CANDU reactors in Ontario have generated approximately 2,762 TWh since Douglas Point connected to the grid in 1967.
The Porter Commission (reported in 1980) demonstrates the decision for supply was between coal and nuclear through the period Ontario Hydro committed to CANDU (Darlington was approved by cabinet around 1975 - so we're 40+ years from the last decision to building new nuclear power plants).
So, for each of those TWh we could assume coal generation was displaced, or about 1 Mt of CO2e for each TWh.
Approximately 2,762 MT of CO2e emissions were avoided, using these assumptions.
This is equivalent to the past 15 years of emissions from all Ontario sectors.