That seems super unlikely to be a problem. For long term storage, I'd expect that we'll dig a deep shaft, or use an existing abandoned mine, and then seal it ba

That seems super unlikely to be a problem. For long term storage, I'd expect that we'll dig a deep shaft, or use an existing abandoned mine, and then seal it back up. Any group of humans who are willing and able to make a hole through several meters of concrete in order to get at the stuff will almost certainly also have the technological sophistication to know what radiation is, and that radioactive isotopes are dangerous. Also, if you do decide to leave a message anyway, you'll just have to make it interpretable to a human, not to any possible "creature". 10 000 years is nothing on evolutionary time scales, so it will still just be us humans around, (or nobody at all, if we happen to go extinct first).