A meltdown is still a big "if" since the actual containment vessel apparently is still intact, but any radioactive particles would be very dispersed after travelling thousands of miles to North America. Imagine pouring a bucket of poison into the Mississippi and trying to measure the level of that poison a thousand miles downstream.

Twenty years after Chernobyl, the best efforts of scientists and medical researchers found no significant increase in cancers in countries downwind of the disaster, like neighboring Belarus, which are much closer than we are to Japan. That's not to say that there were no cancers that weren't caused by Chernobyl, but they couldn't detect any deviation from the normal rate of cancer among people so nothing to definitively attribute to Chernobyl on a population level.

Well, there was one exception--a particular form of childhood cancer, but not a big increase. A highly survivable form of cancer, if I recall correctly.