FORMATION OF ORGANICALLY BOUND TRITIUM IN PLANTS DURING THE 1994 CHRONIC HT RELEASE EXPERIMENT AT CHALK-RIVER

Trace amounts of elemental tritium (HT) were released continuously to the surface atmosphere at a site at Chalk River Laboratories, Canada o ver the 12-day period 1994 July 27 to August 8. The test area consiste d of four agricultural plots, each 5 m square. One plot was left in it s natural state, and the other three were cultivated. Cherry tomatoes, radishes and edible Chinese mustard (Komatsuna in Japanese) were grow n in the three cultivated fields. After the HT gas reached the surface of the field, some portion of it was converted into the water form (H TO) mainly by microorganisms in the soil. Then, plant absorbs HTO from both soil and air. Photosynthesized organic material in plant contain s tritium,also. The main purpose of this research is to analyze the va riation with time of tissue free-water tritium (TFWT) and organically bound tritium (OBT) in several plant species during the HT release. Th e specific activity of HTO in plants increased rapidly after the HT ex posure. The HTO concentration in plant leaves almost attained its maxi mum value in about 10 days, even though there were several precipitati on events, which decreased the soil HTO concentration. The specific ac tivity of OBT in plants increased gradually after the start of the exp osure. The OBT concentration in plants did not attain steady state ove r the whole exposure period of 12 days. The OBT/HTO ratio in plants in creased gradually during the release. The ratio in Komatsuna leaves ch anged from 0.06 to 0.24 between 2 and 11 days. The production rates of OBT in plants are fined to numerical equations.