Piaray Kishen Wattal

Process Development for Bulk Separation of Trivalent Actinides and Lanthanides from Radioactive High-Level Liquid Waste

Abstract In-house R&D studies have resulted in the development of processes for the bulk separation of trivalent actinides and lanthanides from radioactive high-level liquid waste. Synthesis of solvents, namely, n-octyl (phenyl)-N,N-di-isobutyl carbamoyl methyl phosphine oxide and diglycolamide-based tetra (2-ethylhexyl) diglycolamide (TEHDGA), at the required purity has been carried out, and a suitable process for their respective use in actual application has been developed. Inactive scale engineering runs comprised of simultaneous extraction and stripping operations were carried out to establish the process on an engineering scale, including reuse of the solvent system. The composition of surrogate high-level waste (HLW) used at engineering-scale studies corresponds to first-cycle raffinate from reprocessing of long-cooled pressurized heavy water reactor fuel with a burnup of 6500 MWd/tonne. Since trivalent lanthanides and actinides exhibits similar extraction behavior at higher acidity, cerium and lanthanum were only used in making surrogate HLW to represent all the trivalent lanthanides and actinides. Indigenously developed mixer-settlers using a passive system of mixing were used for these runs. Over a period of ˜10 h, ˜300 l of surrogate HLW solutions were contacted with solvent. The results of such repeated trials have shown near-total removal of cerium and lanthanum (>99.8% and 97%, respectively) at aqueous-to-organic ratio of 2.5:1 for a TEHDGA system. As the distribution coefficient values for trivalent actinide (241Am) are found to be significantly higher than those for trivalent lanthanides for both of the solvent systems under consideration, it can be inferred that separation of trivalent actinides along with lanthanides could be feasible using these solvent systems.

Resorcinol-formaldehyde coated XAD resin beads for removal of cesium ions from radioactive waste: synthesis, sorption and kinetic studies

A novel synthetic method was developed to synthesize resorcinol-formaldehyde (RF) resin in spherical form, of required mesh size, using XAD-4 as template beads. The synthesized RF-coated XAD (RF-XAD) beads were characterized, using different techniques. Suitable size and mechanical stability, along with their spherical shape, make these beads most appropriate for column operation. The efficiency of these beads was evaluated for removal of cesium from alkaline medium, in batch conditions, using a radioanalytical technique. The effect of sodium ion concentration, the initial cesium ion concentration and the contact time were also investigated. It was observed that the Kd value for Cs+ ions decreases with increase in Na+ ion concentration. The equilibrium data were fitted into different isotherm models, and were found to be represented well by the Langmuir isotherm equation, with a monolayer sorption capacity of 287 mg g−1. Kinetic modeling analysis, using pseudo first-order, pseudo second-order and intraparticle diffusion equations, shows that the pseudo second-order equation is the most appropriate model for the description of the sorption of cesium ions onto the RF-XAD beads. The rate constants were determined at different initial concentrations. The process mechanism was found to be complex, consisting of both surface sorption and pore diffusion.

Separation and Recovery of Cs from High Active Waste Simulant using Resorcinol Formaldehyde Polycondensate Resin: Batch and Column Studies

Resorcinol formaldehyde polycondensate resin has been extensively used for the separation of 137Cs from low and medium active alkaline waste solutions. The present study examines the suitability of the resin for separation of Cs from high active solutions. Batch uptake of Cs by RFPR has been evaluated from test solutions containing varying Na (0.6–3.0 M) and Cs (1.00 × 10−4 to 0.10 M) concentrations. Batch results have been used to establish Cs exchange isotherms of the resin and were found to follow Freundlich as well as Dubinin-Radushkevich (D-R) isotherm equations. The loading behavior of Cs on RFPR column has been evaluated by conducting five column runs using different Cs bearing solutions. Based on these results, it is concluded that the D-R equation can be used to predict saturated loading of Cs in the column. The saturation loading of Cs on 1 L bed was estimated to be as high as 13 g and 34 g from feed solution containing Cs concentration 2.57 × 10−4 and 4.30 × 10−3 mol/L respectively. Sequential elution of Na and Cs resulted in a high Cs bearing concentrate containing very low concentration of sodium. This study is relevant in the preparation of gamma source for radiation technology applications after immobilization of the concentrate in glass.

Management of spent solvents by alkaline hydrolysis process

This paper deals with the treatment method for the management of spent solvents of reprocessing origin (30% tri-n-butyl phosphate in n-dodecane) using the alkaline hydrolysis process. The consolidated work reported herein has established total conversion of TBP to aqueous soluble reaction products and transfer of near total radioactivity associated with the waste into this aqueous phase. It was also observed that n-dodecane did not take part in the reaction and separated out as the top layer. Repeated runs have established the process on engineering scale with regard to conversion of TBP into sodium salt of HDBP, butanol and recovery of the diluent practically free of TMP. The measured parameters, specifically, temperature and pressure did not deviate from set limits during the entire reaction time indicating safe operation. Experiments were also carried out on immobilization of the aqueous phase resulting from hydrolysis which contains almost all the radioactivity associated with the original spent solvent. These immobilization studies have established compatibility of the process generated aqueous stream with cementitious matrix.

Evaluation of Novel Solvent Systems Containing Calix-crown-6 Ligands in A Fluorinated Solvent for Cesium Extraction from Nitric Acidic Feeds

Several commercial calix-crown-6 ligands such as: calix[4]arene-bis-crown-6 (CC), calix[4]arene-bis-benzo-crown-6 (CBC), calix[4]arene-bis-naphtho-crown-6 (CNC), and bis-(octyloxy)calix[4]arene-mono-crown-6 (CMC) have been evaluated for their Cs uptake ability from nitric acid feed solutions in a novel solvent system. The calix-crown-6 ligands were made soluble in phenyl trifluoromethyl sulphone (PTMS) and 1 × 10−3 M solutions were used for metal ion extraction from nitric acid feed solutions. The extraction efficiency of the calix-crown-6 ligands was found out to be: CBC > CNC > CMC ˜ CC, which was attributed to trends of their partition coefficients. CNC was considered to be the most suitable extractant in view of its better organophilicity. The effects of concentrations of nitric acid and Cs carrier in feed were also investigated. Studies carried out using variation of ligand concentration yielded an extracted species of the type [CsL+][NO3−]org. Stripping of Cs(I) from the loaded organic phase was found to be possible when the organic phase contained minute quantities of Alamine 336. Co-current extraction and stripping studies were also carried out using CNC as the extractant.

Removal of dissolved Tri n-butyl phosphate from aqueous streams of reprocessing origin: engineering scale studies

Abstract Tri-n-butyl phosphate is extensively used in aqueous reprocessing of spent nuclear fuel for separating the fissile elements from fission products and other wastes. Presence of tri-n-butyl phosphate in these acidic aqueous solutions leads to a limitation with respect to evaporation of these streams for achieving volume reduction. Process development for removal of dissolved tri-n-butyl phosphate from acidic solutions has been addressed in the present work using solvent extraction route. Batch studies were followed with continuous runs using Combined Airlift based Mixer-Settler Unit (CAL-MSU). These studies have revealed that the dissolved TBP in aqueous solutions can be brought down from 160 ppm to 15 ppm using dodecane with a maximum loading of 3.6% TBP. All these studies on CAL-MSU have demonstrated the applicability of diluent in removing dissolved TBP from reprocessing aqueous solutions. The results obtained reveal an alternate method for stripping out dissolved TBP from reprocessing solutions...

Denitrification of high sodium nitrate bearing effluents using flow-through bioreactor

ABSTRACT Denitrification of sodium nitrate solution has been studied by isolating biomass, identification of suitable growth medium and finally its use in batch bioreactor. The progress of denitrification of 730 mg/l nitrate as NaNO3 solution has been evaluated for 10 consecutive cycles. The use of harvested cells with gradual increase of nitrate concentration in the batch bioreactor resulted in better acclimatized biomass. Complete denitrifi cation of different concentrations of NaNO3 solution up to 8800 mg/l NO3 – was reached in batch studies. A flow through bioreactor was assembled by growing biomass on to interstices of stainless steel modules and this system was used to study the effects of process variables like C/N ratio, addition of trace elements, etc., on denitrification of high nitrate laden solutions. Based on the gradual acclimatization of biomass to high nitrate bearing solutions, complete denitrification of 12,400 mg/l of nitrate was achieved. Successful demonstration of the process by cont...

Preparation of poly (4-vinylpyridine-DVB) based anion exchangers and its applications on the separation of 99TcO4− ions from reprocessing waste solution

ABSTRACT Three strong-base anion exchange resins were prepared by quaternization of poly(4-vinylpyridine-DVB) with different alkyl (-CH3, -C2H5, and -n-C4H9) halides. The base polymer in granular form was also synthesized in the laboratory. All three synthesized anion exchangers and a conventional strong-base anion exchanger were tested for separation of TcO4 − ions from acidic and alkaline test solutions, each containing 1.0 M NO3 − ions and 99mTc radiotracer. These resins were also evaluated for batch uptake of 99TcO4− ions from actual reprocessing waste solution. Both batch results showed that the resin with n-butyl group on the pyridine nitrogen has higher affinity for 99TcO4 − ions. Further, removal of 99TcO4 − ions from actual reprocessing waste solution was examined using column of poly(4-vinylpyridine-DVB) resin containing n-butyl group on the pyridine nitrogen. Almost complete removal of 99TcO4 − ions was obtained for 150 bed volumes of waste treated, indicating excellent column performance of th...

Catalytic reduction of U(VI) to U(IV) using hydrogen with platinum loaded on alumina and silica

During the reprocessing of spent nuclear fuel, uranium (U) and plutonium (Pu) are together extracted by employing tri-n-butyl phosphate (TBP)/dodecane mixture and their partitioning is achieved by adding uranous nitrate. The partitioning agent, uranous is conventionally produced by the electrolytic reduction of uranyl nitrate. An alternate route for the reduction of U from (VI) to (IV) using hydrogen (H2) as reductant was developed using platinum (Pt) based catalyst. Improvements in the development of the catalyst have been carried out in order to reduce the requirement of Pt without affecting the reduction performance. Experiments using 2 wt% Pt loaded on alumina beads and alumina powder have been performed and results are discussed. As the catalyst supported on alumina was found to be unstable in acidic environment, Pt loaded on silica powder has also been developed. Pt loaded on alumina and silica substrates have been tried to envisage the reduction behaviour using H2 as reductant in presence of hydrazine nitrate which acts as U(IV) stabiliser as well as reductant. Parametric studies have been carried out to optimise the process parameters namely pressure, temperature, U concentration, free acidity, hydrazine concentration and catalyst to U (C/U) ratio. 2 wt% Pt loaded on silica has been selected for further scale up studies for making uranous.

Reduction of uranium (VI to IV) by hydrogenation using Adams’ catalyst

AbstractPlutonium uranium reduction extraction using U(IV) as universal reductant for Pu partitioning is the only technology practiced internationally to recover U and Pu from spent nuclear fuels. Uranous requirement of Indian reprocessing plants is met by the electrolytic reduction of uranyl nitrate with 50–60% conversion. Though the current requirement can be met with this method, it increases the load on uranium purification cycle. In addition, it is a batch process with slow kinetics. In order to achieve higher conversion of uranyl nitrate to uranous nitrate, catalytic reduction method using hydrogen in presence of Adams’ catalyst (PtO2) was tried. Parametric studies have been performed in an autoclave to evaluate the effect of U(VI) concentration, the role of hydrazine nitrate and pressure. It is observed that kinetics is improved at higher pressures. The studies revealed that near total conversion of uranium from (VI to IV) can be achieved by the catalytic reduction route.