Walter Henry Zinn (December 10, 1906 – February 14, 2000) was a Canadian-born American nuclear physicist who was the first director of the Argonne National Laboratory from 1946 to 1956.

[1] In 1939, the Pupin Physics Laboratories at Columbia where Zinn worked were the center of intensive research into the properties of uranium and nuclear fission, which had recently been discovered by Lise Meitner, Otto Hahn and Fritz Strassmann.

The delay between an atom absorbing a neutron and fission occurring would be the key to controlling a chain reaction.

[4] The critical radius of a spherical reactor was calculated to be:[5] In order for a self-sustaining nuclear chain reaction to occur, they needed k > 1.

For a practical reactor configuration, it needed to be at least 3 or 4 percent more;[5] but in August 1941 Zinn's initial experiments indicated a disappointing value of 0.87.

[1] In early 1942, with the United States now embroiled World War II, Arthur Compton concentrated the Manhattan Project's various teams working on plutonium at the Metallurgical Laboratory at the University of Chicago.

[6] Zinn used athletes to build Fermi's increasingly large experimental configurations under the stands of the disused Stagg Field.

This raised hopes that pure uranium would yield a suitable value of k.[7] By December 1942, Zinn and Anderson had the new configuration ready at Stagg Field.

[8] The Army leased a 1,000 acres (400 ha) of the Cook County Forest Preserves known as "Site A" to the Manhattan Project, and "the Country Club" to the hundred or so scientists, guards and others who worked there.

[8] On September 29, 1944, Zinn received an urgent call from Samuel Allison, the director of the Metallurgical Laboratory.

Xenon-135 had a half life close to that, but had not been detected in Argonne or by the X-10 Graphite Reactor in Oak Ridge, Tennessee.

Zinn quickly brought Chicago Pile-3 up to full power, and within twelve hours, had made a series of measurements that confirmed the Hanford results.

[8] Alvin Weinberg characterized Zinn as "a model of what a director of the then-emerging national laboratories should be: sensitive to the aspirations of both contractor and fund provider, but confident enough to prevail when this was necessary.

The Federal government had promised to restore Site A to the Cook County Forest Preserves after the war, and despite intervention from the Secretary of War, Robert P. Patterson, the most the Cook County Forest Preserves Commission would agree to was that the Argonne National Laboratory could continue to occupy a portion of the lease until a new site was found.

He was therefore willing to collaborate with Alvin Weinberg to allow the Oak Ridge National Laboratory to remain involved in reactor design.

[1] By 1948, he had become convinced that it would be unwise to build large experimental reactors near Chicago, and the AEC acquired land around Arco, Idaho, which became an outpost of Argonne.

[24] The BORAX Experiments were a series of destructive tests of boiling water reactors conducted by Argonne National Laboratory in Idaho.

He had the control rods removed to demonstrate that the reactor would shut down without trouble, and it immediately blew up with a loud bang and a tall column of dark smoke, a turn of events that he had not anticipated.

[1] He died in Mease Countryside Hospital in Safety Harbor, Florida, on February 14, 2000, after suffering a stroke.