It would explode, but for any reactor we’re likely to build in our lifetimes, it wouldn’t be anything like a nuclear bomb. We’ve had nastier explosions in natural gas refineries. There would be radiological contamination from the Tritium and any neutron-activated material, but it would be more of an expensive cleanup than an environmental disaster. It would be dirty though, especially if the reactor had been running for a while, building up dust. What wasn’t radioactive would still likely be stuff you don’t want to breathe or ingest.

TL;DR Boom lots of dead people in the containment structure, expensive cleanup, but nothing like a fission disaster. It would be an uninspiring explosion, but it would be dirty.