P. K. Wattal

Recovery of neptunium from highly radioactive waste solutions of PUREX origin using CMPO

The partitioning and recovery of237Np from three types of simulated high level waste solutions originating from PUREX processing of spent nuclear fuels such as sulfate bearing high level waste (SB-HLW), HLW from a pressurised heavy water reactor (PHWR-HLW) and from a fast breeder reactor (FBR-HLW) have been carried out using a mixture of 0.2M CMPO and 1.2M TBP in dodecane. Quantitative extraction of neptunium was possible by either oxidizing it to the hexavalent state keeping K2Cr2O7 at 0.01M concentration or by reducing it to tetravalent state keeping Fe2+ at 0.02M concentration. Stripping of neptunium was carried out using different reagents, such as dilute nitric acid, oxalic acid and sodium carbonate. Almost quantitative recovery of neptunium has been achieved during these studies.

Recovery of actinides extracted by Truex solvent from high level waste using complexing agents

This work deals with the batch studies on stripping of actinides extracted by a mixture octyl(phenyl)-N,N-diisobutyl-carbamoylmethylphosphine oxide (CMPO) and tri-n-butyl phosphate (TBP) in n-dodecane (Truex solvent) from simulated high level waste (HLW) solution. The stripping of americium and plutonium from acid-bearing CMPO-TBP mixture is carried out using a mixture of weak acid, weak base and complexing agent as strippant. A mixture of formic acid, hydrazine hydrate and citric acid appeared to be best suited for efficient stripping of americium and plutonium. With appropriate modifications in the concentration of individual constitutents, this strippant can be used for the recovery of actinides from loaded Truex solvent with any acid content.

STUDIES ON THE PARTITIONING OF ACTINIDES FROM HIGH LEVEL WASTE USING A MIXTURE OF HDEHP AND CMPO AS EXTRACTANT

The paper describes the extraction and stripping behavior of actinides and lanthanides viz. americium, plutonium, uranium, cerium, and europium using a mixture of di-(2-ethylhexyl)phosphoric acid (HDEHP) and octyl(phenyl)-N,N-diisobutylcarbamoyl- methylphosphine oxide (CMPO) in n-paraffin as extractant. The extractant mixture combines the advantages of both the extractants for the partitioning of actinides from PUREX High Level Waste (HLW) solution. The extraction studies have been carried out from nitric acid medium as well as with simulated uranium-lean PUREX-HLW solution as relevant to Pressurized Heavy Water Reactor (PHWR) fuel reprocessing. The extracted actinides are stripped using a mixture of diethylenetriaminepentaacetic acid (DTPA), formic acid and hydrazine hydrate, leaving lanthanides in the organic phase. The lanthanides are subsequently stripped using a mixture of DTPA and sodium carbonate. The behavior of cesium, strontium, ruthenium, and zirconium is also studied using this mixed extractant.

Recovery of actinides extracted by Truex solvent from high level waste using complexing agents: II. Counter-current studies

These studies are an extension of carlier work on the recovery of actinides extracted by Truex solvent from simulated high level waste solution, with a mixture of weak acid, weak base and complexing agent used as a strippant. The effectiveness of the proposed strippant, consisting of formic acid, hydrazine hydrate and citric acid, is tested in a counter-current mode using mixer-settler in the present studies. The studies show that near quantitative recovery of americium and plutonium is feasible from acid-bearing Truex solvent with no reflux (reextration) of activity. Use of this strippant reduces considerably the generation of secondary waste.

Separation and Recovery of Uranium, Neptunium, and Plutonium from High Level Waste Using Tributyl Phosphate: Countercurrent Studies with Simulated Waste Solution

ABSTRACT The present work deals with countercurrent extraction studies on the partitioning of uranium, neptunium, and plutonium using 30% tributyl phosphate (TBP) from simulated high level waste solution generated during reprocessing of spent uranium fuel from pressurized heavy water reactors. The oxidation states of neptunium and plutonium were adjusted either by 0.01 M potassium dichromate or 0.01 M dioxovanadium ion. Neptunium and plutonium, extracted in the TBP phase, were stripped together using a mixture containing 0.05 M ascorbic acid and 0.25 M hydrogen peroxide in 2.0 M nitric acid solution. Although dioxovanadium ion is more effective for proper adjustment of the oxidation states of plutonium and neptunium, subsequent recovery of these actinides from loaded TBP is better if potassium dichromate is used for the valency adjustment. Results of the stagewise analysis of extraction and stripping of actinides using mixer-settlers are presented.

Sorption of radioactive strontium on a silica-titania mixed hydrous oxide gel

A silica-titania (Si−Ti) mixed hydroxide gel with Ti to Si mole ratio of 1∶1 was prepared. Studies on the sorption of radiostrontium from alkaline solutions having various concentrations of sodium were carried out with Si−Ti. The distribution coefficient (Kd) of strontium decreased with increasing concentrations of sodium from 0.1 to 4.0 mol·dm−3. The plot of logKd versus log [Na+] gave a slope of −0.55 as against the theoretical value of −2.0. This suggests that some other paths in addition to the simple stoichiometric ion exchange is also existing for sorption. From pH titrations the pHpzc of Si−Ti was found to be 4.22 coinciding nearly with the pH sorption edge. An attempt was made to correlate the sorption results with the surface characteristics of the sorbent. It was found that sorption is favored when the surface of the sorbent transforms into an anionic form. Sharp increase in the sorption of strontium was observed when the equilibrium pH was between 7 and 10. This behavior could be attributed to the abrupt increase in net negative charge on the surface of the sorbent.

Magnetic Carrier for Radionuclide Removal from Aqueous Wastes: Parameters Investigated in the Development of Nanoscale Magnetite Based Carbamoyl Methyl Phosphine Oxide

Abstract A study on superparamagnetic magnetite polymer composite development was undertaken for application to magnetically assisted chemical separation. Tetramethyl ammonium hydroxide as an alternative to ammonia was used as a precipitation agent to obtain nanoscale magnetite particles. Investigation on stoichiometry control of Fe(III) and Fe(II) ions suggested a correlation between alkalinity and initial Fe(III): Fe(II) ratio for precipitation of magnetite. Studies on polymerization conditions suggested that polymers setting at ambient conditions enable retention of superparamagnetic property of substrate magnetite. Vaporization method for impregnation of solvent extractant CMPO, yielded product that had a high sorption capability for radionuclide europium as compared to wet impregnation method.

Specific adsorption of cesium on silica-titania

The amphoteric nature of hydrous silica-titania gel and its sorption behaviour towards cesium were studied. In NaNO3 solution, the point of zero change (pHpzc) of the gel was found to be 4.22 by pH-titration. The fraction of protonated, deprotonated and neutral surface hydroxyl groups as a function of pH have been computed. Sorption of cesium increased with the increase of pH, reached maximum at a pH of 7 followed by a plateau. Significant uptake of cesium was observed even when the pH was less than pHpzc. Nearly 70% sorption was observed at pHpzc. The free energy of specific adsorption was found to be −18.7 kJ·mol−1.

Radiation effects on silica-titania gel

Laboratory experiments were performed on the sorption of cesium on gamma-irradiated silica-titania gel. The detrimental effects of absorbed dose on the affinity of the sorbent towards cesium was discussed. Data obtained from sorption studies were fit into heterogeneity based Freundlich isotherm. The empirical parameters, thus obtained were used to arrive at the site distribution function. The potential of site distribution function in foreseeing, the reusability and long-term residence of radionuclides in the sorbent has been highlighted.