The answers to your questions are many and varied but all are good news. First, the D2 bottles all have their own main shut off valves at the bottle itself. I never leave this valve open under any circumstances even while running the fusor. (I work off of a small reservoir tank) With this tip you will be hard pressed to have an explosion even if you tried.
I run from the bottle to a regulator gauge group with tiny 1/8" copper line at the full bottle pressure ~700psi. I silver soldered all the copper lines to the gauge set fittings to avoid leaks and used epoxy on all threaded components as seals.
From the gauge, I step the pressure down to 1 psig in the copper 1/8" lines to my little co2 cartridge reservoir. Again, all lines are silver soldered to the fittings and epoxy used to seal the threads. I have a valve on the gas manifold out of the reservoir bulb and this isolates the fusor gas line leak valve and its pipe as well. I evacuate all lines down to near fusor pressure and then fill from the d2 systems gas reservoir.
Maybe once every 3 months, I will crack the bottle's valve to refill the reservoir, but immediately close it off again.
This makes the D2 line pretty free of air and keeps the lines full of D2.
Finally, there is no way with a d2 loaded fusor that a full, large air leak would form any kind of explosive mixture as there just is not enough hydrogen @ 5 microns to generate a sensible explosion.
Almost all of the hazards exist at the high pressure points at the bottle and gauge and the 1 psig points. These hazards are minimal as well, especially if you religiously keep all valves shut when not in use.
There is no such thing as zero hazards with h2 gas systems, but the above fusor gas system I explained comes real close.
Richard Hull
Created on Thursday, May 10, 2001 10:56 AM EDT by Richard Hull