It is used in medical research for a number of applications. For example, one can measure the kinetics of enzyme/substrate binding using tritiated substrates.
It is nice to work with because it has such low energy that the researcher has no worries of the beta particles penetrating the skin.
The amount used in medical research is in the millicurie range. The amount of tritium in the contaminated water stored at Fukushima Dai-ichi is estimated at 3.4 peta becquerels, or 34 with a mind-boggling 14 zeros after it. I reiterate there really isn’t any use for it, but I should qualify saying slthough there is a use for very small quantities, there really isn’t any use for such a vast amount of tritium. US nuclear plants produce hundreds of curies per day that gets released into the environment
Read more at: https://phys.org/news/2016-04-japan-tritium-fukushima.html#jCp