Formation of Precipitate in High-Level Liquid Waste from Nuclear Fuel Reprocessing

Stability of high-level liquid waste (HLW) from nuclear fuel reprocessing was studied by using a simulated HLW. Fundamental works disclosed that precipitates formed during aging at ambient temperature or refluxing the simulated HLW in 2 mol/lHNO₃ solution consist mainly of Mo, Zr and Te contributing significantly to the formation of precipitate. When the simulated HLW was denitrated with formic acid or deacidified with NaOH, fractions of precipitated Mo, Zr and Te increased with pH and amounted to over 85% at pH 0.5, where the fraction of precipitated La was below 0.1%. For further treatment of HLW such as partitioning, denitration of HLW to pH 0.5 might be useful for removing Mo, Zr and Te from the solution without significant contamination with rare earths, Am and Cm.     

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.     

Wide Range Wave-Number Spectra of Place-to-Place Variations in Terrestrial Gamma-Ray Exposure Rate

The precipitation behavior of Pu. Np and Am during the denitration of high-level radioactive liquid waste (HLW) by formic acid was studied using a simulated HLW. The dissolution of the precipitate formed in denitrated HLW was also studied using oxalic acid to recover transuranium (TRU) elements from the precipitate. In the denitration, the precipitated fractions of TRU elements increased with decreasing acidity of the denitrated HLW. In the denitration at [HCOOH]/[HNO₃]=1.5, which was adopted in the partitioning process developed in JAERI, the precipitated fractions of Np and Am were only 0.6% and 0.06%, respectively, whereas that of Pu was 90 %. The precipitation fractions of Pu and Np did not depend on their concentrations in the range of 6x10^?5–6x10^?4 M for Pu and 10^?5–10^?3 M for Np. Plutonium was not precipitated itself by polymerization or hydrolysis but coprecipitated with other elements such as Mo and Zr. It was found that the precipitate formed during the denitration of 1 l of HLW could be dissolved in a 800 ml of 0.5 M oxalic acid solution.     

“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.     

Crud in Solvent Washing Process for Nuclear Fuel Reprocessing

In a solvent washing process for nuclear fuel reprocessing, one of the important problems is a formation of stable emulsions between organic and aqueous phases. These emulsions are called interfacial “crud”. Crud is defined as an emulsion stabilized by finely dispersed solids. These stable emulsions lead to decreased washing efficiency, lower phase separation, disturbance of the interfacial control at the settler of the extractor, and so on. Cruds formed by precipitates of Zr and tributyl phosphate (TBP) degradation products, such as di-n-butyl phosphate (HDBP), mono-n-butyl phosphate (H₂MBP), and phosphoric acid (H₃P0₄) are studied by experiments using a sodium carbonate solution as a washing reagent. Experimental results show that not only pH value of the washing reagent, but also phosphate and zirconium mole ratio (P/Zr) are important in crud formation. Moreover, it is shown that the complex of Zr and HDBP, or Zr and H₂MBP has a significant role in stabilizing emulsions. However, the complex of Zr and H₃P04 is not effective in stabilizing cruds.     

Liquid Metal Extraction for Removal of Molybdenum from Molten Glass Containing Simulated Nuclear Waste Elements

Laboratory-scale experiments for removing Mo and MoO3 from molten borosilicate glass were performed using liquid Cu as an extractant. Removal of Mo from the simulated HLW glass containing oxides of Nd, Fe, Zr, Mo, Sn, Ni, Sr, Cd, Ru, and Se was also performed, and the fractions of these elements transferred into Cu were examined. Mixtures of Cu anda ternary SiO₂-B₂O₃-Na₂O glass containing metallic Mo or MoO₃ were heated in an alumina crucible at 1,673K in an Ar environment. The amounts of Mo and MoO₃ added to 10 g of the ternary glass were fixed at 0.1 and 0.15 g, respectively. As for the glass containing metallic Mo, more than 90% of Mo was extracted into liquid Cu. Spherical Cu metal buttons containing Mo formed on the bottom of the crucible when Cu was added at more than 10 times that of Mo on a mass basis. Removal of Mo from the glass containing MoO3 was also achieved by the addition of Si as a reducing agent for the reduction from MoO₃ to Mo. The fraction of Mo extracted into liquid Cu depended on the molar ratio of Si to Cu added to the glass. The fraction increased up to 84% with an increase in the molar ratio of Si/Cu. However, the excess addition of Si may enhance the chemical interaction between the metal phase and the glass phase, and some of the metal phase containing Mo remained in the glass phase without forming a metal button. The optimum molar ratio of Si/Cu that produces the highest removal fraction was found to be approximately 0.5. Almost the same removal fraction of 88% was obtained from the simulated HLW glass under the condition of Si/Cu = 0.5. Nearly 100% of Ru was extracted into Cu with Mo, while Sr, Zr, and Nd were hardly extracted and remained in the glass.     

“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。     

辐照铀元件溶解液中钼的放化分离

研究了草酸对α－安息香肟－ＣＨＣｌ３萃取＾９９Ｍｏ和＾９５Ｚｒ－＾９５Ｎｂ的影响，寻找了最佳Ｍｏ萃取率和＾９５Ｚｒ－＾９５Ｎｂ去污因数的实验条件。提出了在ＨＮＯ３介质中使用α－安息香肟萃取Ｍｏ的分离程序，可从辐照铀元件溶解液中分离Ｍｏ。     

二(2-乙基己基)磷酸酯从硝酸体系中萃取Mo(Ⅵ)

用低浓缩铀靶代替高浓缩铀靶辐照进行~(99)Mo的生产是一个必然的趋势,但采用低浓缩铀靶辐照后裂变体系的组成可能发生改变,从而影响~(99)Mo的分离提取过程。为此,本工作以低浓缩铀辐照后溶解的模拟溶液为研究对象,在U(Ⅵ)大量存在的情况下,考察了二(2-乙基己基)磷酸酯(P_(204))从硝酸体系中萃取Mo(Ⅵ)的行为,重点研究了不同Mo(Ⅵ)浓度下萃取时间、萃取剂浓度、硝酸浓度、温度、其他主要元素(Cs(Ⅰ),Zr(Ⅳ),Y(Ⅲ),Nd(Ⅲ),Al(Ⅲ))等因素对萃取的影响。实验结果表明,不同Mo(Ⅵ)浓度下,P_(204)-磺化煤油对硝酸体系中Mo(Ⅵ)的萃取行为相似;在相比为1时,φ=10%P_(204)-磺化煤油对Mo(Ⅵ)即有较好的萃取效果;硝酸浓度不大于2mol/L时分配比随着硝酸浓度的增加而减少,但硝酸浓度进一步增大时对萃取无显著影响;萃取反应的ΔH和ΔG均为负值,表明该萃取是一个常温下能自发进行的放热反应;溶液中U(Ⅵ)和本工作考察的其它主要元素存在及其浓度的改变不会显著影响P204对Mo(Ⅵ)的萃取行为,且采用P_(204)可将Mo(Ⅵ)与Y(Ⅲ)、Nd(Ⅲ)、Al(Ⅲ)选择性地分离。     

Influence of Monoisodecyl Phosphoric Acid on Extraction of Neptunium with Diisodecyl Phosphoric Acid

The influence of monoisodecy1 phosphoric acid (MIDPA) on the separation of Np with diisodecy1 phosphoric acid (DIDPA) was studied from the aspects of the extraction rate and the back-extraction with H₂C₂O₄ solution. This study is important for the development of the partitioning method for high-level waste because MIDPA is one of the radiolysis products of DIDPA The increase in the MIDPA concentration was found to accelerate the extraction of Np initially in the pentavalent oxidation state. When the MIDPA concentration was 0.1 M, the extraction rate was ten times faster in the absence of H₂O₂ and twice faster in the presence of 0.5 M H₂O₂. It was also found that the extraction mechanism in the aspect of Np redox reactions did not change even in the presence of MIDPA

On the other hand, Np became less back-extracted with increasing MIDPA concentration. However, the tolerable concentration of MIDPA, determined by the required distribution ratio for the Np back-extraction, is higher than the estimated concentration of MIDPA produced radioiytically in the partitioning process.     

Evaluation study on properties of a macroporous silica-based CMPO extraction resin to be used forminor actinides separation from high level liquid waste

Basic properties of a silica-based octyl(phenyl)-N,N-diisobutylcarbamoyl-methylphosphine oxide (CMPO) extraction resin (CMPO/SiO2-P) was investigated.Adsorption behavior for some rare earth elements (RE) which are constituents of high level liquid waste (HLLW) and the long-term stability of the extraction resin in nitric acid solution were examined.The CMPO extraction resin was significantly stable in 3 mol·L?1 HNO3 solution at 50oC.Furthermore,the RE(III) were efficiently separated from non-adsorptive fission product (FP) elements such as Sr(II) in a column experiment using a highly nitric acid solution.The separation behaviors of the elements are considered to result from the difference in their adsorption and elution selectivity based on the complex formation with CMPO.There was no strong dependency of RE(III) separation efficiency on feed solution flow rate.Only from the perspectives of the acid-resistant behavior of CMPO extraction resin and the elution kinetics for the metal ions with the extraction resin,the CMPO extraction resin can be used in the modified MAREC process for HLLW partitioning.     

Effect of Platinum Group Elements on Denitration of High-Level Liquid Waste with Formic Acid

High-level liquid waste (HLW) from nuclear fuel reprocessing containing platinum group elements could be denitrated effectively with formic acid, and the solution indicated neutral even after the addition of excess formic acid. Platinum group elements, rhodium above all, were found to decompose formic acid and the action was more effective in the decomposition of excess formic acid in HLW. When HLW did not contain platinum group elements, both of unaltered nitric and formic acids remained after addition of stoichiometric amount of formic acid, leaving the solution slightly acidic. Excess formic acid in HLW not containing platinum metals was decomposed almost completely by irradiation with ⁶⁰Co γ-rays up to 2×10⁸R.     

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.     

Conversion of Simulated High-level Liquid Waste to Chloride for the Pretreatment of Pyrometallurgical Partitioning Process

A pyrometallurgical partitioning process is being developed for recovering minor actinides from high-level liquid waste resulting from PUREX reprocessing. Since the high-level liquid waste consists of concentrated raffinate, concentrated alkaline waste and insoluble residues, the various elements in the waste must be converted to chlorides before they can be sent on to the pyrometallurgical partitioning process. The conversion to chlorides is done by a combination of denitration and chlorination. The mass balance of these processes was measured in the present study using simulated high-level liquid waste. The results indicate that almost all of the alkali elements and Re, substituting for Tc, and significant amounts of Se, Cr, and Mo were separated by denitration, and that Cr, Fe, Zr, Mo, and Te were separated by chlorination. The remaining noble metals, Ni, U, and alkaline-earth and rare-earth elements were efficiently converted to chlorides, which were then supplied to the reductive extraction test using a molten salt/liquid-Cd system to demonstrate that the obtained chlorides are appropriate for processing by pyrometallurgical partitioning. In further reduction, noble metals and Ni were reductively extracted into the liquid-Cd phase, and the rare-earth elements and U into the liquid-Cd phase by adding Li reductant. These elements were completely separated from the alkaline-earth elements remaining in the chloride phase.     

DHDECMP-TBP/煤油从模拟高放废液中萃取回收Am-Gd的研究

研究了ＤＨＤＥＣＭＰ－ＴＢＰ／煤油萃取Ａｍ＾３＋、Ｇｄ＾３＋的各影响因素,在单级萃取实验的基础上,用０．６０ｍｏｌ／ＬＤＨＤＥＣＭＰ－１．４０ｍｏｌ／ＬＴＢＰ／煤油为有机相对模拟高放废液进行了逆流串级萃取实验降流串级反萃实验,成功地从模拟高放废液中分离回收了Ａｍ＾３＋和Ｇｄ＾３＋。     

Fractional Extraction of Zirconium,Niobium and Neptunium with Tri-n-butyl Phosphate

Radioactive Zr, Nb and Np were separated from each other by a continuous fractional extraction method.

The extraction of Zr and Nb was carried out with 1 % TBP in chloroform from the 10N hydrochloric acid solution of irradiated U. Under these conditions, Zr, Nb and Np could be extracted continuously with the organic solution, while U and other F.P. remained in the aqueous solution. Finally, Zr in the organic solution was back-extracted with concentrated hydrochloric acid containing 1sim;2 drops of hydrofluoric acid.

The method is convenient for the separation of Zr, Nb and Np from the F.P.

The distribution ratios in the extraction system have also been obtained for a number of radioactive nuclides.     

Modified TRUEX Process for the Treatment of High-Level Liquid Waste

The TRUEX process has been examined to recover Am and Cm from the high-level liquid waste of Purex reprocessing plant. Continuous counter current extraction and back-extraction experiments were carried out by a mixersettler using simulated waste solution for three process flowsheets, i.e. a process flowsheet developed by Argonne National Laboratory and other two process flowsheets which were modified in the scrub stage. The result indicates that the process flowsheet of Argonne National Laboratory cannot be applied for the high-level liquid waste containing high concentrations of lanthanide and actinide elements because of the formation of insoluble salts of these elements with oxalic acid, which is added to restrict the extraction of fission products such as Mo and Zr. A modified process flowsheet, which had only one scrub stage with high concentration of nitric acid, was found to be the best of three process flowsheets examined, where Nd as a simulated element of Am and Cm was sufficiently recovered and any precipitation of oxalate salt was not observed.     

Removal of zirconium from spent fuel solution by alginate gel polymer

The uptake behaviour of zirconium (Zr) in alginate gel polymer and removal of Zr from spent fuel solution have been studied by the batch and column methods. As a first step, alginate gel polymer was synthesized and conditioned. The uptakes of Zr were examined in several concentrations of HNO₃ solution (from 0.01 M to 9 M HNO₃) by a batch method. Stronger affinity of Zr was shown than strontium (Sr), cobalt (Co), uranium (U) and iron (Fe) in 1 M HNO₃. It has been reported that cation binding was stronger with ions of higher charge in the alginate gel polymer. In contrast, the free aqueous ion, Zr⁴⁺, is the dominant form of the Zr species in very acidic solution. Therefore, the strong affinity of Zr is explained. The uptake rate of Zr was also evaluated in 2.6 M HNO₃ solution, which was close to the HNO₃ concentration in actual HLLW from fuel reprocessing. The uptake of Zr reached equilibrium within 2 h. For the column experiment, fission products (FPs) solution containing rare earth elements (REEs), platinum group metals, alkaline metals, alkaline earth metals and the other elements was prepared from actual spent fuel and the concentration of HNO₃ was adjusted to 2.6 M. In the column experiment, the alginate gel polymer was packed into a column, and then a chromatographic experiment was performed using the FPs solution prepared from actual spent fuel. As a result, over 95% of the Zr was removed from the FPs solution. Molybdenum (Mo), technetium (Tc), yttrium (Y), palladium (Pd), tellurium (Te), cesium (Cs) and REEs were eluted by the successive use of H₂O, and 1 M and 3 M HNO₃.     

Extraction of Some Elements by Mixture of DIDPA-TBP and Its Application to Actinoid Partitioning Process

Effect of TBP on the extraction of Pu(IV), Zr(IV), Nb(V), Nd(III) and Am (III) was studied with diisodecylphosphoric acid(DIDPA) as an extractant. Hydrolyzable elements contained in the high-level liquid waste of fuel reprocessing,i.e. Pu(IV), Zr(IV) and Nb(V), could be extracted with mixtures of DIDPA and TBP of different compositions. Addition of TBP to DIDPA causes an increase and a decrease of respectively distribution rate and ratio of Zr(IV). These elements extracted were completely stripped with the aid of oxalic acid. An effect of TBP on separation of transplutonides (III) from lanthanoids(III) in the DIDPA and DTPA extraction system was also studied.

Based on the results, a process flow sheet utilizing the extractant of DIDPA and TBP mixture was contrived for partitioning actinoids in the high-level liquid waste.