A demonstration test was performed to separate minor actinides (MA; Am and Cm) by N,N,N',N',N'',N''-hexaoctyl nitrilotriacetamide (HONTA) as an extractant using mixer-settler extractors installed in ...a hot cell. A high-level liquid waste containing MA, and rare earths (RE; Y, La, Nd, and Eu) was used as the feed. HONTA diluted to 0.05 mol/dm3 in n-dodecane was fed as the organic phase, and a part of the organic phase was reused without solvent regeneration. HONTA effectively extracted MA, whereas RE were less extractable. Consequently, the Y, La, Nd, and Eu ratios distributed to a RE fraction were >99.9%, 99.2%, 61.8%, and 81.4%, respectively. The Am and Cm ratios distributed to an MA fraction were 86.8% and 74.7%, respectively, and a substantial amount of MA (0.12 g) was recovered in the MA fraction by the end of the cumulative duration of 40 h.
Fuel cycle technology for the transmutation of long-lived minor actinides (MAs) using an accelerator-driven system has been developed under the double-strata fuel cycle concept. A mononitride solid ...solution of MAs and plutonium diluted with zirconium nitride is a prime fuel candidate for transmutation of MAs. Pyroprocessing is suitable for recycling the residual MAs in spent nitride fuel with high radiation doses and decay heat. The spent nitride fuel is anodically dissolved, and the actinides are recovered simultaneously into a liquid cadmium cathode via molten salt electrorefining, which is the core step of the process. The process should be designed to achieve the target recovery yield of MAs and the acceptable impurity level of rare earth fission products in the recovered material. In this study, we evaluated the material balance during the pyroprocessing of spent nitride fuel to gain important insight on the design process. We examined the effects of changing the stage numbers in the countercurrent extraction and the zeolite treatment on material flow and quantity of waste. The results show that multistage extraction is necessary to meet the targets and the stage number of the zeolite treatment is recommended to be more than three from the aspect of the amounts of glass-bonded sodalite wastes.
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A continuous counter-current test to separate minor actinides (MAs: Am and Cm) by N,N,N',N',N'',N''-hexaoctyl nitrilotriacetamide (HONTA) as an extractant was performed using mixer-settler extractors ...installed in a hot cell. Nitric acid of 0.08 M (mol/dm
3
) containing MAs and rare earths (REs) recovered from high-level liquid waste was used as the feed, and the experiment was conducted for 14 h. MAs were extracted by 0.05 M HONTA dissolved in n-dodecane, with Am and Cm being recovered at 94.9% and 78.9%, respectively. HONTA hardly extracted Y, La, and Eu from the feed (99.9% for Y, 99.9% for La, and 96.7% for Eu), most of which were distributed to the RE fraction. A portion of Nd was extracted by HONTA, and consequently the ratio of Nd in the RE fraction was 83.5%. HONTA could separate MAs and REs by adjusting the nitric acid concentration without using complexants. In addition, the concentrations of MAs and some REs in each stage were calculated using a simulation code, and the results are consistent with the experimental values. This code indicates that the ratios of MAs in the MA fraction and REs in the RE fraction could be ≥99% by optimizing separation conditions.
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Absorbed-dose estimation is essential for evaluation of the radiation tolerance of minor-actinide-separation processes. We propose a dose-evaluation method based on radiation permeability, with ...comparisons of heterogeneous structures seen in the solvent-extraction process, such as emulsions forming in the mixture of the organic and aqueous phases. A demonstration of radiation-energy-transfer simulation is performed with a focus on the minor-actinide-recovery process from high-level liquid waste with the aid of the Monte Carlo radiation-transport code PHITS. The simulation results indicate that the dose absorbed by the extraction solvent from alpha radiation depends upon the emulsion structure, and that from beta and gamma radiation depends upon the mixer-settler-apparatus size. Non-negligible contributions of well-permeable gamma rays were indicated in terms of the plant operation of the minor-actinide-separation process.
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The Japan Atomic Energy Agency has chosen nitride fuel as the first candidate for the transmutation of long-lived minor actinides (MA) using accelerator-driven systems (ADS). The pyrochemical method ...has been considered for reprocessing spent MA nitride fuels, because their decay heat should be very large for aqueous reprocessing. This study was conducted to investigate the effect of decay heat on the pyrochemical reprocessing of MA nitride fuels. On the basis of the estimated decay heats and the temperature limits of the materials that are to be handled in pyrochemical reprocessing, quantities adequate for handling in argon gas atmosphere were evaluated. From these considerations, we proposed that an electrorefiner with a diameter of 26 cm comprising 12 cadmium (Cd) cathodes with a diameter of 4 cm is suitable. On the basis of the size of the electrorefiner, the number necessary to reprocess spent MA fuels from 1 ADS in 200 days was evaluated to be 25. Furthermore, the amount of Cd–actinides (An) alloy to produce An nitrides by the nitridation–distillation combined reaction process was proposed to be about one-quarter that of Cd–An cathode material. The evaluated sizes and required numbers of equipment support the feasibility of pyrochemical reprocessing for MA nitride fuels.
In this study, we examined the masking effects of 16 water-miscible reagents, on the extraction of Mo, Re, Ru, and Pd. These reagents included soft N- and/or S- donor atoms in their structures and ...they function as multidentate ligands. The extractants, methylimino-N,N-dioctylacetamide (MIDOA) and N,N,N',N',N'',N''-hexaoctyl-nitrilotriacetamide (NTAamide(C8)), show significantly high distribution ratios for these metals(D(M)), were employed in this study. Masking effects were observed as a decrease in D(M) with an increase of masking agent concentration in these extraction systems. The results showed that D(Pd) can be considerably decreased by the addition of thiourea, cysteine, cysteamine, trisaminoethylamine, and diethylenetriamine, and D(Ru) is decreased by the addition of thiourea, trisaminoethylamine, and diethylenetriamine. Pd and Ru can be masked by similar reagents including N- or S- donor atoms, which suppressed the extraction into the organic phase. In contrast, D(Mo) was only slightly masked by the above-mentioned reagents. The masking of Mo was achieved using methylimino-N,N’-diethylacetamide, hexaethyl-nitrilotriacetamide, iminodiacetic acid, and iminodimethylphosphonic acid, which have a central N(CH2C(P)=O)2 framework that is important for this purpose. A masking agent for Re was not found in this study.
A continuous counter-current experiment using mixer-setters was performed to evaluate the separation performance of N,N-dialkylamides for U and Pu in the presence of their degradation products. The ...experiment consisted of two cycles: the first cycle employed N,N-di(2-ethylhexyl)-2,2-dimethyl- propanamide (DEHDMPA) for selective extraction of U(VI) and the second cycle employed N,N-di(2-ethylhexyl)butanamide (DEHBA) for co-extraction of U(VI) and Pu(IV). DEHDMPA extracted most of the U(VI) from the feed, and the ratio of U(VI) recovered from the U fraction was 99.57%. DEHDMPA hardly extracted Pu(IV), and the decontamination factor for U with respect to Pu in the U fraction was 1.1 × 104. The raffinate from the first cycle was used as the feed for the second cycle, and almost all the U(VI) and Pu(IV) were co-extracted by DEHBA. The degradation products of the extractants had no detrimental effect on the two-phase separation and operation of the mixer-settlers.
For the safe storage of zeolite wastes generated by the treatment of radioactive saline water at the Fukushima Daiichi Nuclear Power Station, this study investigated the fundamental properties of ...herschelite adsorbent and evaluated its adsorption vessel for hydrogen production and corrosion. The hydrogen produced by the herschelite sample is oxidized by radicals as it diffuses to the water surface and thus depends on the sample's water level and dissolved species. The hydrogen production rate of herschelite submerged in seawater or pure water may be evaluated by accounting for the water depth. From the obtained fundamental properties, the hydrogen concentration of a reference vessel (decay heat = 504 W) with or without residual pure water was evaluated by thermal-hydraulic analysis. The maximum hydrogen concentration was below the lower explosive limit (4%). The steady-state corrosion potential of a stainless steel 316L increased with the absorbed dose rate, but the increase was repressed in the presence of herschelite. The temperature and absorbed dose at the bottom of the 504 W vessel were determined as 60 °C and 750 Gy/h, respectively. Under these conditions, localized corrosion of a herschelite-contacted 316L vessel would not immediately occur at Cl
−
concentrations of 20,000 ppm.
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The mutual separation of Am, Cm and lanthanides (Ln) by the hydrophilic and lipophilic ligands present in both aqueous and organic phases was studied. The hydrophilic ligands used here are DTPA ...(diethylene-triamine-N,N,N’,N’’,N’’-pentaacetic acid), malonamide, amic acid, TEDGA (N,N,N’,N’-tetraethyl-diglycolamide) and DOODA(C2) (N,N,N’,N’-tetraethyl-3, 6-dioxaoctanediamide) and four types of extractants, TODGA (N,N,N’,N’-tetraoctyl-diglycolamide), TDdDGA (N,N,N’,N’-tetradodecyl-diglycolamide), DOODA(C8) (N,N,N’,N’-tetraoctyl-3, 6-dioxaoctanediamide) and DOODA(C12) (N,N,N’,N’-tetradodecyl-3, 6-dioxaoctanediamide). DOODA(C8, 12) can extract light lanthanides with high D values into the organic phase. On the other hand (TO, TDd)DGA shows the opposite trend for Ln extraction. The high separation factor, over 1000, between D for La and that for Gd can be seen in the extraction using the extractant, TDdDGA, and DOODA(C2) as the masking agent in the aqueous phase. The separation factors for Am/Cm by DOODA(C8, C12) and (TO, TDd)DGA are 1.51-1.94, while values over 3 can be obtained by the conditions introduced in this work.
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