Twenty-nine of the 56 nuclear reactors are currently out of service, and the extreme heat that the country is experiencing is only making matters worse. This summer’s heatwave has led to an unusual increase in the temperature of some water environments. However, a nuclear power plant depends on rivers to cool its reactors.
But once used water is discharged at a warmer temperature, from a few tenths of a degree to several degrees. In order not to overheat the environment and protect aquatic biodiversity, the temperature of the water discharged from power plants is controlled by the Nuclear Safety Agency (ASN) for each nuclear power plant.
But this year, p.bear continue to work at the minimum power level, despite the heat, ASN and the Ministry of Energy have given the green light to some power plants to increase the temperature limit for their water discharges, and this is until July 24.
Possible disturbance to benthic fauna
The impact of this retreat on the aquatic environment is questionable. Dumping too hot water can be a concern for species living in these environments, as Jeremy Laubry, aquatic biodiversity specialist at INRAE, explains to Sciences et Avenir:We assume that the fish may avoid the hottest places. Vice versa, sure there is short-term direct physiological effects on less mobile fauna such as crustaceans, molluscs and polychaete-type invertebrates. These direct impacts could lead to increased local mortality.”
And the consequences could be indirect: when the water is very hot, the microbial loop – incorporation of organic carbon into bacterial biomass – accelerates and consumes a lot of oxygen. “This process causes an oxygen deficiency in the water, and we can reach an oxygen threshold of 5 or even 2 mg/L of water if it goes on too long. This is a biological threshold below which it is very difficult for organisms to breathe.”
Faced with these heatwave episodes, little research has been done so far. After the 2003 heat wave on the Rhone, only a large-scale study over 10 years was carried out, which showed that the average temperature of the water at the plant increased by 1.6°C compared to the natural environment. “This warming has had little effect on bacteria, biofilms. It affected their denitrifying ability”, that is, the ability of bacteria to remove nitrates from water, explains Science et Avenir Christophe Quintin, Chief Inspector of the Office of Nuclear Safety. On the other hand, according to scientists, the fish were not harmed due to the avoidance effect.
Implementation of exceptional follow-up actions
This year, in order to study the consequences of this retreat as closely as possible, ASN organized a more extensive “heat-wave monitoring”. “We’re going to study physico-chemical parameters, do microbiological analyses, plankton analyzes and fish monitoring,” explains Mr. Quintin.
There is long-term scientific monitoring of the impact of power plants on nuclear installations. EDF funds design offices to take samples throughout the year and analyze changes over time. But these observations do not depend on exceptional heat periods.
Thus, this heat wave monitoring is being launched for the first time. Two measurement points must be made: one point a week after the start of the implementation of the retreat, and the second later. “We pay special attention to cyanobacteria. These microorganisms develop when the environment deteriorates and heats up. We carry out analyzes for cyanobacteria every two days from the date of entry into force of the derogation. Usually we don’t have to find them. .”, clarifies the chief inspector of the ASN.
Christoph Kintin also ensures that fish deaths are observed through visual observation: “It’s worth what it’s worth, but it’s not the most important indicator because there are a lot of dead fish that don’t swim. … But if we find dead fish, then action will be taken.”
However, for an INRAE biologist, this kind of monitoring seems insufficient: “As far as fish monitoring is concerned, if we start observing dead fish, something very, very serious is going to happen. Before resurfacing, the fish could be affected in different ways.”
Jeremy Laubry believes that in order to observe the immediate effect, it is necessary to capture individuals and measure physiological stress: hormonal and biochemical. “But to go to the site, conduct observations, take samples, biochemical analyzes and analyze all this in the laboratory. It’s expensive and takes time.”
However, with climate change, heatwaves will become more frequent and intense. Thus, careful monitoring during heat waves is essential to study disturbances in the aquatic environment, especially in the long term. “We must be ready for this.” recognizes Jeremy Lobry.