A UNEX-extractant (0.08 M CCD, 0.02 M Ph2Bu2 and 0.5% PEG-400 in FS-13) has a density of 1.4 g/cm3 and viscosity of 4 mPa*s at 20°C. Stability of all components of the extraction system is rather high for treatment of

HNO3 solutions of radioactive waste. So, CCD in FS-13 on contact with concentrated HNO3 is stable even at elevated temperatures (>100°C). Diphenyldibutyl-carbamoylphosphineoxide and PEG-400 are also stable in contact with HNO3. The UNEX-extractant and diluent FS -13 do not enter into exothermal reactions with HNO3 even at temperatures above 100°C.

Fire safety of the UNEX-extractant depends primarily on the properties of the diluent FS-13, which is the main component of the mixture. The high flash temperature of FS-13 (93°C) is responsible for the fire-proof nature of the UNEX-extractant; for comparison, it should be noted that the flash tem­perature of dodecane (the diluent used in the PUREX-process) is 70°C. Other components of the UNEX-system (CCD, CMPO, PEG) have a high boiling point and are low fire-hazard compounds. As in the two-phase aque­ous-organic extraction process, a temperature higher than 100°C is practi­cally never reached, and the UNEX-process should be considered fire proof.

Specialist studies have revealed the high radiation resistance of FS-13. The total yield of radiation decomposition for pure sulfone and for sulfone in contact with HNO3 was 4.5-5.0 molecules/100 eV. The low yield of fluo­rine-ions in this (0.15 ions/100 eV) makes the UNEX-extractant relatively corrosion-proof when compared to stainless steel. Radiolysis products do not exert any marked effect on the extraction and hydrodynamic properties of the UNEX-extractant. No interphase films, precipitates and emulsions were detected on exposure.

Since the extractant is used over multiple cycles in the process, an impor­tant characteristic is the prolonged stability of its properties, which may depend on differences in the solubility of individual components in the aqueous phase. Polyethylene glycol is the most soluble component during the stripping operation (up to 250 mg/l). Since PEG concentration in the extractant is critical for Sr extraction, this component should be replenished during the course of the process. With the exception of compensation for PEG losses (for example, by its introduction into the stripping solution), the properties of the UNEX-extractant do not practically change over time. This was confirmed by prolonged testing of the UNEX-process at RI and Idaho National Laboratory.