A nuclear device was the first thing I thought of as well, but I'd expect obtaining the parts to be prohibitively difficult for any plausible contacts. Stealing a finished warhead would probably be easier. But it's not that much easier, and either way I can't think of any plausible motive: the wizarding world and the Muggle world are so intertwined that MAD is quite problematic. Nor are there many plausible targets; the wizarding population is so spread out that a single warhead couldn't do much that couldn't be done far more easily, less disruptively, and with less collateral damage using a truck full of ANFO or something. Azkaban may be one; I can't think of any others.
On the other hand, Muggle weapons or controlled substances of some kind (perhaps dangerous chemicals?) seem quite likely. Britain's a wealthy first-world country with plenty of international trade going on; if you've got enough money on hand, about the only reason you'd need to leave the country to get something is if it's not legally for sale.
Azkaban may be one
That would kill the prisoners and the guards, and would not destroy the Dementors.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 98. The previous thread is at nearly 500 comments.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system.
Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: