program, accelerate its nuclear program, without us having any insight into what they are doing. >> reporter: iranian enrichment facilities begin making weapons grade fuel and then an iranian nuclear test. this doctor runs the agency that monitors global nuclear testing. in an exclusive cnn interview he says tehran could not test in secret. >> any country that will try today to hide a nuclear test explosion, we have more than 90% of chance in detecting it. >> reporter: the agency's worldwide sensor network detects seismic activity from underground explosions and the release of radioactive material in the air. there are problems. it took 55 days for sensors to pick up radioactive gases from a north korean nuclear test