Quintessa has recently signed a framework agreement to provide continuing support to EDF across a range of areas associated with continued safe operation of the UK’s fleet of Advanced Gas Cooled Reactors (AGRs) until December 2029.
An important part of AGR safety cases is demonstrating the continued physical integrity of the cores, which comprise a large number of graphite bricks. As the reactor cores age, the properties of the graphite bricks (which provide both the structure in which the fuel resides and acts as a moderator) evolve with time. Safety cases are supported by predictions of what will be seen during graphite inspections, comparison of observations with predictions and forecasts of future conditions for comparison against safety case limits.
Quintessa has supported EDF (and formerly British Energy) since 2003 in making safety cases for the AGRs, with statistical modelling, stress analysis, optimisation, software development and support.
Quintessa uses advanced statistical methods and numerical modelling to predict the evolution of the graphite cores (informed by the history of brick conditions observed to date) around inspections and over reactor lifetimes. Statistical modelling of graphite weight loss and other graphite properties supports comparison of observations and forecasts against safety case limits, as well as supporting stress evolution modelling.
Quintessa has developed and deploys a bespoke code, CrackSim, that employs Monte-Carlo simulations to predict the current state of keyway root cracking and associated damage at each reactor ahead of inspections and to forecast the future evolution for comparison against safety case metrics. This takes account of current knowledge of core state and rates of damage progression to provide forecasts at the high level of confidence required to support the safety cases.
We are also using the COMSOL Multiphysics® and 3DEC™ codes to assess the evolution of the graphite bricks; providing independent assessments for comparison against EDF’s alternative methods and build confidence in predictions.
Quintessa has developed a custom software tool, CHANSELA, which is used to visualise information on the reactor cores and to support decisions on which fuel channels should be inspected to support continued safe operation. CHANSELA is able to suggest the combination of channels that will provide the most useful information subject to user-specified constraints, and to compare this with user-defined selections. The software employs some novel concepts, including a genetic algorithm for optimising channel selections, and allows every stage of the process to be documented automatically, providing a clear and transparent audit trail for minimal user effort.
Other custom tools developed for EDF include ASPECT, used to visualise and interpret information collected from eddy current scanning of the graphite core, and LoTAS, a web-based application used for managing and analysing data recorded as fuel is moved in the reactor.