Masumitsu Kubota

Precipitation Behavior of Platinum Group Metals from Simulated High Level Liquid Waste in Sequential Denitration Process

The precipitation behavior of the platinum group metals from a simulated high level liquid waste (HLLW) was experimentally examined in a sequential denitration process with formic acid. The precipitation fractions of Ru, Rh and Pd were found to depend on pH of the denitrated solution. The precipitation fractions of Ru and Rh increased linearly with pH of the denitrated solution and became more than 98% over pH 7, and almost all Pd was precipitated at pH around 2. Results showed that more then 98% of Rh and Pd and about 80% of Ru could be recovered as precipitates from HLLW, by means of the sequential denitration process.

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.

Recovery of Technetium from High-Level Liquid Waste Generated in Nuclear Fuel Reprocessing

We have been working on partitioning for high-level liquid waste during the past 20 years. In the prescnt study we focus mainly on the four group Separation. The main item to be estabUshed is how to separate transuranic Clements (TRU) and Tc f rom high-level liquid waste (HLLW). The preferential Separation of T R U was considered so as to avoid its spread into many fractions which would be generated in a partitioning process. The Tc Separation was conducted on the rafiinate generated from the T R U extraction with diisodecylphosphoric acid. Two kinds of chemical methods, precipitation and adsorption with active carbon, have been developed for the Tc Separation. The four group partitioning process, including Tc Separation, will be demonstrated by using actual HLLW in the near future. be solidified into a stable form and stored for 30 to 50 years, and then disposed of in a deep geological formation several hundred meters or more underground. Research and development work is underway for this future implementation. By looking into the future possibilities of nuclear technology, attention has been drawn to nucUde partitioning and transmutation. These processes are an effort to provide the benefits of nuclear energy to future generations through long-term basic R and D; they do not seek to serve as short-term alternatives for established or planned back-end policies. However, presently nuclide partitioning and transmutation can be expected to increase the efficiency of HLLW disposal and will allow utilization of existing resources in spent fuel.

Effects of Nitrite on Denitration of Nuclear Fuel Reprocessing Waste with Organic Reductants

Denitration with organic reductant such as formic acid is exclusively used for controlling acidity of the highly radio-active liquid waste (HLW) from nuclear fuel reprocessing. This method, however, has a problem in suppressing a vigorous chemical reaction. The present study aimed at overcoming the disadvantage by the addition of nitrite to nitric acid solution before denitration. As a result, the nitrite addition in the denitration of simulated HLW could reduce the induction period to almost zero and the maximum gas evolution rate to less than 60% of that obtained in the absence of nitrite. Effectiveness of nitrite addition was enhanced especially for HNO3 concentration higher than 8 mol/l. The effective nitrite concentration was over 0.01 mol/l. in the reaction mixture.

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.

Decomposition of a cation exchange resin with hydrogen peroxide

Ion exchange resins are widely used in the field of nuclear industry. The present work aimed at the development of a method for complete decomposition of cation exchange resins with H2O2 in the presence of Fe3+ ion. The decomposition reaction proceeded at ambient temperature and decomposition time was greatly shortened with increasing concentration of Fe3+ ion rather than that of H2O2. The catalytic action of Fe3+ ion was suppressed with increase of HNO3 concentration. As much as 4 g of the air-dried resin could be decomposed with 8 ml of 30% H2O2, and the use of about 60 ml of 30% H2O2 resulted in the complete decomposition of organic carbon to CO2. Absence of any orgnaic carbon in the residual solution will simplify the final disposal.

Synthetic Inorganic Ion Exchangers Showing High Selectivity towards 5f Elements

The selectivity for Pu + , Am + and NpOj has been studied as a function of [HN03] on selected synthetic inorganic ion exchangers at ambient temperature. A large selectivity toward Pu and Am was discovered on tin antimonate ( l . lSn02 · Sb205 · 4.9H20, tetragonal) and titanium antimonate (3.1Ti02 • Sb2Os · 4.9H20, tetragonal) cation exchangers, while they showed low selectivity for 4/elements such as La + , EU and Yb + . The selectivity increased on these ion exchangers in the order of Np0 2 < Am + < Pu + . Log-log plot of the distribution coefficients vs. [HN03] showed a linear relationship with a slope nearly equal to the valence of exchanging ions, indicating that the adsorption predominantly proceeds through ion exchange. The ion-exchange rate in the initial stage of adsorption was about the same as that of a commercial strong acid-type organic cation exchange resin.

Solid formation in simulated high level liquid waste of relatively low nitric acid concentration

Solid formation in a simulated high level liquid waste (HLLW) was experimentally examined at 2M and 0.5M nitric acid concentrations. The precipitation studies were conducted by refluxing the simulated HLLW around 100°C. Zr, Mo, Te and Ru were major precipitation elements in both 2M and 0.5M HNO3 solutions. The amount of precipitate in 2M HNO3 solution decreased with decreasing Zr concentration and no precipitation was found in the solution without Zr. Only about 10% of Zr, Mo and Te were precipitated, if the Mo/Zr ratio in the 0.5M HNO3 solution was kept below 0.5. Complete removal of Zr and Mo was the most effective way to prevent solid formation in the solution with 2M and 0.5M HNO3 concentrations.

Decomposition of oxalic acid with nitric acid

Oxalic acid or oxalate is widely used as a precipitant and a detergent in the field of nuclear energy. The present work aimed at developing a method of decomposing oxalic acid with HNO3 in the presence of Mn2+ ion. The use of Mn2+ ion as low as 10−3 mol/l facilitated the complete decomposition of oxalic acid, and the acidity of the resulting solution became as low as 0.1 eq/1 under the optimum conditions. The decomposition of oxalic acid is a first order reaction and proceeds at temperatures above 80°C; the activation energy of the reaction is 18.6 kcal/mol. This decomposition method is applicable to the dissolution of an oxalate precipitate.

Back-Extraction of Uranium(VI) from Organophosphoric Acid with Hydrazine Carbonate

The back-extraction of uranium(VI) from di(2-ethylhexyl)phosphoric acid (HDEHP) and diisodecylphosphoric acid (DIDPA) was studied by using hydrazine carbonate as back-extractant. U(VI) was back-extracted from n-dodecane solutions of 0.5M HDEHP - 0.2M TBP and 0.5M DIDPA - 0.1M TBP by hydrazine carbonate. The distribution ratios were decreased with an increase of hydrazine carbonate concentration. The back-extraction equilibria were expressed by slope analysis in consideration of neutralization between the extractant (DIDPA, HDEHP) and hydrazine carbonate, which occurred quantitatively during the back-extraction.