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Using N,N-dialkylamides to partition actinides for space power applications
Gogolski, Jarrod
Gogolski, Jarrod
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2020
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2022-04-17
Abstract
Plutonium-238 (238Pu) has been the preferred heat source for radioisotope thermoelectric generators (RTGs), which have been used to power spacecraft for decades. The primary method to produce 238Pu is to transmute neptunium-237 (237Np). Short irradiation times of the 237Np targets are necessary to not have heavier actinides form, such as 239Pu. This results in a mixture of 237Np, 238Pu, and fission products. Efficient separation of 238Pu and 237Np is necessary to reuse unreacted 237Np and produce pure 238PuO2 pellets to be used as the heat source in RTGs. The current extractant used to separate Np and Pu is tributylphosphate (TBP). A major consequence of using TBP is that its degradation products result in phosphorus contamination in both radionuclide products. The excess phosphorus has to be removed, a process that is costly and time-consuming. A workaround for this issue is to use an extractant that does not contain phosphorus, a N,N-dialkylamide. N,N-Dialkylamides (monoamides) are a class of metal extractants that have been considered as replacements for TBP for actinide separation for decades. Four N,N-dialkylamides: N,N-dihexylhexanamide (DHHA), N,N-dihexyloctanamide (DHOA), N,N-diethylhexylbutanamide (DEHBA), and N,N-diethylhexylisobutanamide (DEHiBA) have been selected for study based on their solubility in an aliphatic solvent and promising extraction capabilities for actinide +IV and +VI cations. These prerequisites are part of the current conditions being used at Oak Ridge National Lab (ORNL) for their 237Np(VI)/238Pu(IV) separation. The focus for this research was to ascertain which synthesized N,N-dialkylamide would be ideal to separate Np from Pu so that TBP can be replaced without changing other conditions of the separation (Monoamide Plutonium Neptunium Extraction-MAPNEX). The first area of study was the extraction efficiency of trace and semi-macro quantities of 237Np(IV,V, VI). The favorable N,N-dialkylamides for Np(VI) extraction and Np(V) stripping were DHHA, DHOA, and DEHBA. Also, after studying the extraction of Pu(III, IV, VI) in trace quantities, the same N,N-dialkylamides extract Pu(IV) well and can strip Pu(III) at low acidities. DHHA, which exhibited the best tetravalent and hexavalent actinide extraction overall, was used to successfully coextract Np(VI)/Pu(IV). Separately, these studies additionally showed that DHHA, DHOA, and DEHBA can be used to preferentially extract Np(IV) or Pu(VI) if desired. This study also investigated using a N,N-dialkylamide instead of di-(2-ethylhexyl) phosphoric acid (HDEHP) to purify the initial 237NpO2 targets. The 237Np, which has Th(IV) contaminants, is first procured from Np stockpiles. The purification process using HDEHP results in phosphorus contamination as well. DEHiBA, which can extract Np(VI) decently well and tetravalent actinides poorly, has proven to be a suitable alternative to HDEHP in a simulated study using trace concentrations of Np and semi-macro concentrations of Th. The last area of focus was to understand some of the behavior of the redox agent nitrous acid in N,N-dialkylamide extraction systems. Nitrous acid is used by ORNL to reduce Np(VI) to Np(V) so that the Np can be removed from the Pu. This study found at higher concentrations of nitric acid, nitrous acid will react with N,N-dialkylamides. Fortunately, the process conditions employed at ORNL use nitrous acid to reduce Np(VI) in low concentrations of nitric acid where the adverse effect from nitrous acid was not observed.
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