Vitrification of nuclear waste is a process where nuclear waste is mixed with sand and other chemicals and then baked to melt all the ingredients into glass logs. Those logs are then placed in a geological repository for long term storage. Some countries are using this technique for the disposal of wastes created by the production of nuclear weapons. Other countries use the process for disposing of spent nuclear fuel from nuclear power reactors.
The U.S. has spent millions of dollars on the creation of a vitrification plant at the Hanford Nuclear Reservation to deal with nuclear wastes left over from the creation of the U.S. nuclear arsenal. Work on the plant was halted for a period and millions of dollars were wasted because they started building before they had finished designing the facility.
The justification for this vitrification process to store nuclear waste is based upon the assumption that the glass logs will last for centuries. However, there is some evidence that these glass logs can begin to dissolve eventually and may release the radioactive materials into the environment by the action of ground water. More research into vitrified glass deterioration is needed.
Researchers at the University of Houston, the Department of Energy’s Pacific Northwest National Laboratory (PNNL) and the University of Pittsburgh are currently working on a better understanding of the long-term behavior and deterioration of glass logs. They are researching what chemical/physical processes might cause glass logs to deteriorate quickly after being stable for a long time. The research is supported by an eight hundred thousand dollar grant from the from the Department of Energy.
As time passes, zeolite crystals form in the glass. Zeolites are three-dimensional, microporous, crystalline solids with well-defined structures that contain aluminum, silicon, and oxygen in their regular framework; cations and water are located in the pores. They are commonly used as commercial adsorbents and catalysts. Their formation
The formation of zeolites in glass logs speeds up deterioration. The principle researcher said, “We are looking more closely at the early stage of zeolite formation and thinking about ways to slow or completely impede the process.” They are hoping to find the right combination of ingredients to slow down or stop the formation of zeolite crystals.
There are many types of zeolites. The type that forms in glass logs is one of two polymorphs of the small-pore zeolite P. Polymorphs of a crystalline mineral refer to the exact arrangement of the atoms in the crystal. The more stable of the two possible arrangements is found in the glass logs. The formation of zeolite P is strongly affected by temperature.
The U of H lab will experiment on the formation of zeolites in the glass logs. The PNNL will carry out experiments on glass log deterioration. The Center for Research Computing at the U of P will work on computer models of zeolite nucleation.
A researcher at PNNL said, “We have long observed from laboratory studies that zeolite formation in glass corrosion tests resulted in an increase in the glass corrosion rate. With the expertise of Dr. Rimer, we hope to understand why zeolites lead to an increase in glass alteration rates and why only certain zeolites cause changes, with the long-term goal of preventing their formation.”