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Peilong Dong - Atmospheric Stress Corrosion Cracking of Low Carbon Austenitic Stainless Steels...

Name: Peilong Dong

Title: Atmospheric Stress Corrosion Cracking of Low Carbon Austenitic Stainless Steels used in Dry Storage Casks for Interim Storage of Spent Nuclear Fuels.

Supervisor: Dr Mark R Wenman

Collaborator: EDF

Duration: 01/10/2016 – 01/10/2019 (PhD studentship)

 

Description: One of the major issues with nuclear power generation is the disposal of nuclear waste. In the UK, it has been decided that the ultimate disposal route for high level nuclear waste would be deep geological, however such a facility has not been sited yet and it will be decades before one is constructed and in operation. Previously PWR reactors, for example Sizewell B, used wet storage in the form of fuel ponds, however, these have since reached capacity and alternative methods need to be employed. EDF have decided that further high level waste such as spent nuclear fuels generated at Sizewell B would be stored in austenitic stainless steel dry storage casks.

 

Austenitic stainless steels are known to fail via a localised corrosion mechanism known as stress corrosion cracking (SCC). In low carbon steels, transgranular SCC may occur when a susceptible material is exposed to a specific chemical environment and a tensile stress. This is an issue as dry store facilities are located coastally (on site of nuclear power stations) exposing canisters to an aggressive chloride atmosphere. Residual tensile stresses will also be present in the canisters due to welding. These conditions make failure by SCC a possibility. However, as these canisters have an expected service time of 50-100 years, it is important to find out the SCC susceptibility and the extent of SCC cracking in these materials as a part of the safety case.

 

This project looks at the effect of salt deposition and different type of salts (MgCl2, Synthetic Sea-salt, actual sea-salt composition at Sizewell) as well as different compositions of steels (304L, 316L weld material from an actual Holtec MPC canister, and additively manufactured 316L ). Analysis will be done primarily using optical microscopy, SEM, EBSD and EDX for crack counting, grain and texture information, and qualitative chemical analysis. Atom probe tomography will also be used to try and identify the presence and role of chlorine at/ahead of crack tips using atom probe tomography. 

Dr Claudia Gasparrini - Neutron irradiation damage of nuclear reactor pressure vessel steels

Investigator: Dr Claudia Gasparrini

Supervisor: Dr Mark Wenman

Collaborators: Rolls-Royce Plc, Australian Nuclear Science and Technology Organisation, UK Atomic Energy Authority Materials Research Facility, National Nuclear Laboratory, Culham Centre for Fusion Energy, Oxford University, Manchester University, University of New South Wales.

Duration: 01/12/2017 – 01/12/2019 (Postdoctoral Research Associate)

Description: The aim of this project is to investigate the role of microstructure and processing history on the phenomenon of neutron irradiation embrittlement in reactor pressure vessel (RPV) steels. The materials investigated will all be of the same chemical composition of A508 class 3 ferritic steel but manufactured using different methods.  There will be forged (in current use), hot isostatically pressed and electron beam welded materials.  The position involves help oversee the technical and safety aspects of carrying out irradiations in the OPAL reactor and packaging/transport of the materials to be sent back to the UKAEA Materials Research Facility at Culham.  Micro-mechanical tensile testing and microscopy (SEM and TEM) are performed on unirradiated and irradiated steels in collaboration with the UKAEA Materials Research Facility, Manchester University and the Australian Nuclear Science and Technology Organisation.

Dr Daniel King - Atomic scale modelling of nano-solute-vacancy clusters in reactor pressure vessel steels

Dr Daniel King - Atomic scale modelling of nano-solute-vacancy clusters in reactor pressure vessel steels

Supervisor: Dr Mark Wenman

Collaborators: Rolls-Royce Plc, National Nuclear Laboratory, Culham Centre for Fusion Energy, Australian Nuclear Science and Technology Organisation, Oxford University, Manchester University, University of New South Wales.

Duration: 01/02/2017 – 01/08/2019 (Postdoctoral Research Associate)

Description: The aim of this project is to determine the driving forces and behaviour of nano-scale solute-vacancy clusters, a mechanism responsible for hardening, that occurs due to  neutron irradiation of reactor pressure vessel steels.  The expected outcome is to achieve a mechanistic understanding of this process to support safety cases and models for pressurised water reactor life extensions. This project specifically involves the modelling and prediction of the clustering behaviour of Mn, Ni and Si, in bcc Fe, using density functional theory. Results from these models will be used in a multiscale approach to link fundamental solid state physics calculations with advanced manufacturing of steels for future reactors.  Further, results from this project will allow for less conservative predictions of toughness reduction and support on-going operation for reactors beyond 60-80 years.

Figure
Graphical abstract of a recent publication “D.J.M. King, P.A. Burr, S.C. Middleburgh, T.M. Whiting, M.G. Burke, M.R. Wenman. The formation and structure of Fe-Mn-Ni-Si solute clusters and G-phase precipitates in steels. Journal of Nuclear Materials. 2018. https://doi.org/10.1016/j.jnucmat.2018.03.050” that investigates the structure of FexMn6Ni16Si7 clusters in A508 steel.

Vlad Podgurschi - Modelling the effect of iodine at stress corrosion crack tips in Zr using hybrid quantum mechanics/molecular dynamics simulations

Title: Modelling the effect of iodine at stress corrosion crack tips in zirconium using hybrid quantum mechanics/molecular dynamics simulations

Supervisor: Dr Mark Wenman

 

In pressurised water reactors using unlined zirconium alloy cladding and UO2 fuel, iodine stress corrosion cracking (I-SCC) has been recognised to be the main cause of PCI failures. The complete iodine-SCC mechanism is not yet fully understood; the chemistry of the attacking species and the oxygen’s chemistry are still subject to debate. The I-SCC process has been studied using a novel hybrid quantum mechanics/molecular dynamics technique that allows the exploration of large defects (>10,000 atoms) but maintains high chemical accuracy in regions of interest such as crack tips via density functional theory (DFT) calculations.

Vlad Podgurschi

Jana Smutna - Understanding the role of hydrogen-dislocation interactions...

 

Title: Understanding the role of hydrogen-dislocation interactions in the corrosion and hydrogen uptake of irradiated zirconium fuel cladding alloys

Supervisor: Mark Wenman

Co-supervisor: Andrew Horsfield, Adrian Sutton

Industrial partner: Carrie Miszkowska (Rolls Royce) 

 

Abstract: Zirconium alloys are predominantly used in nuclear fuel cladding. The lifetime of these alloys is limited by the pickup of hydrogen from the surrounding water coolant, and subsequent formation of hydrides. In addition to the alloy composition, the defects, dislocations and dislocation loops caused by radiation damage affect the hydrogen pickup fraction and corrosion rate. The mechanistic understanding of the interactions between hydrogen and radiation damage (especially dislocation loops) requires computational modelling techniques able to simulate thousands of atoms. The empirical potentials available at the moment for the Zr-H system (most notably EAM) do not provide sufficient accuracy, and DFT calculations are too slow for use on the system sizes required. The aim of this project is a development of DFTB (Density Functional Tight Binding) potential for the Zr-H system, where electronic structure is included explicitly. This should provide a model much faster than DFT codes, but more accurate and more transferable than empirical potentials. This will allow for modelling of hydrogen in irradiated zirconium alloys.

Filippo Vecchiato - Microstructural evolution of 316l stainless steel in laser powder bed fusion

Microstructural evolution of 316l stainless steel in laser powder bed fusion

Investigator: Filippo Vecchiato

Supervisors: Dr Mark R Wenman and Dr Paul A Hooper

Duration: 03/10/2015 –31/03/2019 (PhD Studentship)

 

Description: Laser powder bed fusion technology allows for design flexibility, unfeasible with other destructive manufacturing methods. Therefore, it can be used to produce porous structures and lattices with variable mechanical properties. The project has been focused on the control of the microstructural properties of deposited 316L, by controlling the cooling rates involved in the process, changing the laser parameters used during the deposition.

The procedures used variable laser power and laser exposure time, controlling the grain size in the melt pool, producing different cooling rates. The microstructural formation was compared against the investigation of the cooling rates with a multi-channel high speed thermal camera.

Tom Whiting - Atomic-scale modelling of solute clusters in reactor pressure vessel steels

Investigator: Tom Whiting

Supervisors: Dr. Mark Wenman, Prof. Robin Grimes

Collaborators: Rolls-Royce

Duration: 03/10/2016-03/10/2019

 

Description: The lifetime of nuclear power plants is limited by the integrity of the reactor pressure vessel (RPV), that is often constructed out of low-alloy steel. Over time, the mechanical properties of the steel RPV degrade due to neutron irradiation and temperature effects which in particular lead to embrittlement and increase its ductile-to-brittle transition temperature. One of the major contributions to embrittlement of the RPV steel is from clustering of impurities such as silicon, manganese and nickel that could potentially cause a "late-blooming phase"; this would lead to an exponential increase in embrittlement of the steel after around 20-30 years of use.


Computational modelling using density functional theory will be performed to investigate how clustering of the impurities occurs, with particular emphasis on the effect of strain fields caused by dislocation loops and solute interactions in different formations. The overall aim of the project is to examine the extent of the damage caused by neutron irradiation and elevated temperature on steel with the goal of extending the lifetime of RPVs and hence nuclear power plants.