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Materials Performance Centre

Fuels and graphite

Our nuclear graphite research is based in the Department of Mechanical, Aerospace and Civil Engineering (MACE), and is an integral part of our interdisciplinary research portfolio.

Graphite research

The nuclear fuels activity within the MPC has evolved into the National Fuels Centre of Excellence led by Professor Tim Abram. The Nuclear Graphite and Research Group led by Professor Abbie Jones, is an integral part of the interdisciplinary MPC, with key expertise in advanced gas-cooled reactor operations, graphite decommissioning and new graphite for high-temperature reactor and molten salt reactor GenIV systems.

The group was formed with the aim of building-up a strong research activity at The University of Manchester on nuclear graphite technology, so as to develop expertise that can be used to provide independent advice and research to both national and international nuclear organisations and companies. This has enabled the Office for Nuclear Regulation, in particular, to mount robust and constructive challenges of the Licensee’s safety cases.

Our researchers have pioneered novel approaches to nuclear graphite technology to establish microstructural change/irradiation property relationships. These techniques have been applied to both virgin and irradiated graphite and include x-ray microtomography, digital image correlation, Raman spectroscopy, and high-resolution transmission electron microscopy (TEM) made possible by the use of focused ion beam (FIB) techniques to prepare specimens. In addition, numerical and analytical techniques have been developed to conduct multi-scale modelling of irradiated graphite behaviour, to assess the structural integrity of irradiated graphite components, and to evaluate the interaction of the many thousands of graphite components within a graphite core. NGRG has been at the forefront of developing methods for investigating active materials at STFC Facilities (Diamond Light Source, ISIS Neutron Facility).

We specialise in irradiated graphite characterisation and new treatments in order to reduce the isotopic volume, including developing new technologies for the treatment, handling and disposal of irradiated graphite, with emphasis on radioisotopic content, location, mechanism and structural integrity of the final waste form, our research contributions identified areas include: graphite, radionuclides behaviour, and gas research.

The consistently high quality of our activity has been recognised in the awards of the highly prestigious Queen’s Anniversary Prizes for Higher and Further Education in 2011 for Nuclear Engineering and 2013 for X-ray Imaging, and gaining the highest 4* rating of the Impact of the work in the 2015 Research Excellence Framework assessment of university research.

Research highlights

Assessment of the structural integrity of graphite components

The structural integrity of the graphite core is dependent on the structural integrity of its components. Although the core is tolerant to damage, this tolerability is not unlimited. Eventually, the accumulation of damage will limit the reactor life. The graphite components, such as the fuel bricks, are subject to fast neutron damage, thermal gradients and radiolytic oxidation. These are not uniform across the component leading to the generation of internal stresses. The stresses are tensile at the bore and compressive at the keyway of fuel brick in early life. These stresses are reversed as the reactor core ages leading to tensile stresses at the keyways. The geometry of the keyway corners elevates these stresses and may lead to cracking of the fuel bricks.

In fact, such cracking has been observed in some of the leading reactors. Therefore, it is important to understand how damage will progress across the core and the rate of damage as the core ages further. Finite element analysis software, ABAQUS, along with a user subroutine to define the graphite constitutive behaviour, is used to predict the stresses and the deformation as a result of irradiation damage, thermal changes, and radiolytic oxidation. Graphite material property models are written and coded by NGRG in a user subroutine, ManUMAT, along with the constitutive equations applicable to nuclear graphite. The ManUMAT is developed and maintained by the NGRG at The University of Manchester under current and previous funding by the ONR.

Experimental studies of fracture in graphite

The fracture of fuel bricks from keyway corners has been observed in the lead reactors. The NGRG has developed methodologies for both mimicking the effects of irradiation, so that cracks are driven by internal stresses, and observing directly crack nucleation and propagation in active specimens in 3D.

Irradiated graphite property measurement and model development

As the graphite core of an AGR provides channels for control rods and fuel cooling, the structural integrity of the many thousands of graphite components and their assembly is of paramount concern in AGR safety case assessments. Ageing due to fast neutron damage and radiolytic oxidation challenges the structural integrity of these components. ONR have funded the independent development of a comprehensive set of statistical models based on measurements made on Material Test Reactor (MTR) specimens and trepanned AGR graphite specimens. These empirical models are referred to as the NGRG/M and CS models, and during their development they have proven to provide good estimates of the available data.

The Group has performed a world-first experiment to measure the CTE of irradiated graphite as a function of load by synchrotron X-ray tomographic imaging under heating and pressure. This key parameter for structural analysis helps the understanding of the differences between the stresses developed in the reactor operating at power and at shutdown.