Yasuhiro Tsubata

Extraction Behavior of Metal Ions by TODGA, DOODA, MIDOA, and NTAamide Extractants from HNO3 to n-Dodecane

Four novel diamide and triamide extractants developed in our group, TODGA, DOODA, MIDOA, and NTAamide, were examined to see the extractability trends of up to 74 metal ions from nitric acid into n-dodecane. TODGA and DOODA have one or two etheric and two amidic oxygen donor atoms, and MIDOA and NTAamide have a nitrogen donor atom centered in the backbones and two or three diamidic O atoms. The former two extractants are expected to have high extractability for metal ions classified as hard acids and the latter ones have higher extractability for soft acids. TODGA and DOODA have high distribution ratios, D, for metal ions in the 2A-4A groups. On the other hand, MIDOA and NTAamide have high D values for group 5A-7A and 8 metal ions, which follows HSAB theory. The positive relation between the slope values of the extractant dependence and the extraction constant, α, is found, which suggests that a high order of successive formation of metal complexes with extractants gives high extractability.

Novel Extractant, NTAamide, and its Combination with TEDGA for Mutual Separation of Am/Cm/Ln

A novel extractant, N,N,N′,N′,N′′,N′′-hexaalkyl-nitrilotriacetamide (NTAamide), was developed for the extraction and separation of actinides (An) and lanthanide (Ln). NTAamide has a nitrogen atom in the center of a backbone and is bonded to three acetamide groups; the nitrogen and three amide oxygen atoms can work as the donor atoms. NTAamide extracted Am and Cm with a low distribution ratio of Ln in diluted HNO3, and the separation factor (SF) for An/Ln was equal to or higher than 23.6 using 0.5 M NTAamaide/n-dodecane and 0.2 M HNO3. The addition of N,N,N′,N′-tetraethyldiglycolamide as a masking agent to the NTAamide extraction system was effective for separating Am from Cm; the maximum SF was 6.5, one of the highest SF values obtained using the HNO3 and n-dodecane system.

Characterization and storage of radioactive zeolite waste

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 samples 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.

Highly Practical and Simple Ligand for Separation of Am(III) and Eu(III) from Highly Acidic Media.

A new, high-performance, highly practical, simple reagent called alkyl diamide amine (ADAAM) was examined for the separation of Am(III) and Eu(III). ADAAM has three donor atoms, one soft N-donor atom and two hard O-donor atoms, on a central frame. The combination of soft and hard donor atoms affords a tridentate that ensures remarkable extraction ability and selectivity of Am(III) and Eu(III) from highly acidic media (1.5 M HNO3) with a separation factor up to 25.

Continuous extraction and separation of Am(III) and Cm(III) using a highly practical diamide amine extractant

ABSTRACT A highly practical diamide-type extractant, which is an alkyl diamide amine with 2-ethylhexyl alkyl chains (ADAAM(EH)), was investigated for the mutual separation of Am(III) and Cm(III). ADAAM(EH) is a multidentate ligand with one soft N-donor atom and two hard O-donor atoms as part of its central frame. This tridentate arrangement of donor atoms provides selective binding to Am(III) compared to that with Cm(III) in highly acidic media (1.5 M HNO3), resulting in separation factors of up to 5.5. A continuous liquid–liquid extraction and stripping test was conducted using a multistage countercurrent mixer-settler extractor with ADAAM(EH) in n-dodecane. In this test, the separation of Am(III) and Cm(III) was achieved with very high yield.

Uranium and Plutonium Extraction from Nitric Acid by N,N-Di(2-Ethylhexyl)-2,2-Dimethylpropanamide (DEHDMPA) and N,N-Di(2-Ethylhexyl)Butanamide (DEHBA) using Mixer-Settler Extractors

Extraction properties of N,N-di(2-ethylhexyl)-2,2-dimethylpropanamide (DEHDMPA) for nitric acid, U(VI), and Pu(IV) were studied by a batch method using various nitric acid concentrations. The distribution ratio equations for nitric acid, U(VI), and Pu(IV) were derived. A continuous counter-current experiment was performed using mixer-settler extractors to determine the performance of a process that uses two types of monoamides, DEHDMPA and N,N-di(2-ethylhexyl)butanamide (DEHBA), as extractants. This process consisted of two cycles: one dedicated to extraction of U(VI) by DEHDMPA, and the other dedicated to the co-extraction of U(VI) and Pu(IV) by DEHBA. The feed solution used for the continuous counter-current experiment was 4 mol/dm3 nitric acid containing U(VI), Pu(IV), and simulated fission products. DEHDMPA exclusively extracted U(VI) from the feed at the 1st cycle, and the ratio of U recovered in the U fraction stream was 99.93%. The residual U and almost all Pu were extracted by DEHBA in the 2nd cycle, and the recovery of Pu in the U-Pu fraction stream was 99.94%. Concentration profiles of U and Pu in mixer-settlers were calculated using a simulation code, which confirmed that the calculation was effective for estimating the U concentration in the U fraction stream, and the U and Pu concentrations in the U-Pu fraction stream.

Recovery of U and Pu from Nitric Acid using N,N-di(2-ethylhexyl)butanamide (DEHBA) in Mixer-Settler Extractors

The recovery of U and Pu from nitric acid using N,N-di(2-ethylhexyl)butanamide (DEHBA) in mixer-settler extractors was calculated by a simulation code, and a continuous counter-current experiment using mixer-settler extractors was performed. The flow rate, stage number, and nitric acid concentration were chosen as the parameters for the calculation, and the simulation code provided appropriate experimental conditions for separating U from Pu. The continuous counter-current experiment was carried out with three mixer-settler extractors consisted of the following 5 steps: U–Pu extraction (6 stages), Scrub (10 stages), U recovery (6 stages), Pu back-extraction (10 stages), and U back-extraction (16 stages). The results of the continuous counter-current experiment showed that the percentages of U and Pu extracted using 1.5 mol/dm3 (M) DEHBA from 4 M nitric acid were > 99.9% and 97.84%, respectively. Extracted Pu was back-extracted to the aqueous phase via contact with 0.15 M nitric acid, while most of the U content remained in the organic phase. Uranium in the organic phase was then back-extracted via contact with 0.01 M nitric acid, and the percent of U in the U fraction stream was 96.06%. The percentages of U and Pu in the Pu fraction stream were 3.94 % and 97.48%, respectively.

Distribution Behavior of Neptunium by Extraction with N,N-dialkylamides (DEHDMPA and DEHBA) in Mixer-Settler Extractors

ABSTRACT The extraction properties of N,N-di(2-ethylhexyl)-2,2-dimethylpropanamide (DEHDMPA) and N,N-di(2-ethylhexyl)butanamide (DEHBA) for Np(V) and Np(VI) were studied by a batch method using various nitrate ion concentrations. The distribution ratios of Np(VI) obtained with DEHDMPA and DEHBA exceeded unity when the nitrate ion concentration was > 3 mol/dm3, while DEHDMPA and DEHBA barely extracted Np(V). A continuous counter-current experiment using mixer-settler extractors was performed to evaluate the behavior of Np in a process comprising two cycles using DEHDMPA and DEHBA as extractants. The feed was nitric acid containing U, Pu, Np, and several fission products. The results indicated that part of Np(V) changed its valence state to Np(IV) or Np(VI) after the 1st experimental cycle. The recoveries of Np in the streams of U fraction and U-Pu fraction were 63.7% and 29.1%, respectively.

Uranium-based TRU multi-recycling with thermal neutron HTGR to reduce environmental burden and threat of nuclear proliferation

ABSTRACT To reduce environmental burden and threat of nuclear proliferation, multi-recycling fuel cycle with high temperature gas-cooled reactor has been investigated. Those problems are solved by incinerating trans-uranium (TRU) nuclides, which is composed of plutonium and minor actinoid, and there is concept to realize TRU incineration by multi-recycling with fast breeder reactor. In this study, multi-recycling is realized even with a thermal reactor by feeding fissile uranium from outside of the fuel cycle instead of breeding fissile nuclide. In this fuel cycle, recovered uranium and natural uranium are enriched and mixed with recovered TRU to fabricate fresh fuels. The fuel cycle was designed for a gas turbine high temperature reactor (GTHTR300). Reprocessing is assumed as existing reprocessing with four-group partitioning technology. As a result, the TRU nuclides excluding neptunium can be recycled by the proposed cycle. The duration of potential toxicity decaying to natural uranium level can be reduced to approximately 300 years, and the footprint of repository for high-level waste can be reduced by 99.7% compared with the standard case. Surplus plutonium is not generated by this cycle. Moreover, incineration of TRU from light water reactor cycle can be performed in this cycle.