Session: CS-15-03 Mechanical Properties of Nuclear Graphite and Their Implementation in Codes and Standards (Joint with MF-29) – 3

Paper Number: 106367

106367 - Structural Integrity Assessments of Nuclear Graphite: Managing Uncertainty and Variability 

The cores of EDF’s fleet of UK based Advanced Gas-cooled Reactors (AGR) contain a network of keyed graphite bricks to provide both the core structure and the required moderation. These reactors have now seen extensive service and two of the key aging mechanisms of these graphite components are fast neutron damage dose and radiolytic oxidation caused by the carbon dioxide coolant. This aging leads to significant changes in the bricks’ structural performance, including internal stress generation, deformation and material properties evolution.

Graphite is an inherently variable material, which in conjunction with the systematic variation in reactor power across the core extent result in substantial uncertainty. Hence, within the fleet, each core and indeed each graphite brick there is significant variability and uncertainty in their start of life (unirradiated) and irradiated states. Assuring their behaviour, and critically the bricks’ ability to perform their required functions, is therefore inherently probabilistic.

Probabilistic methods are used to fit the models that describe the evolution of the bricks’ aging and degradation, from measurements made from the AGRs and Material Test Reactor (MTR) experiments, capturing uncertainty and variability. These models are then used to make predictions of the core state to substantiate safety cases.

A damage mechanism of particular concern is known as keyway root cracking. This is where the larger “fuel bricks” that contain the channels for the fuel stringers and keyways which engage with other graphite components develop internal stresses during the later operational lifetime, which are concentrated at the keyway root and can drive cracking through to the brick bore over the full height of the brick. Whilst the safety justification is robust to limited cracking, in extreme cases, it is predicted to degrade the core’s seismic performance.

A probabilistic forecast of individual brick cracking assesses the model variability and uncertainty, combined with a simple feature strength assessment criterion to predict both onset and rate of keyway root cracking. This irradiated feature strength uses the evolution of flexural strength, which can be measured from small samples of graphite removed from the AGRs and MTRs, to scale the unirradiated feature strength measured on full-size mock ups. 

Whole-core assessments are also conducted to assess the reactor’s ability to fully insert the control rods to shutdown and hold down under normal operation, and fault and seismic conditions. Due to the size and complexity of the AGR core, and the number of bricks metrics have been developed to transpose brick distortion and property evolution at the detailed individual brick level to whole core assemblies. This includes metrics to account for various aspects of the key-keyway capacity, i.e. strength evolution, irradiated modulus redistribution and remanent capacity.

In both these examples, the brittle fracture of gilsocarbon graphite is assessed using a simple feature strength model rather than recourse to more detailed methods like a fracture mechanics approach. The anisotropic heterogeneity nature of graphite makes traditional code-based bounding, deterministic assessments impractical.

This paper provides a summary of the methods described above with a particular emphasis on managing uncertainty and variability, and the transferability of approaches developed and demonstrated successfully in the licensing of UK plant to future graphite moderated reactor designs.

Presenting Author: Graeme Horne Frazer-Nash Consultancy

Presenting Author Biography: Graeme is a Senior Consultant at Frazer-Nash Consultancy. He has studied and worked in the UK nuclear industry since 2009 including research on residual stress and fracture, and the analysis of nuclear graphite.

Authors:

Graeme Horne Frazer-Nash Consultancy
Amanda Young Frazer-Nash Consultancy
Daniel Kent Frazer-Nash Consultancy
Richard Gray Frazer-Nash Consultancy
Mark Joyce Frazer-Nash Consultancy