I was not aware of the Santa Susana Field Laboratory disaster until I caught Modern Marvels: Engineering Disasters 19, which looks at the little-known meltdown at the experimental facility in 1959:
The reactors located on the grounds of SSFL were considered experimental, and therefore had no containment structures. The reactors and highly radioactive components were housed without the large concrete domes surrounding modern power reactors.Numerous accidents occurred at the site. On July 26, 1959, the Sodium Reactor Experiment (SRE) suffered a power excursion, in which power production from the reactor rose out of control. With significant effort, the reactor was shut down. However, a few hours later it was restarted, without the cause of the incident having been determined. The reactor continued to operate for several more weeks, with high radiation readings and other signs of problems, until it was shut down at the end of the month.
After a full shut down was completed, the reactor operators discovered that a significant fraction of the fuel had suffered melting. Tetralin, a coolant used for the pump seals, had leaked into the sodium coolant of the reactor. Carbonaceous material formed, blocking the coolant channels, preventing the sodium coolant from reaching the reactor core, which in turn caused the fuel to overheat and melt.
Approximately one-third of the fuel experienced melting.
Radioactive gases were released from the reactor into holding tanks and then bled into the atmosphere over a period of weeks. The extent of the radioactive releases remains uncertain to this date, but some estimates put the amount at more than 260 times the amount of radiation that was released at the Three Mile Island facility.
I’d like to reiterate that the reactor used sodium as a coolant. Hey, what could go wrong?