Abstract: In order to study uptake and formation of tritiated water (HTO) and organically bound tritium (OBT) in carrots following acute exposure of airborne HTO, potted carrot plants were exposed to evaporated HTO for 1h in an exposure box at four different growth stages. The concentrations of tissue free water tritium (TFWT) and OBT in top and root were measured at the end of exposure and at harvest, respectively. The results showed that in most cases, it usually takes shorter than 1 h for the TFWT concentrations in the whole carrot top to reach equilibrium with the HTO concentration in the ambient air moisture. The TFWT concentration in the carrot top and root at harvest were lower than those at the end of exposure. In the carrot top, the relative TFWT concentrations at harvest represented about 1/389—1/86 of those obtained at the end of exposure; while these values were found to be about 1/116—1/3 in the root. In the carrot top and root, the exposure at the vigorous growth period generally ended with a higher OBT concentration at the end of exposure and at harvest. The OBT concentrations in the carrot top and root at harvest were lower than those at the end of exposure. When the exposure was conducted at the fleshy root expending stage, no obvious difference was observed for the OBT concentrations in the carrot top and root between at harvest and at the end of exposure. The concentration ratios of OBT to TFWT at harvest ranged from 2.79 to 20.73 in root, it is estimated that OBT would contribute much more to the ingestion dose than TFWT does.

Key words: HTO vapor, carrot, short-term exposure, tritium concentration