Congratulations on a fun mod.

I think the description under "The CHALLENGE!" on the main page may be outdated. It currently reads:

"This will take 1000 reactors about 7 hours to produce the tritium, and needs to be detonated ~70km away - this is a 15 minute journey by nuclear-fuel train, or about 30 minutes by car (presuming no turning, stopping, etc.).
You also need to get 1000 stacks (25 full train carriages) of tritium there... and escape..."

I detonated a gigaton device and it only required 20000 units of tritium, which is 400 stacks. At a distance of about 73km from my main base, it did pretty significant damage. About half of my machines were wiped out, and my nuclear power plant detonated in sympathy on the first try. I tried again, deconstructing all the reactors first to keep them safe, and that went better. The message shown when configuring the test site mentions much larger safety radii for a device with this yield.

I am playing with the Nuclear Fuel mod, which replaces uranium-235 by plutonium for most purposes, including True Nukes fission weapons. Is it deliberate that tritium production still requires uranium-235 and not plutonium? This requirement has the important consequence of making mining the rate-limiting step for large fusion devices, because uranium-235 can't be made in reactors.

So in theory, the 70km is kinda the minimum safe distance, which I think I calculated to be broadly recoverable - although that might have been a bit too much damage - I'll update it.
Yeah, I think I halved the tritium use, and didn't update the challenge - I'll redo it.
The U-235/Plutonium wasn't exactly deliberate, although I'll have to look into what to do to make it reasonable - its done in a kinda generic way across the mod, so I'll have to work out a system, and I'm gonna be busy, so I might delay doing that.

The challenge should be updated now

I managed to recover at 73km, but it was more damage than I expected from the "Information" page and maybe less damage than I expected from the message shown while configuring the test site. As for the U-235/plutonium for tritium production, I can see that both ways - having tritium mass production require a lot of mining was somewhat interesting from a game play perspective, but I'm not sure it's easily justifiable from the underlying physics.