Table of Contents
Advanced Gas-Cooled Reactors
Advanced Gas-Cooled Reactors, or AGRs, are a fleet of nuclear power reactors that are unique to the UK.
The UK once boasted 14 AGRs that provided a peak of roughly 16% of the UK energy demand in 2020. Now, in the UK, the reactors that once made up the AGR fleet are coming to the end of their operation and all are currently planned to close by the year 2030.
The map below shows the locations where AGRs are still operating, as of March 2024.
Torness
Hartlepool
Heysham 1 & 2
AGRs were the successor to the Magnox generation of reactors, with changes to the fuel and design leading to superior efficiency.
The changes to fuel were facilitated by the UK discovering how to enrich uranium efficiently; therefore, AGRs were designed to use enriched uranium oxide fuel, as is also used in PWRs. Compared to PWRs, AGRs have a greater thermal efficiency of 41 % (compared with 34 %).
AGRs lack thermal feedback, a form of passive safety present in PWRs where an increase in coolant water temperature decreases the criticality of the reactor – failing to safe. However, AGRs have a much larger reactor pressure vessel and thus a lower power density. Despite the absence of thermal feedback, upon a criticality failure it takes several days before the reactor will heat beyond cooling capability.
How do AGRs work?
- Simplified Primary System for an AGR. Image: World Nuclear Association.
As with all nuclear reactors, a lot of heat is generated by the nuclear fuel inside the reactor core. The heat is generated by the nuclear fission of uranium-235. This heat is taken away by the coolant in the primary circuit. For AGRs, the primary coolant is carbon dioxide gas. The gas can reach very high temperatures typically over 350oC and in modern AGRs temperatures of 650oC can be reached. This is much higher than temperatures seen in PWRs and Magnox reactors. The higher temperature results in AGRs being much more efficient than Magnox reactors.
To make use of all this heat, a secondary circuit is used to generate the electricity. In the secondary circuit water is pumped into one side of the steam generator and the hot gas enters the other. Heat is exchanged between the hot gas and the cold water. In this process the gas is cooled back down so it can remove more heat from the nuclear reactor core. As well as this, the water fed to the steam generator is heated up sufficiently high that it evaporates and forms high energy steam.
The steam then uses its energy to turn a turbine connected to an electrical generator. The steam then enters the condenser, condensing back to liquid water.
Fuel
Fuel
Moderator
Moderator
Coolant
Coolant
The UK's AGR Fleet
Decommissioning the UK's AGR Fleet
Whilst some of the UK’s AGR fleet are still producing power as of November 2023, decommissioning is the next stage of the nuclear life cycle and is imminent for these reactors. This is due to AGRs being life-limited by cracking of graphite within their cores.
The decommissioning strategy of the UK AGR fleet follows the following structure:
- Defuelling: This process will take 3-10 years whilst the spent fuel from the reactor is removed and stored in cooling ponds. This stage handles 99% of the radioactivity on the entire site.
- Deconstruction of the Site: This process will take 10-15 years where all non-essential buildings (excluding the reactor) and structures are decontaminated and deconstructed.
- Safe Store Period: The remains of the site such as the reactor will be left covered for 70-85 years while the activity level naturally drops to much safer levels.
- Final Destruction: This process will take roughly 10-15 years were all remaining equipment, such as the reactor, is dismantled and processed.
- Final Remediation: After all equipment and building have been deconstructed and disposed, final clean up takes place restoring the site back to the green field it once was.
Louie McConville
Energy Encyclopaedia – Gas-cooled Reactor (GCR) and Advanced Gas-cooled Reactor (AGR) (https://www.energyencyclopedia.com/en/nuclear-energy/the-nuclear-reactors/gas-cooled-reactor-gcr-and-advanced-gas-cooled-reactor-agr)
National Audit Office – Decommissioning of the AGR Nuclear Power Stations (https://www.nao.org.uk/wp-content/uploads/2022/01/The-decommissioning-of-the-AGR-nuclear-power-stations.pdf)
ONR – Operational Reactors (https://www.onr.org.uk/civil-nuclear-reactors/index.htm)
UK Government – Nuclear Electricity in the UK (https://assets.publishing.service.gov.uk/media/5c9a5d37ed915d07b20fa2ba/Nuclear_electricity_in_the_UK.pdf)
World Nuclear Association – Nuclear Power in the United Kingdom (https://world-nuclear.org/information-library/country-profiles/countries-t-z/united-kingdom.aspx)
World Nuclear Association – Nuclear Power in the United Kingdom (https://world-nuclear.org/information-library/country-profiles/countries-t-z/united-kingdom.aspx)
World Nuclear Association – AGR (https://world-nuclear.org/information-library/country-profiles/countries-t-z/united-kingdom.aspx)
Wikimedia Commons – Dungeness B (https://commons.wikimedia.org/wiki/File:Dungenesspowerstation.JPG)
Wikimedia Commons – Hunterston B (https://commons.wikimedia.org/wiki/File:Hunterston_B_nuclear_power_station_(with_sheep).jpg)
Wikimedia Commons – Hinkley Point B (https://commons.wikimedia.org/wiki/File:Hinkley_Point_B_Power_Station_-_geograph.org.uk_-_1951621.jpg)
Wikimedia Commons – Hartlepool (https://commons.wikimedia.org/wiki/File:Hartlepool_nuclear_power_station_2022.JPG)
David Merrett – Heysham 1 & 2 (https://www.flickr.com/photos/davehamster/8903456468…)
Wikimedia Commons – Torness (https://commons.wikimedia.org/wiki/File:Torness_Nuclear_Power_Station_-_April_2016.jpg)
