Getting off topic but I'm well aware of how CANDU's work, and the radiological side of hydrogen, but what I was talking about was on the conventional side of things. To give a high level overview, the heat from the reaction is transferred to heavy water (under high pressure), which is then transferred to light water turning it into steam which spins the turbines and ultimately the generator. Now what most people don't realize is the generator is hydrogen cooled, for many reasons including its heat capacity and density (its lighter than air requiring less energy for it to circulate) and many more. It is used at such a high purity level in a sealed system that ignition is not possible. So yes, on site there are large quantities of gaseous hydrogen stored and being transported. Page 17 of this document is the first one that comes to mind for a good read.
D2O, yes it is stored but in much lesser quantities.