Shigeru Dojiri

Incorporation of Radioactive Spent Ion Exchange Resins in Plastics

Experiments were undertaken on the incorporation in plastics—polyethylene in particular—of radioactive spent ion exchange resins produced in nuclear power plants. The resulting polyethylene products burdened with radioactive resin were tested to ascertain the properties considered important for radioactive waste management. The items chosen for testing were mechanical strength, leachability of radionuclide and radiation resistance. Polyethylene products burdened with 50w/o of resin were found to possess an impact strength of 10kg-cm/cm and a compressive strength of 300 kg/cm2, which values do not indicate any appreciable decrease in mechanical strength compared with polyethylene unburdened with resin. The leaching rate of 137Cs from the resin- burdened polyethylene product was very small—only 0.1% leached out in one year. In respect of decomposition by radiation, the amount of gases evolving upon absorbing a dose of 109 rad was 10 ml/g. The effect of radiation on the mechanical strength was also studied. ...

A comparison of the acid digestion of spent ion exchange resins using H2SO4-HNO3 and H2SO4H2O2

An alternative acid digestion system, H2SO4H2O2, to a conventional reaction system, H2SO4HNO3, has been proposed to reduce the volume of spent ion exchange resins generated at nuclear power plants. A comparative study on both reaction systems has been carried out to obtain the relationship between the reaction conditions and the conversion of the resins, the coprecipitation behavior of 60Co and 137Cs, and the release rate of the radionuclides from a reaction vessel, and to elucidate the feasibility of the system proposed. The mole ratio of an oxidant and carbon contained in the resins, required to digest the resins, was comparable between both systems, while the H2SO4H2O2 system gave higher conversion than the H2SO4HNO3 system. Some degradation products of an anion exchange resin were relatively stable against the oxidation, and required excessive amounts of the oxidant and a higher reaction temperature to digest, especially in the H2SO4HNO3 system. The coprecipitation behavior of the radionuclides in both systems was almost identical; 60Co coprecipitated effectively with Fe(III) sulfate while 137Cs did not coprecipitate. The release rates of the radionuclides were on the order of 10−3%/hr.

Separation of Pu-Am from the Leachant of a Deposit in an Acid Digestion Solution by Calcium Oxalate Coprecipitation

Abstract Pu and Am were recovered from the deposit in a synthetic acid digestion solution. After being leached out from the deposit with water, they were separated from the resultant solution by Ca-oxalate coprecipitation. The alpha radioactivity in the solution was reduced from about 4 mCi/L to 0.1 μCi/L by the technique. The precipitate obtained was dissolved in 7 M nitric acid solution, from which Pu-Am were separated by the use of anion and cation exchange resin columns, respectively. The coprecipitation technique was also utilized for the concentration of Am in the effluent of the anion column prior to its separation by a cation column. The overall recovery efficiencies of Pu and Am were about 80 and 85%, respectively.

Solidification of powdered ion exchange resins with polyethylene

The adaptability of polyethylene solidification method to spent powdered ion exchange resins (Powdex), one of the low-level radioactive wastes, has been examined. The polyethylene product incorporating the wastes was prepared using a batch type melter on a laboratory scale. The limit of the waste content is 40–50 wt.%, so the volume reduction effect of this method is about 3–4 times higher than that of a cement solidification one. The product with 50 wt.% waste has a density of 1.05 g/cm3, the compressive strength of 230 kgf/cm2, enough toughness and low leachability (diffusion coefficient of 60Co is 10−10-10−12 cm2/day). The compressive strength of the products are not reduced by the irradiation with absorbed dose of up to 1.4 × 109 rad. The formation rate of hydrogen by radiolysis which occupies 70%–90% of total gas evolved is (2.0–2.4) × 10−2 cm3/g · Mrad. The water resistance of products depends not only on the ratio of carbon to anion resins therein.

Incorporation of an evaporator concentrate in polyethylene for a BWR

Abstract The adaptability of polyethylene solidification method to an evaporator concentrate produced in BWR has been examined. The polyethylene product incorporating sodium sulfate as a main constituent of the concentrate was prepared using a batch-type melter. The product was tested with respect to homogeneity, density, mechanical property, water resistance and leachability, in order to evaluate the safety of the product. The product can incorporate the waste (sodium sulfate) up to 70 wt.%, but it may be recommended through tests of water resistance and leachability that the maximum waste content in the product is 50 wt.%. The product with 50 wt.% waste has the density of 1.28 g/cm 3 , the compressive strength of 213 kgf/cm 2 , enough toughness, low leachability (diffusion coefficient of sodium is 10 −7 −10 −6 cm 2 /day) and excellent water resistance. This solidification method offers about 5 times larger volume reduction effect than a cement solidification does.

Improvement of bitumen-waste product in leachability—Leachability of bitumen product containing BWR's evaporator concentrate

Abstract The method has been developed to improve the bitumen product which incorporates an evaporator concentrate from a BWR, with respect to the swelling and leaching. The leachability of the product has been measured by the method recommended by the IAEA. The swelling of the product is successfully prevented by the addition of calcium chloride. The specimen containing the waste up to Wa/B (Weight ratio of Na 2 SO 4 + CaCl 2 and bitumen) = 60:40 shows no pronounced swelling, when it is immersed in water. The cumulative fractions of 137 Cs and 60 Co leached from a specimen which does not contain CaCl 2 are 0.65 and 0.2 at the leaching time of 30 days. On the hand, the corresponding value at 100 days for the specimen with calcium chloride addition is 5 × 10 −4 for 137 Cs and 1 × 10 −4 for 60 Co. The coating of the specimen surface with a fresh bitumen (5 mm thickness) reduces the leachability further. These results indicate that this method is effective to improve the bitumen product incorporating BWRs evaporator concentrate.