[ Note: this is definitely not my field; apologies if I get this wrong. ]
For what it's worth, there are some designs of nuclear reactors that are fairly safe. Yes, they're operating in "critical", but it's unlikely that they will go super-critical quickly.
The two broadest, relevant questions about reactor designs include: how stable is the speed of the nuclear reaction? and, if it becomes unstable, does the speed tend to increase or decrease?
Chernobyl uses a fairly unstable design that tends to get hotter if it gets out or control. A counter-example are the CANDU reactors, which are pretty stable and safe.
There are even better designs which have not yet been implemented, such as CAESAR. As I understand it, this design uses depleted, non-radioactive Uranium as fuel. Steam moderates the speed of neutrons to the precise speed where they will cause depleted Uranium to split. If the reactor overheats or underheats, the density of the steam changes, neutrons are no longer moving at the speed necessary to sustain the reaction, and the reaction stops. The advantages of using depleted Uranium as fuel include the ability to have Uranium rods which are 100% fuel, instead of around 5% in traditional reactors, which means ~40 years of power without replacing the fuel rods. Also, the fuel rods are not usable for nuclear weapons either before or after they are used; we'd have the option of building these reactors in unstable countries without increasing nuclear proliferation.
Again, this is definitely not my field, so please forgive me (and correct me) if I'm wrong.
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