Flattop is a benchmark

critical assembly In nuclear engineering, a critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its atomic nucleus, nuclear properties (specifically, it ...

that is used to study the nuclear characteristics of

uranium-233 Uranium-233 (233U or U-233) is a fissile isotope of uranium that is bred from thorium-232 as part of the thorium fuel cycle. Uranium-233 was investigated for use in nuclear weapons and as a reactor fuel. It has been used successfully in ex ...

uranium-235 Uranium-235 (235U or U-235) is an Isotopes of uranium, isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile ...

, and

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 ...

in spherical geometries surrounded by a relatively thick

natural uranium Natural uranium (NU or Unat) refers to uranium with the same isotopic ratio as found in nature. It contains 0.711% uranium-235, 99.284% uranium-238, and a trace of uranium-234 by weight (0.0055%). Approximately 2.2% of its radioactivity comes ...

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 ...

. Flattop assemblies are used to measure

neutron activation Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus decays immediately by emitti ...

and reactivity coefficients. Since the neutron energies gradually decrease in the reflector, experiments may be run in various energy spectra based on the location in which they are placed.

Specifications

Flattop is a natural-uranium-reflected, benchmarked, fixed-geometry critical assembly machine that can accommodate plutonium or uranium cores. The fast neutron spectrum is used to provide benchmarked neutronic measurements in spherical geometry with different fissile driver materials. Key missions for Flattop include fundamental reactor physics studies, sample irradiation for radiochemical research, actinide minimum critical mass studies, detector calibration, and training. The U-233 core is no longer usable because of its high

gamma-ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically sh ...

activity. The experiment was originally located at the

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 ...

Critical Experiments Facility (LACEF) located at the Los Alamos Pajarito Site, otherwise known as Technical Area 18. In 2005 the Pajarito Site started to shut down and nuclear material was moved to the National Criticality Experiments Research Center (NCERC) which is located at the

Nevada National Security Site The Nevada National Security Site (N2S2 or NNSS), known as the Nevada Test Site (NTS) until 2010, is a United States Department of Energy (DOE) reservation located in southeastern Nye County, Nevada, about 65 miles (105 km) northwest of the ...

. However, NCERC continues to be operated by the Los Alamos National Laboratory. The core capabilities at NCERC include Flattop along with three other critical assemblies, Comet, Planet, and Godiva-IV and a significant inventory of nuclear material items available for experimental use. NCERC critical operations commenced in 2011 and continue to be operational today.

Space power research

In 2012, Flattop was used for key demonstration of the use of nuclear power for space applications. The Demonstration Using Flattop Fission, or DUFF, test was planned by Los Alamos National Laboratory to use Flattop as a nuclear heat source. A team from the NASA Glenn Research Center in partnership with the LANL reactor design team designed, built, and tested a heat pipe and power conversion system to couple to Flattop with the end goal of demonstrating electrical power production using technology applicable to space application.

Controls

Flattop consists of a hemispherical fixed reflector and two movable quarter-spheres of reflector that can close down on the central core. One movable reflector is controlled by hydraulic pressure, while the other is actuated by a motor.

References

{{Reflist Nuclear technology Nuclear research reactors