Therefore , to make a weapon , either uranium must be enriched , or plutonium must be produced .
Plutonium is almost nonexistent in nature , and natural uranium is about 99.3 % uranium 238 ( U-238 ) and 0.7 % U-235 .
To make a weapon , either uranium must be enriched , or plutonium must be produced .
Plutonium is almost nonexistent in nature , and natural uranium is about 99.3 % uranium 238 ( U-238 ) and 0.7 % U-235 .