Nuclear Containment

Not sure if this has been posted before if so can admins please delete.

I was thinking about Situation X and what happened if the power grids went down and all the nuclear reactors cooling failed. I know the are backups in place but at some stage human intervention would be required, such as re-fueling the backup pumps. Im guessing that couldnt be relied upon during an all hell breaks loose situation.

If that happened, am i correct in assuming they would go critical?

I dont have figures or locations on where the worlds nuclear reactors are but Im assuming that it would mean large areas of the world would become toxic wastelands and down wind of the reactors would become highly radioactive.

That would mean anyone planning to bug out and hold up somewhere may need to consider the nuclear fallout from reactors not just possible missle strikes.

No expert here but wouldn't they just use the power there generating to power there own colling system. I believe they are not dependent on the out side grid to power themselves internally. I may be wrong but it makes sense.

Nuclear reactors have many safety systems that can operate without mains power, and indeed power at all. This was all learnt after Chernobyl.
Once mains power goes, plants have both power internally produced by the reactor and power provided by conventional-fueled generators.
The primary concearn with managing nuclear reactors is to stop the core from overheating. Usually a normal shutdown of the reactor would suffice, but in the worst case scenario, I would think that a SCRAM would be initiated, where emergency rods are dropped into the reactor core to stop the neutrons that sustain a nuclear reaction. But even after that happens, there is still a copious amount of heat in there. If the cooling system couldn't handle it, or if the cooling system failed, or if the SCRAM was only partially successful, one of the emergency core cooling systems would be activated. ECCS's really depend on the type of coolant used. One used for a liquid sodium reactor would be different to a PWR, but they all have the same purpose, to cool the core down.
Apart from all the active safety measures, a lot of reactors also have passive safety stuff built in. For example, in PWR's pockets of steam in the water appear if there is excessive heat, which allow neutrons to pass through them uninhibited and hence the neutrons are moving too fast to collide with other atoms and sustain the reaction.

Of course, if everything goes to ----, the containment buildings around reactors are practically unbreachable (learnt from Chernobyl, too). They can hold internal pressures of about 200psi, and are usually tall cylindrical steel buildings attached to concrete missile shields. You get into them though hatches like in a ship.

A lot of people don't realize that there have been about 10 recorded nuclear meltdowns, instead of 3 Mile Island and That One In The Ukraine, except the safety systems were good enough and - more importantly - were managed well enough and implemented soon enough to stop radioactive contamination.