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Nuclear energy development is essential to meet our increasing energy demand due to the insufficient energy resources in Korea, although the radiation effect from use of nuclear fuel may become major public concern. The amounts of radioactive wastes from nuclear power plants and institutional and industrial activities are increasing rapidly these days. Soils around the radioactive waste storage areas can be highly contaminated with radioisotopes such as ^(137)Cs and ^(90)Sr. Thus, in situ vitrification (ISV) and phosphate treatments will be discussed at techniques for immobilization of radioisotopes in contaminated soils. A field demonstration of ISV was carried out at the Oak Ridge National Laboratory (1) to evaluate the operational performance of ISV within the local geologic and hydrologic regime; (2) to determine the soil retention factors for Cs and Sr; and (3) to determine leach resistance and durability of the resulting ISV product. A trench (1.0 m × 9.2 m × 1.5 m) was constructed and chemical additions of Cs₂ CO₃ and SrCO₃, were used to simulate the ^(137)Cs and ^(90)Sr contained in the contaminated soil. Four molybdenum electrodes were placed in the soil formation around the demonstration trench and a specifically fabricated off-gas hood placed over the trench. Electrical power (up to 400 ㎾) was applied to the electrodes for 110 hours. The ISV product was found to be a mixture of microcrystalline and glass phases. The retentions of Sr and Cs in the vitrified zone were$gt;99.999% for Sr and 99.88% for Cs, respectively. Tests for leach resistance and durability showed that both the crystalline and glass phases were as durable as several vitreous forms considered for high-level radioactive waste. The feasibility of in situ phosphate and metal (caicium, aluminum, and iron) solution treatment for ^(90)Sr immobilization was investigated. Batch and column experiments were performed to find optimum conditions for coprecipitation of ^(90)Sr with calcium-, aluminum-, and iron-phosphate compounds in contaminated soils. Separate columns were packed with artificially ^(90)Sr-contaminated acid soil as well as ^(90)Sr-contaminated soil from the Oak Ridge Reservation. After metal-phosphate treatment, the columns were then leached successively with either tap water or 0.001 M CaCI~ solution. Most of the ^(85)Sr coprecipitated with the metalphosphate compounds. Immobilization of ^(85)Sr and ^(90)Sr was affected by such factors as solution pH, metal and phosphate concentration, metal-to-phosphate ratio, and soil characteristics. Equilibration time after treatments also affected ^(85)Sr immobilization.
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