User:Maury Markowitz/Sandbox

The X-10 Graphite Reactor at Oak Ridge National Laboratory in Oak Ridge, Tennessee, formerly known as the Clinton Pile and X-10 Pile, was the world's second artificial nuclear reactor (after Enrico Fermi's Chicago Pile-1) and was the first reactor designed and built for continuous operation. The pile was a prototype breeder reactor designed to produce plutonium for use in atomic bombs as part of the World War II Manhattan Project. The name refers to both the construction of the reactor, and its location on a plot of land code-named X-10. Operated by the University of Chicago, the site later became known as the Clinton Laboratory.

While Chicago Pile-1 (CP-1) demonstrated the feasibility of nuclear reactors in general, the Manhattan Project's goal of producing enough plutonium for weapons required reactors a thousand times as powerful, along with facilities to chemically separate the plutonium from leftover uranium and other fission products. The original plans called for the reactor to be the first example of a production design using helium gas as a coolant in a reactor using nuclear graphite as a neutron moderator, and pure natural uranium in metal form for fuel. Data from CP-1 in late 1942 and early 1943 demonstrated new ways to build a production reactor, and the helium cooled design was abandoned in favor of a water cooled one.

Although it would no longer be prototype of the production plants, the experimental plant would still serve the useful purpose of testing the extraction line and being an intermediate step between CP-1 and the much larger design being considered. To further simplify it, the design was modified from helium to air cooling. After some debate on the location of the test site, DuPont began construction of the chemical semiworks at Oak Ridge in February 1943, and the reactor itself that April. The reactor went critical on November 4, 1943, and the first plutonium was extracted in early 1944. It supplied the Los Alamos Laboratory with its first significant amounts of plutonium, and its first reactor-bred product. Studies of these samples heavily influenced bomb design.

X-10 ended its plutonium production role and turned to research in January 1945, after the water-cooled B Reactor began operation at the Hanford Site in Washington state. X-10 reactor continued breeding radioactive isotopes for scientific, medical, industrial and agricultural uses. It was shut down in 1963, and was designated a National Historic Landmark in 1966.

Origins
The discovery of nuclear fission by German chemists Otto Hahn and Fritz Strassmann in 1938, followed by its theoretical explanation (and naming) by Lise Meitner and Otto Frisch, opened up the possibility of a controlled nuclear chain reaction with uranium. At Columbia University, Enrico Fermi and Leo Szilard began exploring how this might be done. Szilard drafted a confidential letter to the President of the United States, Franklin D. Roosevelt, explaining the possibility of atomic bombs, and warning of the danger of a German nuclear weapon project. He convinced his old friend and collaborator Albert Einstein to co-sign it, lending his fame to the proposal. This resulted in support by the U.S. government for research into nuclear fission, which became the Manhattan Project.

In April 1941, the National Defense Research Committee (NDRC) asked Arthur Compton, a Nobel-Prize-winning physics professor at the University of Chicago, to report on the uranium program. His report, submitted in May 1941, foresaw the prospects of developing radiological weapons, nuclear propulsion for ships, and nuclear weapons using uranium-235 or the recently discovered plutonium. In October he wrote another report on the practicality of an atomic bomb. Niels Bohr and John Wheeler had theorized that heavy isotopes with odd atomic numbers were fissile. If so, then plutonium-239 was likely to be.

Emilio Segrè and Glenn Seaborg at the University of California produced 28 μg of plutonium in the 60-inch cyclotron there in May 1941, and found that it had 1.7 times the thermal neutron capture cross section of uranium-235. At the time only such minute quantities of plutonium-239 had been produced, in cyclotrons, and it was not possible to produce a sufficiently large quantity that way. Compton discussed with Eugene Wigner from Princeton University how plutonium might be produced in a nuclear reactor, and with Robert Serber how the plutonium produced in a reactor might be separated from uranium.

The final draft of Compton's November 1941 report made no mention of using plutonium, but after discussing the latest research with Ernest Lawrence, Compton became convinced that a plutonium bomb was also feasible. In December, Compton was placed in charge of the plutonium project, which was codenamed X-10. Its objectives were to produce reactors to convert uranium to plutonium, to find ways to chemically separate the plutonium from the uranium, and to design and build an atomic bomb. It fell to Compton to decide which of the different types of reactor designs the scientists should pursue, even though a successful reactor had not yet been built. He felt that having teams at Columbia, Princeton, the University of Chicago and the University of California was creating too much duplication and not enough collaboration, and he concentrated the work at the Metallurgical Laboratory (Met Lab) at the University of Chicago.

Reactor design
By 1942 the theory behind the physics of the chain reaction was understood in broad terms. Generally put, the neutrons released by fission of U235 had high energy, and could cause other U235 atoms to undergo fission as well. However, in natural uranium ore, U235 atoms make up only 0.711% of the mass, meaning that the chance of those neutrons encountering another U235 atom and keeping the reaction going is very low. Calculations demonstrated that a mass of uranium ore could not sustain criticality. This could be addressed by concentrating the U235. Another solution relied on the fact that those same atoms are more sensitive to the reaction when the neutrons are moving slower, so-called thermal neutrons. This can be accomplished with the use of a neutron moderator to slow the fast neutrons from the reactions down to thermal energy.

In a moderated reactor using thermal neutrons, the numbers of neutrons available would be more than enough to keep the reaction going. This meant that there were leftover neutrons in the system, and it was these neutrons that the plutonium breeding concept relied on. Natural uranium consists mostly of the non-fissile U238, but when struck by a fast neutron, before moderation, it can undergo a reaction that leads to it being converted to Pu239, after a short delay. The problem was ensuring that there were enough neutrons to keep the chain reaction going while still having enough left over that the losses to U238 would provide a reasonable rate of production of Pu239. This introduced a geometric limitation on the design; the fuel elements have to be large enough that the fast neutrons inside them have a reasonable chance of undergoing the breeding reaction, while being small enough that enough reach the surrounding moderator to keep the reaction going. Only a certain combination of moderator, fuel element and support structures would have all the required features.

Of the known moderator materials, two stood out. From a technical perspective, heavy water is the best solution as it is both a superb moderator as well as a practical cooling fluid. Unfortunately it was available in only minuscule quantities, and when Fermi and Leo Szilard were discussing the issue, Szilard suggested it would not be possible to get enough of the material to build a reactor in a reasonable time. Instead, Szilard suggested using carbon, specifically graphite, as the moderator. When the two visited the National Carbon Company, Szilard discovered that commercial graphite contains small quantities of boron, a neutron absorber, and arranged for the company to provide them with a boron-free version.

With the basic concept set, Fermi's team left their labs at Columbia University to join Compton at Chicago's Met Lab. Compton agreed with the design concept outlined by Fermi and Szilard, and selected a construction site in the Argonne Forest Preserve, about 25 miles southwest of Chicago. A series of problems delayed this work, and while they waited, Szilard discovered that it was possible to operate a reactor that would create a chain reaction while operating slightly below criticality, about 3%. Doing so greatly reduced the rate that power changed within the reactor. This meant there would be a delay of as much as several minutes between a neutron spike and the resulting increase in reactivity, compared to microseconds for one operating at criticality. Based on this discovery, Fermi convinced Compton that the reactor design was safe enough to build at the university. Construction of the Chicago Pile-1 (CP-1) was allowed to go ahead in their improvised laboratories in a squash court under Stagg Field.

While work on CP-1 began, a group led by Compton's chief engineer, Thomas V. Moore, began exploring the design of a production reactor with the specific goal of producing plutonium on an industrial scale. To produce the material at reasonable rates, the neutron flux would have to be very high, which implied both a very large fuel mass as well as considerable waste heat. Since the fuel capsules were to be clad in aluminum, the operating temperature of the reactor could not exceed about 200 C. CP-1 generated only a few hundred watts and was cooled by air convection on the outside of the reactor, but such a solution would not offer nearly enough cooling for a production design. The obvious solution would be to run water through the design, but as water is a neutron absorber, this would make it difficult to maintain a chain reaction. Heavy water solves this problem, but supplies remained limited, and any design using it would ideally dispense with the graphite entirely. Moore's team finally concluded that the best solution would be to use highly pressurized gas as the coolant, with hydrogen and helium both being essentially inert at the atomic level, and helium being inert chemically and thus not a fire risk.

On 25 September, Moore reported to Compton on the resulting design. It consisted of a block of graphite with vertical holes drilled through it that the fuel was dropped into. Helium would be blown through and around the tubes to cool the reactor. Their design was not the only one presented; Eugene Wigner and Gale Young proposed a design using normal water for cooling, while Szilard suggested using liquid bismuth metal for the coolant, which would be so much more dense than helium or water that the resulting reactor design would be much smaller. Fermi added to the confusion by suggesting they they build an intermediate design, essentially a larger CP-1 with external cooling, before making the jump to a production design.

All of the designs had powerful backers and good technical arguments to support them, and the decision on which one to build dragged on. Concerned about the delay, Szilard complained to Leslie Groves, who arrived in Chicago on 5 October 1942 and ordered Compton to make a decision within one week. A compromise position was developed; Fermi would build his intermediate design and operate it until 1943, when it would be disassembled and the small amount of plutonium generated would be extracted. Simultaneously, Moore's helium cooled design would be built as the prototype for a series of a full-scale reactors, and along with it, a complete production-quality chemical plutonium separation line would be built.

Chemical separation, DuPont enters
While the Met Lab was working on the reactor design, Glenn T. Seaborg was leading efforts to design a chemical extraction system. Using lanthanum fluoride and natural uranium ore, Seaborg isolated a weighable sample of plutonium in August 1942. Others explored alternative solutions; Isadore Perlman and William J. Knox worked with peroxide, John E. Willard explored concepts using various metals that precipitated plutonium out on their surface, Theodore T. Magel and Daniel K. Koshland, Jr., worked on various solvent-extraction processes; and Harrison S. Brown and Orville F. Hill explored volatility reactions.

Even before Seaborg's successful extraction, Groves had been talking to DuPont about designing and building an industrial scale system. Stung by allegations of profiteering during World War I, the company agreed to work on the project for the fixed price of $1 over cost, and stated their intent to turn over all intellectual property to the government and exit the business at the end of the war. When Groves agreed to these terms, the company officially joined the project on 3 October 1942, given not only the task of designing the chemical separation plant, but also handed the role of overall management of the entire plutonium extraction effort.

By late 1942, both DuPont and Seaborg were relatively certain that either the original lanthanum fluoride process or a new one based on bismuth phosphate would work. In May 1943, with the site of the plant already being cleared, DuPont's Greenewalt pressed Seaborg to make a decision. He couldn't; both processes had basically the same chance of being successful in the end. Greenewalt eventually chose the bismuth phosphate path because Seaborg was confident that 50% of the plutonium in the fuel cans could be extracted with this method, enough for the process to be practical. Seaborg began to focus on refining the process, while Greenewalt began design of a plant based on it.

Site selection
Plutonium was not the only route to a nuclear weapon; highly enriched U235 was a usable material as well and had been the focus of much early consideration by both the US and UK. U235 had the advantage of being available in natural uranium ore, and did not require nuclear reactors to be built to supply it. The problem is that the U235 is mixed with a much larger amount of U238, and would have to be extracted and purified. This task fell to a team led by Ernest O. Lawrence of the University of California at Berkeley, who proposed using machinery adapted from his cyclotron to act as mass spectrometers and separate the U235 atom-by-atom. To do so at a reasonable rate would require enormous numbers of these calutrons, and a large power plant to run them. Looking for locations, on June, 25 1942 the Office of Scientific Research and Development (OSRD) S-1 Executive Committee decided to build the plant at a remote location at the newly created Oak Ridge, Tennessee. It was initially decided that site would also host the production plutonium plants, setting aside a 83000 acre site for this purpose.

Another meeting of the S-1 committee on September 13 and 14 was held to consider the site of the pilot plutonium plant. Compton pressed for it to be built at his previously selected location in Argonne, which would keep it close to the Met Lab offices. However, by this time it was becoming obvious that the plutonium extraction equipment would be too large to be easily built at the 1000 acres site that had been rented from Cook County. The decision was made to build Fermi's testbed reactor at Argonne, and Moore's production prototype at Oak Ridge. DuPont, however, expressed concerns with the location at Oak Ridge. They felt the area set aside was not large enough, and that expanding it would encroach on the other sites at Oak Ridge. In contrast to the uranium extraction processes, plutonium required complete nuclear reactors, and they were also concerned that Knoxville was uncomfortably close in case of an accident.

On 16 December, a team including Grove's staff and three engineers from DuPont scouted six locations in the Pacific northwest before selecting a location near Hanford in Washington state as the location of the production facilities, which became known as Site W. Compton took the opportunity to reopen the issue of where to build the prototype production plant, once again pressing for it to be built at Argonne. By this time, DuPont had taken over management of the construction effort, and been selected to build the chemical extraction side of the plant. Their extremely capable manager, Roger Williams, argued that there was simply not enough room at Argonne, and with room already set aside at Oak Ridge, there was no reason to reconsider the location. They also expressed concern that the close location to the Met Lab would lead to the scientists interfering with the work. The decision was made final on 12 January 1943.

With that decision made, Compton and Groves proposed that DuPont operate the semiworks. Williams counter-proposed that the semiworks be operated by the Metallurgical Laboratory. He reasoned that it would primarily be a research and educational facility, and that expertise was at the Metallurgical Laboratory. Compton was shocked. The Metallurgical Laboratory was part of the University of Chicago, so the university would be operating an industrial facility 500 mi from its main campus. James B. Conant told him that Harvard University "wouldn't touch it with a ten-foot pole", but the University of Chicago's Vice President, E. T. Filbey took a different view, and told Compton to accept. When University President Robert Hutchins returned, he greeted Compton with "I see Arthur, that while I was gone you doubled the size of my university".

Changing design
Fermi's CP-1 achieved critically for the first time on 2 December 1942. Over the next months a wealth of information started flowing from its test runs. Among these was the clear indication that the calculated value for k was considerably lower than what was being achieved in the reactor. This suggested that there would be more than enough left-over neutrons even after breeding and other losses to keep the reactor critical, enough that some additional losses could be accepted. This led to the possibility of using water cooling, as the losses to the water would not be great enough to stop the reaction. The cost advantages and mechanical simplicity of such a solution was overwhelming compared to the gas cooled solution.

Crawford Greenewalt, head of DuPont's reactor review team, continued to support the original helium design for production models. Fermi's new value of k not only allowed for a helium cooled design, but an air cooled one as well. Greenewalt pointed out that the same improved values of k that allowed for a water cooled design also implied the helium could be replaced by air. Such a reactor would not have the cooling performance of the pressurized helium design, and thus would not be suitable for large scale production, but it would still produce enough plutonium to test the separation process. Most importantly, it could be easily adapted from the helium design, and be built before the water cooled designs were complete. He argued that building a prototype extraction plant centered around such a reactor would still be worthwhile even if that design was not used in production. His arguments carried the day, and in early 1943 the company outlined a complete experimental site based on an air cooled design.

The final reactor design was essentially a scaled up version of CP-1. It consisted of a large graphite block surrounded by several feet of concrete. Hundreds of horizontal holes would be cut through the entire reactor from front to back to hold the fuel. The fuel, in individual metal cans, would be pushed into the holes, or fuel channels, from the front of the reactor. They would eventually be pushed all the way through the channels by new fuel can being inserted behind them. When a can reached the back of the reactor it fell out into a pool of water, where it sat and cooled off for several weeks while some of the shorter-lived nuclear products burned off in spontaneous radioactivity. The cans would then fished out of the pool to be processed.

Construction
Although the design of the reactor was not yet complete, DuPont began construction of the plutonium semiworks on February 2, 1943, on an isolated 112 acre site in the Bethel Valley about 10 mi southwest of Oak Ridge officially known as the X-10 area. There was a chemical separation plant, research laboratories, waste storage area, training facility for Hanford staff, and administrative and support facilities that included a laundry, cafeteria, first aid center and fire station. Because of the subsequent decision to construct water-cooled reactors at Hanford, only the chemical separation plant operated as a true pilot. The semiworks eventually became known as the Clinton Laboratories, and was operated by the University of Chicago as part of the Metallurgical Project.

Construction work on the reactor had to wait until DuPont had completed the design. Excavation commenced on April 27, 1943. A large pocket of soft clay was soon discovered, necessitating additional foundations. Further delays occurred due to wartime difficulties in procuring building materials. There was an acute shortage of both common and skilled labor; the contractor had only three-quarters of the required workforce, and there was high turnover and absenteeism, mainly the result of poor accommodations and difficulties in commuting. The township of Oak Ridge was still under construction, and barracks were built to house workers. Special arrangements with individual workers increased their morale and reduced turnover. Finally, there was unusually heavy rainfall, with 9.3 in falling in July 1943, more than twice the average of 4.3 in.

Some 700 ST of graphite blocks were purchased from National Carbon. The construction crews began stacking it in September 1943. Cast uranium billets came from Metal Hydrides, Mallinckrodt and other suppliers. These were extruded into cylindrical slugs, and canned by Alcoa, which started production on June 14, 1943. The fuel slugs were canned primarily to protect the uranium metal from corrosion that would occur if it came into contact with water, but also to prevent the venting of gaseous radioactive fission products that might be formed when they were irradiated. The cladding had to transmit heat but not absorb too many neutrons. Aluminum was chosen. General Electric and the Metallurgical Laboratory developed a new welding technique to seal the cans airtight. The new equipment was installed in the production line at Alcoa in October 1943.

Construction commenced on the pilot separation plant before a chemical process for separating plutonium from uranium had been selected. Not until May 1943 would DuPont managers decide to use the Bismuth-phosphate process. The plant consisted of six cells, separated from each other and the control room by thick concrete walls. The equipment was operated from the control room by remote control. Work was completed on 26 November 1943, but the plant could not operate until the reactor started producing irradiated uranium slugs.

Operation
The X-10 Graphite Reactor was the world's second artificial nuclear reactor after Chicago Pile-1, and was the first reactor designed and built for continuous operation. It consisted of a huge block, 24 ft long on each side, of nuclear graphite cubes, weighing around 1500 ST, that acted as a moderator. They were surrounded by 7 ft of high-density concrete as a radiation shield. In all, the reactor was 38 ft wide, 47 ft deep and 32 ft high. There were 36 horizontal rows of 35 holes. Behind each was a metal channel into which uranium fuel slugs could be inserted. An elevator provided access to those higher up. Only 800 (~64%) of the channels were ever used.

The reactor used cadmium-clad steel control rods. Three 8 ft rods penetrated the reactor vertically, held in place by a clutch to form the scram system. They were suspended from steel cables that were wound around a drum, and held in place by an electromagnetic clutch. If power was lost, these rods would drop into the reactor, halting it. The other four rods, were made of boron steel and horizontally penetrated the reactor from the north side. Two of them, known as "shim" rods, were hydraulically controlled. Sand-filled hydraulic accumulators could be used in the event of a power failure. The other two rods were driven by electric motors.

The cooling system consisted of three 55000 cuft/min electric fans. Because they used outside air, the reactor could be run at a higher power level on cold days. After going through the reactor, the air was filtered to remove radioactive particles larger than 0.00004 in in diameter. This took care of over 99 percent of the radioactive particles. It was then expelled back into the air through a 200 ft chimney. The reactor was operated from a control room in the southeast corner on the second floor.

In September 1942, Compton asked a physicist, Martin D. Whitaker, to form a skeleton operating staff for X-10. Whitaker became the inaugural director of the Clinton Laboratories, as the semiworks became officially known in April 1943. The first permanent operating staff arrived from the Metallurgical Laboratory in Chicago in April 1943, by which time DuPont began transferring its technicians to the site. They were augmented by one hundred technicians in uniform from the Army's Special Engineer Detachment. By March 1944, there were some 1,500 people working at X-10.

Supervised by Compton, Whitaker and Fermi, the reactor went critical on 4 November 1943 with about 30 ST of uranium. A week later the load was increased to 36 ST, raising its power generation to 500 kW, and by the end of the month the first 500 mg of plutonium was created. The reactor normally operated around the clock, with 10-hour weekly shutdowns for refueling. During startup, the safety rods and one shim rod were completely removed. The other shim rod was inserted at a predetermined position. When the desired power level was reached, the reactor was controlled by adjusting the partly inserted shim rod.

The first batch of canned slugs to be irradiated was received on December 20, 1943, allowing the first plutonium to be produced in early 1944. The slugs used pure metallic natural uranium, in air-tight aluminum cans 4.1 in long and 1 in in diameter. Each channel was loaded with between 24 and 54 fuel slugs. The reactor went critical with 30 ST of slugs, but in its later life was operated with as much as 54 ST. To load a channel, the radiation-absorbing shield plug was removed, and the slugs inserted manually in the front (east) end with long rods. To unload them, they were pushed all the way through to the far (west) end, where they fell onto a neoprene slab and fell down a chute into a 20 ft deep pool of water that acted as a radiation shield. Following weeks of underwater storage to allow for decay in radioactivity, the slugs were delivered to the chemical separation building.

By February 1944, the reactor was irradiating a ton of uranium every three days. Over the next five months, the efficiency of the separation process was improved, with the percentage of plutonium recovered increasing from 40 to 90 percent. Modifications over time raised the reactor's power to 4,000 kW in July 1944. The X-10 semiworkls operated as a plutonium production plant until January 1945, when it was turned over to research activities. By this time, 299 batches of irradiated slugs had been processed. A radioisotope building, a steam plant, and other structures were added in April 1946 to support the laboratory's peacetime educational and research missions. All work was completed by December 1946, adding another $1,009,000 to the cost of construction at X-10, and bringing the total cost to $13,041,000. Operational costs added another $22,250,000.

X-10 supplied the Los Alamos Laboratory with the first significant samples of plutonium. Studies of these by Emilio G. Segrè and his P-5 Group at Los Alamos revealed that it contained impurities in the form of the isotope plutonium-240, which has a far higher spontaneous fission rate than plutonium-239. This meant that it would be highly likely that a plutonium gun-type nuclear weapon would predetonate and blow itself apart during the initial formation of a critical mass. The Los Alamos Laboratory was thus forced to turn its development efforts to creating an implosion-type nuclear weapon—a far more difficult feat.

The X-10 chemical separation plant also proved the bismuth phosphate process that was used in the full-scale separation facilities at Hanford. Finally, the reactor and chemical separation plant provided invaluable experience for engineers, technicians, reactor operators, and safety officials who then moved on to the Hanford site.

Peacetime use
After the war ended, the graphite reactor became the first facility in the world to produce radioactive isotopes for peacetime use. On August 2, 1946, Oak Ridge National Laboratory director Eugene Wigner presented a small container of carbon-14 to the director of the Barnard Free Skin and Cancer Hospital, for medical use at the hospital in St. Louis, Missouri. Subsequent shipments of radioisotopes, primarily iodine-131, phosphorus-32, molybdenum-99/technetium-99m and carbon-14, were for scientific, medical, industrial and agricultural uses.

The X-10 Graphite Reactor was shut down on November 4, 1963, after twenty years of use. It was added to the National Register of Historic Places on December 21, 1965, and was designated a National Historic Landmark on October 15, 1966. In 1969 the American Society for Metals listed it as a landmark for its contributions to the advancement of materials science and technology, and in 2008 it was designated as a National Historic Chemical Landmark by the American Chemical Society. The control room and reactor face are accessible to the public during scheduled tours offered through the American Museum of Science and Energy. During 2015 tours were part of a general three-hour tour of the Clinton Engineer Works facilities, and were conducted on Mondays through Fridays at noon, from June 4 to September 30, except on July 4 and 5.

Similar reactors
With the construction of the water cooled B Reactor at Hanford, the air cooled design became relatively rare. The inherent risk of fire in the graphite with forced air ventilation was considered a major disadvantage of the design, and later graphite moderate reactor designs generally used either water as the coolant, or used a more inert gas. Gas cooled examples have been built using nitrogen, helium and carbon dioxide. Most other plutonium breeding processes have used entirely different designs, with heavy water moderators being common.

When the United Kingdom made the decision to begin its own plutonium extraction process and placed a strict timeline on the efforts, they did not have the time to use these alternate designs and had to choose between the gas and water cooled designs they were familiar with from the Manhattan Project. John Cockcroft considered the issues in depth, and considered that a loss of coolant event on the liquid cooled B Reactor would be much more dangerous than a loss of forced air in the air cooled design, which could be passively cooled using the chimney effect. Accordingly, the design of the Windscale reactor featured a very large chimney and large open channels that maximized natural convection. After construction started he demanded the addition of complex filters on the chimney, nicknamed by the operators "Cockcroft's follies", which proved to be anything but folly when the reactor caught on fire and the filters greatly reduced the ultimate nuclear release. Windscale would be last major air cooled plutonium producing reactor, the UK's follow-on Magnox designs used carbon dioxide.

The only other large air cooled reactor of the X-10 type was the Brookhaven Graphite Research Reactor (BGRR). This was the first nuclear reactor to be constructed in the United States following World War II. Led by Lyle Benjamin Borst, the reactor construction began in 1947 and reached criticality for the first time on August 22, 1950. The reactor consisted of a 700 ton, 25 ft cube of graphite fueled by natural uranium. Its primary mission was applied nuclear research in medicine, biology, chemistry, physics and nuclear engineering. One of the most significant discoveries at this facility was the development of production of moybdenum-99/technetium-99m, used today in tens of millions of medical diagnostic procedures annually, making it the most commonly used medical radioisotope. The graphite reactor was shut down in 1969 and fully decommissioned in 2012.

, another reactor of similar design to the X-10 Graphite Reactor is still in operation, the Belgian BR-1 reactor of the SCK•CEN, located in Mol, Belgium. Financed through the Belgian uranium export tax with the help of British experts, the 4 MWth research reactor became critical for the first time on May 11, 1956. It is used for scientific purposes, such as neutron activation analysis, neutron physics experiments, calibration of nuclear measurement devices and the production of neutron transmutation doped silicon.