"Thorium metal scrap" can explode? (1956 pic)

How did this happen? What did Sylvania do with thorium?

Thorium is highly flammable if it is finely powdered, or in small turnings. Maybe that’s what happened.

Thorium was a fuel used in some early commercial reactors:

Sylvania was involved in developing commercial-scale thorium fuel.

Richard Nixon made the call to focus on development of Uranium 235 for Nuclear power because a byproduct of the reaction is Plutonium 239 which can be used to make bombs. The Uranium design uses boiling water under pressure for cooling. The Thorium design uses a molten salt.