September 2000


Reactor veteran recalls account of the birth of a key word in the nuclear vernacular

Editor’s note: Edwin Blackburn, a millwright in the Research Reactor Division’s HFIR Shop, for a time worked alongside the late Wallace Koehler. Koehler, a renowned physicist who designed and built ORNL’s Small Angle Neutron Scattering Facility, was one of three technicians assigned to man one of the buckets on top of the Stagg Field pile reactor when it first went critical on Dec. 2, 1942. He worked on the Manhattan Project until September 1948 and was a researcher in ORNL’s Solid State Division from 1949 until he died in 1986.

Edwin was working on the Small Angle Neutron Scattering Facility when he asked Koehler how he became a physicist.

“He told me he guessed it just happened due to his college days and I inquired just where he attended college. He said the University of Chicago. I said, ‘Isn’t that where they first split the atom?’ He said, ‘Yes, as a matter of fact I was there. As a matter of fact, I was standing directly on top of the pile when it first went critical.’

“I asked then, excitedly, ‘You mean you met Fermi?’ He explained that he was a technician for Fermi. He then immediately asked me, ‘Do you know what scram means?’ I said no, and I guessed two or three things. He told me this story; I’ve never forgotten it.”

Webster defines Scram as “A rapid emergency shutdown of a nuclear reactor.” To most of us in the nuclear business this term means “to place the reactor in a safe condition.”

Scrams are usually activated or “tripped” by electronic means through some of the hundreds of safety sensors and systems of modern nuclear reactors, but they still have a manual scram. Once this is initiated it occurs in fractions of a second. The term can always be found on any reactor control desk, adjacent to a large red button labeled “scram”.
After a few test runs, the logger complained to Enrico that his neck was getting stiff. Could he just holler out loudly, "Cut the rope"?

The first sustained nuclear chain reaction was accomplished during the early days of the Manhattan Project. This event, at the University of Chicago’s squash courts under Stagg Field’s bleachers, was destined to change the world forever.

Italian physicist Enrico Fermi headed up a group of technicians, scientists and others who constructed a “pile” of his design to safely prove that splitting the atom could be accomplished and controlled by the critical spacing, configuration and shapes of the required uranium slugs, graphite blocks, cadmium sheets and cadmium rods. The cadmium rods or strips absorbed neutrons, which enabled them to raise or lower the activity of energy emitted during operation.

Fermi needed absolute certainty that this “test” would not go awry. The experiment was originally planned to be performed in the Argonne Forest, some 28 miles out of Chicago. Because of construction overruns and the all-out push from the administration, a decision was made to go ahead at Stagg Field as long it could be done safely. It was through Fermi’s confidence in his calculations with the earlier construction and testing of 30 other smaller “piles” that they did so.

Fermi added two more safety devices. The first was to place three technicians (history records them as physicists, one of which was Wally Koehler) always at the ready to pour buckets of cadmium sulfate down through the pile if Fermi gave them hand signals, which were pantomime motions of dumping a bucket. The other device was already in place but lacked the speed he knew it must possess. This was a centrally located cadmium strip vertically suspended by a cable with a lead weight on the bottom, which was raised and lowered by a hand-operated winch.

Fermi informed the Army’s top liaison officer that he needed the services of an expert axeman as soon as possible. A professional logger from the woods of Washington or Oregon was hurriedly whisked to the site.

To the amazement of this Northwest woodsman, the success of this top secret operation and the safety of all of them rested truly on his shoulder, upon which also rested an axe, a fireman’s type with one blade, a pointed end and a wooden handle, all painted red. With it the woodsman would cut a heavy rope placed between the cable to the winch and the cable to the pile. It was strung against a piece of railroad tie placed vertically—a chopping block.

This was the first line of defense for an emergency “shutdown,” followed by, if needed, the dumping of the liquid cadmium solution into the pile.

The axeman received his hand-signaled instructions from Fermi, who stressed the speed necessary by holding one hand flat and depicting a chopping motion with the other.

After a few test runs, the logger complained to Enrico that his neck was getting stiff and that he really needed to keep a keen eye on the rope to ensure that he did not miss, and could he just holler out loudly “cut the rope” for his cue, which would result in a much faster process.

The following morning Fermi gathered the group together and informed them of the following emergency shutdown plan:

Safety was high on Enrico Fermi's (top) agenda when he oversaw the Stagg Field experiment in 1942. The late Wallace Koehler (right) was stationed above the pile with a bucket of liquid cadmium.
“Insert the control rod into the pile, unlatch the winch lock and reinsert the vertical cadmium back into the pile as quickly as possible. If I yell the word ‘scram,’ the axeman will swiftly cut the rope and let the heavy weight rapidly pull the poison cadmium rod into the pile by gravity until it falls into position. If all this fails to shut down the reaction, I will make the dump-the-bucket-signal and all three buckets will be poured into the pile and we will all exit the facility. Are there any questions before we proceed?”

One hand at the back slowly raised from the assembled group. Enrico Fermi was asked by the burly logger, “Sir, just what does ‘scram’ mean?”

Fermi’s reply to the assembled group was, “ Safety Cut Rope Axe Man.”