Fracture of Graphite Fuel Bricks

The graphite core in nuclear Advanced Gas cooled Reactors (AGRs) provides channel for fuel cooling and shutdown/
control rod insertion. Damage tolerance assessments need to determine that adequate margins remain for the
integrity of the fuel and the safety systems proper behaviour. For UK existing plants, graphite bricks' cracking is a key
question regarding lifetime management decisions. For the next generation plants, it has an influence on expected lifetimes
and on investment plans.
The objective of this proposal is to gain a good understanding and evaluation of fracture in graphite components by
complementary numerical and experimental approaches. An advanced modelling tool allowing to simulate automatically
crack propagation in graphite bricks will be developed and transferred to industry end-users. An innovative experimental
method will be elaborated to validate these models. Uncertainties on the whole core mechanical behaviour will be
assessed.

The structural integrity of nuclear graphite is fundamental to the continued safe operation of the UK's nuclear fleet which
provides about 20% of the UK's electrical energy.The safety assessments performed by the Licencees (Magnox, EdF
Nuclear Generation) are used to justify continued operation to the Office for Nuclear Regulation. These assessments have
several independent legs, some of which require a fundamental understanding of crack growth and stress analysis of the
core. This is particularly true of graphite where the properties degrade through lifetime operation. The project may
ultimately lead to improved structural integrity assessment methods that will more accurately define the margins of
continued safe operation, enabling reliable life extension of the current nuclear fleet. The impact of the work, therefore, is in
two parts: socioeconomic, and scientific. In socioeconomic terms, by supporting the lifetime extension of existing AGRs, the
project will benefit the whole of UK industry by maintaining a secure supply of low-carbon electric power until new plant become available. This helps industry plan for growth, stimulating the economy, establishes a cap on energy price rises for
the population, and aids the Government's Climate Change obligations. Scientifically, the development of robust modelling
approaches to complex, multi-length scale problems is key in a number of areas of materials science, physics, chemistry,
and engineering. The added value here for this project is not only that the codes developed will be open access, allowing
their adoption, adaption, and use by many researchers internationally, but that they will be maintained and supported into
the future by EdF R&D rather than residing within a single HEI. EdF's subcontractors are among the largest contract
research organisations. The codes will be adopted by them and used in a number of related structural integrity issues (for
example, degradation of concrete structures) enabling the impact of the research to be felt outside of the reactor core. In
addition, the younger researchers employed on the project will acquire valuable skills in a research area where
experienced scientists are approaching retirement.