Possible Clues:

• A mob, misguided, going round grave for explosive device
• It was used to discover fermium
• A grave old doctor's frightening killer
• Weapon that might cause a grave mob riot?
• Go off after 22 across in a very big way
• Destructive weapon
• The first fell on Hiroshima
• Mushroom cloud producer
• A grave mob shattering explosive device
• A grave situation - order second category weapon