Here is the link to the Japanese language Sankei Shimbun story about a Japanese government study on developing nuclear weapons.
The story doesn’t have the document, but reproduces the “essence” in bullet points:
一、小型核弾頭試作には最低でも３～５年、２０００億～３０００億円かかる
一、核原料製造のためウラン濃縮工場拡張は非現実的。軽水炉使用済み燃料再処理をしても不可能
一、黒鉛減速炉によるプルトニウム抽出が一番の近道
I gather (from a friendly translation) that this boils down to three points:
一 To trial manufacture a miniaturized nuclear warhead, the minimum time required is 3-5 years, and the cost is 200-300 billion yen.
一 To produce fissionable material, the expansion of uranium enrichment facilities is unrealistic; it is also impossible to reprocess spent fuel from light water reactors.
一 The extraction of plutonium from graphite-moderated reactor is the quickest route.
That’s not meant to be a literal translation and Japanese speaking readers are encourage to do better.
More later …
Robot Economist provides an “on the fly” translation in the comments section.