“The Third Phase” of Extraction Processes in Fuel Reprocessing, (I)

Formation conditions, compositions and structures of precipitates or “the third phase” were systematically examined in the systems of Zr and radiation depleted products of TBP, such as HDBP, H₂MBP, H₃PO₄, by means of elemental analysis, X-ray diffraction, infrared spectra and ¹H-NMR. It was confirmed that one of the most important origins for the third phase is a complexation between Zr ion and the depleted products of TBP. Followings were also elucidated:

(1) When the depleted products co-exist with each other, the cooperative effects on the precipitate formation appear in low acid solutions.

(2) Precipitate formation depends on the mole ratio of HDBP/Zr. The amount of precipitate reaches the maximum at the mole ratio of approximately 2 and decreases with increasing concentration of HDBP and finally disappears at approximately 10.

(3) Precipitate formed at the mole ratio of approximately 2 has the chemical formula, Zr (NO₃)₂ (HDBP)₂ (OH)₂.

(4) Precipitates of the Zr-H₂MBP system begin to appear at the concentration of H₂MBP in one order of magnitude smaller than that of HDBP in Zr-HDBP system.

(5) Precipitates of Zr-H₂MBP system have no NO₃ ion and a basic structure of Zr-(HMBP)₂(OH)₂ with an interlayer distance of 16 Å.     

乏燃料后处理萃取过程中的界面污物

在乏燃料后处理萃取工艺工程中，萃取剂和溶剂的辐解以及料液中不溶性固体微粒的存在导致产生界面污物。界面污物严重影响萃取柱的正常操作。文章就有关界面污物的研究状况进行概要评述。目前，普遍认为，在Purex流程萃取过程中，尤其是在一循环中，界面污物的产生与Zr和TBP降解产物HDBP、HzMBP、H3PO4形成的沉淀以及料液中存在的不溶性RuO2、Pd等微粒的表面化学现象有关。沉淀是否产生以及形成的界面污物类型与HDBP／Zr摩尔比和水相条件密切相关。此外，煤油等稀释剂的降解产物也是形成界面污物的一个不可忽略的因素。从萃取设备中排出的界面污物可用Na2CO3或草酸进行处理。在界面污物模拟实验中，需同时考虑Zr与TBP降解产物HDBP、H2MBP、H3PO4形成沉淀和不溶性微粒RuO2、Pd的影响等多种因素。     

锆与H2MBP生成萃取界面污物的行为研究

观察了ＴＢＰ的降解产物磷酸一丁酯（Ｈ２ＭＢＰ）与锆在３０％ＴＢＰ正十二烷／ＨＮＯ３体系中产生界面污物的现象；进行了锆浓度，Ｈ２ＭＢＰ浓度以及硝酸浓度对界面污物生成的影响，并对界面污物固体进行了红外光谱分析和Ｘ射线荧光分析。     

锆与HDBP形成萃取界面污物的行为研究

研究了锆与ＨＤＢＰ形成界面污物的行为，分析了Ｚｒ－ＤＢＰ界面污物的基本构成，并就观察到的ＨＤＢＰ与锆形成界面污物过程出现的“异常”现象进行了探讨，提出了产生这种现象的可能机理。     

Separation of Precipitate-forming Elements during High Level Waste Treatment Using Acidic Zirconium Salt of Dibutyl Phosphoric Acid

Iron extraction and the limits of iron dibutyl phosphate precipitation were investigated for use in TPE/RE recovery and partitioning with the use of the acidic Zr salt of dibutyl phosphoric acid (ZS HDBP, Zr:HDBP = 1:9), dissolved in 30% TBP with Isopar-L. The presence of Mo and the solvent loading with RE affected the Fe extraction in the opposite ways. Slow kinetics and process irreversibility were found for the Fe extraction with the ZS HDBP solution. To increase acceptable Fe concentration in a HLW, reduction of Fe(III) by ascorbic acid (AA) was studied to be carried out continuously in the feed flow, just before entering the head contactor of the partitioning extraction cycle. The Mo extraction is of the same order as that of TPE and RE; so its selective stripping prior to the TPE/RE separation is required. This can be done using the DTPA or H₂O₂ solution in diluted nitric acid. The effectiveness of the process was verified by laboratory scale trials on the centrifugal contactor rig using simulated HLW. The Zr-to-HDBP ratio of 1:6 was found to be useful to decrease the Fe and Mo extractability for their better backwashing with complexants.     

“The Third Phase” of Extraction Processes in Fuel Reprocessing, (III)

Abstract

The coordination and complexation behavior of Zr-dibutylphosphate was investigated by ³¹P-NMR measurement, in order to clarify the mechanism of the previously reported phenomena that the precipitates once formed at low HDBP/Zr mole ratio of 2 in an organic solution disappears with increasing HDBP/Zr. As the results of ³¹P-NMR spectrum measurements, following two steps were identified. (1) Species having Zr:DBP=1:3 immediately reacts with free HDBP to form species having Zr : DBP=1:4 with increasing HDBP/Zr, (2) Under the presence of higher HDBP/Zr concentration, species having more DBPs than 4 to one Zr are observed. DBP^? (or HDBP) ligands in these species bridge two Zr ions to form three-dimensional net-work structure and have an interaction with HDBP in the solvent. Consequently, the mechanism was elucidated for the re-dissolution of Zr-DBP precipitates with sufficient free HDBP producing highly viscous oilyliquid.     

Magnetic Properties of Uranium Carbides after Neutron Irradiation

Fundamental investigations on valence control and solvent extraction of americium were carried out to develop a method for americium separation from reprocessing solution. In order to adjust americium valency from III to IV and VI, (NH₄)₁₀P₂W ₁₇O₆₁ synthesized was used as complexant stabilizing Am(IV). Oxidation behavior of americium was investigated as a function of (NH₄)₁₀P₂W ₁₇O₆₁ americium ratio. Using 0.1M (NH₄)₂S₂O₈ and 0.01M AgNO₃ as oxidation reagent, Am(IV) was obtained quantitatively at the ratio of 15. On decreasing the ratio to 0.6, 92% of americium was adjusted to Am(VI). The concentration of (NH₄)₂S₂O₈ could be reduced to 1/15 compared to the previously reported method in which no complexant was used. Americium(IV) was also prepared by reacting O₃ and AgNO₃ but no Am(VI) was obtained even at low (NH₄)₁₀P₂W ₁₇O₆₁ to americium ratio.

Americium(VI) could be extracted by tri-n-butyl phosphate stably without influence of (NH₄)₁₀P₂W ₁₇O₆₁. The distribution coefficient of Am(VI) was 4 between 100% tri-n-butyl phosphate and 1 M nitric acid, and separation factor from Nd(III) was 50.

With regard to the americium separation method which implemented valence control followed by extraction, adding (NH₄)₁₀P₂W ₁₇O₆₁ led to minimization of waste volume and improvement of extraction efficiency.     

A contribution on the mechanism of plutonium nitrate extraction by mono- and dibutyl phosphates

In a previous communication [1] we examined the effect of monobutyl phosphate (MBP) and dibutyl phosphate (DBP) on the extraction of plutonium nitrate by tributyl phosphate (TBP).In the present work the mechanism of plutonium extraction by MBP and DBP is elucidated. An investigation was made of the mechanism of plutonium nitrate extraction by MBP and DBP with an ionic strength equal to 6. It was shown that plutonium is extracted in the form of PuK₄, where K is [(C₄H₉)₂PO₄ ^– or [C₄H₉HPO₄]^–. It was calculated that the equilibrium constant for the reaction of plutonium nitrate with MBP equals (1.5 ± 0.25)·10³ and with DBP, (6.15 ± 0.85) ·10³.     

The Effect of Hydrogen Bond Formation between Tri-n-Butyl Phosphate and Carboxylic Acid on Uranium Distribution

The distribution ratio of uranium between tri-n-butyl phosphate and water decreased with the addition of water-insoluble carboxylic acids such as benzoic, lauric, oleic and stearic acids at various concentrations of nitric acid. It is suggested that the decrease of uranium distribution ratio may be due to hydrogen bond formation between tri-n-butyl phosphate and carboxylic acid. It was indicated from infrared spectrometry that a strong hydrogen bond is formed between tri-n-butyl phosphate and carboxylic acid. In other cases, water-soluble carboxylic acids such as acetic, adipic and oxalic acids had no effect at all on the distribution ratio of uranium at various concentrations of nitric acid.     

Pulsed Neutron Experiment in Fast Natural Uranium Systems

Separation of fission products was carried out by solvent extraction using tri-n-butyl phosphate (TBP). ^{144, 144}Ce, ⁹¹Y and ⁹⁵Zr and ⁹⁵Nb were extracted with TBP from a freshly prepared nitric acid-potassium bromate solution. These nuclides in TBP were successively back-extracted with various aqueous solutions: ^{144, 144}Ce with hydrogen peroxide and nitric acid solution, ⁹¹Y with hydrochloric acid solution, and ⁹⁵Zr-⁹⁵Nb with oxalic acid solution. The other nuclides were not extracted by the TBP and remained in the nitric acid and potassium bromate solutions.     

Removal of Fission Products and Their Complexing Agents from Degraded Solvent by Ion Exchange Method

Removal of fission products and their complexing agents from chemically degraded TBP/kerosene and particularly from the kerosene diluent, was studied. With the use of column technique with mixed anion and cation exchange resins, more than 80% of the fission products and their complexing agents were removed. The carboxylic acids, which constitute one of the degradation products bringing about decrease of uranium extractability, were completely removed together with the complexing agents thereof by anion exchange resin. Removal was also obtained of monobutyl and dibutyl phosphoric acids (H₂MBP and HDBP), which can retain fission products and which in actual practice are liable to be formed by hydrolysis of TBP under intensive irradiation with concentrated nitric acid and/or at the elevated temperatures.     

Extraction kinetics of ruthenium in the mixture of tri-n-butyl phosphate and n-dodecane

The extraction kinetics of ruthenium in the mixture of tri-n-butyl phosphate and n-dodecane has been investigated experimentally, using a constant interfacial area stirred cell. Effect of stirring speed, temperature, specific interfacial area and concentrations of reactant and nitric acid on the extraction rate of ruthenium were studied to identify the reaction location, extraction reaction regime and also to determine the intrinsic rate constant of the reaction and mass transfer coefficients. A systematic approach was adopted for the determination of rate parameters. Based on experimental results it is concluded that the reaction with the extractant takes place at interface and the overall rate is controlled by both the intrinsic kinetics and the mass transfer of the species involved.     

Purification of Water–Salt Solutions by Ti(IV) and Zr(IV) Phosphates

The sorption of - and -emitting radionuclides from processed solutions of liquid wastes with a salt background, using powdered titanium and zirconium phosphates with the composition TiO(OH)_2(1–x)(HPO₄) _x ·nH₂O, where x = 0.23–1, and Zr(HPO₄)₂·nH₂O, was studied. The decontamination process was most efficient on titanyl hydrophosphate in the range pH = 4–6. It was found that stagewise sorption of radionuclides under static conditions and with flow of solution and sorbent is best. It was found that for decontamination of liquid wastes an amorphous sorbent based on titanium (IV) hydroxide-phosphate matrix makes it possible to remove radionuclides and simultaneously petroleum products from the processed solutions.     

Group Type Analysis of Carboxylic Acids Formed by Degradation of i-Dodecane and Kerosene with Nitric Acid

Quantitative analysis of carboxylic acids formed by diluent degradation with nitric acid was investigated by group type analysis.

Based on Beer-Lambert's law and alkaline titration, the apparent molar extinction coefficient (E^a _max) for C=0 stretching vibration (v _c=0: 1,694 cm^?1) of carboxylic acids in degraded diluents was determined to be 620±20 mole^?l·/·cm^?1. Using this value, the concentrations of carboxylic acids formed under various nitric acid concentrations, temperatures and contact times were determined, and some of the results were compared with the measured data from alkaline titration.

The method might also be used to determine the concentration of carboxylic acids hydrogen-bonded with tri-n-butyl phosphate (TBP) in degraded solvent.     

Theoretical calculation of neutron cross sections for 90,91,92,94,96Zr in the incident energy range between 200 keV and 20 MeV

Neutron cross sections of ^{90,91,92,94,96}Zr were calculated in the incident energy (E_n) range from 200 keV to 20 MeV for the revision of the 4th version of the Japanese Evaluated Nuclear Data Library (JENDL-4.0). The calculation was carried out by using conventional nuclear reaction models such as the spherical optical model, the distorted wave Born approximation, preequilibrium models, and the multi-step statistical model. Parameter values of these nuclear models were adjusted with the aid of experimental cross sections which were published after the JENDL-4.0 evaluation. Cross sections were computed for total, elastic and inelastic scattering, (n, γ), (n, 2n), (n, p), (n, α), (n, nα), and (n, x) = (n, d) + (n, np) reactions, and they were almost consistent with the experimental data. The cross sections were also estimated for the metastable states with the half-life larger than 1 sec. The obtained results well reproduced measured cross sections for the reactions ⁹⁰Zr(n, 2n)^89mZr, ⁹¹Zr(n, x)^90mY and ⁹¹Zr(n, nα)^87mSr.     

Continuous separation of molybdenum and zirconium from simulated high-level liquid waste with a Taylor–Couette contactor

Reprocessing of spent nuclear fuels generates high-level liquid waste (HLLW) which undergoes vitrification into borosilicate glass before final geological disposal. To ensure the quality of the glass, control of the concentration of chemical species such as molybdenum (Mo), which has an adverse impact on the vitrification process, is critical. Also, zirconium (Zr) can cause crud in washing process and Zr-93 is a long-lived fission product needed to be separated. In this study, a liquid–liquid countercurrent centrifugal contactor with Taylor–Couette flow (TC contactor) was applied to practical multi-species cases. Continuous separation of Mo and Zr from a simulated HLLW with bis(2-ethylhexyl) phosphoric acid (HDEHP) as extractant has been performed. Among a variety of metals in simulated HLLW, Mo, Zr, Y, and Fe are extractable, Mo and Zr were separated from HLLW by equilibrium, and Fe/Y separation was achieved by the effect of non-equilibrium state in TC contactor. Addition of tributyl phosphate could improve extraction of Mo. This study has expanded the scope of the TC contactor to multi-species separation processes.     

Relative Reactivities of Radioiodine Released from U3O8 in Oxygen and Helium Atmospheres toward Propane

Chemical form of radioiodine released from U₃O₈ will be UI_x in helium and I₂ in oxygen atmospheres. Relative reactivities of these iodines toward organic compounds have been studied at a temperature range of 250~500°C, by choosing propane as a reactant. The UI_x has a higher reactivity than I₂, particularly at a temperature range of 300~400°C. Above that temperature, both iodine species show a similar reactivity. In the reactions, CH₃I, C₂H₅I, i, and n-C₃H₇I are formed: relative yields depend on both temperature and atmosphere. Two reaction processes participate in the formation of organic iodides. One is an iodine-initiation reaction and is relatively important at a lower temperature. The other is a radical-initiation reaction and become predominant as the temperature rises.     

在Na2CO3溶液中季铵盐萃取Zr机理的研究

用红外光谱分析和季铵盐萃取研究了Zr—DBP界面物在碳酸钠溶液中的溶解机理。结果表明,Zr—DBP界面物在碳酸钠溶液中生成了能溶于水的配合阴离子Zr(CO3)4^4-或ZrO(CO3)3^4-,Zr—DBP界面物溶解前后与纯HDBP的红外谱图相比,溶解液中DBP的P—O键的特征峰与纯HDBP的更为接近。说明界面物在溶解Na2CO3过程中,破坏了原有界面物的配合,DBP不再与Zr配合,而以自由离子形式存在。     

Separation of Uranium from Fission Products

A simple procedure has been developed for separating U from F.P. and neutron-irradiated Th. The separation is performed with tri-n-butyl phosphate in a system of dodecane-mixture of sulfuric acid and aluminium nitrate.

Uranium dioxide was irradiated with 20 MeV bremsstrahlung, which produced both ²³⁷U and F.P. The target was dissolved in dilute nitric acid and U was extracted into the organic phase of the above mentioned system. Finally, U in the organic phase was back-extracted into an aqueous phase. The γ-ray spectrum and decay curve of the separated U fraction show no radioactive nuclides other than U isotopes and its decay products.

This method can also be applied to the preliminary separation of ²³⁸U from neutron- irradiated Th.

The distribution ratios (Kd) for U, Th and some other elements in the extraction system are also given.     

过氧化氢湿法氧化模拟有机污物的研究

用过氧化氢湿法氧化处理有机污物,研究了过氧化氢的投加量和投加方式,催化剂以及pH值等对反应的影响。结果表明,为提高H2O2的利用率,H2O2应以间歇式投加,选取Fe^2 作催化剂,体系pH值为1～2。为保证反应快速启动和平稳进行,反应时将催化剂总投加量的1／3提前与要分解氧化的物质混合一段时间,剩余2／3在反应进行中连续定量滴加,并于反应结束前半小时投加完毕。正交试验表明,分解2．00g界面污物的最佳实验条件为：pII-1．10mL30％H2O2溶液,10mL 0．1mol／L Fe^2 溶液。反应结束时,反应釜底部的白色残余物经红外分析和元素分析表明主要是磷酸盐。H2O2湿法氧化能够有效分解TBP／OK,HDBP／OK溶液和界面物。