Investigating the Effect of Chemical Species on Pellet Cladding Interaction in Pressurised Water Reactor Fuel using Hybrid Quantum Mechanics Molecular

Lead Research Organisation: Imperial College London
Department Name: Materials

Abstract

In nuclear fission reactors, especially pressurised water reactors (PWR), pellet-cladding interaction (PCI) is a phenomenon in nuclear fuel rods whereby the outer surface of a fuel pellet is in contact with and interacts with the inner surface of the fuel pin cladding. It occurs during power transients, in incidental transients, fuel manoeuvring as well as during steady state operation. PCI is developed by fuel-pellet relative motion, and can also manifest itself through stress-corrosion cracking (SCC) of the cladding in the presence of volatile fission products such as iodine (I-SCC). In PWRs using unlined Zircaloy cladding and UO2 fuel, iodine stress corrosion cracking (I-SCC) has been recognised to be the main cause of PCI failures. For I-SCC to occur, however, there are several requirements that must be met. A critical stress is needed in order for the crack to propagate. The critical strain is needed in order to induce cracking of the oxide film covering the cladding. A critical iodine concentration at the crack tip is required in order to allow continuous diffusion of iodine into the crack. Finally, a minimum time is required in order for the system to come to chemical equilibrium before I-SCC can occur.

The aims of the PhD project are to investigate the effects that iodine and other halogens have on the stress corrosion cracking process in pressurised water reactor fuels. The exact chemistry of the attacking species is not yet known. In addition, there is debate surrounding the role of oxygen in the presence of iodine and whether it inhibits or enhances the I-SCC process. The project examines the fracture process of Zirconium and how iodine and oxygen affect that process.

Existing techniques of investigating fracture processes use either empirical potentials for describing the interaction between limited types of atoms in large systems or use quantum mechanics techniques that allow chemical accuracy but are limited to very small systems. Using a recently developed hybrid quantum mechanics molecular mechanics technique, the investigation of large systems is possible while maintaining high chemical accuracy at crack tips via density functional theory (DFT) calculations. Various types of fracture simulations including molecular statics, molecular dynamics and nudged elastic band simulations are performed in order to determine what the effect of iodine is on the process.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509486/1 01/10/2016 30/09/2021
1857821 Studentship EP/N509486/1 01/10/2016 31/03/2020 Vlad Podgurschi
 
Description Iodine defects in zirconium are more difficult to form when the oxygen content increases, suggesting that the oxygen has a protective effect against iodine stress corrosion cracking. At the same time, if the material is strained, higher oxygen content zirconium structures are more negatively affected than lower oxygen content zirconium structures indicating that for deformed zirconium, oxygen has a negative impact on its performance. The two competing effects were previously observed but the current work is a first step towards understanding that interaction. The work has opened up a new question: characterising the structures encountered at a crack tip during the iodine stress corrosion cracking process and how they affect the stress corrosion rate.
For the deformation of zirconium, the rate at which the material is strained and the temperature it was strained at were dependent on each other for a specific orientation (basal) but not for another orienation (prismatic). That dependence or lack of dependene (dependign on the orientation of the material) allows designing systems to ensure that they are not in condition that could allow cracks to grow. The work opened up new questions: how are the deformation processes that are encountered created and how can the presence of iodine have an effect on them?
Computational methods were developed that allow the investigation of fracture in pure single crystal materials for a variety of conditions.
Exploitation Route The outcomes of this funding would allow the iodine stress corrosion cracking process that occurs in pressurised water reactors to be better understood. Consequently, that can provide new ways of safe guarding against it that would allow a more efficient use of nuclear reactors. The outcomes can be further studied both experimentally and using modelling techniques in both academia and the industrial sector.
Sectors Construction,Energy