Dr Claire Corkhill
26th February 2019 – Trinity College Dublin
28th February 2019 – Portsmouth
8th March 2019 – Durham
? – Derby
? – Manchester
Lecture A: The Mineralogy of Nuclear Meltdowns
In April 1986, Reactor 4 of the Chernobyl nuclear power plant underwent a catastrophic failure. This nuclear incident resulted in 31 direct deaths and caused a radioactive plume to spread across much of Europe and the former USSR. During the meltdown of the reactor core, temperatures reached >1600°C from the uncontrolled decay heat of the fission products, causing the nuclear fuel and zirconium cladding to melt together. More than 100 tons of molten reactor core magma moved through the sub-reactor rooms, incorporating structural building materials such as steel, concrete and sand. The minerals that formed, known as lava-like fuel containing materials (LFCM), are highly crystalline, radioactive, glass-like slags.
25 years later, in 2011, an earthquake off the eastern coast of Japan resulted in a loss of coolant accident at the Fukushima Daiichi Nuclear Power Plant. After the partial meltdown of Reactor Units 1 to 3, at temperatures in excess of 2000°C, UO2 fuel pellets reacted with zircaloy fuel cladding. Since it has not yet been possible to retrieve any of the materials from within the reactor buildings, due to the extreme levels of radioactivity, the precise mineralogical details of the melted core are not known. However, it is thought that a mineral phase with a composition of U1-xZrxO2 was formed, containing a variety of fission products and minor actinides, and it is possible that glassy lava-like minerals are also present.
The removal of these materials from the reactors and the sub-reactor buildings is vital to the remediation of the sites. This is a critical step on the pathway to allowing re-habitation of the local areas, which can only be completed once a thorough understanding of the composition and microstructure of the lavas has been accomplished. This talk describes the research being performed at the University of Sheffield, with our collaborators in the UK, Ukraine and Japan, to understand these fascinating nuclear meltdown minerals.
Lecture B: Digging Deep, Aiming High? Nuclear Waste Evolution in a Geological Disposal Facility
Nuclear waste—the radioactive by-product from nuclear power generation, nuclear weapons and medical isotope production—is one of the most challenging types of waste for our society to manage. Its high radioactivity requires that it be safely isolated from humans and the environment until it no longer poses a hazard; of the order of a million years. There is international consensus that the safest option is to remove nuclear waste from the dynamic surface of the Earth, where human intrusion, climate change and tectonic processes may disturb it, and to place it within an underground storage facility several hundreds of metres or more below ground. In a stable rock formation, the environment will remain largely unchanged over the 10,000 to 1 million years required to allow the waste to safely radioactively decay, isolated from the biosphere. This concept is known as the geological disposal of nuclear waste.
Inside the geological disposal facility, a series of engineered barriers, which can be likened to a set of Russian dolls from the inside to the outside, will be used to slow down the ingress of groundwater and the release of radionuclides to the biosphere. As groundwater interacts with each of these materials, e.g. clay, cement, Fe-canisters, it creates micro-environments with unique geochemical conditions. Building an understanding of how nuclear waste minerals and materials behave in these mini-radioactive worlds is crucial if we are to reduce uncertainty in the long-term behaviour of the geological disposal facility. This talk will describe how a range of geo- and radio-chemistry techniques, complemented by state-of-the-art synchrotron spectroscopy, are being applied to predict the behaviour of nuclear waste over 100,000 year time scales.
Dr Sarah Gleeson
7th February 2019 – NUI Galway
7th March 2019 – Leicester University
8th March 2019 – University College London
10th April 2019 – Brighton University
4th October 2019 – Liverpool
Lecture C: Diagenesis, sulphur and giant Zn deposits
Zinc is among the top 4 mined metals in the world and societal demands require a long-term stable supply for a growing market. The largest Zn deposits in the world are massive sulphide deposits formed in carbonaceous mudstones; many are of Paleoproterozoic and Lower Paleozoic age. In the deposits Lower Palaeozoic deposits of the North American Cordillera, the sulphide bodies are spatially associated with stratabound barite.
The commonly accepted genetic model for these deposits involves the exhalation of a metal-bearing hydrothermal fluid onto the seafloor where it mixes with reduced and oxidized sulphur in a stratified water column (in a restricted basin). In this presentation I will show that, in fact, barite and pyrite are formed in the sediment by diagenetic processes that pre-date the hydrothermal system. The diagenetic assemblage forms below the seafloor at the sulphate methane transition zone. The hydrothermal system dissolves barite, and in situ S isotopic data from pyrites show that anaerobic oxidation of methane plays an important role in the generation of a sulphide “trap”. The hydrothermal system is superimposed on this diagenetic environment but, typically, does not exhale onto the seafloor. As a result, complex biological-chemical-physical interactions in time and space in the diagenetic environment have an important control on the genesis and size of these important deposits.
Lecture D: Pyrite as ore and gangue in hydrothermal systems: potential and pitfalls
Pyrite is the most common sulphide mineral in the Earth’sc crust, and is found in igneous, metamorphic and sedimentary rocks. Pyrite is also a common gangue mineral in hydrothermal ore deposits formed in sedimentary basins, and in one type of Au deposit, the Carlin type deposits (Nevada), arsenian pyrite is the primary ore mineral. In order to assess how Au is sequestered from hydrothermal fluids into As rich pyrite, experiments have been conducted at conditions similar to those in which Carlin type Au deposits form. The sequestration of gold into pyrite appears to be dependent on the concentration of As in the fluid; at high As concentrations, Au is strongly partitioned into pyrite. This suggests that simple partitioning (and the underlying process of adsorption) is the major depositional process in forming these giant deposits.
In sediment hosted Zn deposits, pyrite is a major pre–, syn– and post–ore phase, and may often be the most abundant sulphide mineral present. As such, differentiating between background and ore-stage pyrite is critical for understanding the footprint of the deposit. For example, pyrite enrichment has previously been considered to be a distal expression of exhalative hydrothermal systems, and used as an exploration vector to mineralization. More recently, however, the coupling careful petrography with in situ techniques (LA-ICP-MS and SIMS) has revealed a more nuanced story. In some of the better-preserved deposits, it is clear that hydrothermal mineralization post-dates early diagenesis and pre-ore pyrite formation. As such, the distribution of hydrothermal pyrite around deposits (‘pyrite halo’) formed in the sub-surface is more restricted than previously thought. Moreover, trace element maps of hydrothermal pyrite in large ore deposit are complex and highlight the challenge we face in scaling up micro-analytical data to 3d volumes of crustal rocks as represented by economic ore deposits.