Yasuji Morita

Actinide partitioning from HLW in a continuous DIDPA extraction process by means of centrifugal extractors

ABSTRACT An experiment on actinide partitioning from real high level waste (HLW) was performed in a continuous process by extraction with diisodecylphosphoric acid (DIDPA) using a battery of 12 centrifugal extractors installed in a hot cell. The HNO3 concentration of the HLW was adjusted to 0.5xa0M by dilution. The extraction section had 8 stages, and H2O2 was added to extract Np effectively. After extraction, Am and Cm were back-extracted with 4xa0M HNO3 in 4 stages and Np and Pu were stripped with 0.8xa0M H2C2O4 in 8 stages. The actinides, except Np, were extracted from HLW with a very high yield. Although only 84 % of the Np were recovered in the present experiment, the recovery would be improved to 99.7 % by increasing the temperature to 45°C, the number of stages from 8 to 16 and the H2O2 concentration from 1xa0M to 2xa0M. Long-lived Tc and the main heat and radiation emitters Cs and Sr were not extracted and were thus separated from the actinides with high decontamination factors. About 98 % of Am and Cm were...

Effect of DTPA on the Extractions of Actinides(III) and Lanthanides(III) from Nitrate Solution into Todga/n‐Dodecane

Abstract For the separation of trivalent lanthanides [Ln(III)] and actinides[An(III)], the chelating agent, diethylenetriamine‐N,N,N′,N″,N″‐pentaacetic acid (DTPA), was used to decrease selectively the distribution ratios (D) of An(III) between TODGA/n‐dodecane and nitrate solution. The D values of An(III) and Ln(III) decrease with an increase of the pH of the aqueous phase. Their dependencies for light Ln(III) are different from those for medium and heavy Ln(III), which implies that the complexing ability of DTPA5− with Ln(III) is different from each other. The calculation of D values was performed using dissociation constants of H5DTPA and the formation constants of Ln–DTPA. Their results suggested the existence of Ln(III) ions without any complexation at the intermediate stage of the extraction reactions and demonstrated the curvilineal relation of D with pH in Ln–TODGA–DTPA extraction system. Americium(III) and Cm(III) form more stable 1:1 complex with DTPA5− and show lower D values than light Ln(III), it is expected to have a high separation against light Ln(III). The separation factors (SF), D Ln/D Am, and D Ln/D Cm, determined varied within the range of 1–60.

EXTRACTION OF NEPTUNIUM WITH DI-ISODECYL PHOSPHORIC ACID FROM NITRIC ACID SOLUTION CONTAINING HYDROGEN PEROXIDE

ABSTRACT Extraction of Np with di-isodecyl phosphoric acid (DIDPA) from HN03 solution containing H2O2> was studied from the aspect of the extraction mechanism and the enhancement of the extraction rate. Neptunium initially in the pentavalent oxidation state can be extracted into DIDPA even without any redox agents, but the extraction rate is rather slow. In the present work, H2O2 was found to accelerate the extraction. The rate was almost proportional to the 1/2-power of H2O2 concentration. Dependence of the extraction rate on HN03 concentration was also examined. Neptunium extracted into DIDPA from 0.5 M HN03 solution containing H2O2 was found to be in the tetravalent state in the organic phase, whereas neptunium in aqueous phase was in the pentavalent state. Reduction of Np(V) with H2O2 occurs rapidly in the presence of DIDPA. When the concentration of HNO3 was 4 M. the neptunium extracted was also tetravalent, but the reduction of NP(V) in an aqueous solution was also observed.

Extraction of Pentavalent Neptunium with Di-Isodecyl Phosphoric Acid

As a fundamental study for separating neptunium from high-level radioactive waste, the mechanism and the rate of extraction of pentavalent neptunium with DIDPA were investigated. From the analysis of oxidation state of neptunium in organic phase, it was proved that disproportionation reaction of Np(V) was concerned in the extraction. The reaction order of the extraction with respect to neptunium concentration was larger than unity. The extraction rate was much reduced by the decrease of DIDPA concentration. The dependence of the rate on nitric acid concentration and on the temperature was also examined.

Electrochemical and spectroscopic studies of Pu(IV) and Pu(III) in nitric acid solutions

Electrochemical and absorption spectroscopic properties of Pu(IV) and Pu(III) in nitric acid have been investigated by using cyclic voltammetry (CV) and UV–Visible spectroscopy. CV using a glassy carbon electrode suggested that the electrochemical reaction of Pu(IV) nitrate complexes were found to be a quasi-reversible reduction to Pu(III) species. The formal redox potentials (E0) for Pu(IV)/Pu(III) couples were +0.721, +0.712, +0.706, +0.705, +0.704, 0.694, and +0.696xa0V (vs. Ag/AgCl) when nitric acid concentrations are 1–7xa0M nitric acid solutions, respectively. These results indicate that the reduction product of Pu(IV) is only Pu(III). Further details for reaction mechanism of Pu(IV) were discussed on the basis of digital simulation of the experimental cyclic voltammograms. The absorption spectroscopic properties of Pu(III) and Pu(IV) in nitric acid solutions were investigated with UV–Visible spectrophotometry. As a result, it was founds that the intensities of the characteristic absorption peaks of Pu(III) and Pu(IV) tend to decrease with increasing nitric acid concentration for 1–8xa0M, and the peaks positions shifted longer or shorter wavelengths depending on the complex-forming abilities of Pu(III) and Pu(IV) with an increase in the nitric acid concentration.

Behavior of Neptunium in Chemical Process of Partitioning Long-Lived Radionuclides from High-Level Waste

The partitioning process has been developed in Japan Atomic Energy Research Institute for the removal of long-lived radionuclides from high-level waste (HLW), and was tested with actual HLW. But, the separation behavior of neptunium was not examined in the developed process. In the present study, the behavior of neptunium in the partitioning process was examined with actual HLW by means of the analysis of 237Np. At the first step of the partitioning process, 86% of neptunium was not extracted with TBP. It was suggested that most of neptunium was pentavalent in feed solution. At the following step, more than 90% of neptunium was extracted with DIDPA and was not stripped with 4 M nitric acid. It was proved in tracer experiments that most of neptunium was pentavalent before DIDPA extraction and Np(V) could be extracted with DIDPA by a slow rate. Neptunium which was extracted from pentavalent state was hardly stripped with nitric acid, which indicated that the extraction of Np(V) was an irreversible process. ...

An investigation into dissolution rate of spent nuclear fuel in aqueous reprocessing

A simple equation was proposed for the dissolution rate of spent LWR fuel, of which the change in the dissolution area was estimated by taking into account of the area of the cracks in the pellet caused by thermal stress during irradiation. The applicability of proposed equation was examined using LWR spent fuel dissolution test results in the present study as well as the results obtained by other workers. The equation showed good agreements with the dissolution test results obtained from spent fuel pellets and pulverized spent fuel. It was indicated that the proposed equation was simple and would be useful for the prediction of dissolution of spent LWR fuels.