it is. B/c again, after Fukushima(2011), civilian probing of released cloud exposed far exceeding concentrations of thorium, far beyond Japanese non-proliferation obligations.
Concentration of throrium means reactor was used to synthesize uranium-233. It's a weapons grade material(it produces worse bombs, and it makes sense mostly if you can't have normal enrichment, but they work). Simple as that.
Of course, maybe it was the only reactor of the exact same type in Japan used for this exact activity, and it's all gone. Ponies shit rainbow, too.
What's the source for this claim that thorium was found in "civilian probing of released cloud"?
FYI, U233 is a very poor choice for a nuclear weapon. There's always contamination with U232 which has a hard gamma ray emitter in its decay chain (Thallium 208). Weapons grade Pu is defined as >93% Pu239, that's contamination measured in the parts per hundred. Contamination in the parts per million of U232 is sufficient to make a bomb both unworkable and the activity around it highly detectable.
When people talk about Japanese proliferation, they mean Japan's stockpile of reprocessed reactor grade Pu. It's not something a nuclear weapons state like the P5 or even a serious illicit weapons program like the DPRK's would be interested in, but it's workable enough for a crude nuclear weapon. It would be less reliable because of fizzling (premature criticality because of a high neutron background from spontaneous fission) and the heat from Pu240 decay would need to be dealt with.
Your argument that China wouldn't do anything if Japan moved toward a nuclear weapon because Japan's already doing so falls flat on its face. Your "thorium cloud" claim is hearsay, U233 is useless as a weapons material, and even if China wouldn't react with a prompt nuclear strike or threat of nuclear use, it would make very vocal and visible diplomatic moves if it had intelligence that this was happening.