Metropolitan Edison Company

The Three Mile Island Unit 2 accident was initiated by equipment trouble, but it became severe inside an operating system that repeatedly converted physical evidence into misleading or unusable information. A relief valve remained open while its control-room light implied closure. The pressurizer appeared full while the reactor vessel was losing usable coolant. More than 100 alarms arrived without meaningful priority, essential indications were out of sight or off scale, and training had prepared operators to protect pressurizer level more readily than to diagnose an unfamiliar small-break loss-of-coolant accident. This reconstruction treats the control room as an accountability boundary: a place where equipment design, operating experience, training, management, emergency planning and regulatory oversight had to combine into reliable control. They did not. The analysis first establishes chronology, then separates physical cause, operational action, institutional responsibility and legal findings.