The giant space
A- (for entertainment value and Mara Corday), D- (for just about everything else)
The filmmakers' grasp of the physics of radiation and radioactivity is poor, but the movie is actually pretty good. The only real misstep is the mid-movie creature rampage, in which the monsters supposedly cause a number of disasters such as train crashes and airplane explosions. The scene is composed of stock footage mayhem and superimposed footage of the creatures from the film's climax (a fight between the creatures, soldiers, and the police). The stock and reused footage, as well as the fact that the creatures were explicitly said to be small in number and limited in how long they can function (their brains tend to fail after a few days) make me think that this was a last minute addition to make the creatures seem more threatening.