Most of Ontario's power comes from nuclear sources currently and a good number of the plants are nearing the end of their safe life spans and will require further investment or to be closed down which will increase the cost of electricity to homes and business moving forward.

Also, although, the power generation of nuclear produce electricity is "green" not everyone would agree that the disposal of the spent fuel rods is the same category.  See the town of Port Hope and the clean up required to make it relatively safe for the citizens as an example.