Insects like cockroaches can withstand doses of radiation three times greater than the level that would kill a human. In the 1960s when concerns about an all-out nuclear war ran high, biologists liked to say that insects and grass would survive, but we would not.

The subject is very big, but here are some links that might get you started:

http://www.lowdose.energy.gov/99meeting/abstracts/joiner.html

insect radiation resistance
http://www.madsci.org/posts/archives/2003-12/1072227809.Zo.r.html

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=384122

Also, even though you are interested in insect welfare vis-a-vis nuclear plants, you should know that the National Institutes of Health National Cancer Institute did a study of populations living around nuclear facilities and found that they did not have higher rates of cancer than populations who did not live near nuclear facilities.
http://www.cancer.gov/cancertopics/factsheet/Risk/nuclear-facilities