Supplementary written evidence submitted by Office for Nuclear Regulation


Thank you for the opportunity to provide evidence to the Committee at the hearing on Monday 28 February 2022 regarding critical national infrastructure and climate change adaption.


During the session our inspector from the Office for Nuclear Regulation (ONR), Andria Gilmour was asked:

Do you envisage a realistic possibility that there could be climate conditions in which it was not safe for nuclear energy to make a contribution and it might have to go out?


Her response referenced the risk of high temperatures and the impact on cooling, noting circumstances like this have arisen in Europe within the last 3-5 years. Specifically, she said:

Absolutely. The most obvious example, and something that Europe has come close to, is high temperature, when the cooling function could be compromised or the margins reduced. Certainly in parts of Europe in the last few years there have been times when operators have either reduced power output or even switched off reactors to ensure safety. In the UK context we could foresee that happening within the sorts of periods that you describe as medium term, five years.


We wanted to provide some additional context to her answer, given the complexity of the topic in discussion. We hope this will be useful to inform your evidence gathering and would be happy for this to be published alongside our written evidence.


Our inspector’s reference to European examples comes from a report published by the Nuclear Energy Agency Organisation for Economic Co-operation and Development (OECD) called Climate Change: Assessment of the Vulnerability of Nuclear Power Plants and Approaches for their Adaptation (NEA No. 7207, 2021), which may be a useful resource to you. This report explains that climate change could affect power generation systems through its impact on cooling water availability and quality, which given the greater cooling needs in a nuclear reactor needs careful consideration and planning for within systems and operations.


In recent years, due to heatwaves in 2018 and 2019 in parts of Europe, there were higher river and canal temperatures and restricted water supply, which led to output reduction or outage at some nuclear power plants. Output is reduced and/or outage occurs to avoid overheating the inland water source (such as a river or lake) when the water is returned out of the reactor. This is because warm water can damage biodiversity, leading to mass fish die-offs and other potential environmental impacts. 


The position in the UK though is different to Europe and some other countries. Our nuclear reactors take their cooling from the sea, rather than inland sources, and so we have a much more stable source of cooling water in the UK. This means they are far less vulnerable to temperature variations and/or restricted water supply or elevated cooling water temperatures, since the sea level will not reduce (and tidal changes are accounted for), and the sea temperature responds much more slowly to extended periods of elevated temperature.


As the UK’s independent regulator for nuclear safety, ONR requires all of its nuclear site licensees to demonstrate resilience to the impacts of climate change in a safety case, and provide evidence (through their safety cases, which we assess) that they can continue to operate safely in the most extreme (one in ten thousand year) hazards, such as extremes of temperature and sea-level changes.


ONR monitors the latest climate change information and predictions, requiring all of the nuclear site licensees to update their safety cases on a regular basis, to ensure they remain current against the latest evidence.


We also require licensees to modify their plants and facilities, where necessary to ensure they can continue to withstand extreme hazards. For example, as part of the learning post Fukushima, flood defences improvements were made at Dungeness B in Kent. For all new build projects within the UK (such as Hinkley Point C), we consider the impacts of climate change for the life of the facility that ensure significant margins against external hazards, demonstrating the safety of the reactors in all conditions.


The current fleet of nuclear reactors in the UK have all provided robust justifications that underpin their continued safe operation in extreme conditions, and we remain satisfied that there will be no need for them to shutdown prematurely due to the impact of predicted climate changes on external hazards.


This also includes the environmental impact of higher sea temperatures, which can lead to an increase in the prevalence of some marine vegetation and animals that have the potential to restrict the cooling water flow at the intake screens on reactors. 

For example, it is possible for seawater cooling system intake screens and/or filters to become clogged with seaweed. These screens and filters are routinely maintained during planned outages, but at certain times of year, under particular weather conditions such as prolonged warm weather, seaweed volumes can increase and enter the power station's cooling water intake system.


In such incidences, it may be necessary for a plant to reduce power or temporarily shutdown, as a safety precaution in accordance with their safety case, to allow for such vegetation to be cleared, as well as minimise any risk to fish and other ecology.

There are also multi-layered safety systems that monitor such conditions and inbuilt safety mechanisms would take a unit offline automatically if no action was taken.


Through her answer our inspector intended to articulate that should we see hotter and longer summers in the UK within the next 3-5 years due to extreme weather conditions, such as those seen in parts of Europe, that could lead to some output reductions and/or outage, as part of standard safety procedures. Such precautionary arrangements are already considered within safety cases for UK reactors, and we continue to keep those arrangements under review in line with our regulatory processes set out above.


In summary, at this time, as the nuclear safety regulator we remain satisfied that there will be no need for the UK’s current reactor fleet to shutdown prematurely because of the impact from predicted climate changes on external hazards.


Katie Day

Director of Policy and Communications


14 March 2022