A country can renege on it's debts to outside sources. It will cause many problems, but it is possible. Especially considering how powerful Japan is (and her military also, btw).
But a government can NEVER renege on debts to it's citizens or it can kiss getting re-elected goodbye and no one would EVER trust government bonds again. Which would be disastrous.
Look at Greece; far and away most of it's debt is owed to foreigners. And there has already been lots of talk in Greece of simply defaulting those debts, leave the Euro and go back to the drachma. And there is much support in Greece for that.
Sure, they will have great trouble getting more foreign loans. But it IS an option (and I believe the one they will eventually have to take) that is supported by the people (for obvious reasons).
Also, if they threaten default, all the foreign banks and institutions they owe money to will surely offer them great deals not to default since if they do not they risk losing the entire loan.
But none of this is an option if the money is owed to Greeks/Greek institutions.
You simply cannot default on loans from your countries people/institutions.
Out of the question - both politically and practically.