powder keg 
noun
1A barrel of gunpowder.Heavy was the powder keg in the padded backpack he possessed-he'd taken all he could comfortably carry upon his back....
- It is no coincidence that, in a significant addition to Jonson, he turns Morosus into a man damaged in conflict, his aversion to noise the result of his eardrums being perforated when the powder keg of his ship exploded during battle.
- Stepping forward and receiving the half-empty powder keg from her subordinate, the General suppressed an inward shiver of fear as she recognized the contents.
1.1A dangerous or volatile situation: the place had been a powder keg since the uprising...
- A powder keg situation at the best of times, the game has added spice as the teams occupy the top two spots in the division with both promotion and the title still to play for.
- She becomes a powder keg of emotion, especially in situations where tact would seem to be the rule.
- He becomes obsessed with them, and when he develops photos that threaten their happy marriage, he becomes a powder keg waiting to explode.