hague andSellafield 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 occurringbackground radiation. According toGreenpeace, this additional radiation is small but not negligible.[15]
^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.Bibcode:2004JRNC..261..485P.doi:10.1023/B:JRNC.0000034890.23325.b5.S2CID94173845.
^Goldemberg, Jose’ (2009).Interactions: Energy / Environment. Oxford: EOLSS Publications. p. 227.ISBN978-1-84826-540-0.
^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.
2(OP(OR)3)2.svg)
