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

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.

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.