From the reactor to the cooling pool
When the fuel can no longer effectively sustain a nuclear reaction, it is removed from the reactor. Here it is extremely hot, both thermally and radioactively. The fuel elements are moved from the reactor along water channels so that the spent fuel is always shielded for storage in a cooling pool and other spent fuel.
Solid pools
A cooling pool is about 12 meters deep. The walls are constructed of reinforced concrete, typically 1.2 and 2.4 meters thick. The pools contain a 6 to 13-mm stainless steel liner attached to the walls with rivets embedded in the concrete. [1]
Like the nuclear power plant, the cooling pool is designed to withstand extreme events such as earthquakes, floods, hurricanes, tornadoes, and more.
The pools’ Function
The pools cool and remove the residual heat from the spent fuel over time, and the fission products with the shortest half-lives decay.
The water shields from the radiation the used fuel produces so that you can walk in the hall in normal clothes without any radiation danger. The water is clear but tinted a Ty-D-Bol shade of blue because plant workers have added boron, a chemical that absorbs stray neutrons. (Boron is not toxic; it is the main ingredient in boric acid eyewash.).
You can take a swim in such a pool. But it’s a bad idea to dive down and touch the fuel. [2]
The water in the pool is cooled through circulated water and heat exchangers, which remove the heat from the water. It is continuously filtered out through various mechanisms, such as filters.
After approximately five years in the cooling pool, the materials with the shortest half-life have disappeared, and the heat production of the used fuel is far down. The fuel can then be moved to dry storage.
Source:
1. https://nap.nationalacademies.org/read/11263/chapter/23#43
2. https://what-if.xkcd.com/29/
