Reply #21: fissile U-235

[divvy]

in a nutshell, when fissile U-235 breaks into two pieces and releases energy, the two pieces are unstable - they have too many neutrons for their new size. So the neutrons decay, emitting beta and gamma rays.

The fission products are normally isolated from the environment by three barriers: the zirconium cladding around the fuel elements (which are uranium oxide pellets), then the reactor vessel itself, then the steel and reinforced concrete containment structure.

If water cooling inside the reactor is lost, the reactor will be shut down instantly by inserting the control rods, which shut down the chain reaction. The fission of fuel ceases instantly. The problem, however, is this: the existing fission products contained in the fuel continue to release a substantial amount of heat, and with no heat transfer medium to carry that heat away, the zirconium cladding will melt, releasing the uranium dioxide fuel pellets and the fission products (which are both solid and gaseous) into the reactor coolant system. If the reactor coolant system has been breached, then these products will be released into the reactor building itself.