I once had a lock which the key broke inside it. That meant I could any key (or flat head screwdriver) to open the lock, since the key was stuck completely inside. Did you try that before destroying your U lock?
I broke a screwdriver trying that. The key was a terrible design where right in the middle of the part that hits the tumblers there were two deep cuts exactly opposite one another, and not surprisingly this was where the key broke with minimal torque. As a result only half of the needed part of the key was left behind, and a barb protruding from the top blocked me from using the other half of the key to turn the lock. This combined with the fact that the locks were chronically freezing and jamming made me switch away from U-locks to shackle locks which have been exceedingly more weather resistant and reliable.