The Three Mile Island accident was primarily caused by what?

• A. A cooling malfunction causing major core damage.
• B. A deliberate act of sabotage.
• C. A volcanic eruption.
• D. A software error in reactor protection.

Answer: (A)

The key idea here is that removing heat from the reactor core is what keeps it safe; if cooling is lost, the fuel overheats and can be damaged. At Three Mile Island, the cooling system couldn’t maintain adequate removal of heat. A malfunction in the cooling hardware, including a relief valve that moved coolant out of the primary system, reduced the coolant inventory. Because operators also misread indicators, the situation wasn’t corrected quickly enough. As heat built up, some fuel rods overheated and the core sustained damage.

There isn’t any real basis for attributing the accident to sabotage, a natural disaster like a volcanic eruption, or a fault in reactor protection software. The defining problem was the failure to keep the core properly cooled, with contributing human/indicator interpretation factors, leading to partial core damage.