Claire Corkhill on Geological Radioactive Waste Disposal
Listen to the podcast here or wherever you get your podcasts.
Scroll down for podcast illustrations.
Storing radioactive waste safely over timescales of millennia to hundreds of millennia raises many questions, both technical and human. In the podcast, Claire Corkhill discusses the geology such storage sites require, some new materials that can confine radioactive isotopes over extremely long timescales, and the kind of hazards, including human, we need to guard against. She appears in the photo in a new, not yet radioactive “hot cell” for remotely manipulating spent fuel.
Claire Corkhill is Professor of Mineralogy and Radioactive Waste Management in the School of Earth Sciences at the University of Bristol.
Podcast Illustrations
Courtesy of Claire Corkhill unless otherwise indicated.
Loading a fuel assembly for nuclear power station. Nuclear fuel rods contain the uranium dioxide fuel whose fission products create the high-level radioactive waste that needs to be disposed of safely over very long timescales.
Each individual fuel rod has several tens of uranium dioxide fuel pellets stacked up directly on top of each other. Depending on the type of reactor, 8 - 20 fuel rods are arranged in an assembly. In the picture, newly prepared fuel rods are being loaded into a fuel assembly surrounded by a graphite sleeve, which acts as a moderator. A number of assemblies are loaded into the reactor core in a series of vertical channels that are surrounded by some means of cooling the fuel, such as CO2 gas or pressurized water.
Courtesy of the Nuclear Decommissioning Authority
Stainless steel canisters used to store high-level radioactive waste that has been vitrified, i.e., combined with borosilicate glass. As explained in the podcast, having no long-range order in the arrangement of their atoms, glasses are less vulnerable to radiation damage that can disrupt crystalline material.
The molten glass is poured directly into the containers, and it is left to cool down naturally. The steel has some resistance to radiation damage, but it is expected that it will completely corrode in a geological facility before radiation severely damages it. The corrosion (by groundwater) will likely take around 1,000 to 2,000 years.
Courtesy of Sellafield, Ltd.
A cartoon illustrating Claire Corkhill’s research. The process starts with problematic radioactive materials from fuel reprocessing and nuclear decommissioning (top left). The research attempts to work out how best to turn these materials into a glass or ceramic waste material (bottom left, yellow container), using natural glasses and minerals (magnifying glass) as a template. Corkhill and her team then study the glass structure to learn how it will behave over long timescales buried in a geological disposal facility (right).
Drawing by Alys Mordecai
Geological Disposal Facility at Onkalo, Finland
Radioactive waste disposal vault 475 meters underground at the Finnish geological disposal facility at Onkalo. The site will store 3,250 of the radioactive waste containers shown at left below. One of the disposal holes that will house one of the waste containers is just visible on the floor behind the fence.
Spent fuel container for the Onkalo geological disposal facility. The individual fuel rods are placed in the array of channels in a strong boron-steel insert (right), which is inserted into a copper tube (left) and provides stability to the radioactive waste package. The copper tubes are 5m long, 1m in diameter, and 5cm thick.
Mockup of the disposal system for Onkalo. The copper containing the array of spent fuel rods is deposited in a vertical borehole and surrounded by rings of bentonite clay. The bentonite protects the containers and the spent fuel from groundwater. It has a layered crystalline structure, which can take water into its interlayers. As such, when it gets wet it swells, forming a tight seal around the container, thus preventing radioactive elements from escaping.
Underground rock laboratory at Bure, France, where researchers are testing methods of constructing a geological disposal facility in the Callo-oxfordian clay. This type of tunnel is designed for intermediate-level waste (not heat-generating), which will be placed in large boxes and stacked on top of one another.
Simulant high-level radioactive waste glass (without radioactivity) being poured into a bucket of water to make small glass fragments. These fragments have high surface area, which is useful for making a glass powder whose properties can be characterized, or for undertaking glass leaching experiments that are accelerated by the high surface area.
Crystal structures of zirconolite, a very durable naturally-occurring mineral. As Corkhill explains in the podcast, this mineral can accommodate uranium (U) and thorium (Th) within its crystal lattice where they replace either calcium (Ca) or zirconium (Zr) atoms in the lattice, thus trapping them over a period of hundreds of millions of years, even as the crystal is damaged by radiation. The two different structures are different forms (polytypes) of the same mineral – the 3T (on the left) is generally understood to be more resistant to radiation damage than the 2M (on the right).
Photograph (left) and scanning electron microscope (bottom right) images of a one-billion-year-old titanite mineral containing uranium and exhibiting hydrothermal alteration. In the podcast, Corkhill discusses this ancient sample, which comes from the MacArthur River province in Canada. The area highlighted in the red square includes a U- and Th-rich titanate mineral together with a zircon crystal, which has been used to provide an age of around 1 billion years. The electron microscope image shows a series of veins running through the titanate mineral (large paler region), which were caused by hydrothermal alteration. Despite the fact that radiation has made the mineral completely amorphous (it is metamict), and it has been leached by hot fluids, it still contains most of its original U content.
Graph showing the radioactive decay of spent nuclear fuel. Since there are many different isotopes with different half-lives depicted in this curve with some of the daughter isotopes having higher radioactivity than the parent isotopes (e.g., radium is more radioactive than its parent, U), the curve is not a simple decaying exponential. It will take 10,000 years for the radioactivity to return to the same level as that of the naturally occuring uranium ore from which the fuel originated. This is still fairly radioactive, so it is desirable to let the waste decay for two more orders of magnitude, or one million years. For reference, homo sapiens first walked the Earth around 300,000 years ago.