THEORETICAL AND PRACTICAL ASPECTS OF A FREE-ATMOSPHERE TRITIUM EXPOSURE SYSTEM

A free-atmosphere release system was designed to expose an experimenta l area continuously to tritiated hydrogen (HT) over periods of days to weeks. The system was constructed of four 19-m-long line sources arra nged in a square to fumigate the central al en containing plants and s oil, irrespective of wind direction. The line sources were operated bz the upwind direction only to conserve HT, and the release rate was ad justed in proportion to the wind speed. Both the wind-direction and wi nd-speed adjustments were controlled by a datalogger in the field and were updated every 5 min. Sulphur hexafluoride (SF6) was also released as a tracer to measure spatial and temporal variability. The system o perated satisfactorily over a 12-day experimental period and achieved a relatively uniform concentration of HT across the experimental plot. The temporal variability was about 25% based on daily averages, but 4 -h averages varied mor e. Mean horizontal spatial variability was abou t 20% across the plot, and there was little vertical variability at lo wer heights where soil and vegetation were exposed. Measured ratios of HT to SF6 din not vary much over time, but were about 20% less than c alculated from the soul ce term. The experimental data were used to st udy the oxidation of HT to tritiated water vapour (HTO) by by micro-or ganisms in the soil and the subsequent environmental transfer of the H TO. Crown Copyright (C) 1997 Published by Elsevier Science Limited.