Mechanisms of Retention and Transport of Fission Products in Virgin and Irradiated Nuclear Graphite

Lead Research Organisation: Loughborough University
Department Name: Chemistry

Abstract

The UK has long experience in the operation of Advanced Gas-cooled Reactors, which rely many tonnes of nuclear-grade graphite which has a dual role, firstly to slow down (moderate) neutrons to enhance nuclear fission and secondly to provide structure in the rather extreme environment in the reactor core, comprising high temperatures and intense radiation.

Designs of reactor for the next generation of nuclear reactors being developed in the USA, such as high temperature gas reactors and molten salt reactors also rely on graphite, and will require a good scientific understanding of its properties under irradiation, particularly under higher temperature conditions. Therefore, fundamental studies are required to reveal the mechanisms underlying graphite behavior, before these new reactor concepts can be taken through the design process and be licensed for operation.

A big issue for these new reactor designs is the retention of activated fission products within graphite, and their subsequent potential release during decommissioning. This includes the complex graphitic matrix material used in fuel pebbles for Pebble Bed Modular Reactor designs.

This is a joint experimental-computational approach to measure the diffusivities of fission products (FPs) - Iodine (I), Cesium (Cs), Krypton (Kr), Strontium (Sr), Ruthenium (Ru) and Silver (Ag), and Europium (Eu) in four graphite grades - HOPG, NBG-18, PCEA and IG-110, and uncover the mechanisms of transport using multiscale simulations involving electronic structure, atomistic, and phase field methods

The UK teams at Manchester and at Loughborough will be working with the USA groups based in University of Central Florida, North Carolina State University and Oak Ridge National Laboratory on this problem. Manchester will be contributing experimental measurements on legacy graphite from the AGR and Magnox reactor programmes and Loughborough will be using theoretical methods to elucidate electronic structure and energy landscapes of the FPs within realistic models of the graphite at the beginning of service, and graphite after decades of exposure to neutron and gamma radiation.

Planned Impact

The results of the work proposed here will be published in international journals and disseminated at international conferences, connecting with both the international carbon community and the international nuclear graphite community and the International Atomic Energy Agency.

Modelling and measuring segregation and diffusion of chemical impurities of widely different characters, including intercalants, and how they interact with complex microstructure of the virgin and radiated graphite is key to understanding nuclear graphite performance in fault conditions and legacy graphite, such as that stored at Sellafield, which has been stored in complex, poorly understood, often aqueous, environments. The impact will be felt by those responsible for waste (the Nuclear Decommissioning Authority and the licensees: Magnox and EDF Energy Generation) and will inform the work of the regulator (the Office of Nuclear Regulation).

A spin off impact will be improvement in techniques and understanding of the complex structure of virgin and irradiated graphite. It will assist the work of the licensee and the regulator in contributing to understanding the evolution of physical property changes with age in the reactor core. There will be a positive impact in the confidence of graphite ageing predictions, which will enable better management of the UK's AGR fleet toward their end of life. Apart from benefits in managing the national grid and in managing extra investments in generating plant, there is a huge potential win in better management of fuel shuffling between reactors.

Publications

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Description A key finding is that the fission products (for example: silver, ruthenium or europium) associated with nuclear reactors can diffuse through graphite. The diffusion is strongly dependent upon the microstructure of the graphite. Point defects in the graphite such as atomic sized holes strongly effect this diffusion as they act as sites that more strongly bind to the fission products compared to a pristine graphene plane. Furthermore, fission products that form small clusters of 3, 4 or more atoms were found to diffuse more readily than single atom inclusions.

Many methods and approximations within the density functional theory (DFT) computational method were tested as part of this work. We found that for DFT calculations of fission products with graphite surfaces and bulk structures, van der Waals corrections are required for accurate modelling of adatom adsorption. Standard methods such as Local-density approximations (LDA) and generalized gradient approximation (GGA) fail to predict experimental values.
Exploitation Route It is expected that the diffusion of fission products will be very important to consider for the creation of new synthetic graphites for use in the new generation 4 nuclear reactors. Therefore, graphite manufacturers and nuclear power plant operators such as EDF can use the computational modelling protocols developed to model and predict these properties accurately. Energy barrier calculation results in the papers published from this work, could be used by them in more coarse grained models, enabling more accurate modelling of larger structures.
Sectors Energy

 
Title Supplementary information files for Stacking-mediated diffusion of ruthenium nanoclusters in bilayer graphene and graphite 
Description Supplementary files for article Stacking-mediated diffusion of ruthenium nanoclusters in bilayer graphene and graphite The diffusion, penetration and intercalation of metallic atomic dopants is an important question for various graphite applications in engineering and nanotechnology. We have performed systematic first-principles calculations of the behaviour of ruthenium nanoclusters on a graphene monolayer and intercalated into a bilayer. Our computational results show that at a sufficiently high density of single Ru atom interstitials, intercalated atoms can shear the surrounding lattice to an AA stacking configuration, an effect which weakens with increasing cluster size. Moreover, the interlayer stacking configuration, in turn, has a significant effect on cluster diffusion. We therefore find different trends in diffusivity as a function of cluster size and interlayer stacking. For monolayer graphene and an AA graphene bilayer, the formation of small clusters generally lowers diffusion barriers, while the opposite behaviour is found for the preferred AB stacking configuration. These results demonstrate that conditions of local impurity concentration and interlayer disregistry are able to regulate the diffusivity of metallic impurities in graphite. 
Type Of Material Database/Collection of data 
Year Produced 2022 
Provided To Others? Yes  
URL https://repository.lboro.ac.uk/articles/dataset/Supplementary_information_files_for_Stacking-mediate...
 
Title Supplementary information files for Stacking-mediated diffusion of ruthenium nanoclusters in bilayer graphene and graphite 
Description Supplementary files for article Stacking-mediated diffusion of ruthenium nanoclusters in bilayer graphene and graphite The diffusion, penetration and intercalation of metallic atomic dopants is an important question for various graphite applications in engineering and nanotechnology. We have performed systematic first-principles calculations of the behaviour of ruthenium nanoclusters on a graphene monolayer and intercalated into a bilayer. Our computational results show that at a sufficiently high density of single Ru atom interstitials, intercalated atoms can shear the surrounding lattice to an AA stacking configuration, an effect which weakens with increasing cluster size. Moreover, the interlayer stacking configuration, in turn, has a significant effect on cluster diffusion. We therefore find different trends in diffusivity as a function of cluster size and interlayer stacking. For monolayer graphene and an AA graphene bilayer, the formation of small clusters generally lowers diffusion barriers, while the opposite behaviour is found for the preferred AB stacking configuration. These results demonstrate that conditions of local impurity concentration and interlayer disregistry are able to regulate the diffusivity of metallic impurities in graphite. 
Type Of Material Database/Collection of data 
Year Produced 2022 
Provided To Others? Yes  
URL https://repository.lboro.ac.uk/articles/dataset/Supplementary_information_files_for_Stacking-mediate...
 
Description Oral Presentation at the 20th International Nuclear Graphite Specialist Meeting 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact A talk given to an audience of academics as well as industry experts and policy makers in the field.
This lead to fruitful collaborations with the research groups in Manchester and the USA.
Year(s) Of Engagement Activity 2019
URL https://ingsm2019.sckcen.be/-/media/Files/INGSM2019/Monday/12-40-JamesMcHugh-INGSM.pdf
 
Description Oral Presentation at the APS March Meeting 2019 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact An international audience of at least 60 people attended the talk. This sparked questions from other researchers which lead to debate and progress in the research.
Year(s) Of Engagement Activity 2019
URL https://ui.adsabs.harvard.edu/abs/2019APS..MARS04009M/abstract