My grammar diary #4 how to avoid using a repeating noun?

As a non native speaker, I feel incovenience when using repeating a noun in a sentence, for example:

As a non-native speaker, I feel uncertain when I use "repeating nouns " in a sentence . Below are the examples :

1. My car is at the corner: the red car. I think it's better to rewrite , so I change it to " My car is at the corner-- the red one".

The em dash can be used in place of a colon when you want to emphasize the conclusion of your sentence.
 
2. The constructions of new nuclear power plants and the disposal of toxic waste from existing ones are two concerns raised by environmentalists.


"One" should be replaced by "ones" because "ones" is a plural noun whereas "one" is singular . 

Examples:

a. new nuclear power plants and
b. the disposal of toxic waste .

If the sentence not using pronoun "ones", then the sentence become:


2. The construction of new nuclear power plants and the disposal of toxic waste from existing (power plants and toxic waste) are two concerns raised by environmentalists.
  >Wrong assumption .Existing ones means that they are more than one plant now . Existing one means only one plant now --that's about it.
>>>without bracket

Your understanding of when to use one and ones is correct, however, I think you have a mistake in the full sentence:

Another way to think about it is in the first half you are talking about "new nuclear power plants." The existing _______ refers to those power plants. (New/existing)

However, "power plants" is still plural, so you are still correct in using "ones."