Experimental

2.1. Lab Scale SFE System Employed for the Recovery of Plutonium from Actual Waste

The SFE system (M/S Jasco) used consists of two reciprocating pumps, one with peltier cooling for pumping carbon dioxide and another pump for the delivery of modifier, a constant temperature oven (±1˚C) and a backpressure regulator. Carbon dioxide gas used was of supercritical grade i.e. 99.9%. The SFE system was setup in a glove box for the recovery of plutonium from waste matrices [20]. Extraction vessels of 1 mL and 0.1 litre capacities were employed for loading the sample. The 0.1 litre capacity (11 cm height × 4 cm dia) extraction vessel was designed and fabricated at our laboratory. The glove box facility was developed incorporating modifications required for handling high pressures and isolated electrical connections. A secondary container accommodated the primary extraction vessel so that any accidental pressure release can be contained causing no disturbance to the maintenance of an appropriate negative pressure of the glove box. Various in-built safety measures and associated alarm annunciation facilities were established to avoid over-pressurising of the system during its operation [20]. The two pumps used for the delivery of liquid carbon dioxide and the modifier (extractant in methanol) were placed outside the glove box. However the backpressure regulator and heating oven, which accommodated the extraction vessel were kept inside the glove box. The “extracted sample collection system” was also arranged inside the glove box and plutonium was sampled for assay.

In the initial experiments, 1 mL capacity extraction vessel was employed to optimize the SFE conditions for efficient recovery of plutonium from waste matrix. The cellulose based matrix containing plutonium in its nitrate form was cut into small pieces, typical dimension being 5 × 10 cm. The cut pieces were inserted into a 1mL extraction vessel, 1N HNO₃ (100 mL) was sprinkled and extraction was carried out under typical conditions viz., pressure: 250 bar, temp: 45˚C, Sc-CO₂ flow rate: 3 mL/min, methanol flow rate: 0.15 mL/min; ligand content: ~5 wt% ligand in methanol, about 1.8 mL of HNO₃ added to 100 mL solution of methanol containing suitable ligand. The extraction was carried out for a period of 2 hrs. The plutonium present in extract was collected in small volume of dil.HNO₃ + isopropanol medium. The plutonium contents of the extract after the SFE experiment was assayed by liquid scintillation technique. The remains i.e. un-extracted plutonium in cellulose waste matrix was estimated by leaching with 4N HNO₃. The leaching efficiency using ultrasonication was established earlier in our laboratory and was found to be >98% [19]. After the initial studies, the plutonium bearing waste was taken after cutting it into small pieces in to a 0.1 litre SFE vessel and extraction was carried out.

Date: 2015-01-02; view: 1105