system
Given the following post, respond as if you're commenting on Reddit:
human
CMV: Nuclear waste isn't really a big problem.

So my train of thought goes as follows: The uranium mined for nuclear energy is already radioactive to begin with. In the power plant, it decays into much more radioactive isotopes, but these have really short half-lives, so it will just take a couple months for them to decay into isotopes just as weak as the uranium that was originally mined.

So why do we need final disposal sites where this weak radioactive material can stay for thousands of years? If we just dumped them deep into the ocean for example, what would happen? Wouldn't they just dilute to the point of not being noticeable at all?

It's really hard to get information on this online, but I gotta be wrong in some way, otherwise people wouldn't have this argument, right?
gpt
It's true that the diffusion of radioactive matter into the sea will dilute it, but it comes from a very concentrated source to begin with, thus harming aquatic life nearby. It's not so much that entire oceans are affected significantly, but the immediate area is. 

However, like you say, encasing waste in concrete is a pretty inexpensive and safe way to dispose of the waste, so there shouldn't be much of an incentive (if any) to just dump it in the sea.