Japan is screwed. There's no way in hell they can pay back that debt, or even meaningfully reduce it. Their economy has "grown around" massive government deficits for 20 years, and is now structurally dependent on continued government spending. If the deficit-spending is withdrawn, they'd have a severe recession.
Even if the budget could be balanced, the Japanese savings rate is tipping over into negative territory, and they'll soon be forced to get funding from international capital markets. They sure won't get it at 1.5%. I read that if the average interest rate on Japanese debt rose to 4%, debt service alone would consume 100% of tax revenues.
It's going to be quite a fireworks show, that's for sure.