Transport of HTO in the atmosphere

Most of the T from atmospheric tests is formed as HTO. Presumably, the heat of the fireball ensures oxidation. Ehhalt (1966) noted an increase in the atmospheric HT before the series of large thermonuclear tests in 1962 and 1963, but observed no increases correlated with individual tests. Mason Ostlund (1979) analysed samples of stratospheric air after the Chinese tests of November 1976 and observed a big increase in HTO but not in HT. HT may, however, be released following underground tests. 

Tritium is also formed as a product of ternary fission in power reactors. Yields in thermal fission of 235U and 239Pu are about 1 x 10-4 and 1.5 x 10 4 respectively (NCRP, 1979). Most of the tritium is retained in the fuel, but some may be released to atmosphere as HTO during reprocessing. At present, no fuel is reprocessed in the USA. The NCRP (1979) report included speculative estimates that reprocessing in Europe, excluding the USSR, may release 0.4 kg a-1. This is small compared with the release in atmospheric thermonuclear tests. 

The pre-bomb HTO concentration in rain at Valentia, on the S.W. coast of Ireland, was 2.5 TU. In European and American wine (as measured later) it was 3 to 5 TU (Schell et al., 1974). By 1963, the concentration in rain at Valentia had reached 1,000 TU. It then declined rapidly, though Chinese and French tests reduced the fall after 1967. The reduction from 1963 to 1967 was by a factor of 12. Allowing for 

Transports of HTO vapour and H20 vapour to and from surfaces are controlled similarly by eddy diffusion in the free air and molecular diffusion across the viscous boundary layer near the surface. There is also a liquid phase boundary layer, and diffusion through this is a limiting resistance to the transport of sparingly soluble gases such as H2 or HT. For HTO, the liquid film resistance is negligible (Slinn et al., 1978). When the concentration gradients are in opposite directions, transport of HTO to a water surface can proceed simultaneously with evaporation of H20. 

The factor/3, which is about 1.1 (Sepall Mason, 1960), allows for the difference in vapour pressure of HTO compared with H20. 

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