Can I use my clear retainers as my whitening tray with the UV light or would it harm my retainers?

Great question. 
The clear retainers that your orthodontist gave you should be fine with whitening gel and the UV light as long as you make sure that you thoroughly wash them afterwards and leave no residual whitening gel in the trays. If they are not properly cleaned, the gel can harden and build up causing damage to the trays as well as causing them to not fit properly.
Best of luck!
Dr. Neil Hadaegh
#beverlyhillsdentist
#cosmeticdentist 

Hi,
  I would say the ortho retainers are custom made meant to hold your teeth in their proper alignment and usually fit very snug.. Placing the gel in them may cause the gel to squish out onto your gum tissue which may cause irritation, and discomfort. Bleach trays are made of a softer material enabling the bleach to stay in the tray and if loaded properly will stay only on the teeth. 

They can be used however, over time it could effect the structural strength of the retainer with using the light.