What’s the problem with Mactaquac Generating Station?
March 10 2016, 14:29 PM
We have a big decision to make about the future of the Mactaquac Generating Station. The station is nearing the end of its life, and NB Power is considering what to do next. Whatever is decided will have a big impact on the environment and the people who live and work near the station. That’s why NB Power is working with experts and the public to find out what those impacts might be, and how to lessen them. NB Power will recommend a path forward in 2016. This blog post is the first in a series that will introduce you to each aspect of the process of finding a recommended solution.
Since the 1980s, the concrete structures at Mactaquac have been expanding due to a chemical reaction known as alkali-aggregate reaction. But what is alkali-aggregate reaction (AAR) and what does it do?
Alkali-aggregate reaction is a chemical reaction. As you may know, concrete is a mix of cement, rock, sand and water. When alkalis in the cement react with silica in the rock, it produces silica gel. Silica gel absorbs water and swells. This swelling gel causes concrete to expand.
Hundreds of hydro stations, bridges and other structures around the world have been affected by AAR. It also affects the concrete portions of the Mactaquac Generating Station (highlighted in yellow).
The earthen dam that retains Mactaquac headpond is a rock-filled structure sealed with clay and does not have AAR problems.
Where did the concrete come from?
When the dam was built, the rock was crushed and made into concrete at the site. It’s similar to rock that was used at that time to make concrete in the Fredericton area. But once AAR was discovered at Mactaquac, Fredericton area concrete plants had to find alternative sources of concrete aggregate.
How much has the concrete at Mactaquac expanded?
The concrete expansion varies widely. The most extreme numbers are a rate of 0.12 mm for every meter of concrete. While this doesn’t seem like much, for a structure that is 42 meters tall, it adds up to 5mm/year. The movement of the concrete slowly shifts embedded equipment such as turbines, generators, gates and pipes. This must be addressed, because it can affect the operation of this equipment.
What’s done to keep the station operating?
NB Power employees at Mactaquac operate and maintain the station 24/7 year round. NB Power has different solutions to manage the effect of AAR, for example, slot cuts were made into the concrete with diamond wire.
The earthen dam that retains the Mactaquac headpond is a rock-filled structure sealed with clay and does not have AAR problems. This massive rock structure relies on its weight to resist the force of the river while protecting the clay core that prevents water coming through.
How do you know the dam will reach the end of its service life by 2030?
Since 1994, there have been regular engineering studies that looked at when the concrete main spillway, diversion sluiceway and the powerhouse will need to be replaced. These studies have shown that, without significant repairing or rebuilding, the existing structures will need to be replaced by 2030.
Can you extend the life expectancy of Mactaquac to avoid a massive project or until a better solution comes up?
We are examining if we can extend the life of the station beyond 2030. We work with independent experts to see if we can make more repairs to the concrete or partially replace some of the concrete parts. This work is being done in parallel with the studies being conducted to determine the best option for replacement, should it be required. All this work must be completed so that a decision can be made at the end of 2016 and work completed by 2030.
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