A reactor scram isn't necessarily enough -- you still have decay heat to worry about. In the case of Fukushima, the fission chain reaction was stopped but without cooling pumps the decay heat was still too much.
It seems like you should build a water reservoir at a higher elevation than the core and then apply a similar principle where valves regulate the water stream, but if the valves lose power they fail open. The reservoir can be built so that there is always enough water to cool the core.
For light water reactors this basically just amounts to a large pool up a nearby hill or in a water tower.
That is easier said than done - modern reactors are in the 1000 MW+ electrical power range, which means about 3x as much heat needs to be generated to get this much electricity - say 3000 MW.
Even when you correctly shut down the chain reaction in the reactor (which correctly happened in the affected Fukushima powerplant) a significant amount of heat will still be generated in the reactor core for days or even weeks - even if it was just 1% of the 3 GW thermal load, that is still 30 MW. It will be the most intense immediately after shutdown and will then trail off slowly.
The mechanism for this is inherent to the fission reactors - you split heavier elements into lighter ones, releasing energy. But some of the new lighter elements are unstable and eventually split to something else, before finally splitting into a stable element. These decay chains can take quite some time to reach stable state for a lot of the core & will still release radiation (and a lot of heat) for the time being.
(There are IIRC also some processes where neutrons get captured by elements in the core & those get transmutated to other, possibly unstable elements, that then decay. That could also result add up the the decay heat in the core.)
And if you are not able to remove the heat quickly enough - the fuel elements do not care, they will just continue to heat up until they melt. :P
I am a bit skeptical you could have a big enough reservoir on hand to handle this in a passive manner. What on the other hand I could image could work (and what some more modern designs include IIRC) is a passive system with natural circulation. Eq. you basically have a special dry cooling tower through which you pass water from the core, it heats up air which caries the heat up, sucking in more air (chimney effect). The colder water is more dense, so it sinks down, sucking in more warm water. Old hot water heating worked like this in houses, without pumps.
If you build it just right, it should be able to handle the decay heat load without any moving parts or electricity until the core is safe.
Yea, it seems like you could design a cooling loop that runs just off the latent heat. Im sure somebody in reactor design has sketched it out.
Some napkin math based upon heat capacity of water and assuming a 20 degree celsius input and 80 degree celsius output and 30MW heat results in about 120 liters per second of water flow needed. That is about 10 million liters of water per day, or about 4 olympic sized swimming pools. I don’t know how long you need to keep cooling for, but 10 million liters of water per day seems not insane and within the realm of possibility.
If you allow the water to turn into superheated steam you can extract much larger amounts of heat off the reactor as well.
there are reactor designs that work that way, but most civilian power plants are pressurized water reactors. it is important that the water stays pressurized or you get a chernobyl
Fukushima was based on a Westinghouse BWR (Boiling Water Reactor) design, so pressurization was not that much of an issue - if enough of sufficiently cold water was provided, there would be no meltdown.
