powder keg

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A small cask for holding gunpowder or other explosives.
• noun A potentially explosive situation or thing.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A barrel containing gunpowder.
• noun An explosive or otherwise volatile situation.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a potentially explosive state
• noun keg (usually made of metal) for gunpowder or blasting powder