Haruto Nakamura

Radiation effects on the separation of lanthanides and transplutonides by the TALSPEAK-type extraction

Effects of radiation on the extraction system composed of DEHPA extractant and DTPA aqueous solution containing nitrate were studied by measuring distribution ratios of Am(III) and Nd(III) with the extractant and DTPA solution either one of which was irradiated with60Co γ-rays or the organic—aqueous mixed phase irradiated under continuous stirring. The irradiation causes an increase of Df and a decrease of the Nd/Am separation factor β, to an especially large extent in the mixed phase system. These effects are due firstly to the radiolytic decomposition of DTPA and secondarily to the formation of MEHPA. The replacement of nitrate with lactate stops the degradation of DTPA and DEHPA resulting in the retardation of increase of Df and a decrease of separation factor. The DEHPA-DTPA-lactic acid system is concluded to sustain absorption of radiation at an absorbed dose up to 200 Wh·I−1 in the partitioning of transplutonium elements in HLW.

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.

Application of synergistic MEHPA-DEHPA mixed solvent to the extraction of lanthanides(III)

The distribution ratio of neodymium(III) in extraction with mono(2-ethylhexyl)-phosphoric acid was determined as a function of extractant and Nd(III) concentrations, and effect of co-existing DEHPA was studied. A characteristic extraction of a tracer quantity of Nd(III) was found at an extractant concentration of approximately 0.01 M. Di(2-ethylhexyl)phosphoric acid depresses the extraction of Nd(III) in MEHPA of a concentration of 0.01M, but enhances it at higher concentrations of MEHPA. Owing to the high Df values against lanthanides(III) and an excellent radiation stability, the synergistic combination of MEHPA-DEHPA, preferably at a DEHPA/MEHPA mole ratio of 3, was concluded to be applicable to the isolation of lanthanides (and actinides) from highly acidic solutions.

Preparation of Tc-99m by Direct Adsorption from Organic Solution

Purification of 99mTc extracted into methyl ethyl ketone from an alkaline solution of neutron irradiated MoO3 was studied in order to establish a rapid method for producing 99mTc.After removal of the organic solvent by evaporation, 99mTc could be purified from remaining traces of Mo and other contaminants by adsorption in an alumina column from an aqueous solution and by subsequent elution with 0.1N HCl or with physiological saline solution.Organic solvents containing 99mTc could also be treated directly in an alumina column to purify the 99mTc: this saved the time required for the evaporation of the solvent.Problems essential to the adsorption of 99mTc on alumina in organic media were studied to determine the optimum conditions for the adsorption.A standard procedure for 99mTc preparation would be: Extract 99mTc into methyl ethyl ketone from alkaline solution. Pass organic phase through cation exchange column to remove Na and then through alumina column. Finally, elute 99mTc from alumina column with 0.1N HCl or saline solution.The process can be finished within 1hr. About 500mCi of 99mTc is obtained with a chemical yield higher than 95%, and the product contains less than 10-4% of radiochemical impurities.

A study of radiation effects on the extraction of americium(III) with acidic organophosphates

Radiation effects on the extraction of americium(III) with several dialkylphosphoric and monoalkylphosphoric acids have been studied comparatively. The extractants were exposed to60Co γ-rays in the absence and presence of diluent and HNO3. The Df of Am(III) was measured and the yield of radiolytic products was determined after γ-irradiation as a function of the absorbed dose. Di(hexoxyethyl)phosphoric acid was the most susceptible to radiation among the extractants studied and its extractability was weakened by radiation regardless of the presence or absence of diluent or HNO3. Diisodecylphosphoric acid itself showed an enhancement upon irradiation but incurred a depressant effect upon intimate contact with HNO3·Mono(2-ethylhexyl)phosphoric acid showed a decrease of the Df of Am(III), similarly to DIDPA in the presence of HNO3 during γ-irradiation. All the above extractants gave H3PO4 as the principal radiolytic product upon γ-irradiation upon intimate contact with HNO3 causing decreasing solely the extraction of Am(III). Di(2-ethylhexyl)phosphoric acid proved to be the most stable extractant to radiation among those studied.

Volatilization of cesium from nuclear waste glass in a canister

Volatilization of /sup 134/Cs from simulated high-level waste glass in a canister during several reheatings up to a maximum of 1000/sup 0/C was examined. The results showed that the temperature dependence of the amount of /sup 134/Cs suspended in the air inside the canister could be divided into two categories. As the temperature was increased above 500/sup 0/C, the amount of /sup 134/Cs suspended in the air inside the canister also increased. On the other hand, for temperatures <500/sup 0/C, the amount of /sup 134/Cs suspended in the air inside the canister had an almost constant value after several reheatings up to a maximum of 1000/sup 0/C. In this case, the air contamination by cesium-bearing material inside the canister is considered to be significant even at waste storage temperatures <500/sup 0/C.

A new method using titanium hydride for fabrication of Synroc

Synroc of high density and low leachability was formed by using titanium hydride which provides sufficiently reducing conditions during calcination and hot pressing without additional reducing gas and metal powder. The amount of titanium hydride sufficient to keep the reducing conditions was at the most30 wt% (in titanium dioxide equivalent). Omission of the gas flow into the calciner pot prevented powder carry-over up to the pot lid during calcination. The Synroc using titanium hydride consisted of three main phases, namely, hollandite, perovskite and zirconolite, plus Ti3O5 which formed due to dissociation-oxidation of titanium hydride. Leach rates of Ca, Cs and Na from this Synroc were similar to those from the Synroc in the conventional process. These results indicate that the method employing titanium hydride can be adopted for fabrication of Synroc, especially in a hot cell.

Radiation stability of macro-porous and gel-type cation exchangers

Macro-porous cation exchange resin Diaion CPK-08 and gel type cation exchange resin Dowex 50WX8 were irradiated with γ-rays from60Co, while soaked in distilled water, 0.5M HNO3 or 4M HNO3, and the ion-exchange properties, such as strong- and weak-acid capacities, moisture content and wet resin volume, were examined in relation to absorbed dose. There was no appreciable difference between the radiation stabilities of the two cation exchangers. Increase of HNO3 concentration reduced the loss of strong-acid capacity and increased the decross-linkage and the weak-acid capacity. Elution characteristics of137Cs and90Sr from columns packed with γ-irradiated resin were examined and the column distribution ratio of these radionuclides and the theoretical plate number were calculated. These values decreased with the increase of absorbed dose. Diaion CPK-08 was packed into a pressurized column and irradiated with γ-rays at a dose rate of 2·106 R/hr, while water was passed through the column at a constant flow rate. The greatest change of the resin properties was observed at an upper stream position from the position of the highest radiation dose of 2·106 R/hr.

Leaching of SYNROC-C: Relation to Microstructures

Soxhlet-type leaching experiments were carried out for SYNROC-C specimens synthesized by three different methods; hot uniaxial pressing, hot isostatic pressing, and atmospheric sintering. The leaching solutions were analyzed by inductively coupled plasma spectroscopy and atomic absorption spectroscopy. When elements such as sodium, cesium, and molybdenum contained in the glass phase of SYNROC-C, they are leached preferentially at the initial stage of leaching. The difference in elemental mass loss between the three SYNROC-C SPECIMENS (10 TO 10/sup 2/ g/m/sup 2/) depends mostly on the amount of preferential leaching. The release of the above elements is controlled by preferential leaching at the initial stage and then by diffusion through the host crystalline phases. The other elements not found in the glass phase can mainly be controlled by diffusion. The leaching mechanisms of waste forms are important for predicting long-term leach rates that can serve as input data for the safety evaluation for high-level waste (HLW) disposal.

Radiation Damage of Organic Extractant in Partitioning of High-Level Liquid Waste, (II): Calculation of Radiation Dose Absorbed in Organic Solvent in Extraction Process

Radiation dose absorbed in the organic extractant was estimated for the counter- current extraction with mixer-settler in the partitioning of the high-level liquid waste of fuel reprocessing. The radiation dose due to the absorption of α and β-particles was calculated to be 10, 5.0 and 0.42 Wh/l for 150 days, 1 year and 10 years after fuel discharge from the reactor, respectively. About 70–90% of the total dose was due to the absorption of β-particles. These values were discussed in comparison with the published data for the fuel reprocessing. On the basis of the calculated data, the radiolytic effects upon the extraction of strontium, lanthanoids and transplutonium elements in partitioning were estimated be sufficiently small.