It was generated at the Graphite Reactor in 1948.
Others claim credit for generating the first electricity through nuclear power, but it happened at ORNL first, a few years after the accomplishment of the Manhattan Project mission at X-10 and the end of World War II.
Einstein's famous letter to FDR mentioned "vast amounts of power" from uranium, which, besides eliciting the most pressing fear of being channeled into a weapon by an enemy state, also kindled ideas for peaceful uses such as electric power generation.
The Clinton Pile by 1948 had long since ended its wartime work and was well into its storied career in scientific research and isotope production. The operators at the Graphite Reactor, as it came to be called, are now known for their ability to improvise for the sake of science, such as adapting the agitator mechanism on a washing machine into a "pile oscillator" for neutron irradiation experiments.
The "washing machine" experiments, originally proposed by Eugene Wigner and conducted by Herb Pomerance in 1947, helped solve a concern related to zirconium's supposed property as a neutron absorber. The element was proposed as a high-temperature replacement for aluminum as fuel element cladding. But zirconium was believed to be a neutron absorber, and would thus "poison," or slow, the reactions.
The reactor operators and likely some ingenious crafts workers rigged the washing machine agitator into an oscillating mechanism, which exposed the samples to irradiation at intervals necessary for the experiment. The poisoning was actually found to be due to the almost chemically identical element hafnium, a much more efficient neutron absorber, which frequently contaminates zirconium.
Ensuing efforts to separate hafnium from zirconium, a feat of chemistry at the time, became an enabling technology for the nuclear navy and nuclear power generation.
On the latter subject, the next year, in 1948, Logan Emlet, who worked in the X-10 operations organization, suggested two other reactor operators, Mansell Ramsey and Charles Cagle, hook a toy Jensen steam engine to a tube of water that was connected to 10 uranium fuel slugs that were placed inside the Graphite Reactor through one of its ports.
Graphite Reactor veteran, the late Art Rupp, describes in the ORNL Oral Histories the experiment that resulted in the illumination of a small Christmas tree light bulb.
Well, Logan, who had charge of the pile operations, and a few of his associates just wanted to demonstrate the production of electricity by the Graphite Reactor, the world's first continuously operated reactor. After all, everybody had been talking about nuclear power. So, they got the idea of putting an aluminum trombone tube in a graphite stringer and placing it into the reactor. A stringer is a piece of graphite that can be slid into the side of the reactor. The little steam engine was attached to a little device that generates electricity.
So, the heat from the reactor easily generated some steam to turn the toy engine, producing a small amount of electricity. It was purely a demonstration. Logan, himself, knew that it might come under the heading of a "stunt."
But, nevertheless, it was an interesting, very practical demonstration for a lot of people who saw or heard about it. Logan really did make some steam from the reactor, demonstrating the possibility of nuclear power. So, it was not meant to be any kind of a scientific experiment. It was actually just a demonstration of the possibilities in a very graphic way.
The "stunt" is said to have displeased officials at the Atomic Energy Commission. It was the previous "garlands dreary" Christmas, 1947, that the AEC had decided to centralize reactor development at Argonne National Laboratory. So not much more was said about ORNL's glowing bulb.
Three years later, in late 1951, it was ceremoniously announced that the Experimental Breeder Reactor-1 at what is now Idaho National Laboratory produced the first "usable nuclear electricity."
In all fairness, the Idaho reactor generated enough power to light the building, much more than a tiny, single bulb. But first is first. The toy steam engine is still on display at the Graphite Reactor Museum on the ORNL site. Staff members can drop by to see it any time.