Meltdowns won't happen with gravel bed reactors, but that doesn't make them safe. Sellafield, which produces plutonium for the ministry of defence, never had a meltdown, but it did manage to pollute the west coast of southern Scotland and Northern England with plutonium over a period of 30 years to the point where shellfish from those areas are no longer safe to eat, and will be banned from sale under proposed EU restrictions on radioactivity in food. Ireland complained to the UK about it polluting their shores and fisheries too, but it's still allowed to dump reduced amounts of plutonium into the sea. Other nuclear plants around the country have similar histories of repeatedly leaking radioactive materials into the surrounding environment.
Nuclear power could be great if managed properly, but so far it's been horribly over-budget with an appalling safety record. That's not to say that it's worse than coal-fired powerstations; they pump huge quantities of heavy metals and radioactivity into the atmosphere, but the nuclear industry seriously needs to clean up it's act before it should be allowed to expand significantly.