It's not about writing, this isn't like "Hey if we write in big letters surely they can read it" because there is no reason to think people will have English or even writing at such a distance in time. These are NON-LINGUISTIC messages. Their expression as written text is for our convenience in assessing the project, which is to deliver that message without language.
"This place is best shunned and left uninhabited" isn't a useful thing to write, but it's a useful sentiment to evoke for this purpose. Construction of deliberately hostile landscapes was one potential implementation. If I can live anywhere, why would I choose to live on the vast, too hot, practically indestructible black rock the ancients have inexplicably built here ?
One of the potential options for this project was very brutal. Just deliberately leave the surface of the site slightly radioactive. People who try to live there will discover they get cancer and die, it doesn't matter whether they go "Oh, it's radioactive here, that's not good" or "The ancient curse, don't live there because the Ancients have cursed that place" it has the same functional effect which is they stop living there and don't return.
Yeah, I have a master's in philosophy and semiology, I know exactly what they're getting at, but at the same time almost any kind of message of warning is going to be misconstrued and inverted anyway. One may as well write: "Extremely radioactive waste" in English and a few other languages - the future culture will either be able understand that because they're still at our level, or they won't at all and have to work it out for themselves. Virtually everything we've found with an ancient curse we've completely disregarded, same for dynamiting our way into the Great Pyramid, and also "honour" isn't what it used to be, and as a concept regarded wholly differently by almost every society on earth, even between individuals in the same society, so I wouldn't even bother invoking a concept like that in a warning.
The idea of poisoning the perimeter a radioactive site with radiation is similarly bizarre. If there's been some kind of catastrophic "fall of man" then we may as well leave them to it. If they're approaching our level, then their (reinvented) Geiger counters will tell them what they need to know.
The entire premise is an exercise in fear mongering, not aimed at people 10,000 years in the future but rather at people in the present. The message is that we must not use nuclear power because the waste is so dangerous that we have to go to elaborate theatrical lengths to prevent cave men in the future from killing themselves with it.
The most sensible way to deal with the waste, after burning up transuranic waste in nuclear reactors (common sense), is to put it somewhere that the effort to find and retrieve it is so great that only people who already knew what they were doing could manage to get their hands on it. The most obvious candidates for such a method are dumping it into the ocean. If you want to get fancy it could first be dissolved into water, or instead vitrified and sealed. Either way, it could be placed directly on the sea floor in deep parts of the ocean, or dropped into subduction zones, or buried beneath the sea floor. Any of these would be adequate. Another option is horizontal boreholes a few kilometers deep, but that's a bit less practical.
Of course people today lose their lids at the thought of a little tritium getting dumped into the ocean; this is the result of the same sort fear mongering campaigns that "this is not a place of honor" comes from. But this is a sociological problem deliberately created by activists, not a technological problem. A great deal of nuclear waste, including spent fuel and reactor components, was dumped into the ocean in the 20th century and nobody crying about Japan's tritium loses much sleep over that waste. China pretends that Japan's tritium will ruin fish for China, but they're not acting concerned about the several hundred TBq of nuclear waste the Soviet Union dumped into the Sea of Japan. Their objections to Japan dumping their tritium are political theater. The impact this oceanic dumping has had since it was done decades ago can be studied, and the answer is that it's not an issue. Water is a really great shielding material and very deep water is really good at keeping meddling idiots away from things. Much better than any spooky monument or deliberately poisoned land.
At the same time those activists likely don't want oil, gas, or coal either. They probably wouldn't enjoy the countryside being covered with solar panels and wind farms. Not sure how you'd win if it's from an activist perspective with some kind of escatological focus ten-thousand years hence used as a mode of attempting social change today.
I don't see what the major issue is with burying radioactive waste inside a mountain and then dynamiting and sealing the entrance so it can't be accessed without serious excavation machinery. (Finland is doing this). Uranium is naturally occurring, we've just condensed it a bit. The civilisation that encounters it would need decent technology to dig it up, and if they've gone backward to the point they don't then not much semiology is going to reach them.
We also have a reasonable grasp of some of the meaning and motivations inherent in rock art from Aboriginal cultures and Lascaux. Much of that is 10,20,30 thousand years old. Communicating at that distance isn't some impossible task of semiology, but the more one tries to abstract a message into symbolic meaning, the greater the chance of it being misunderstood which is why I'd stand by my point of writing it in English and other languages to give a key. Could carve it into granite alongside a well-rendered image of humans dying and in pain. Humans are pretty smart particularly when determined to understand something. If they're completely illiterate it might even inspire them to realise that language can be encoded as symbols, but I don't know what would have to come about whereby we'd lose that ability now. And if it's got that bad, then a mountain full of uranium is going to be the least of their problems.
