Nuclear Case Study: The Three Mile Island Wake-Up Call

Summary

On March 28, 1979, the Unit 2 reactor at Three Mile Island in Pennsylvania suffered what would become the most serious accident in U.S. commercial nuclear history — a partial core meltdown. It began with something that looked routine: a secondary cooling pump tripped. But a small mechanical fault — a pressure relief valve that stuck open — allowed coolant to escape. In the control room, operators were faced with confusing, incomplete data. They believed the system had too much water when in fact it was losing coolant fast. Their well-intentioned interventions disabled the emergency cooling system, setting the stage for significant core damage.

This wasn’t simply a story of equipment failure — it was a human-systems failure. What unfolded revealed the danger of designing systems that outpace the ability of people to understand them.

ICAM Analysis

Absent/Failed Defences (A/FD):Critical information was missing. The control panel didn’t show whether the pressure relief valve was open or closed. Nor could operators see real-time water levels inside the core. Those blind spots left them operating partly on assumption.

Individual/Team Actions (I/TA):The operators manually shut down the automatic emergency cooling system. Based on the indicators in front of them, they believed the core was overfilled. In reality, they were working from misleading feedback loops — a perfect storm of trust in faulty instruments.

Task/Environmental Conditions (T/EC):Procedures at the time didn’t prepare teams for this exact kind of failure. The loss-of-coolant protocols assumed a clean break, not a stuck-open valve slowly bleeding the system. The situation didn’t fit any familiar pattern.

Organisational Factors (O/F):Training and simulation lagged behind the complexity of the technology. Operators were trained for textbook failures, not the grey areas that live between mechanical and human error. The industry, at the time, underestimated just how confusing a “novel event” can be when you’re under pressure.

Executive Insight

Three Mile Island is a reminder that resilience isn’t built on redundancy alone. It’s built on understanding. The best defences in any system are the ones that connect human intuition with reliable feedback — clear data, realistic procedures, and leadership that invests in diagnostic skill, not just compliance.

In other words: technology can’t save you from poor system design, and procedures can’t save you from what you don’t understand.

Source: President’s Commission on the Accident at Three Mile Island (Kemeny Report, 1979)