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Why is spent nuclear fuel stored underwater before permanent disposal?

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Labeled diagram showing the process of a nuclear power plant with a pressurized-water reactor. Water circulates through the reactor core, heats to produce steam in a steam generator, which drives a turbine connected to an electric generator. Steam is then cooled in a condenser using water from a cooling tower, which releases nonradioactive water vapor. Control rods, pumps, and transmission lines are also indicated. — nuclear power Schematic diagram of a nuclear power plant using a pressurized-water reactor.

Spent nuclear fuel is stored underwater in cooling pools immediately after being removed from a nuclear reactor primarily for two reasons: shielding and cooling.1

Shielding: Spent fuel remains extremely radioactive upon removal. The water acts as an effective shield, protecting personnel from radiation exposure.12
Cooling: The fuel continues to generate significant energy, known as decay heat. Water serves as a medium to remove this heat through natural circulation.2
Criticality Control: The pool water typically contains dissolved boric acid, which acts as a neutron absorber to ensure the fuel assemblies do not go critical while in storage.1

After the fuel has been stored in these pools for two or three years, its radioactivity and heat generation decrease significantly. At this stage, it may be transferred to air-cooled spent-fuel storage casks for long-term dry storage before eventual waste disposal.12