I just looked it up and found this wiki page about it:
https://en.wikipedia.org/wiki/Nuclear_and_radiation_accidents_and_incidents
It describes the same incident as a “partial meltdown”
>https://psmag.com/environment/50-years-after-nuclear-meltdown-3510<
There are 3 telling statement:
"In fact, from July 12 through July 26, 1959, an unknown amount of radioactive gases were intentionally vented to prevent the Sodium Reactor Experiment from overheating and exploding."
"We know there was a fuel meltdown," said William Taylor, the current spokesman for the U.S. Department of Energy. "We don't know how much [radiation] or if any was released."
"According to an analysis of a five-year study by a panel of independent scientists convened years after the incident, the SRE accident spit out up to 459 times the amount of radiation released during the 1979 meltdown at Three Mile Island."
This is important because the TMI incident was also a big nothing! And 459 X 0 = 0.
The point is that just because the sodium coolant leaked and caught fire, doesn't mean the fissile material leaked.
It may indeed have melted, but that doesn't mean it was released.
The "radioactive gases" they refer to were from a secondary source, that of the coolant sodium.
"A 2006 report by David A. Lochbaum, the nuclear safety engineer with the Union of Concerned Scientists, determined that up to 30 percent of the reactor's radioiodine and cesium could have vaporized during the accident."
As is made clear here:
"One of several findings in the report read, "In spite of the cladding failure to 13 fuel elements and the release to the primary coolant of several thousands of curies of fission product activity, no radiological hazard was presented to the reactor environs. Recovery operations were conducted by SRE operating crews, working within standard AEC regulations on radiation exposure."