Japan may be considered a "virtual" nuclear weapons state because it already has delivery systems, separated plutonium, and the industrial/technical capabilities to quickly weaponize that plutonium should it need to.
Japan's plutonium is reprocessed from commercial reactors and is not "weapons grade." This is a bit of a misnomer, however: weapons grade plutonium is really good for making weapons because it has a low concentration of the isotope plutonium 240. Pu-240 undergoes spontaneous fission frequently enough that it makes a pre-detonation "fizzle" (bomb blows itself too quickly to achieve useful yield) likely in a simple bomb design like that used during World War II. "Supergrade" plutonium, especially pure Pu-239, is also desirable for submarine-launched weapons because it reduces crew exposure to radiation from the stored warheads.
It is however technically feasible to make effective weapons from "reactor grade" plutonium containing more Pu-240 if the weapons designer invests in more sophisticated features like deuterium-tritium boosting of the fission device.
To quote Carey Sublette's excellent Nuclear Weapons FAQ,
"The fusion reaction rate typically becomes significant at 20-30 million degrees K. This temperature is reached at very low efficiencies, when less than 1% of the fissile material has fissioned (corresponding to a yield in the range of hundreds of tons). Since implosion weapons can be designed that will achieve yields in this range even if neutrons are present a the moment of criticality, fusion boosting allows the manufacture of efficient weapons that are immune to predetonation. Elimination of this hazard is a very important advantage in using boosting. It appears that every weapon now in the U.S. arsenal is a boosted design."
(My emphasis.)
https://nuclearweaponarchive.org/Nwfaq/Nfaq4-3.html
Japan could produce its own tritium by irradiation of natural lithium targets in commercial power reactors. It takes maybe 3 grams of tritium to make an efficient boosted weapon. Japan has enough plutonium for perhaps a thousand boosted-fission weapons in the 100 kiloton yield range, comparable in yield (if not efficiency) to France's latest submarine-launched warheads. I don't think it can produce tritium fast enough to supply a large number of weapons, given tritium's short half life and Japan's small number of operating reactors, but many reactors idled post-Fukushima are still in working condition and could be started up again in an emergency. Further, an opponent that does not consider dozens of nuclear weapons a rational deterrent is unlikely to be deterred by hundreds either, so Japan may not need many weapons to demonstrate a credible deterrent.