Oh, My Darling Clementine: A History of the Los Alamos Plutonium Fast Reactor

By Hannah Patenaude | February 1, 2022


In a cavern, in a canyon excavating for a mine dwelt a miner forty-niner and his daughter, Clementine

One ‘miner,’ physicist Philip Morrison, brought to life the world’s first fast nuclear reactor, his daughter Clementine. She was constructed on the side of a Los Alamos canyon in the immediate aftermath of World War II and christened by Morrison after the hit gold rush folk song, “Oh My Darling, Clementine.” It was a particularly fitting theme given its location near the Water Boiler reactor at the Omega Laboratory (TA-2), which required moderate excavation into the canyon wall. Additionally, because the code for plutonium-239 in written correspondence was the combination of its atomic number (94) and mass number (239), ‘49,’ team members on this project were aptly referred to as ‘miner forty-niners.’

Also called the Los Alamos Plutonium Fast Reactor , Clementine “was to be different in principle from all other existing reactors,” and supported the need for high-intensity, high-energy neutrons towards weapons development. The project simultaneously investigated the prospect of using fast neutrons and breeding fissile material for commercial power production. Since then, no reactor has been cooled by mercury, given the development of other heavy metals such as lead and alloys like sodium-potassium. Additionally, metallic fuels have steered away from plutonium-gallium and toward ternary eutectics such as U-Pu-Zr. However, Clementine provided a wealth of fundamental data that paved the way for modern and advanced reactor technology in a variety of areas within the nuclear energy industry. Specifically, Clementine influenced the design of the Omega West reactor, which both physically and technologically replaced the fast reactor. The data also informed work at the Experimental Breeder Reactor I (EBR-I), which carried the torch across the finish line in 1953 to produce fissile material in situ. Finally, the results gave both a foundation for liquid fast reactors cooled by liquid heavy metals and a strong proof-of-concept for fast neutron spectrum reactors.

An early timeline of Clementine

When Enrico Fermi moved to Los Alamos from the Chicago Metallurgical Laboratory—where progress on EBR-I was being quickly made—he carried with him an interest in pursuing a breeder reactor design, ultimately inspiring Morrison’s concept for Clementine (Morrison previously worked for Fermi in Chicago). It was intended to operate at low power (10–25 kW) and serve as an experimental fast neutron facility that could also demonstrate plutonium’s use as a fuel.

Fig 1 Both
Figure 1. Construction of the new Clementine building next to the Omega Laboratory at the TA-2 site, nestled within Los Alamos Canyon.

Records were well kept in the many letters sent between Manhattan Project participants around the country regarding the development of the fast reactor. The first written discussion relevant to Clementine came at the end of 1944, when Bernard Feld wrote to Leo Szilard about the results from an investigation into fast neutron energy distributions. The following year came with more specific correspondence between Percible King and John Manley, outlining ways to plan construction around the existing Water Boiler reactor and any new critical experiments coming to Omega Site in the canyon.

By the fall of 1945, Morrison, George Placzek, and Louis Slotin began considering the possibility of redirecting those critical experiments toward the development of a device capable of generating a fast neutron spectrum. By November 10, a team of experts was assembled for the first of four ‘Power Reactor Notes’ meetings for preliminary discussions and planning for the development of Clementine. Less than two weeks later, on November 23, 1945, Los Alamos Director Norris Bradbury wrote to Lieutenant General Leslie Groves, officially requesting a non-weapons grade quantity of plutonium to construct a fast reactor. Approval came in extraordinary speed from Washington, D.C. on December 17.

Vigorous planning began in early January, 1946, including requests for uranium from Madison Square Area* and thorium from Iowa State College. Communication and collaboration with Wally Zinn’s EBR-I team at Chicago was strengthened, and designs for fabricating the plutonium fuel slugs were in full swing by the end of the month. Mercury circulation pumps were deliberated in March. By the end of May, a letter to Morrison from his team described many technical specifications in great detail, including tamper material, safety and control devices, sample and experimental holes, tamper cooling, shielding, slug information, reactor instrumentation, beta and gamma measurements, mercury vapor detection, nuclear measurements leak detection, and mass estimations.

This pace was maintained over the summer. By the fall of 1946, Clementine was adopted by husband-and-wife team David and Jane Hall when Morrison accepted a position at Cornell University, but close correspondences and visits continued with Morrison. On September 12, Darol Froman sent Lieutenant C. O’Brien a finalized list of individuals who were cleared to receive active material (plutonium) from the security guard, namely: Morrison, the Halls, King, and Jerome Kellogg. The “dream team” had been fully assembled, and next up would be the plutonium. When the team assembled the active material, Clementine was brought to life. She operated only for three days, however, before a tamper malfunction required disassembly that lasted for approximately two months. In their downtime, the team produced a list of planned experiments. Irradiation testing was orchestrated through the Hanford site with the manufactured plutonium slugs because the flux at the Water Boiler next door was not high enough. The critical assembly of Clementine was reconstructed on November 21, and some preliminary results were sent from the Halls to Morrison in December.

An entire reactor had been built and assembled within a single year—clearly, an unimaginable amount of work was accomplished in an incredibly short amount of time. More than 20 years after her disassembly, Norris Bradbury reflected on his tenure as Director of the Laboratory throughout Clementine’s lifetime:

We made the first plutonium reactor, called Clementine, and it was the first reactor that operated upon a fast neutron spectrum. I only pause to note that today it would take you ten billion dollars and fifty thousand volumes of environmental reports and nobody’d let you do it anyway, but we just did it. And it worked beautifully; we finally shut it down when it became clear that some of the components were a little tired.
— N. Bradbury, Reminiscences of Los Alamos 1943–45, 1980.

Initial operation and experimentation

According to a letter from the Halls updating Morrison, critical assembly measurements began November 19, 1946, and the experiments warranted a biblical reference: “We concluded from our estimates of control rod effect which were based on multiplication measurements on the way up to critical, that about 23.6 slugs were critical. (Obviously demanding the sixth verse of the Twenty-third Psalm.).”

Low power measurements were conducted until February, 1947, then the power was increased to 10 kW for twenty-one months while the core was still being referred to as a ‘critical assembly,’ during which they determined factors such as critical mass versus core configuration, effectiveness of reactor control, reactor temperature coefficient, effect of temperature and reactivity changes in reactor operations, and neutron energy distribution in the center of the reactor. Additionally, the team employed the Danger Coefficient Method of introducing various materials into the core to monitor reactivity changes, thus providing novel insight into the neutron cross-sections of over forty isotopes. The term “danger coefficient” was likely a result of the growing concern over criticality accidents, two of which had occurred in recent years in Morrison’s group.†

It was not until January of 1949 that the final assembly was complete, and Clementine came to full power in March. She operated for a year before issues arose with function of the control and safety rods, resulting in the first of two failures that occurred throughout the fast reactor’s lifetime. The reactor was shut down and opened for investigation in May, 1950.

Fig 2
Figure 2. (a) Partially constructed uranium tamper showing the locations for the thorium window (left) and uranium window (right). (b) Thorium window and (c) uranium window within the uranium tamper cube. The face with the uranium window shows a set of experimental holes used to access the active region. (d) Aluminum jacket for tamper water cooling system that removed heat generated by the uranium blocks (right). (e) Lead bricks surrounding the tamper.
Fig 3
Figure 3. Complete structure of the Clementine reactor with stair access to the pile. Inset: Commemorative plaque on the stairs leading to the pile.

Fig 4

 Figure 4. Schematic cross-section of Clementine.

Failure 1: Wedged slugs

After exploring the impacts of temperature and reactivity on the operation of the reactor, it was concluded in 1948 that blistering of the safety block and rods was “not expected to cause trouble during the reactor’s life-time.” Yet, this was, in fact, the first failure experienced by Clementine. After approximately one year of operation, it was estimated that the fuel rods were thermally cycled between 500 and 800 times from 25 to 150°C. For 450 g plutonium slugs, it was estimated that approximately 7.5 × 1019 fissions had occurred in this time, and 1.4 × 1019 for 580 g uranium slugs (4% due to uranium-235).

When the control and safety rods became frozen in their channels in March 1950, the reactor was shut down for examination. It was discovered that two of the uranium slugs were wedged in the cage along with one of the plutonium fuel slugs. Of the two uranium rods, #141 showed a significant crack in the steel jacket along with several surface blisters (Fig. 5a), and #205 had a single blister on the surface of the jacket. They were taken for further inspection and were not returned to the reactor.

The wedged plutonium slug was not identified numerically, other than its location being adjacent to uranium slug #141, but reportedly suffered no visible damage. However, X-ray radiography performed on the plutonium rods while the core was disassembled exposed an interesting phenomenon that warranted further analysis. It appeared as though gaps were present between the uranium wafer and plutonium rod that were not observed in 1947 before irradiation in the pile.‡ The change in total gap spacing around the plutonium slug was recorded based on X-ray imaging and compared to information recorded before reactor operation. The height of the uranium wafer was unchanged, and it was concluded that the original density of plutonium correlated with the observed shrinkage of the active material. This change was thought due to the dimensional destabilization of the δ-phase plutonium, transitioning to a higher-density phase from heat cycling and/or irradiation.

Fig 5

Figure 5. (a) Cracked steel can for uranium rod #141 removed after the first failure. (b) Irradiated plutonium rod #66 laying on a bed of cheese cloth with shriveled spot and crack located under the arrow. (c) Ruptured plutonium fuel rod responsible for Clementine’s second failure.

A seemingly undamaged fuel rod was chosen to examine for any other evidence of evolution in the material. Although the steel jacket was unchanged and not distorted, there were three longitudinal cracks found in the nickel coating of the plutonium-gallium rod. The cladding was removed and uncovered many small cracks in the plutonium, itself. Additionally, there was one large round spot that was described as “shriveled and cracked” (Fig. 5b).

The core was slightly reconfigured in order to resume operation. A new basket with fewer holes (37 instead of 55) was fabricated. In this case, only 32 of the holes would contain plutonium slugs and the five remaining would be filled with steel cans, each with a small hole in the bottom and top so the mercury could flow slowly through minimizing disruption of the coolant flow pattern within the cage. Again, each of the plutonium rods were radiographed, cleaned, and tested for leaks with a mass spectrometer leak detector before reassembly.

This new critical assembly was completed on June 27, 1950, and welded closed two days later. A power level of 25 kW was resumed on July 17, with the temperature of the central plutonium slug measured at 170°C, compared to 155°C from the original loading. However, this was expected considering there are 32 instead of 35 fuel slugs, therefore the heat exposure per slug has increased. Additionally, a slight increase in reactivity was reported, as would have been suggested from previously noted nuclear reactivity changes, but Jane and David Hall believed that it fell well within their control limits and “no hazard appears to exist.”

Failure 2: Alpha-contaminated coolant

Clementine operated for nearly two more years before the second failure occurred. By the second half of 1952, there were two apparent issues that would become problematic in future operation: the safety block was sticking and failing to seat, and the control rods were binding in their operating passageways. They concluded that both problems were a result of fission product build-up and warpage due to abnormal crystalline growth of the uranium blanket, including the safety block. However, this was not the failure that concluded Clementine’s operation.

For the last two years of operation, regular monitoring for alpha contamination was conducted in samples of the mercury coolant in order to detect any ruptures of plutonium fuel rods. Unfortunately, on Christmas Eve of 1952, considerable quantities of alpha activity were found in the mercury and operation of the reactor was immediately stopped for the final time. Disassembly began soon after.


The disassembly of Clementine was no trivial task. The mercury coolant was now heavily contaminated with alpha radiation in the form of plutonium. Additionally, they calculated the gamma radiation from each of the 32 fuel rods would be 10–20 Ci. It was necessary to conduct disassembly procedures under an inert, dry atmosphere in order to prevent contamination of surfaces in the room that housed Clementine. To reduce radiation exposure for the workers, remote-control handling was required for certain operations.

Disassembly was to be completed in a specific sequence of steps: (1) removal of mercury coolant, (2) cutting of mercury coolant pipes, and (3) removal of fuel rods and “top tamper” assembly that filled the pot above the assembly. Step (1) required two separate operations: first, draining of the supply tank, pump, heat exchangers, and plumbing into the mercury cabinet through the petcock which was installed for that purpose. Second, the coolant trapped in the pot and any connected pipes would have to be removed in conjunction with steps (2) and (3). All containers required appropriate shielding for radiation purposes, in addition to proper sealants to prevent the loss of plutonium into the environment.

Fig 6
Figure 6. Removing the remainder of the tamper required the welding of two 4” I-beams to the supporting plate below the tamper unit. The beams were welded onto skids made of larger I-beams so the assembly could slide across the floor and onto the adjacent loading dock, where it was loaded onto a truck for transportation to the South Mesa contaminated area and covered with a wooden shack for protection against the weather.

Two of the fuel rods were especially difficult to remove, not coming loose even with hard, two-handed pulling. It eventually became necessary for the team to saw sections off the top plate of the cage. Although one of the stuck rods showed no visible damage, the other was badly ruptured and was clearly the source of plutonium contamination into the coolant system (Fig. 5c).

Fig 7






Figure 7. Left: Philip Morrison, father of the Clementine design, in 1945. Right: Jane and David Hall, adopted parents of Clementine, the Los Alamos Plutonium Fast Reactor.



Clementine, the Los Alamos Plutonium Fast Reactor, was one of the most radical and rapidly developed reactors ever built. She created the foundation upon which the nuclear energy industry still operates today, and the surplus of data that came from those few short years has proven invaluable. This data included key characteristics of dozens of material nuclear cross-sections, which furthered the understanding of reactor design and performance and pushed innovation in the area of reactor safety and decommissioning. The accomplishments of Clementine span multiple subfields within the nuclear industry, and in particular form a proof-of-concept for fast spectrum neutron reactors, metal-fueled designs, and those with metal-coolant systems. Additionally, Clementine served as Louis Rosen’s first experimental neutron source, 10 years before he established the Los Alamos Meson Physics Facility which developed into today’s Los Alamos Neutron Science Center (LANSCE).  This work supported the advancement of neutron diagnostics to be employed in later bomb development tests.

Now that the industry is well past the days of danger coefficients, many consider Clementine’s operation as merely a small point along the timeline of nuclear technology development. However, pioneers like Philip Morrison, Jane Hall, and David Hall walked so we could sprint toward solutions for global threats such as nuclear security and climate change. Although lost and gone forever, as the song goes, there is no dreadful sorrow for Clementine given all that she provided from the peak of the Atomic Age through today. She lives on through the archives of national laboratories, technical summaries and retrospectives, and the designs of advanced reactors being constructed today.

Further reading:

1. H.K. Patenaude, F.J. Freibert; “Oh, My Darling Clementine: A Detailed History and Data Repository of the Los Alamos Plutonium Fast Reactor” submitted for publication in Nuclear Technology, 2022.

2. J.H. Hall, “Plutonium Fast Reactor at Los Alamos,” LAMS-567, 1947.

3. J.H. Hall, “Fast Plutonium Reactor Experimental Facilities,” LAMS-908, 1949.

4. J.H. Hall, “Modification of the Los Alamos Fast Plutonium Reactor,” LA-1163, 1950.

10. E.T. Jurney, D.B. Hall, J.H. Hall, “The Los Alamos Fast Plutonium Reactor,” LA-1679, 1954.

Meet The Writer
Hannah Patenaude was a Seaborg Graduate Research Assistant for the summer of 2022 studying the Clementine Reactor under mentor Franz Freibert. She is currently a PhD student at the University of Nevada, Las Vegas in the Radiochemistry Program. Her dissertation research focuses on actinide molten salt electrochemistry.