There are pools and pools and there are leaks and leaks. Anybody that has built an aquarium knows that a 80cm high design is much more complicated to made leak-proof than a 40cm high design holding the same water.
A shallow pool would not be enough to contain a small nuclear plant, so you need a non standard bigger pool. Higher the pool, higher the weight of water column, the force pressure against the walls will increase, and the leak will be much faster because the water weight in the upper level of the pool will force the water in the low levels to go out. If your leak is in the upper side of the pool will be a small self contained problem but if the bottom leaks is a different thing. As the bottom needs to support much more force against it and there is a weak area when walls meet bottom, is more probable to fail first.
If our smallest manmade lakes (swimming pools) can leak a lot, and our largest manmade lakes (dams) can also leak a lot the idea there's an in-between size that doesn't leak might need a bit of elaboration.
The safety requirement here is not "doesn't leak", it's "holds most of the water for 30 days (after which water is not required)". You would have to get an implausibly-large leak, during a situation where nobody can add more water for a month.
We should be orders of magnitude more worried about all the carbon dioxide we're dumping into the atmosphere, than the failure modes of an engineered hole in the ground.
Maybe a particular reactor design could use such a mechanism as one failsafe, but that alone is not enough, and no design is perfect, and the people operating it are not perfect.
I think some of the risks of nuclear are acceptable, I am actually very pro nuclear, but we should acknowledge them instead of pretending they don't exist. The only way risk can be properly managed is if it's acknowledged.
I would prefer to see inherently safe designs like LFTR gain traction, but NuScale has one of the few designs likely to be built in the near term, where you could SCRAM and take a vacation without causing a meltdown. That is a major advancement in safety; let's not let perfect be the enemy of good.
Unless we plan on getting the pope to build nuclear power plants, which does sound cool.
People are fallible on the best days, assuming everyone did their very best from nuclear physicists to construction workers, mistakes are made. You take steps to reduce the risk. Research gets review. Engineering schematics get review. Construction gets inspection. Still some mistakes will get through.
And people always act their very best all the time right?
You can even have a perfect design, perfect construction, that is mismanaged years after it's built, after the original engineers and bureaucrats lose control.
The same people problems apply to basically every human endeavor, but nuclear's capability to cause accidents that have a lasting impact is pretty scary. You don't feel even a twinge of existential dread when you think about? If you don't, then I don't think I want you working on a reactor.