There's a significant amount of conversion from hydrogen to deuterium. The likelihood of neutron absorption by deuterium is much smaller (by a about a factor of ten) than the likelihood of neutron absorption by plain hydrogen, but it still happens since there's a lot of neutron flux.
Canadian reactors (the CANDU design) use heavy water as moderator because it doesn't absorb neutrons as readily, and the improvements in neutron economy allow them to avoid enriching their fuel.
Radiation from tritium is definitely something that we worry about as far as worker safety near piping containing irradiated water, but it's not something that concerns us too much as far as being vented into the environment (we try to minimize that, but there's only so much you can do, especially with a BWR).
Canadian reactors (the CANDU design) use heavy water as moderator because it doesn't absorb neutrons as readily, and the improvements in neutron economy allow them to avoid enriching their fuel.
Radiation from tritium is definitely something that we worry about as far as worker safety near piping containing irradiated water, but it's not something that concerns us too much as far as being vented into the environment (we try to minimize that, but there's only so much you can do, especially with a BWR).