This involves storing  spent fuel rods under at least 20 feet of water. The water provides adequate  shieldingfrom the radiation for anyone near the  pool. It also provides cooling for the fuel. The decay of  fission products produces heat in the fuel (called  decay heat) even after the reactor has ceased operation. This heat necessitates the storage of  spent fuel in a pool to allow for heat removal by water. The rods are moved into the water pools from the reactor along the bottom of water canals, so that the  spent fuel is always shielded to protect workers. The quantity of fuel that can be stored is constrained by the size of the pool.

(Source: US NRC)