Why can the fireball from a nuclear explosion reach extremely high temperatures?

The fireball from a nuclear explosion reaches extremely high temperatures—often millions of degrees—because of the sudden, massive release of thermal energy generated during the nuclear fission process.1

When a fission chain reaction occurs, the nuclei of heavy elements like uranium-235 or plutonium-239 split into lighter fragments. This process releases an enormous amount of energy relative to the mass of the material involved, as described by the Einstein relation, $E=m{c}^2$. In an uncontrolled atomic bomb detonation, this energy is released almost instantaneously, heating the bomb's materials to temperatures comparable to those found at the center of the Sun. This intense thermal energy creates the characteristic fireball, which subsequently drives the blast wave and thermal radiation effects associated with the explosion.123