The reactor fuel rods are occasionally replaced by removing them from the top of the containment vessel.
Because they are hot both radioactively and thermally, this is done via cranes and under water. For this reason the spent fuel storage pools are above the reactor in typical installations. They are shielded by borated water several times their height, and stored in rigid arrays in which their geometry is controlled to avoid criticality. In the Fukushima reactor incident this became problematic because water was lost from one or more spent fuel pools and the earthquake could have altered the geometry. The fact that the fuel rods' cladding is a zirconium alloy was also problematic since this element can react with water at extreme temperatures to produce hydrogen and oxygen, and can ignite in air. Normally the fuel rods are kept sufficiently cool in the reactor and spent fuel ponds that this is not a concern, and the cladding remains intact for the life of the rod.
Famous quotes containing the word systems:
“Before anything else, we need a new age of Enlightenment. Our present political systems must relinquish their claims on truth, justice and freedom and have to replace them with the search for truth, justice, freedom and reason.”
—Friedrich Dürrenmatt (19211990)