Are you aware that our whole planet is warmed by the decay of thorium and other heavy elements below the crust? Geothermal energy is nuclear energy.
To respond to your question, Earth would get over it, and humanity would survive even 507 meltdowns. Fukushima was an exceptional case on a 40-year-old design, so we wouldn't have 507 Fukishimas (keep in mind how many people died from the giant tsunami that caused the Fukushima meltdown).
I'm no expert, but I suspect there's a big difference between the planet's naturally occurring radiation and the high volume of unstable particles spewing out from the Fukishima meltdown everyday.
What people never seem to understand is that the proximity of the source matters.
Compare a tiny amount of radiation coming from a long distance (say the core of the earth) and a tiny amount of radiation coming from speck of decaying radioactive material released by Fukushima that is floating around in the air of your office room. You measure the radiation in the room and sure enough the radiation is very weak. You conclude "See? Fukushima is no worse than natural radiation". Now what happens if you're so unfortunate to inhale that speck of material and it settles in your lungs? Then the picture changes completely because some cells of your lungs will now be very close to the source of radiation and these cells -- and always the same cells -- will be subjected to a long period of intense radiation.
> a tiny amount of radiation coming from speck of decaying radioactive material released by Fukushima that is floating around in the air of your office room.
By that logic, we should panic if we inhale the sweet scent of someone eating a banana, or walk by a soy farm with the obvious smell of fertilizer. What people actually never seem to understand is that you live in a world already filled with natural radionuclides, they are not simply buried deep in the Earth's mantle, they are everywhere around us.
What airborne radioactivity is floating in your office from Fukushima, by the way? And what type of radiation does that radioactive contamination emit; alphas and betas would effect local tissue, but neutrons and the much much more common gammas would have their interactions occur over a much much greater volume (possibly outside the body entirely).
Some of the potassium in your cells is radioactive and decaying right now at a rate of 4900 decays per second[0]. That's about as close as you can get to the source, and it's not a big deal.
You ignore the fact that the radioactive potassium will be distributed more or less evenly over your body.
Secondly, and more importantly, you bring up this number of 4900 decays per second as if it means anything by itself. Those 4900 decays per second are caused by the presence of 18 milligram of Potassium-40 in your body. One of the elements released by Fukushima is Iodine-131. If you ingest 1 microgram of Iodine-131, this speck will generate 4,6 BILLION decays per second. So yeah, you're right that 4900 decays per second is not a big deal. But then you conveniently forgot the fact that the material released by Fukushima is orders of magnitude more radioactive than Potassium 40.
To respond to your question, Earth would get over it, and humanity would survive even 507 meltdowns. Fukushima was an exceptional case on a 40-year-old design, so we wouldn't have 507 Fukishimas (keep in mind how many people died from the giant tsunami that caused the Fukushima meltdown).