KSP 1.9.1.2788
KSPIE 1.25.30

Despite having all tech nodes researched, and 2 tons of tritium onboard, tritium-tritium fusion cannot be selected in the reactor control window.

Test craft appears to be able to select all other fuel modes for each reactor that each has listed, such as deuterium-tritium, but not tritium-tritium.

Exactly what reactor are you using?

All of the following:

Magnetized Target Fusion Reactor
MFC Spherical Tokamak
Plasma Jet Magneto Inertial Fusion Reactor
Stellarator Fusion Reactor

For context I was making a spreadsheet of most of the fusion modes (i left a few out deliberately) and the ratios at which they consume and produce various resources and that's how I noticed that tritium-tritium wasn't selectable for some reason, which is why it's currently not in my spreadsheet.

I suspect its some safety feature that kicks in to prevent the reactor from choking itself. T-T fusion is normally only used when you don't have anything better.

I just tested. Even if only tritium is available, the reactor won't run T-T.

Also, while testing that, I found that the Magnetized Target Fusion Reactor actually doesn't have all the modes as listed in the part info. With further testing, it only has 4 modes. D-T, cold D-D, D-D, he3 catalyzed D-D. I think it's because all the modes that are listed in part info but don't actually work are largely Charged Particle reactions but the MTF can only use Thermal modes.

Well I do remember there are some min and max neutronic constraint on some reactors, also it the fusion mode produces too little power it might be constrained at low tech levels. Did you test in sandbox on campaign/science mode?

Tested in both campaign and sandbox. And yes, I made sure to check the "enable part upgrades" in game settings for sandbox.

I checked WarpPlugin\Resources\ReactorFuels.cfg and I think it might be neutron constraints as you mentioned. D-T has the highest neutron ratio out of almost any of the reactions at a 0.8, but T-T has a ratio of 3.2398

Then we have our solution. We have to Fix that number. I believe it should be (1 - 1 / 5 =) 0.833. I will apply the fix for next release

Fixed in eb30968

btw, I noticed Tritium-Helium3 Fusion is missing. I will add it for the next release. It quite useful as it produces more heat than pure He3 fusion

Did it work well now it the latest beta?

https://www.curseforge.com/kerbal/ksp-mods/ksp-interstellar-extended/files

How to access latest beta?

Tritium-Tritium and Tritium-Helium3 are selectable in all 4 discussed reactors in the 1.26.3 beta

so we can close it?

Aww, but I like talking to you.

(yes, I'll close it now)