Most nuclear reactors of the era used ordinary water to cool their cores. The Sodium Reactor Experiment tried something more audacious: liquid sodium metal, heated to hundreds of degrees, flowing through the core as both coolant and heat transfer medium. Built by Atomics International at the Santa Susana Field Laboratory about 30 miles northwest of downtown Los Angeles, the reactor represented a genuine gamble—a test of whether sodium cooling could make nuclear power more efficient and economical. The facility sat in Area IV, a mountaintop section of the sprawling SSFL complex, chosen for its remoteness from populated areas. Between 1957 and 1959, the experiment generated useful data and produced real electricity. It seemed, for a time, to be a genuine success.

In July 1959, thirteen of the reactor's 43 fuel elements partially melted. The cause was a coolant problem: tetralin, an organic solvent used in the system, had decomposed and coated the fuel elements, disrupting heat transfer. Radioactive gases were released during the incident and during the subsequent cleanup, some of which escaped into the atmosphere. The reactor was taken offline, decontaminated over the following months, and eventually restarted—only to be shut down permanently in 1964. But the full story of what had happened was not told publicly. Internal documents were kept from the public, and it was not until investigative reporting in the 1970s and subsequent government investigations that the extent of the incident became clear. It was, investigators eventually concluded, the first partial meltdown of a commercial nuclear power reactor in the United States—years before Three Mile Island.

The Department of Energy's handling of the 1959 incident became, in retrospect, a case study in the institutional instinct to minimize and suppress. For decades, communities surrounding the Santa Susana Field Laboratory had no way of knowing what had occurred on the hill above them. Workers at the site were not fully informed. Regulatory frameworks that might have required disclosure did not yet exist in their modern form. By the time the full picture emerged, the hill had also been contaminated with chemical solvents and other radioactive materials from the facility's broader operations. The question of health effects on nearby residents and former workers remains contested and painful—a reckoning that began too late for many families.

The Sodium Reactor Experiment site is now part of the long, unresolved cleanup of the Santa Susana Field Laboratory. The reactor building itself has been demolished. What remains is the question of what was released, where it went, and who was affected. The experiment's original achievement—first commercial nuclear power in California—has been largely overshadowed by what followed. In the history of nuclear power, the SRE occupies an uncomfortable position: pioneer and cautionary tale at once, a reminder that the same ambition that drives progress can, when paired with secrecy, leave lasting damage.