Well, in the case of radiation levels, the science goes back far enough, and with enough duplication/replication that it is as solid as anything that’s an ongoing endeavor gets.
Like, everything is unreasonable technically going to be “to the best of current knowledge” because science is a process, and even when there’s mountains of evidence, there could be newer evidence that contradicts previous conclusions.
But the general dosage limits have been in place and matched predictions for at least my lifetime (around 50 years), since those standards were used by my dad at that time and are still the same. A lot of the nuclear stuff wasn’t done for profit, nor were the standards. So it’s a tad bit better than something like petrochemical data.
I’d phrase it like this; I wouldn’t want to go swimming in the tank the water is stored in, but I wouldn’t worry about swimming in the ocean a few days later at all. The levels are just so low at that point that any danger is a non issue compared to things like smog.