Radionuclide decontamination separation

The measure of purification is the decontamination factor, DF, defined as the concentration of the interfering radionuclide before separation divided by its concentration after separation. The required DF depends on the initial concentrations of the interfering radionuclide and the radionuclide of interest, and the extent of acceptable contamination when counting the emitted radiation. The DF must be large if the radionuclide of interest is a small fraction of the total initial radionuclide content. The interfering radionuclide that remains in the source that is counted... 

There are certain unique features to the chemical separations used in radiochemistry compared to those in ordinary analytical chemistry that are worth noting. First of all, high yields are not necessarily needed, provided the yields of the separations can be measured. Emphasis is placed on radioactive purity, expressed as decontamination factors rather than chemical purity. Chemical purity is usually expressed as the ratio of the number of moles (molecules) of interest in the sample after separation to the number of all the moles (molecules) in the sample. Radioactive purity is usually expressed as the ratio of the activity of interest to that of all the activities in the sample. The decontamination factor is defined as the ratio of the radioactive purity after the separation to that prior to the separation. Decontamination factors of 105-107 are routinely achieved with higher values possible. In the event that the radionuclide(s) of interest are short-lived, then the time required for the separation is of paramount importance, as it does no good to have a very pure sample in which most of the desired activity has decayed during the separation. 

The extraction cycles lead to the production of various aqueous and organic waste solutions, mainly from solvent clean-up and washing. Some of the aqueous LLW solutions may be released directly into the environment if their activity is low enough. Others are decontaminated by precipitation, coprecipitation, ion exchange or sorption procedures. The general tendency in handling liquid wastes is to reduce the volume as far as possible and to transform LLW into MLW, as already mentioned. Liquid organic wastes are either incinerated or the radionuclides contained therein are separated by precipitation or other procedures. 

The RO process was implemented at the Institute of Atomic Energy, Swierk. The wastes collected there, from all users of nuclear materials in Poland, have to be processed before safe disposal. Until 1990 the wastes were treated by chemical methods that sometimes did not ensure sufficient decontamination. To reach the discharge standards the system of radioactive waste treatment was modernized. A new evaporator integrated with membrane installation replaced old technology based on chemical precipitation with sorption on inorganic sorbents. Two installations, EV and 3RO, can operate simultaneously or separately. The membrane plant is applied for initial concentration of the waste before the evaporator. It may be also used for final cleaning of the distillate, depending on actual needs. The need for additional distillate purification is necessitated due to entrainment of radionuclides with droplets or with the volatile radioactive compounds, which are carried over. 

A purification procedure can be considered appropriate if separation from other radionuclides is so effective that, in the absence of the radioelement of interest, no radioactivity is detected in the purified sample. That is, the measured radioactivity is zero within the measurement uncertainty. If the radioelement of interest is present, then contaminants should not observably increase its measured activity. To plan the procedure, the amount of each contaminant radionuclide that remains after applying the separation methods is calculated in terms of its decontamination factor (DF) (see Section 3.1). Meeting the stated criteria depends on the initial concentrations of the radioelement of interest and of each contaminant radionuclide in the sample, and on the achieved DF. Because these values generally are not known initially, reasonable concentrations must be assumed and subsequently revised when initial measurement results become available. 

Flocculation of added FeCh solution by proper addition of Ca(OH)2 removes the bulk of the radionuclides from the spent decontamination solution decontamination factors of up to 500 can be achieved by an appropriate performance. While the resulting volume of precipitate is comparatively high, it can be reduced by separation using a centrifuge-decanter system (Gregoire et al., 1989). Several attempts have been made to process spent decontamination solutions by inverse osmosis, but so far these attempts have been unsuccessful, mainly due to rapid plugging of the membranes. 

Besides the metallic structures of the circuits and components, nuclear power plants contain huge masses of concrete, a small fraction of which is activated and/ or contaminated. In order to keep the resulting waste volumes as small as possible, the fraction which contains radionuclides has to be separated during dismantling from the non-radioactive bulk of the material the techniques used for this will not be discussed in what follows. Likewise, decontamination of building walls and surfaces will not be treated here. 

A SLM consists of an organic liquid solution (mixture of an organic diluent and a carrier) absorbed by capillary forces onto a microporous support separating two aqueous solutions [6-8] the first solution, referred to as the feed solution containing the permeating ions, is the solution of waste to be decontaminated, and the second solution, referred to as the stripping solution, initially free of ions (demineralized water) receives and concentrates the selectively transported radionuclides. 

The isotope separator has been applied to iodine isotopic analysis (Rook et al., 1975). In this method, neutron activation is used to produce radioactive iodine activities. The isotope separator serves to mass-separate the radioiodine and also to decontaminate the irradiated sample from other interfering radionuclides such as %r. 

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