the NRC requires 7 days of fuel for the back-up generators on site; and 28 days worth readily accessible. If the nuclear power generator(s) shut down, the core of nuclear fuel and the storage pools for spent fuel still need back-up electrical power from elsewhere to continue to circulate water to keep the zirconium cladding on the fuel rods from overheating and melting, as happened at Fukushima.

The robustness of the tanks and their mountings, of the piping running from the tanks to the generators, and of what do if roads to the nuclear plant are severed, are other issues.

I'm a bit hazy on the details of nuclear power plant specs, but I believe that the Fukushima reactors do/did have passive (and pump-less) means to cool the reactors even without power, but I believe that they all failed eventually. Unit 1 had something called an isolation condenser (IC). Unit 4's is called a reactor core isolation condenser (RCIC). Not sure about Units 2 and 3. I don't know if they would be able to cool the reactors enough all by themselves all the way to cold shutdown or whether they are just stop gap measures.

As for #2, I suspect that the practicalities of needing room to periodically load/unload fuel rods from the top of the reactor vessel is why the control rods come up from the bottom.

Good point. It would make it more difficult from above. I'll be the first to admit I don't know much beyond the basics about nuclear reactors. It just seems strange that an electricity producer would have to have grid power to operate.

Under normal operating conditions, a nuclear power plant needs no external source of electricity. The cooling water pumps and other auxilary machinery are powered by using a small proportion of the output power of the reactor.

If however the reactor is shut down, then no power is produced, but the reactor core still produces heat for some time.
Under these conditions power must be either produced on site or imported from the grid, unless the reactor is designed for passive cooling.

The heat produced from a shut down reactor is not constant but steadily reduces, eventually to a level low enough for passive cooling.

Here in the UK a friend has restored to usable condition an abandoned cold war bunker.
Some years ago they purchased land that included a small bunker and originally only regarded this as an interesting relic of the past.

Recently they have cleared out all the junk, steam cleaned and repainted and stocked the shelter.
These shelters are quite basic and rather small, about 7 feet high, 7 feet wide, and 18 feet long.
No air filtration or specific radiation protective measures are installed, but the simple fact of being 20 feet below ground is said to reduce the radiation to one five hundredth of that received on the surface.
Originally no mains services were installed, but I have added grid electricity. This is unlikely to be available after TSHTF, but whilst times are normal grid powered lighting, a dehumidifier and a battery charger are useful.

(BTW, I know that this is a very old thread, but the points raised are still relevant and I saw no point in starting a new thread on the same subject)

How is air supplied? Can a filtration system be added? 20 feet below ground I would be concerned about air supply & quality after the lights go out.

A filtered air supply is required as is a 2" pipe the runs from 1/4" from the lowest part of the floor and then runs straight up to the outside with a flapper valve on top. The 1st brings air in, the 2nd forces CO2 out.

The existing ventilation arrangements are entirely passive. At one end of the shelter is the access shaft and immediately adjacent to this is an air shaft, a second air shaft is at the other end.
This works OK but does not filter the air.

Lighting is battery powered, with candles in reserve.
Any ground water that enters is removed by a small hand pump.
No plumbing is provided, just a chemical closet.

This virtual tour gives a good idea of the design,
video tour of a bunker

Considering the air supply, I'd dump the candles and replace whatever lights you have with low wattage LED's. Candles will literally burn up your air supply. LED's will allow the batteries to last much longer.

I'm assuming the shelter design in the video is more generic than specific to your friend's bunker. If it is just like that with the above ground structures, armored hatch and air vents, very cool.

Were I to have a nice deep fallout shelter, I'd build a greenhouse over it and install an air filter and a small air pump to ensure a good air supply to the bunker during a possibly long period underground. Since there might be an EMP associated with whatever I intended to shelter against, I'd have a couple solar panels in the bunker which would be brought to the surface after the EMP threat passed and would be used to keep the batteries charged, air flowing and lights on. With an EMP in mind I might even consider taking the shelter/greenhouse off-grid so that street power/grid electricity didn't destroy the electric system which would kill the lights and turn off the air. But that's just me thinking without seeing the layout of the subject bunker -- where it's located relative to London or other juicy targets. Worth about $.02 or less...

The virtual video tour is an accurate representation of my friends bunker, many hundreds were built to the same design.

The lighting is all 12 volt fluorescent and LED and with several hundred AH of battery capacity it is most unlikely that the candles would be needed, but it seems sensible to keep them just in case.
BTW, the original lighting was one bulb of 3 watts, that was it in total, no light in access shaft or the chemical closet room.