If the symbols are the problem, why not discard them entirely and demonstrate the actual danger in increments? What if we create concentric rings of gradually lower levels of radiation around the site? The closer you approach, the more you notice the effects of radiation -- lack of bio-matter in the soil, radiation sickness, etc. Of course, it won't be a zero-casualty solution. A few people might become sick or die by the time they figure out the danger. But for a 10,000 period, (high probability * low exposure) seems better than (low probability * catastrophic exposure).