Uranium is a heavy metal. The fire will heat it just, because it is have low temperature in order to melt uranium (1132°C). It is not explosive unless very specific conditions are met. The critical mass needed for explosion. Unfortunately, he does not have the critical mass. As you know, not everyone is able to maintain the uranium nuclear chain reaction. Uranium-238, which accounts for 99.28% of the natural mixture of isotopes, it is not able to. Due to the fact, and is converted into plutonium 238, a natural mixture of uranium isotopes tend enrich the isotope uranium-235, the ability to share with thermal neutrons.