Sorry if I get this wrong, but I'm still trying to sift through your code. But the way I see it, your ambient temperature is set to 20°C, which is normal, but you set a "cryogenic lower limit" for fuel and core to 0°C.

I would assume that if you would set a logical lower limit it should be ambient temperature, since apart from ice blocks and cryotheum none of the coolant materials has an innate temperature below that.

So why did you decide to use the 0°C when it is only directly reachable with one single block in the game (I guess cryotheum would drop far below the 0°C)?

That's largely a leftover from the original equations in BR 0.1, which used a lot of logarithmic functions. Logarithmic functions don't like numbers below 0.

I could've tracked temperature internally for Kelvin, but that would break a fair amount of existing BR code and also break any Computercraft/OpenComputers programs out there. I don't see any gameplay benefit to permitting ultracold reactors right now, so I've just not bothered to convert.

Besides, it's all fake-physics/fake-math anyway. :)

Of course it's fake, but you want to make reactors more realistic in minecraft and you are on a very good way, but in my mind it would be nice to set the lower temperature limit to the ambient temperature, but that might also be annoying perfectionism on my side. :)