Except Michio Kaku is not a nuclear scientist or engineer or energy policymaker (fundamental physics is not nuclear physics), he is not Japanese (perhaps relevant - he is born and bred American), and he most certainly is biased. Apparently that reasoning didn't stop ABC's Weekend Edition from having Kaku on as "our expert ... all through this crisis".
His lack of expertise makes commenting as an expert irresponsible - granted, we in physics often feel all-powerful in understanding everything, but when laypeople actually take us seriously, we have to give every possible inch of humility in our conclusions. His comparisons to Chernobyl (the comparison was even worse the next day) are sensationalist, as the disasters are nothing alike in mechanics or magnitude.
But he is not only irresponsible, but unethical, in that he never reveals his quite-relevant 30-year NNP activism for a complete dismantling of nuclear energy on Earth. See a response from Nuclear Dreams with full quotations given in a 2008 interview with the India Times. In addition, I refer you to the following quotation from a commentary Kaku wrote for The Guardian ("Ban Nuke Power, Ban Nuke Weapons") in 1979 while a highly-public activist:
"Nuclear power and nuclear weapons are two sides of the same coin. They are controlled by the same people, produced by the same corporations and serve the same political and financial interests. They give off the same radioactive poisons, generate the same deadly waste that nobody yet knows what to do with. And both threaten catastrophic destruction. The people who brought us Hiroshima now bring us Harrisburg."
Scientists grow up with all sorts of biases and shouldn't have to cite them in their work if they have sound reasoning. However, Kaku has a history of vocal political activism in this subject, but rather than giving a disclaimer or stating up-front his views on the complete dismantling of nuclear power, he uses his scientific authority (in spite of his field being completely unrelated) to spread hyperbole and misinformation -- in effect, fear-mongering.
His is an extreme case, but scientists in general have to be very careful about the topics on which they are sought for comment or authority. News reporters often think they can write accurately on biology, economics, and psychology, but admit ignorance on physics, seeking the most famous name they can find to comment on other people's research or topics outside their field. For those sought, famous or not, it is absolutely unethical to exert argument by authority alone. You are a journalist within science - speak carefully and cite your facts.