Going Critical: Averted. In four ways:
Fission reactors in the universe are very well designed, ideally – although not always – to keep messy things like prompt criticality out of the possible performance envelope. Some of them, the higher-power ones, can still quietly melt down (giving you basically a corium puddle in a highly refractory can to dispose of, but no major problems outside that), but most of them – like the ones they use in vehicles, for example – are pebble-bed designs that can’t even do that.
Fusion reactors depend on the continuous operation of their support systems to maintain the conditions that make the fusion reaction possible. If they go wrong, even for extreme values of going wrong, what you get is a fizzle as the fusion plasma expands, loses its heat and pressure – all the more so if it escapes the envelope and touches the surrounding environment – and quenches. A worst-case crash shutdown will screw up the inside of the reactor vessel, forcing you to replace the lining before you can restart, but it won’t penetrate it.
And no, they can’t go runaway. There is a clever device built into the deuterium, etc., feed lines to stop that from happening. It’s called a valve, which is attached to a big purely mechanical lever, which is labeled “IF SHIT HAPPENS, PULL”.
Matter/antimatter reactors by and large don’t do the equiavlent of going prompt critical, mostly because as long as you can pull the equivalent of said lever, the ambiplasma in the reactor vessel will quench much like the fusion reactor case. (Remember, this isn’t Star Trek engineering – there’s always much more matter being fed in than antimatter, because it’s a lot easier to extract energy from hot plasma than from photons. Thus, necessarily, no excess antimatter floating around inside the reactor core waiting to cause trouble.) The remaining loose antiparticles that are there will chew the crap out of the inside of the containment, definitely, but it’s even heavier-duty than the fusion containment is, being designed for essentially this case.
Now, the storage cryocels where the antimatter’s stored, they can explode with great verve and drama, but that’s called “losing containment”, not “going critical”.
Singularity inductors don’t go critical because if the mini-black-hole falls out of the field knot and then through the containment, there’s usually stuff around for it to eat which will prevent it from going all Hawking-evaporatey on y’all. Of course, you do then have a loose singularity chewing its way through your ship, station, habitat, or possibly even planet, so it’s not like your day isn’t going to suck anyway… but it won’t go critical.
At least not until it’s run out of stuff to eat.