Graphite distortion at UK AGRs investigated

25 February 2016

A graphite irradiation research project being carried out by an alliance of EDF Energy, NRG of the Netherlands, and Frazer Nash Consultancy and Atkins of the UK, will investigate the ageing management of advanced gas-cooled reactor (AGR) units in the UK, NRG said on 18 February. Phase three of 'Project Blackstone' will run for five years and provide data that can be used in safety cases to support AGR life extensions.

The work will focus on the graphite from the Heysham B and Torness reactors in the UK and is a follow-up to the first two phases of Project Blackstone in which graphite from other AGRs was characterised to support lifetime extension. Phase 1 aimed to show that graphite samples could be irradiated in the High Flux Materials Test Reactor, operated by NRG at Petten, and then subjected to non-destructive and destructive post-irradiation examination. A database of the results was compiled. Phase 2 developed and expanded that database by obtaining end of lifetime data for graphite cores.

In phase 3, graphite material extracted from AGRs will be irradiated in the Petten reactor so that the in-core neutron and oxidation damage mechanisms that take place in an AGR are simulated and accelerated. Following the irradiation, graphite specimens will be characterised in NRG's hot cell laboratories to provide material behaviour data, which is used to assess the future structural integrity of AGR cores.

AGR reactors feature a graphite moderator and are cooled using carbon dioxide. The graphite blocks cannot be replaced or repaired during the operating life of the reactors. However, radiation damage changes the shape and size of the crystallites that comprise graphite, a process known as dimensional change, which then degrades the mechanical properties of the graphite. For continued operation, it is necessary to demonstrate that the graphite can still perform its intended role irrespective of the degradation.

On 16 February, EDF Energy announced that the scheduled closure dates for its Heysham 1 and Hartlepool plants had been extended by five years to 2024, while those of Heysham 2 and Torness had been extended by seven years to 2030. The announcement followed life extensions at EDF Energy's other AGR NPPs.

Russia has been working for several years to restore the graphite stacks at its RBMK reactors to make possible life extension of the units. In 2011-2012 cracking and distortion of the graphite stacks was detected at units 1 and 2 at the Leningrad and Kursk NPPs. Technology was developed to restore the stacks and in 2013 it was used at unit 1 of the Leningrad plant. Kursk 2 underwent graphite restoration in 2013/2014 and Kursk 1 is currently closed for similar work.



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