How the X-10 Graphite Reactor Was Controlled and Cooled

A reactor has to be stoppable. At Oak Ridge's X-10 Graphite Reactor, operating from 1943 to 1966, the answer was seven control rods, each eight feet long, that could soak up neutrons and prevent fission — stopping a spontaneous chain reaction or turning the reactor's output down. An exhibit panel at the reactor explains how they worked.

The "Graphite Reactor Design" exhibit panel. Credit: U.S. Department of Energy.

The control rods

To start the reactor, operators pulled the three vertical rods out of the core so fission could begin, then withdrew two of the horizontal rods part or all of the way to set the power.

Keeping it cool

Fission made tremendous heat, which had to be carried off before it melted the reactor's structure. X-10 was cooled by air pushed through the core by huge fans. The air was then filtered — removing 99 percent of its radioactive particles — and sent out through a 200-foot stack. The uranium fuel slugs were kept to a maximum of 536°F, and the graphite moderator to 280°F.

What the panel shows

• A drawing of the reactor, cut away to show the loading face with its channel openings, the tubes leading into the core, and the side holes for experiments and control rods.
• A man at a worktable facing a wall of dials and controls, beside signs reading "Lethal Hazard" and "Danger, X-rays."
• Two men at an experiment port: one, in coveralls, lifts a small cylinder with long-handled tongs; the other holds a device shaped like a handgun with a large cylinder where the barrel would be.
• A small aluminum cylinder beside a plaque: "ORNL Graphite Reactor AEC, 1943 – 1966. Dedicated as a National Historic Landmark, September 13, 1966."

Visiting

The X-10 Graphite Reactor, part of Manhattan Project National Historical Park, stands on the secure grounds of Oak Ridge National Laboratory. In-person visits are allowed only on guided tours.

Sources

Based on "Oak Ridge X-10: Graphite Reactor Design, Panel 2," Manhattan Project National Historical Park, National Park Service; a work of the United States government in the public domain.