CS217764B1 - A method of separating radiocesia from waste gases - Google Patents

Abstract

The purpose of the invention is the effective removal of radiocesium from waste gas generated during the vitrification of highly radioactive waste. The essence of the invention is the use of zinc ferric cyanoate, which enables the effective separation of radiocesium. The said substance is prepared by precipitating 0.5 M sodium ferric cyanoate at a temperature of 24 ± 2 °C with 0.5 M zinc chloride, which is in a 10% excess of the stoichiometric amount. After precipitation, the precipitate is decanted, filtered, washed and after drying, a fraction with a grain size of 0.2 to 0.8 mm is used. The sorbent prepared in this way, used in the column, removes radiocesium from waste gas from vitrification with an efficiency of three orders of magnitude. The said sorbent can be used for the removal of radiocesium from gases generated during the reprocessing of spent nuclear fuel and for the removal of radiocesium from gases generated during the combustion of radioactive waste.

Description

BACKGROUND OF THE INVENTION The present invention relates to a process for the separation of radiocesium from waste gases using zinc cyanide.

The gases generated by the vitrification of highly active wastes contain a long-term radionuclide of ¹³⁷ Gs. So far, attention has been paid only to the capture of radioruthenium (RuOi) from the gas phase, also occurring in this process. Radioruthenium is mainly removed by sorption on inorganic compounds, predominantly on silica gel. However, radioruthenium trapping agents are insufficiently effective for radioclesl [JD Christian,

Pence, T. T., PNL-2486 Report (1977)].

According to the available literature, a procedure intended solely for the effective capture of radiocesium from the waste gas from the vitrification has not been used so far to guarantee that the radioceslum will not pass through the used cleaning systems to an unauthorized extent.

The principle of the radiocesium separation process according to the invention is based on the use of zinc ferrocyanide having a grain size of 0.2-0.8 mm as the sorbent p-ro for the capture of radiocesia, prepared as follows: 500 ml of 0.5 M sodium ferrocyanide is precipitated at 24 ± 2 ° C for 30 minutes with 0.5 M zinc chloride, which is in a 10% excess of stoichiometric amount. After the precipitation was complete, the precipitate of zinc ferrocyanide was decanted with 1000 ml of distilled water and allowed to stand for twelve hours. It is then filtered, washed with 500 ml of distilled water and dried at 130 + 1 ° C to constant weight. The dried precipitate is crushed and a 0.2-0.8 mm fraction is separated for use.

This preparation procedure differs from that described in the literature [S. Kawamura, H. Kuřák-, K. Ku-rotaki: Anal. Chim. Acta 49,

317 (1970); V. Smirim, J. Rais, B.Million: J. Inorg. Bored. Chem. 26, 1111 (1964)].

As can be seen from the example, zinc cyanoferrate (II) prepared according to the process of the invention exhibits a higher efficiency for the removal of radiocesia from the gas phase than zinc cyanol-ironeate prepared according to the literature. The advantage of using said sorbent is that it extends the function of the high efficiency aerosol filter downstream of the cleaning system. Placing said s-orbent in the column in front of the high efficiency filter will reduce clogging of the filter and thereby increase its lifetime.

Example

Exhale from vitrification vysoceaktivníh-model of the waste -containing radiocesia trace concentrations (of the order of 1O ^-7 GCS / 1) was contacted with 3.1 g of zinc ferrocyanide in box, prepared according to the present invention. The gas flow rate through the column was about 17 cm @ ^-1 . Under these conditions 99.92% -radiocesia was removed from the gas. !

On the other hand, when contacting an exhalate from vitrification of a model high-activity or waste stream containing trace concentrations of radiocesia (of the order of 10 ^-7 g Cs / l) with 3.1 g of zinc ferrocyanide in a column prepared according to the literature, ca 17 cms ^-1 removed only -99,10 o / _o radiocesia from gas.

The example shows that the sorbent prepared according to the invention reduced the concentration of -radiocesia in the waste gas from the vitrification by three orders of magnitude, while the sorbent prepared according to the literature by only two orders of magnitude.

Claims (2)

Process for separating radiocesium from waste gases, characterized in that these gases are passed through an orbent, which is zinc cyanoferrate, prepared by precipitation of 0.5 M sodium ferrocyanide at 24 ° C. ± ± 2 ° C 0.5 M zinc chloride, which It is in a 10% excess of these metric quantities, the resulting zinc cyanoferrate is decanted with distilled water, filtered off, washed with distilled water, dried at 130 ± 1 ° C to constant weight and used in a fraction of 0.2 to 0.8 mm.