The Ultra-High Temperature Reactor Experiment (UHTREX) was an experimental gas-cooled nuclear reactor run at Los Alamos National Laboratory between 1959 and 1971The Bradbury Years
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Los Alamos Science ''Los Alamos Science'' was the Los Alamos National Laboratory's flagship publication in the years 1980 to 2005. Its main purpose was to present the laboratory's research and its significance to national security to the scientific community, and U ...

'' Winter/Spring 1983 (

Los Alamos National Laboratory Los Alamos National Laboratory (often shortened as Los Alamos and LANL) is one of the sixteen research and development Laboratory, laboratories of the United States Department of Energy National Laboratories, United States Department of Energy ...

).The Agnew Years
from ''Los Alamos Science'' Winter/Spring 1983 (Los Alamos Scientific Laboratory). as part of research into reducing the cost of nuclear power.''The Atom'' Volume 11 #51 (Jan-Feb 1974)
/ref> Its purpose was to test and compare the advantages of using a simple fuel against the disadvantages of a contaminated cooling loop.''ULTRA HIGH TEMPERATURE REACTOR EXPERIMENT (UHTREX) HAZARD REPORT''
Los Alamos Science Document #LA-2689 (1962). It first achieved full power in 1969.Milestones in the history of Los Alamos National Laboratory
Los Alamos Science Document Number 21-1993. The experiment was a spin-off from Project Rover, intended to develop a

nuclear thermal rocket A nuclear thermal rocket (NTR) is a type of thermal rocket where the heat from a nuclear reaction replaces the chemical energy of the rocket propellant, propellants in a chemical rocket. In an NTR, a working fluid, usually liquid hydrogen, is ...

Reactor core design

The UHTREX core was composed of a vertical hollow rotating cylinder (turret) constructed of solid

graphite Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...

UHTREX Reactor Core Vertical Cross Section 1

The cylinder was 70 in. OD x 23 in. ID x 39 in. high. The core had 312 fuel channels. The channels were equally spaced radially around the core at 15 degree intervals arranged in 13 separate layers of 24 channels each. Each channel held up to 4 fuel elements and extended completely through to the inside of the cylinder. The core could be refueled remotely while at full power. Refueling involved rotating the core to the channel containing the element requiring replacement and pushing in a new element. The used element would be pushed out into the center and fall to the base of the reactor to be collected. At full power the reactor used up 1 to 6 fuel elements per day depending on enrichment and porosity of the fuel element. It produced 3MW of thermal energy.

Advantages

A typical

nuclear reactor A nuclear reactor is a device used to initiate and control a Nuclear fission, fission nuclear chain reaction. They are used for Nuclear power, commercial electricity, nuclear marine propulsion, marine propulsion, Weapons-grade plutonium, weapons ...

prevents the coolant medium from directly contacting the fuel pellets either by cladding the fuel pellets, sealing the fuel pellets inside a fuel rod or running the coolant through separate piping. This prevents contamination of the cooling medium. One of the disadvantages of a sealed fuel assembly includes buildup of fission products inside the fuel element. Some of these products

poison A poison is any chemical substance that is harmful or lethal to living organisms. The term is used in a wide range of scientific fields and industries, where it is often specifically defined. It may also be applied colloquially or figurati ...

the reaction ultimately leading to poor efficiency well before a significant portion of the fuel is used up. At this time the reactor requires re-fueling. Keeping the fuel and coolant separate can present significant design challenges as well. For example, the metal tubing required to do so cannot be operated at temperatures above its melting point which is generally significantly lower than the fuel pellet. This lowers the maximum theoretical

thermal efficiency In thermodynamics, the thermal efficiency (\eta_) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc. For ...

of the reactor. The UHTREX used un-clad porous carbon extruded fuel elements each shaped like a long hollow cylinder. The fuel elements were manufactured by vacuum impregnating the porous carbon cylinders with aqueous uranyl nitrate solution then air drying and baking them in a furnace ultimately producing a uranium oxide coating tightly held in a porous graphite matrix. This fuel was expected to be substantially less expensive to manufacture than other types of fuel at the time. The primary advantages of this type of fuel was that the porosity of the pellet in addition to the high temperatures achievable would allow most of the poisons created by the fission products to migrate out of the fuel. The poisons would then be carried away by the coolant stream for eventual filtering out and removal. This allows a higher percentage of fuel to be burned up before the pellet needed replacement (up to 50%).

Disadvantages

The major disadvantage to porous reactor fuel is that the entire primary cooling loop including all the

pump A pump is a device that moves fluids (liquids or gases), or sometimes Slurry, slurries, by mechanical action, typically converted from electrical energy into hydraulic or pneumatic energy. Mechanical pumps serve in a wide range of application ...

s, compressors and

heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...

s would become highly contaminated with fission products. Contamination caused by a potential contaminated

coolant A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corr ...

leak would pose a significant danger to personnel and the environment. The high contamination level precludes being able to open the reactor vessel for the eventual refueling. Therefore, the reactor was designed for remote online refuelling.

Specifications

UHTREX had following specifications: *Fuel - highly enriched uranium *Rated power - 3 MW (thermal) *Core construction material - graphite *Moderator - graphite *Reactor vessel -

carbon steel Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states: * no minimum content is specified or required for chromium, cobalt ...

sphere 13 ft. 2 in. diameter 1.75 inches thickness. *Fuel channels - 312 channels. Each one is 1.1 in. ID, 23.5 in. long and holds up to 4 fuel elements. *Fuel element - 1 in. OD, 0.5 in. ID and 5.5 in. long (25.4 mm x 12.7 mm x 139.7 mm). *Core power density - 1.3 W/cc *Fuel utilization - up to 50%. *Coolant -

helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

at 500 psi (3.45 MPa) *Coolant temperature - inlet 1600 °F, Outlet 2400 °F (871 °C and 1316 °C). *Coolant flow rate - 10,250 pounds per hour (1.294 kg/s) where ID and OD are the inner and outer diameter, respectively.

References

External Links