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This is fantastic news. Currently Ontario gets well over 1/3 of its power from Nuclear power. However these plants are getting old and near time where they must be decommissioned or substantially refurbished to run for longer than original designed.
We also have a lot of hydro which provides roughly 1/4 of our power but still rely on lots of gas (a bit over 1/4). Nuclear definitely isn't the full solution to energy in Ontario but if we lose our current plants you can bet that they are going to be mostly replaced by gas and other environmentally damaging solutions. Having a strong base load provided by nuclear is incredibly valuable and the time is running out to replace these plants.
https://www.ieso.ca/power-data
Ontario is also doing lots of experimental work with small modular reactors but we would be very wise not to put all of our eggs in one basket. We have really stagnated too long with nuclear technology and moving towards a slow and incremental progression is a huge deal. We have gone so long without a nuclear project that they have become incredibly expensive due to lack of experience both technically and politically. Getting these projects moving forward makes the next one so much easier.
I know that some people are concerned with nuclear power and possible dangers. But the fact is that if you take the worst estimates of worldwide nuclear casualties it is incredibly safe. If you measure casualties per Wh of power nuclear is the second safest (only behind solar). If you take more reasonable estimates of casualties it is very close to solar. Solar and wind are great technologies and will definitely help make up our grid, but they don't replace nuclear (and gas) as a base load. We need both. I think nuclear is like a lightning strike, it is very scary, but incredibly improbable to actually hit you. Also even though scary when the problem is known it can be safely designed around, like that Airplanes are frequently hit by lightning but it is a solved problem.