On July 11, 1946, the Argonne laboratory officially became the Argonne National Laboratory, with Zinn as its first director.
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."

One of the first problems confronting Zinn was that of accommodation.
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.
Zinn rejected alternate sites outside the Chicago area, and the Army found a new site for the laboratory's permanent home about 5 miles (8.0 km) away in DuPage County, Illinois, which became known as Site D.
Under Zinn, the Argonne National Laboratory adopted slightly more progressive hiring practices than other contemporary institutions.
Three African American women and seven men, six of whom had worked on the Manhattan Project, were employed in research at Argonne at a time when the Los Alamos National Laboratory had no African American scientists.
Argonne also appointed women to positions of authority, with Maria Goeppert-Mayer as a section leader in the theoretical physics division, and Hoylande Young as director of the technical information division.

The  Atomic Energy Commission (AEC) replaced the Manhattan Project on January 1, 1947, and on January 1, 1948 it announced that the Argonne National Laboratory would be "focused chiefly on problems of reactor development."
Zinn did not seek the additional responsibility, which he realised would divert the Laboratory away from research, and divert him from other responsibilities, such as designing a fast breeder reactor.
He even obtained a written assurance from Carroll L. Wilson, the AEC's general manager, that it would not.
He was therefore willing to collaborate with Alvin Weinberg to allow the Oak Ridge National Laboratory to remain involved in reactor design.
Nonetheless, reactor research accounted for almost half the laboratory's budget in 1949, and 84 percent of its research was classified.

Zinn did not get along well with Captain Hyman G. Rickover, the US Navy's Director of Naval Reactors, but nonetheless Argonne assisted in the development of nuclear marine propulsion, eventually producing two reactors, a land-based prototype Mark I and a propulsion reactor, the Mark II.
The STR (Submarine Thermal Reactor) pressurized water reactor designed at Argonne powered the first nuclear-powered submarine, USS Nautilus, and  became the basis of nearly all the reactors installed in warships.

The other branch of reactor development at the Argonne National Laboratory, and the one closer to Zinn's heart, was the fast breeder reactor.
At the time it was believed that uranium was a scarce resource, so it would be wise to make the best use of it.
The breeders were designed to create more fissile material than they consumed.
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.
The Experimental Breeder Reactor I (EBR-I, but known at Argonne as "ZIP" — Zinn's Infernal Pile) was the first reactor to be cooled by liquid metal, and the first to produce electricity.
It proved the breeder concept.
AEC Chairman Gordon Dean described it as a major milestone in nuclear history.

The BORAX Experiments were a series of destructive tests of boiling water reactors conducted by Argonne National Laboratory in Idaho.
The BORAX-1 test was conducted under Zinn's supervision in 1954.
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 h
On July 11, 1946, the Argonne laboratory officially became the Argonne National Laboratory, with Zinn as its first director. 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."
One of the first problems confronting Zinn was that of accommodation. 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. Zinn rejected alternate sites outside the Chicago area, and the Army found a new site for the laboratory's permanent home about 5 miles (8.0 km) away in DuPage County, Illinois, which became known as Site D.
Under Zinn, the Argonne National Laboratory adopted slightly more progressive hiring practices than other contemporary institutions. Three African American women and seven men, six of whom had worked on the Manhattan Project, were employed in research at Argonne at a time when the Los Alamos National Laboratory had no African American scientists. Argonne also appointed women to positions of authority, with Maria Goeppert-Mayer as a section leader in the theoretical physics division, and Hoylande Young as director of the technical information division.
The  Atomic Energy Commission (AEC) replaced the Manhattan Project on January 1, 1947, and on January 1, 1948 it announced that the Argonne National Laboratory would be "focused chiefly on problems of reactor development." Zinn did not seek the additional responsibility, which he realised would divert the Laboratory away from research, and divert him from other responsibilities, such as designing a fast breeder reactor. He even obtained a written assurance from Carroll L. Wilson, the AEC's general manager, that it would not. He was therefore willing to collaborate with Alvin Weinberg to allow the Oak Ridge National Laboratory to remain involved in reactor design. Nonetheless, reactor research accounted for almost half the laboratory's budget in 1949, and 84 percent of its research was classified.
Zinn did not get along well with Captain Hyman G. Rickover, the US Navy's Director of Naval Reactors, but nonetheless Argonne assisted in the development of nuclear marine propulsion, eventually producing two reactors, a land-based prototype Mark I and a propulsion reactor, the Mark II. The STR (Submarine Thermal Reactor) pressurized water reactor designed at Argonne powered the first nuclear-powered submarine, USS Nautilus, and  became the basis of nearly all the reactors installed in warships.
The other branch of reactor development at the Argonne National Laboratory, and the one closer to Zinn's heart, was the fast breeder reactor. At the time it was believed that uranium was a scarce resource, so it would be wise to make the best use of it. The breeders were designed to create more fissile material than they consumed. 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. The Experimental Breeder Reactor I (EBR-I, but known at Argonne as "ZIP" — Zinn's Infernal Pile) was the first reactor to be cooled by liquid metal, and the first to produce electricity. It proved the breeder concept. AEC Chairman Gordon Dean described it as a major milestone in nuclear history.
The BORAX Experiments were a series of destructive tests of boiling water reactors conducted by Argonne National Laboratory in Idaho. The BORAX-1 test was conducted under Zinn's supervision in 1954. 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 h
Walter Zinn