The technology is complicated, but the concept behind gaseous diffusion is easily understood: separate the two uranium isotopes by weight. All uranium atoms have 92 protons, but U-238 has 146 neutrons, giving it an atomic weight of 238. U-235 has three fewer neutrons than U-238, which accounts for its lower atomic weight (235).

To take advantage of this difference, the engineers and scientists developed the gaseous diffusion process. They started by turning uranium into a gas by reacting it with fluorine. This formed uranium hexafluoride, the only uranium molecule volatile enough to behave as a gas at room temperatures.

Then they pumped the gas through a diffusion barrier, a thin, tubular metal membrane with millions of nanoscale holes (diameters equal to 1/5000 thickness of sheet of paper) perforating its surface. Because the U-235 gas molecules were 1.3 percent lighter than those containing U-238, they moved slightly faster and were more likely to contact and diffuse through the barrier’s pores and into a chamber on the other side. This created a second stream of uranium hexafluoride that was very slightly richer in U-235 than the stream that went straight through the pipe without diffusing.

To create a stream of gas rich enough in U-235 to use in fission, this process was repeated thousands of times. The equipment to do that was housed in the K-25 Plant at Oak Ridge, Tenn., the first U.S. gaseous diffusion facility. When it opened in 1945, the mile-long, U-shaped, four-story-high plant was the largest roofed building in the world. It was followed by the Paducah, Ky., facility in 1952 and the Portsmouth, Ohio, unit in 1955. These three gaseous diffusion plants formed what is known as the U.S. gaseous diffusion complex.

Gaseous Diffusion Was Built Using Incredible Technologies in the 1940’s

Nuclear fission was only one year old when the U.S. government began formulating plans to develop an atomic bomb. In 1938, two German chemists, Otto Hahn and Fritz Strassman, and a physicist, Lise Meitner, of the Kaiser Wilhelm Institute in Berlin discovered that bombarding U-235 atoms with neutrons would split the atoms and release massive amounts of energy. It was clear to many physicists that this reaction could produce a weapon of incredible power, and that Nazi Germany had the lead in developing it. In 1939, several leading U.S. physicists, including Albert Einstein, wrote a letter warning President Franklin Roosevelt of the danger. Their intervention led the President to launch to Manhattan Project to develop an atomic bomb.

The development of the mass spectrometer, which measured the weight of individual atoms, helped prove the existence of isotopes, atoms with the same number of protons but different numbers of neutrons. By 1938, scientists knew that some uranium isotopes underwent fission, but they did not know which one. That year, American physicist Alfred Nier at the University of Minnesota used the technique to identify U-235 and show that there is only one U-235 atom for every 139 atoms of U-238. This 139:1 ratio occurs no matter where in the world uranium is mined, and is the reason uranium must be enriched.

Uranium hexafluoride is one of the most reactive and highly corrosive gases known. Unchecked, it would have eaten away the thousands of miles of steel piping in the Oak Ridge K-25 gaseous diffusion plant. To prevent this, researchers developed a nickel plating process that protected the pipes. Similar coatings were later used to protect equipment in chemical and food processing plants as well as jet engines.

The first use of Teflon was to prevent uranium hexafluoride from leaking in gaseous diffusion plants. Teflon was discovered in 1938 by a DuPont scientist looking for a new fluorine-based refrigerant. This fluorine polymer so highly inert, it appeared slippery. It was used to seal thousands of diffusion pipes and valves, preventing dangerous uranium gas from escaping into the environment. To produce Teflon, engineers first had to develop a process to mass-produce fluorine, which is used today in a wide range of Teflon and other industrial products.

The diffusion barrier was the Manhattan Project’s single greatest technical challenge. To make gaseous diffusion work, the diffusion barrier had to allow only some uranium fluoride molecules to pass while blocking most of the others. This meant finding a way to produce millions of 10-25-nanometer holes only slightly larger than the uranium hexafluoride molecules themselves. The pores ran through an ultrathin metal tube that still needed high mechanical strength. The holes also had to resist corrosion and plugging by any contaminants in the gas stream.

Gaseous Diffusion Helped End World War II

During WW II President Roosevelt assigned the atomic bomb project to the Army Corps of Engineers, which had a huge budget and could conceal the bomb’s development costs and progress from U.S. enemies. It was named the Manhattan project because the Corps of Engineers’ District in charge of the project was located in the Manhattan borough of New York City.

Without the gaseous diffusion process, the atomic bomb in needed quantities could not have been made. The consensus among historians is that America’s use of the atomic bomb shortened the war in the Pacific and potentially saved millions of lives by making a ground invasion of Japan unnecessary.

Why the “Cold War” Required Three U.S. Enriched Uranium Gaseous Diffusion Plants

Although the United States and the Soviet Union were allies during WW II, tensions between the two nations rose over the political future of Europe. Those tensions escalated in August 1949, when the U.S.S.R. exploded its first atomic bomb using a design stolen from the United States. This ushered in the Cold War, an open but restricted rivalry that threatened direct nuclear confrontation between the world’s two superpowers. The nuclear threat continued until the early 1990s, when the U.S.S.R. finally collapsed.

In July 1950, the National Security Council (NSC) directed the Atomic Energy Commission (AEC) to find a second site (beyond the Oak Ridge K-25 Plant) to produce enriched uranium used in nuclear weapons by “no later than October 1st, 1950.” A primary reason for the expansion was the threat of military intervention by China into the Korean Conflict.

The NSC letter included a familiar plant site checklist, such as adequate labor supply, local air transportation, and river access. It also added unique criteria, calling for a site more than 500 miles from the reach of submarine-launched missiles and more than 100 miles from cities with known communist subversive activity.

The AEC was given only three months to find a site for the plant. To meet that deadline, the NSC recognized that it would have built on existing U.S. Government property. Paducah, Ky., was chosen as the site best to meet all these criteria.

As the Cold War intensified, a third diffusion plant was opened in Portsmouth, Ohio, in 1955 to increase the nation’s nuclear materials output. Located at least 150 miles from both Oak Ridge and Paducah, the plant increased the likelihood of diffusion capability surviving if nation was every attacked. Any two of the three existing plants were able to meet total U.S. production needs for the U.S. military.

Gaseous Diffusion Fueled the U.S. Nuclear Navy

Gaseous diffusion provided all the fuel for the nuclear reactors that propelled the U.S. nuclear navy. The fleet began in 1954, when the U.S. Navy launched the world’s first nuclear submarine, the Nautilus. Nuclear reactors went on to power a fleet of submarines, cruisers, and, beginning in 1960 with the Enterprise, aircraft carriers. Today, the U.S. Navy’s nuclear powered fleet includes 72 submarines and 11 aircraft carriers.

The ship-based nuclear reactors can run for 20 to 25 years without refueling. This gives the U.S. nuclear fleet a major tactical advantage, enabling to deter aggression while rapidly projecting military power all around the globe.

Gaseous Diffusion Enabled Peacetime Electric Power

The first nuclear power plant, the Shippingport Atomic Power Station in western Pennsylvania, opened in 1958. Today, nuclear power plants generate 800 billion kilowatt-hours of electrical power, or 20 percent of all electricity produced in the United States.

Gaseous diffusion was an energy-intensive process. Yet for every 1 kilowatt-hours (kWh) of electrical energy used to enrich uranium, the enriched fuel generates approximately 54 KWH of electricity in a nuclear power plant.

A single enriched uranium fuel pellet (0.5 inches diameter, 0.5 inches long) contains the energy equivalent of 2,000 pounds of coal. A typical nuclear power plant uses about 18 million of these small pellets at a time, equivalent in power to 170,000 railcars loaded with coal.

In 1964, the U.S. government ceased production of weapons-grade uranium after deciding that it had enough enriched uranium in inventory and could recycle fissile material found in older nuclear weapons. At that point, all gaseous diffusion production was diverted to the U.S. Navy and peacetime nuclear power plants.

Gaseous Diffusion Became an International Business

In 1969, the United States began a toll enrichment program, using gaseous diffusion plants to enrich nuclear fuel for U.S. and foreign operators of nuclear power reactors. Under the program, customers would provide unenriched uranium hexafluoride containing 0.7115 percent U-235 to the Department of Energy. It then enriched the material in its gaseous diffusion plants to between 3 percent and 5 percent U-235 and returned the enriched fuel to its customers.

Toll enrichment was not a novel concept. It could be traced back to a centuries-old practice of the miller’s toll, where the owner of a mill would grind wheat or corn in exchange for a portion of the ground grain as payment. Similarly, in uranium toll enrichment, the customer pays a fee, or toll, depending on the degree of enrichment.

Within two years of the program’s start, Union Carbide, which ran the DOE’s gaseous diffusion plants, had signed global toll enrichment contracts worth $218 billion in today’s dollars.

Over time, the major nuclear powers and several other countries that operated nuclear power plants built their own enrichment facilities. By the mid-1990’s, the two remaining DOE gaseous diffusion plants could no longer compete with these overseas facilities. In 1998, Congress passed a law to privatize the still-operating Paducah and Portsmouth facilities, believing that a privately held company would prove more competitive with foreign producers. The United States Enrichment Corporation (USEC) was formed to operate the two facilities.

While the transition to private ownership was under way, the newly formed USEC negotiated a deal with the Russian Federation to buy enriched uranium contained in approximately 10,000 Russian nuclear warheads. Weapons grade uranium is typically 90 percent U-235. USEC blended it with depleted (mostly U-238) uranium until it reached the 3 percent to 5 percent level needed for nuclear reactors.

While this was a great step for nuclear disarmament, it further drove down the demand for enriched uranium from the remaining U.S. gaseous diffusion plants. By 2013, all U.S. diffusion-based uranium enrichment production ceased. The United States is currently operating on its reserve enrichment inventory for military needs.

Demand remains high for enriched uranium for nuclear power plants. Today, this need is satisfied by centrifugal enrichment. This process that spins uranium hexafluoride gas in a cylindrical tube at speeds in excess of 1,000 revolutions per second, driving the heavier U-238 gas to the edge of the tube. The lighter U-235 gas remains closer to the center, where it is removed and purified again and again. The URENCO plant, owned by a European consortium and located in Eunice, New Mexico, is the only centrifugal enrichment plant operating in the United States.

Written by Steve Polston, Lockheed Martin Paducah Plant Manager 1990-98; Alan Brown, technical editor.

*Original full-text article online at: https://www.paducahsun.com/news/what-is-gaseous-diffusion/article_69e2005c-0a7d-55c2-baeb-4903f8752453.html