Aqueous Homogeneous Research Reactors
An In Depth Look into an Early Fission Technology
Based on this Paper: Aqueous Homogeneous Research Reactors
Introduction
Aqueous homogeneous research reactors (AHRRs) are a specialized class of nuclear reactors that once garnered significant interest due to their inherent safety characteristics and versatile experimental capabilities. Developed primarily in the mid-20th century, these reactors use a solution of uranium salts dissolved in water as both the fuel and the moderator. This unique configuration offers several advantages, particularly in research and training settings.
Historical Context
The concept of aqueous homogeneous reactors dates back to the early 1950s. By the time of the first International Conference on the Peaceful Uses of Atomic Energy in 1955, four AHRRs were operational in the United States. These included the SUPO reactor at Los Alamos, the NCSC reactor at North Carolina State College, and the WBNS and L-3 reactors in California. Since then, numerous advancements have been made, largely driven by Atomics International, a division of North American Aviation
Design and Safety Features
AHRRs are notable for their inherent safety features. The large negative temperature and power coefficients of reactivity ensure that any uncontrolled power excursion will naturally lead to a shutdown, preventing catastrophic failure. This safety mechanism is facilitated by the reactor's fuel configuration: a solution of uranyl sulfate in water, which can safely accommodate fuel additions and offers high radiation stability.
Core and Fuel Characteristics
The core of an AHRR is typically a spherical vessel made from stainless steel, with an internal cooling system comprising distilled water circulated through cooling coils. The fuel solution—uranyl sulfate—provides several benefits over other compounds like uranyl nitrate, including greater radiation stability and lower neutron absorption. Impurities in the fuel are strictly controlled, with allowable limits for elements like iron, boron, and cadmium detailed in reactor specifications.
Reactor Variants
AHRRs come in various sizes and power levels, each designed for specific applications:
L-54 Reactor: Operating at 50 kW, this reactor features a large graphite reflector and multiple thermal columns. It includes gamma ray exposure facilities, making it suitable for a wide range of experiments.
L-55 Reactor: Designed for up to 1.5 kW, this reactor has a large vertical column useful for exponential experiments. It utilizes a recombination system to manage radiolytic gas production.
L-77 Reactor: A compact, 10-watt reactor intended for training purposes. It features a vapor-pressure recombiner and a three-region composite shield for enhanced safety and ease of use in educational settings.
Experimental Capabilities
AHRRs are highly versatile and can be equipped with various experimental facilities, including horizontal and vertical beam tubes, thermal columns, and gamma exposure ports. These features allow researchers to conduct a wide range of experiments, from neutron flux measurements to radiation exposure studies.
Gas Handling and Recombination
One of the critical challenges in AHRR operation is managing the radiolytic gases—hydrogen and oxygen—produced during reactor operation. Atomics International developed the Wet Type Gas Handling System (WTGHS) to address this issue. The WTGHS continuously recirculates water through a catalyst bed to recombine hydrogen and oxygen into water, preventing the accumulation of explosive gas mixtures. This system operates effectively at power levels up to 50 kW, with safety measures in place to handle gas concentrations well below explosive thresholds.
Fuel Stability and Corrosion
The stability of the fuel solution and the integrity of reactor materials are paramount. Extensive testing has shown that stainless steels, particularly types 304, 316, and 347, exhibit excellent resistance to corrosion in uranyl sulfate solutions. The presence of fluorides and chlorides is carefully controlled to prevent localized corrosion. Long-term operational data from reactors like those at the University of California Radiation Laboratory confirm the durability of these materials, with minimal corrosion observed over several years of operation.
Conclusion
Aqueous homogeneous research reactors represent a pinnacle of safety and versatility in nuclear research technology. Their unique design, inherent safety features, and wide range of experimental capabilities make them invaluable tools for both research and education. As advancements continue, these reactors will likely play a crucial role in the development of nuclear science and technology.
