The reactor's pressure vessel is typically housed inside a concrete liner that acts as a radiation shield. That liner is housed within a much larger steel containment vessel. The steel containment vessel is intended to prevent leakage of any radioactive gases or fluids from the plant. Finally, the containment vessel is protected by an outer concrete building that is strong enough to survive such things as crashing jet airliners. The absence of secondary containment structures in Russian nuclear power plants allowed radioactive material to escape in an accident at Chernobyl.
Steam rises from the cooling tower at the Harris plant.
Workers in the control room at the nuclear power plant can keep an eye on the nuclear reactor and take action if something goes wrong. Uranium-235 is not the only possible fuel for a power plant. Another fissionable material is plutonium-239. If there are no other U-235 atoms in the area, then those free neutrons fly off into space as neutron rays. The mass will exist at a stable temperature.