Radionuclide decontamination

Phytoremediation methods for radionuclide decontamination do not involve hyper-aeeumulators, except possibly for uranium. Plants require a long period of eontact with a contaminant to evolve the ability to hyper-aeeumulate, and most uranium ores are located underground and so are not in eontaet with plants. Soils with high concentrations of uranium are present only where uranium is or has been mined or processed, but these have only been in existenee for a few deeades. 

External decontamination procedures are designed to minimize internal contamination of patients and the individuals providing care. Radionuclides on the intact skin surface rarely produce a high enough absorbed dose to be hazardous to patients or to medical staff. However, this is not the case for chemical hazards. Prior to implementing decontamination procedures, a patient should be moved to an upwind position outside of the area of contamination. 

The sharing of practical inside and very specialised information on subjects like decontamination of radionuclides and ammunitions or propellants... 

Owing to their high surface area and affinity for many ions, Fe oxides are used in water treatment processes (see Chap. 21). Coprecipitation of radionuclides with Fe oxides at pH > 10 can be used to decontaminate low level, radioactive waste. 

In 1997, the U.S. Department of Energy (DOE) released bench-scale results of an integrated TechXtract system for decontaminating surfaces contaminated with radionuclides. The overall economics of the system were evaluated. Costs for the system compared favorably with existing options for radioactively contaminated scrap metal (D177859, p. 3). 

CORPEX Technologies, Inc., offers CORPEX technology for the decontamination of undesirable and toxic ions or radionuclides from contaminated surfaces and coatings. The vendor states that the process can operate as either a batch or semicontinuous process. The commercially available CORPEX technology uses patented, innovative chelation chemicals to control and recover radioactive and other types of hazardous metal ions from soils, concrete, steel, and other materials. 

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. 

Among common radionuclide sources are the natural environment, fallout from nuclear weapon tests, effluents from nuclear research laboratories, the nuclear power fuel cycle, radiopharmaceutical development, manufacturing, and various application, teaching and research uses. Decontamination and decommissioning activities at former nuclear facilities and the potential of terrorist radionuclide uses are current topics of interest for radioanalytical chemistry laboratories. Simplified information on the numerous radionuclides is conveniently found in Charts of the Nuclides such as Nuclides and Isotopes (revised by J. R. Parrington, H. D. Knox, S. L. Breneman, E. M. Baum, and F. Feiner, 15th Edition, 1996, distributed by GE Nuclear Energy). 

A particular aspect of water treatment is the rehabilitation of accidentally contaminated soils by radionuclides. This is well illustrated by the works carried out after the Cernobyl catastrophe. The incorporation of clinoptilolite into contaminated soils reduced the transport of heavy metals and radionuclides from soils into ground water and biomass (7). Union Carbide s IONSIV EE-95 (CHA) and A-51 zeolites (LTA) with excellent Cs+/Na+ and Sr2+/Na+ selectivities, respectively, have also been employed for decontamination of high activity level water in the reactor containment building from Cs+ and Sr2+ after the accident at Three Miles Island (5). The radioisotope loaded zeolites were then transformed into glasses for ultimate disposal. 

The module operation efficiency is determined on the one hand, by selectivity of membrane elements and sorption characteristics of ion-exchangers and on the other hand, by physico-chemical and radionuclide composition, concentration of suspended particles, salts and radionuclide activity levels. The integral decontamination-purification coefficients Kpurj t, depending on LRW radionuclide, physical and chemical composition, vary within 10 - 10 . Depending on composition of initial LRW and in compliance with on-line control data, MMSF can operate either imder the full-cycle mode involving all basic modules or imder reduced-cycle mode using only some of modules. 

At the second work phase a Laser Decontamination laboratory was established at Nuclear Physics Institute (Gatchina, Leningrad region), and a hot chamber was made for works with contaminated details. The hot chamber is equipped with two laser installations to perform studies. Laser light enters the chamber via inlet window and has a possibility of two-coordinate scanning with the help of mirrors guided from processor. As established in experiments, when processing contaminated surfaces, the contamination level by individual radionuclides decreases by 70% and more (Fig. 4). 

To work the decontamination procedme through, real samples of equipment and systems of nuclear submarines under dismantlement were used along with samples of oil-gas pipes with depositions containing natural radionuclides Th and Ra), Fig. 6. 

Multidentate synthetic chelating agents are used for the decontamination of nuclear reactors and for nuclear waste processing, because they form stable soluble complexes with many radionuclides. Unfortunately, their joint disposal has sometimes resulted in increased radionuclide mobility, with the concomitant contamination of groundwater. An attempt to lessen this problem involves degrading the chelating agents with bacteria (see Chapter 11). 

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. 

If the activity of each isotope discharged within a given period is divided by the activity in the reprocessed fuel, a factor is obtained which crudely represents the relative ease with which a species is discharged. This release factor will be specific to the Sellafield site, which passes the effluent through a variety of decontamination procedures prior to discharge including storage whilst short-lived radionuclides decay. 

For example, in fission product effluents where radio-contaminants having longer half-lives are present, the emphasis should be on very high volume reduction with permissible/acceptable decontamination factors so that the permeate could be directly discharged. For effluents contaminated with radionuclides of short half-lives, a good decontamination with reasonable volume reduction may be acceptable because the concentrate could be stored tiU the activities decay before discharge. The radioactive effluents requiring treatment may vary with respect to the type of radionuclide, its chemical nature, concentration, pH, concentration of inactive solutes, and presence of suspended matter. 

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. 

The laboratory experiments showed the influence of the total concentration of ballast non-active salts on decontamination factors. As the concentrations of total solute in permeate from the first and third stages were very low a decrease of retention of radionuclides was observed. To improve the efficiency of radioisotopes removal additional salt injection took place before the second stage. 

There are assessments predicting the use of reverse osmosis for the processing of the wastes from medical application [36,37] and for the removal of caesium-137 from decontamination wastes after accident in the steel production factory [38]. RO is considered as a method for removal of radioactive pollutants from contaminated water (removal of Cs and °Sr) in the vicinity of atomic power plants [39], as well as for removal of small quantities of radionuclides ( Rn, Ra) from... 

The experiments were conducted in the temperature range 35°C-80°C at feed inlet, and 5°C-30°C at distillate inlet, and with feed and distillate flow rates up to 1500 dm /h. Under these conditions permeate stream was 10-50 dm /h (60-300 dm /m day). During the experiment run the activity of the distillate was stable on the level of natural background radioactivity and the concentrating of radioactive compounds took place in retentate. Retention of radioactive ions in retentate was almost complete (decontamination factors oo. Table 30.13). Most of radionuclides were not detected in distillate only trace amounts of Co-60 and Cs-137 were present. Also retention coefficients of non-active ions were high (Table 30.14). 

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