Until now, even mentioning the word "nuclear has been taboo in Japan. This is because of the experience of the actual use of nuclear weapons in Hiroshima and Nagasaki.
However, with North Korea developing more and more nuclear missiles and Putin unhesitant to use nuclear weapons in Ukraine, even Japan has begun to discuss the pros and cons of nuclear armament and, if so, what form it should take. So let me try to summarize some basic points here.

　If Japan were to arm itself with nuclear weapons, it would have to drop the "three non-nuclear principles"; (not possessing, not producing, and not allowing the introduction of nuclear weapons). This is neither a constitution nor a law, but a resolution of the Diet, so it is not difficult to change. So, if we drop this principle and allow Japan to possess nuclear weapons, what is the next step?　

First, there is the question of whether Japan can easily produce nuclear weapons.
There is a theory saying that "Japan has 50 tons of plutonium, which can be used to make nuclear weapons, so it can build 6,000 nuclear bombs". But, as it turns out, the story is not so simple. Eighty percent of Japan's plutonium is deposited with the United Kingdom and France. And the 10 tons of plutonium in Japan is still in the waste from the light water reactors and contains too many impurities to make a nuclear bomb.
　
One more thing. There is an option to make a nuclear bomb using uranium as material. But for this, a large amount of uranium 235, which is contained in uranium ore only by 0.7%, would have to be produced, and Japan does not have the facilities or technology to do so. Even if it did, the problem of how to secure uranium ore imports would arise.

This is where the issue of the NPT (Nuclear Non-proliferation Treaty) weighs in. This treaty prohibits countries that do not possess nuclear weapons from developing them. It is theoretically possible to get an exception, but Russia, China, and South Korea, as well as the United States, are unlikely to recognize Japan's nuclear armament. Japan could withdraw from the treaty and go nuclear, but then its uranium ore imports would be cut off.
　
"Nuclear Sharing" with the U.S.

So, shall we have the U.S. deploy its nuclear weapons in Japan? In that case Japan would drop one of the three non-nuclear principles: "Do not allow to bring nuclear weapons into the country". But right now, there are no U.S. nuclear weapons that can be brought into Japan. In the past, U.S. Navy nuclear submarines equipped with Tomahawk cruise missiles with nuclear warheads sailed around Japan. However, from the Bush administration to the Obama administration, the U.S. pulled out the Tomahawks and scrapped the nuclear warheads.

Therefore, the only deterrent that could dissuade China or North Korea from launching a nuclear attack on Japan is the nuclear bombs carried by bombers deployed on the island of Guam. This is not very promising.

The U.S. military is currently developing intermediate-range missiles with nuclear warheads and has announced its intention to deploy them in Asia as well. Russian Foreign Minister Lavrov and others have already warned against deploying these missiles in Japan and other countries. What to do about this?　

First, there will be a huge debate in Japan over whether to accept or not. Even if they decide to accept it, no local government will accept its deployment. Even if they do, the only places they would admit them deployed would be on uninhabited islands or submarines. And Japan does not yet have such a submarine; normally, such missiles would be mounted on nuclear submarines, which can remain submerged for months at a time.

There is an argument, then, that these missiles be mounted on U.S. nuclear submarines, etc., for "joint use" (i.e., sharing) with Japan. Japan would propose "A country so-and-so is doing something wicked, let's shoot them with nuclear missiles", and if the U.S. says "Okay", the missiles would fly, or the U.S. would propose "Let's shoot that guy", and Japan would agree.

You may think this is a very cumbersome and unthinkable approach for the U.S. military, but that is what Germany is doing with the US. Germany has about 20 small U.S. nuclear weapons - called tactical nuclear weapons -deployed in Germany, and if the enemy attacks Germany, they will explode right under the noses of their large forces (but in German territory). This is called the "Dual Key" method. Both Germany and the U.S. have to agree to "turn" the key.

For Japan, however, it is very hard to realize a sharing of missiles on board submarines. There are limits to communication with submarines in the water. If sharing is to be done, the new medium-range missiles being developed by the U.S. military would be deployed to a remote islet and shared with Japan.

I can think of no other scenario in which Japan could go nuclear. Perhaps Japan has the guts not to care, even if China, North Korea, or Russia threaten to use nuclear bombs - threats don't work on those who are not aware of them - or it could develop laser or electromagnetic weapons that would certainly bring down their missiles. The only other option would be to develop laser or electromagnetic weapons.