A storage facility near it contains another 6000 spent mox fuel rods. The smoke of the fire is plutonium oxide and chloride which is fatal to humans at doses of 1-10 micrograms.
There is little doubt that if that happens at Fukushima the fallout would be carried by the jetstream over the US and, eventually the entire Northern hemisphere.
How many tons is that 6000 spent rods? Then remember exactly how big the Pacific Ocean is and how large, comparatively, a microgram is. A microgram is only 10^-12 of a ton, area crossed is a square fall off rate.
Could it immediately pollute the ocean and cause problems? Sure! Would the fall-out in the ocean cause a long term problem? Not unless there is way more than I expect from those fuel rods; the ocean is huge! One third of the Earth's surface, over half of the salt water on Earth; and you are worried about the toxins that humans failed to plan for when a volcano that's been dormant for a long history suddenly might be a little closer to eruption? Be worried about the loss of life from the volcano going off, and the loss of life from the climate change that a large eruption would cause (famine, loss of utilities, etc).
Or, if you must be scared of nuclear stuff, be scared of the fall out from all the nuke warheads and fuel stored close enough to Yellowstone that would be vaporized in the expected eruption.