
Clementine was the code name for the world's first
fast-neutron reactor
A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons (carrying energies above 1 MeV or greater, on average), as opposed ...
, also known as the Los Alamos fast plutonium reactor. It was an experimental-scale reactor. The maximum output was 25 kW and was fueled by
plutonium
Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhib ...
and cooled by liquid
mercury. Clementine was located at
Los Alamos National Laboratory
Los Alamos National Laboratory (often shortened as Los Alamos and LANL) is one of the sixteen research and development laboratories of the United States Department of Energy (DOE), located a short distance northwest of Santa Fe, New Mexico, i ...
in
Los Alamos, New Mexico
Los Alamos is an census-designated place in Los Alamos County, New Mexico, Los Alamos County, New Mexico, United States, that is recognized as the development and creation place of the Nuclear weapon, atomic bomb—the primary objective of the ...
. Clementine was designed and built in 1945–1946 and first achieved criticality in 1946 and full power in March 1949.
The reactor was named after the song "
Oh My Darling, Clementine." The similarities to the song were that the reactor was located in a deep canyon and the reactor operators were 49'ers, as 49 (last digits of element 94, isotope 239) was one of the code names for plutonium at the time.
The primary goal of Clementine was to determine nuclear properties of materials for nuclear weapons research after the
Manhattan project
The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project w ...
. A number of other experiments were performed at the reactor, including investigation of the feasibility of civilian breeder reactors, and measuring
neutron cross section
In nuclear physics, the concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus. The neutron cross section σ can be defined as the area in cm2 for which the number of ...
s of various materials.
Core design

The core was contained in a long mild steel cylinder that had an inside diameter of and a wall thick. The fuel assembly was in diameter tall and contained 55 fuel elements. Each fuel element was composed of ''δ''-phase
plutonium-239
Plutonium-239 (239Pu or Pu-239) is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 is also used for that purpose. Plutonium-239 is also one of the three main ...
. They were each diameter and long. The fuel elements were clad in thick plain carbon steel. The core was located at the bottom of the steel cylinder.
The core was cooled by liquid
mercury. The maximum thermal output was 25 kW. The mercury was circulated through the core and out to a mercury-water heat exchanger at a maximum flow rate of by an induction type
electromagnetic pump with no moving parts.
Shielding and support structure

The reactor core was wrapped in a series of neutron reflectors and shielding structures, starting with a 15 cm (6 in.) thick cylindrical blanket of natural uranium immediately surrounding the core. This blanket was open at the top and bottom and could be moved up and down. Next was a 15.2 cm (6 in.) thick steel reflector and 10 cm (4 in.) of lead. Finally, most of the reactor was surrounded by multiple laminations of steel and boron plastic. This entire assembly was surrounded and supported by a thick
concrete
Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most ...
shell that provided additional shielding. A number of holes ran through the shielding to provide fast neutrons to the various
physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which rel ...
experiments.
Reactor control
The reactor was the first reactor to demonstrate reaction control via control of
delayed neutrons,
this was more of a function of being one of the very early reactors, rather than a special design feature. Control was accomplished by several means. The uranium blanket described above could be raised and lowered.
238U is a good
neutron reflector
A neutron reflector is any material that reflects neutrons. This refers to elastic scattering rather than to a specular reflection. The material may be graphite, beryllium, steel, tungsten carbide, gold, or other materials. A neutron reflect ...
, so the position of the blanket controlled the number of neutrons available to the reaction. When the blanket was raised, more neutrons were reflected back into the core, causing a greater number of fissions and, consequently, greater power output.
Additionally, there were two shutdown/control rods composed of natural uranium and boron that was
enriched in the
boron-10
Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...
isotope.
10B is a very effective
neutron poison
In applications such as nuclear reactors, a neutron poison (also called a neutron absorber or a nuclear poison) is a substance with a large neutron absorption cross-section. In such applications, absorbing neutrons is normally an undesirable eff ...
that could be inserted to control and shut down the reaction.
Shutdown of the reactor involved simultaneously dropping the uranium blanket and inserting the two control rods into the center which, absorbed neutrons and poisoned the reaction. Up to 20 other holes were available in the core for experimental configurations or additional control or fuel rods.
Use and shutdown
Clementine operated successfully from 1946 until 1950 when the reactor was shut down to correct a problem with the control and shim rods. During this shutdown it was noted that one of the natural uranium rods had ruptured. It was replaced and the reactor was restarted.
It was again operated successfully until 1952 when the cladding on one of the fuel rods ruptured. This caused contamination of the primary cooling loop with plutonium and other
fission products
Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the relea ...
. At this time it was decided that all the primary objectives of Clementine had been achieved and the reactor was permanently shut down and dismantled.
Results of the Clementine experiment
The experience and data provided by operating the Clementine reactor was very useful for both military and civilian applications. One of the notable achievements of the Clementine project included measurements for the total neutron cross sections of 41 elements to a 10% accuracy. Additionally, Clementine provided invaluable experience in the control and design of fast neutron reactors. It was also determined that mercury was not an ideal cooling medium for this type of reactor due to its poor heat transfer characteristics.
Specifications
*Type:
fast neutron reactor
A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons (carrying energies above 1 MeV or greater, on average), as opposed ...
*Fuel:
plutonium-239
Plutonium-239 (239Pu or Pu-239) is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 is also used for that purpose. Plutonium-239 is also one of the three main ...
*Coolant: mercury at maximum.
*Moderator: none
*Shielding: multiple layers of uranium-238, steel, lead and boron-impregnated plastic.
*Power: 25 kW maximum.
*Core temperature: inlet , outlet , max core
See also
*
Omega West Reactor (OWR)
The Omega West Reactor (OWR) was an experimental nuclear reactor located at Los Alamos National Laboratory in Los Alamos NM. OWR was completed in 1956 and primarily used for scientific scale nuclear research until it was fully decommissioned ...
*
UHTREX
The Ultra-High Temperature Reactor Experiment (UHTREX) was an experimental gas-cooled nuclear reactor run at Los Alamos National Laboratory between 1959 and 1971
*
Liquid metal cooled reactor
A liquid metal cooled nuclear reactor, liquid metal fast reactor or LMFR is an advanced type of nuclear reactor where the primary coolant is a liquid metal. Liquid metal cooled reactors were first adapted for nuclear submarine use and have been ...
References
{{DEFAULTSORT:Clementine (Nuclear Reactor)
Nuclear research reactors