Cooling considerations

NUCLEAR UNITS, LIKE OTHER THERMAL power plants, circulate immense volumes of water through their systems to carry excess heat away and dissipate it in the environment. The volumes required can present problems when the limitations of the geography collide with the inherent variability of the natural environment.

Plants that use river water for cooling are most vulnerable here. A dry year and low river levels may reduce water flows to a level that it is not possible to abstract the full amount required for cooling. But it is more likely that there may be too little flow in the river to dissipate the volumes of warmed water being discharged from the power plant. River temperatures around cooling water outflows are strictly policed to maintain water quality and protect plants and animals in the river, and there have been numbers of occasions when plants have been forced to shut down for days so as not to breach water temperature limits.

This issue is one that also presents long term risk to thermal plant, if the local river environment is vulnerable to climate change affecting river flows.

All at sea

For power plants sited on the coast, different problems arise. Unlike an inland plant with its cooling towers, the large and complex infrastructure used to cool the plant may not be obvious to the casual observer. But for the designer and operator, undersea elements of the ‘balance of plant’ raises problems in ensuring that aquatic life is protected.

It is a problem that has to be solved in a very testing environment, where the subsea conditions may include strong tides, abrasive sands — and even a shifting sea floor. In an update last year1, the Electric Power Research Institute (EPRI) noted that even monitoring the temperature in the ‘heat plume’ around an outfall can present a risk to worker safety, as it is usually done by boat. EPRI is researching the use of air and aquatic drones to map heat plumes instead.

The sea is a huge heat sink and dissipation is less of an issue than in rivers, although it remains material. It raises several issues. One is an overabundance of life, because the slightly warmed water immediately around the outfall has proved to be very attractive to some animals. The zebra mussel is one example, a species that has become invasive around power plant outfalls where huge numbers can accumulate and can eventually cause blockages.

For most aquatic life elevated temperatures present a threat. But EPRI also notes that there is a balance to be struck in protecting fish from a plant’s thermal discharge. Operators may install large underwater structures to help fish and other organisms avoid heated waters after entering an intake. But a bigger return structure also creates more potential for injury or mortality — and it incurs greater construction, operation and maintenance costs.

Finally, of course, considerable attention has to be paid to measures that will ensure sea life is not swept up and into the cooling circuit along with the water. Again EPRI research shows that there is no simple solution. 

EPRI said it had examined the effect on fish of screens used at water inlets to prevent debris from blocking and reducing cooling water flow. They are also designed to stop debris from entering the cooling system and damaging equipment. In the 1970s, according to EPRI, operators began making the screens more fish-friendly by equipping them with fish-lifting buckets, gentle low-pressure sprays and troughs to return organisms to the water source.

In some cases, EPRI reports, screens were modified with fine mesh (less than 2mm) screens so fish and shellfish eggs and larvae could be collected and returned. But that was not the outcome when it tested screens at Con Edison’s East River Generating Station in Manhattan, New York, that were modified to filter fish, fish eggs, and larvae from the intake. In fact, the team found that eggs and larvae channelled through the cooling system and back into the river had higher survival rates than with those removed by the screens.

Taking decisions

The trade-off between maintaining the aquatic environment, ensuring worker safety and managing costs is well illustrated by decisions being taken for EDF Energy’s Hinkley Point C plant in the UK.

This EPR, with two 1600MW units, is sited in western England on the south side of the Severn estuary (known as the Bristol Channel at this point), which has the UK’s biggest tidal range.

When the plant was granted a Development Consent Order (DCO) it had three fish protection measures: low velocity side entry (LVSE) intake heads, a fish recovery and return system (FRR) and an acoustic fish deterrent system. Now EDF Energy is seeking to change that part of its consent, and it is carrying out a consultation2 with a view to cancelling plans for the acoustic system.

EDF Energy says acoustic systems, which use sound to divert fish from the intake region, can deter 95% of some fish species from entering the tunnels. That sounds like an effective measure. But EDF says that is only the case where it is in a suitable location. In the Bristol Channel it presents problems of design, construction and maintenance — and also raises risks to workers. EDF admits that removing the measure will cause higher fish mortality, but it maintains that the difference is very small in local populations and the overall benefit of the acoustic system is limited. It is consulting on the arguments — which have already encountered opposition from some local groups — until June and plans to apply for a change in its consent in the second half of the year.

Each nuclear unit will abstract water at a rate of 132m3 per second, through an intake pipe 6m in diameter and extending 3.3km into the Bristol Channel. The 7m diameter outfall tunnels extend nearly 2km into the channel.

The LVSE intake head at each intake is a rectangular structure 35.5m long, 10m wide and 2.8m deep. The intake heads are designed to standards set by the Environment Agency, the UK’s environmental watchdog, to minimise sealife entry. For example, they are in deep water and near (but not on) the sea bed because many fish are in the middle of the water column and crustaceans are on the sea floor. The water intake is side entry because fish are able to escape horizontal currents more easily than vertical ones and they are perpendicular to the tidal current with vertical bars 30cm apart.

Onshore a 29m-deep ‘forebay’ structure allows hydraulic energy from the water to dissipate before it is pumped into the cooling system, and here marine life is collected on filters and returned to the sea, via dedicated channels (the recovery and return system).

The acoustic deterrent would use sound projectors to deter fish from approaching the intake heads. But EDF now argues that the acoustic deterrent only works on certain species of fish. And although the system was regarded as emerging best practice at the time of the original application in 2011, detailed design for the Hinkley Point system had still to take place.

It now appears that the system would require 288 projectors to be installed directly on the intake heads (which are safety-critical components that consequently cannot be impeded, either by the projectors or by work on them). The location is an area of high wave heights and frequent storms, where the water has high sediment levels and low visibility. The high tidal range and fast tides mean practicable underwater maintenance periods will be limited to 30-60 minutes between tides.

Is it worth the cost and risk? EDF has taken advice from research and consultancy organisation the Centre for Environment, Fisheries and Aquaculture Science (Cefas) and believes that although the acoustic system does affect fish kills, the effect is negligible when it is compared to the catch taken by commercial fishing in the area.

EDF says in its submission to National Infrastructure Planning that the total amount of fish estimated to be killed by the operation of HPC without the AFD system has been predicted by Cefas to be around 56t in a year. “An impact of this magnitude can be compared to that of one small fishing trawler. This compares with approximately 650,000t commercially fished in the UK in the same year assessed,” it said. That has not convinced some local sea anglers who took their concerns to the local newspaper as EDF began its consultation.

The company will consult locally until June. Assuming the application to vary the Development Consent Order (DCO) is submitted after that it will be of great interest to UK developers for all types of development, and by planning lawyers — as well as sea fishers.

That is not about the fish, but about the UK’s framework for granting consent for large infrastructure projects. That framework was put in place a decade ago, in an attempt to make the UK’s notoriously slow and uncertain planning regime more streamlined and less risky. It has met with some success and this will be the first project where the developer returns to National Infrastructure Planning to make the case for a ‘material change’ in the DCO.

Both process and outcome will be closely watched.  

References

1. Shiel, Tom. “A Deep Dive on Fish Protection.” EPRI Journal, May 2018, eprijournal.com/a-deep-dive-on-fish-protection/

2. “Consultation on Proposed Planning Change.” EDF Energy, www.edfenergy.com/energy/nuclear-new-build- projects/hinkley-point-c/about/acoustic-fish-deterrent