When is the Best Time to Take a Nuclear Power Plant Offline?

By Erin Polka
Communications Officer
Canadian Nuclear Association

What happens to greenhouse gas emissions when a nuclear power plant goes offline? Let’s look at the Bruce Power complex in Kincardine, Ontario. On April 15, Bruce Power shut down the four reactors in its B building to enable a vacuum building outage (VBO). The vacuum building, which is an essential safety feature, needs regular maintenance that should last about a month.

Shutting down Bruce B means some 3,268 MW of generating capacity needs to be replaced with some combination of hydro, gas and wind. Which combination is better for the environment?

Hydro capacity is highest in the spring, as winter snows melt and rivers run high. So it stands to reason that hydro power will make up for some of the shortage. (And, yes, the VBO was timed to match the availability of hydro.)

What about wind? Not as much help. Wind provides only four percent of Ontario’s electricity on average. Whether it could provide more would depend on whether the wind blows longer and stronger. Maybe it will, and maybe it won’t – hardly the reliability needed to replace the steady nuclear workhorse.

And then there’s gas. It can be fired up quickly and easily, it runs reliably, and it doesn’t cost all that much more than nuclear power – about twice as much.

In the best-case scenario, hydro would replace the power from the four Bruce B reactors. It’s the best case because hydro, like nuclear, generates no greenhouse gases. But there’s a problem. Hydro in Ontario is quite limited as a result of the province’s geography, and the province lacks sufficient transmission lines to import replacement power from Quebec. Also, even if the lines did exist, Quebec doesn’t have a spare hydro dam to match the output from the four reactors.

The next-best scenario would use all the available hydro power, keeping cost and emissions down, and use gas for the rest. Very likely, hydro could replace half the nuclear energy from Bruce B, and natural gas would replace the other half.

Is that a problem? After all, Ontario businesses and residents will still get steady, reliable electricity – just as they did with the Bruce reactors. But here’s the thing – natural gas emits greenhouse gases, especially carbon dioxide, which is primarily responsible for climate change.

Replacing half the nuclear output with gas means the province’s gas plants will emit an additional 295,095 tonnes of carbon dioxide. For perspective, that’s the weight equivalent of about 300,000 adult giraffes.

What else would produce 295,095 tonnes of CO₂?

• Driving a car 35,563 times around the Earth’s equator
• Taking 82,394 round-trip flights from Toronto to Sydney

And that’s not all. Unlike nuclear and hydro, gas also emits nitrogen oxides (NO_x), sulphur oxides (SO_x), and particulate matter (PM) during operation. These “other” greenhouse gases cause lung and heart disease, and make these conditions worse. They can also harm plants and animals on land and in the sea. And they can even cause building materials to deteriorate and weaken.

Of course, if hydro weren’t able to stand in for the offline nuclear plants, then Ontario would need to use gas alone. And that would mean the weight of another 300,000 giraffes in greenhouse gas emissions, or another 35,563 trips around the world (“Are we there yet?”), or another 82,394 round trips to Sydney.

So, timing is everything. Scheduling the VBO in spring, when hydro reaches its peak performance, was a wise decision. Just how much hydro will be available, and how much gas is actually used, remains to be seen.

You can track the results on the CNA website, if you like. Check our emissions tracking.