There's some way to see it in the project window.  I believe it tops out at 30%, which accords nicely with the 0.1*modifier number you give.  (I was checking for a friend who wanted exploding ships.  No, I don't know why.)

It's actually kind of amusing, as most nuclear weapons are actually really difficult to set off on accident in the real world. Hitting a nuke with a bomb is going to more likely make it unable to detonate, rather than cause a detonation.

Except anti-matter weapons. If those get damaged, containment would probably be lost causing the whole thing to go boom. And regular nuclear munitions may go as well as the force from a reaction (enemy weapons fire) may cause a chain reaction in your own munitions.

I agree. The way understand it. When X-points of damage penetrates shield\armor.
1. A random component type is picked from DAC (aka probability table total ship size to total component size)
2. If one of its component isn't damage, X damage is applied against that component HTK.
4. Queue FUN stuff for secondary exposition chance.
5. Back to step 1 with X minus HTK .

What confuse me is that note in tutorial 15 about internal armor and HTK calculation.

1.) Engines, Powerplants and Magazines all have an explosion chance that you can see when designing them.

I was afraid of that, I hopped for some rare\crazy events on top of that. Anyway, while the Engine explosion chance has been described in Steve's tutorials from a few years back, in the create new project window you can see\test it only for Powerplants and Magazines. (And unfortunately Missiles/Magazines seems a bit overpowered in comparison)

3.) If you don't have enough maintenance supplies to repair a component that breaks it is instead destroyed, with a chance as in 1.) for secondary explosion.

If so, than I guess it comes back to how secondary explosions work.

For engines and powerplants I'm pretty sure it scales with their power ( so indirectly with size ). Since it's internal it also ignores armor (naturally)

Magazines should also scale with their capacity, but I'm not sure if it takes into account if they are loaded and/or with what they are loaded. That is an interesting question I'd also like answered.

1. Any ideas on how secondary explosions work? What can cause it (any weapon, mesons, shock damage) what is formula?
3. How maintenance failure cause ship to be destroyed?

1.) Engines, Powerplants and Magazines all have an explosion chance that you can see when designing them. This is the chance to trigger a secondary explosion when the component is destroyed. In the overall ship design it's given as "Exp 5%" for reactor and "Exp 2%" for engine in above example. Low tech magazines and high power reactors and engines have higher chance to trigger secondary explosions when destroyed.

3.) If you don't have enough maintenance supplies to repair a component that breaks it is instead destroyed, with a chance as in 1.) for secondary explosion.

If we wanted to build the toughest ship on a given tonnage, yes.

I've been focusing more on ships consisting mostly of very low-power engines: Cheaper per HTK, and more likely to be hit.
Add enough base-tech railguns (capacitors are expensive and can explode) to handle moderate ASM attacks and you end up with something promising.
Exceptionally crummy for its size...but sturdy, costing about 1/5 of a respectable warship, uses practically no fuel and it can rebuild itself multiple times over (all components are dirt-cheap, 1000MSP from a storage bay last forever).

Why not use armored gauss turrets for even more extreme internal durability? Granted, they'll be kind of big for the PD they provide, but they'll actually be able to track incoming missiles and tank hits rather well.
Maybe a good mission-purpose of a craft like this would either be an escort, or a mothership-support ship hybrid.

If we wanted to build the toughest ship on a given tonnage, yes.

I've been focusing more on ships consisting mostly of very low-power engines: Cheaper per HTK, and more likely to be hit.
Add enough base-tech railguns (capacitors are expensive and can explode) to handle moderate ASM attacks and you end up with something promising.
Exceptionally crummy for its size...but sturdy, costing about 1/5 of a respectable warship, uses practically no fuel and it can rebuild itself multiple times over (all components are dirt-cheap, 1000MSP from a storage bay last forever).