Tritium emanation

A range of experiments were set up that used active desiccant (310,000Bq/g), concrete (8,950 Bq/g) and metal (llOBq/g) as tritium emanating source materials. RO water, silica gel, cellulose filter paper, plastic, metal, and various desiccants were used as potential receivers. The interaction with drying agents such as silica gel, zeolites and Drierite (CaS04) were also examined. 

Experiment 2 Tritium emanation from active concrete, desiccant and metal... 

Experiment 4 Tritium emanation from active desiccant... 

Different materials became contaminated to different extents when stored with a tritium emanator such as active concrete (Figure 3). The absorbers used, silica gel, zeolite and Drierite (CaS04), were contaminated to a greater extent than were metal and plastic, especially in an open scintillation vial. Zeolite and Drierite (CaS04) were more contaminated in the fridge (Figure 3). Plastic and metal, however, were relatively little contaminated which implies physical and compositional controls on the extent of contamination [5, 9]. All samples stored in closed vials showed very low contamination. 

Active concrete powder ( 9 kBq of tritium) was placed in a sealable polythene bag and put in one of eighteen Kilner jars which had very effective rubber lid seals. An open and a closed scintillation vial containing 10 ml each of RO water were added to each of the jars which were then stored in a fridge, a freezer and at room temperature for 1 day, 5 days, 10 days, 15 days, 20 days and 30 days, respectively. At the end of each pre-defined emanation time the respective Kilner jars were opened and the contents removed. 8 ml of each RO water vial (open and closed) was taken and transferred to clean scintillation vials with 12ml of scintillation cocktail (Goldstar) and counted by LSC for 1 hour. Similarly 50 ml of tritium-free RO water was added to the Kilner jar and shaken to collect any tritium (as HTO) that had adsorbed on the walls of the glass jars. An 8ml sample of this was also mixed with scintillant and counted. 

Figure 4. Variation of cross contamination of silica gel and cellulose filter-paper exposed to an active desiccant emanator (nominally 158 kBq tritium) with time. Storage occurred in a fridge, a freezer and at room temperature (Expt. 4)...

Emanation of tritium was most significant from metals (nominally 30% loss over 2 weeks storage) and to a lesser extent eoncretes stored at room temperature (6% loss) and in a refrigerator. Emanation from desiccants was considerably lower. Again storage in sealed containers and in a freezer effectively eliminated the potential for cross contamination. 

The contribution of inhalation to the internal doses to the critical group is substantial for radioactive gases and vapours (e.g. radon or tritium oxide) and for radionuclides with low solubility and low mobility in food chains (e.g. actinides and transuranics), especially for persons working in the open air and in dusty conditions. The special case is that of long term residence in areas with elevated concentrations of natural uranium and radium resulting in the emanation of radon. 

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