THOREX

THOREX
Name	THOREX Process
Type	Nuclear reprocessing
Inventor	Oak Ridge National Laboratory
Developed	Late 1940s
Industry	Nuclear fuel cycle
Related	PUREX, UREX

THOREX. The THOREX process is a solvent extraction method used in the nuclear fuel cycle to separate thorium, uranium, and fission products from spent nuclear fuel. Developed primarily for the utilization of thorium-based fuels, it is a chemical cousin to the more widely deployed PUREX process. The technique enables the recovery of valuable fertile and fissile materials for potential reuse in advanced nuclear reactor designs.

Overview

The process was conceived to address the specific chemistry of thorium dioxide and its irradiated products, differing from uranium-plutonium fuels processed by PUREX. Its development was largely driven by research into breeder reactor concepts and the potential of the thorium fuel cycle. Key facilities involved in its development and testing included the Oak Ridge National Laboratory and the Idaho National Laboratory. The process shares fundamental principles with other hydrometallurgical techniques used in the nuclear industry.

Process Description

The THOREX process typically begins with the dissolution of spent thorium fuel in nitric acid, forming a feed solution. This solution is then subjected to multi-stage solvent extraction using a mixture of tri-n-butyl phosphate (TBP) diluted in an organic diluent like kerosene. The extraction chemistry is carefully controlled by adjusting the concentration of nitric acid and salting agents such as aluminium nitrate. Equipment like pulse columns or mixer-settlers, similar to those used at the Savannah River Site, are employed for the separation stages. Subsequent stripping steps recover purified thorium and uranium-233 streams.

Chemical Reactions

The core extraction involves the formation of complexes between metal ions and tri-n-butyl phosphate. The major extraction reaction for thorium can be represented as Th(NO₃)₄•2TBP, while uranium extracts as UO₂(NO₃)₂•2TBP. The presence of fission products like zirconium, ruthenium, and neodymium complicates the chemistry, requiring precise control to minimize their co-extraction. The process also manages the behavior of protactinium-233, an intermediate in the decay chain to uranium-233. Research into alternative extractants has been conducted at institutions like the Julich Research Centre.

Applications

The primary application is the reprocessing of fuel from reactors utilizing thorium, such as the Light Water Breeder Reactor or the Advanced Heavy Water Reactor developed in India. It is integral to closing the fuel cycle for proposed molten salt reactor designs like the LFTR. The process enables the recovery of uranium-233, a fissile isotope, for use in new fuel assemblies. Projects in nations like India and China have investigated THOREX for their respective nuclear power programs.

Advantages and Limitations

A key advantage is its ability to facilitate the thorium fuel cycle, which offers greater abundance of thorium compared to uranium and reduced plutonium production. It can also partition uranium-233 for efficient fuel recycling. However, significant limitations include the high gamma radioactivity from uranium-232 decay products, complicating handling. The process generates high-level waste streams containing fission products and actinides, requiring management akin to PUREX waste. Its economic viability is challenged by the current dominance of the uranium fuel cycle.

Historical Development

Development began in the late 1940s at the Oak Ridge National Laboratory as part of the Atoms for Peace program and research into breeder reactor technology. Pilot-scale demonstrations were conducted during the 1960s. The process was tested with fuel from the Indian Point Energy Center and other experimental reactors. While never deployed commercially on a large scale, it informed later partitioning processes like UREX. Ongoing research continues in countries with active thorium programs, such as India and Norway.

Category:Nuclear reprocessing Category:Nuclear technology Category:Radiochemistry