Simplify the following text for a Grade 9 native English-speaking person.
Original: Additionally , nuclear testing has often been used as an indicator of scientific and military strength , and many tests have been overtly political in their intention ; most nuclear weapons states publicly declared their nuclear status by means of a nuclear test .
Simplified: A nuclear explosion is energy being released from a very fast nuclear reaction . It can be caused by nuclear fission , nuclear fusion , or both .