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Why must used nuclear reactor fuel be handled carefully?

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nuclear fission Italian-born physicist Enrico Fermi explaining a problem in physics, c. 1950. He was awarded the 1938 Nobel Prize for Physics.

Used nuclear reactor fuel must be handled with extreme care because it is highly radioactive and generates significant heat, making it dangerous to both the public and the environment.12

When fuel is removed from a reactor, it remains a source of intense radioactivity and heat for several hundred years. To manage these risks, the fuel must be continuously shielded and cooled. This is typically achieved by placing the spent fuel in water-filled storage pools, which provide both cooling and radiation shielding. The water often contains boric acid to absorb neutrons and prevent the fuel from going critical.13

After several years, as radioactivity and heat generation decrease, the fuel may be transferred to air-cooled, corrosion-resistant storage casks. Because this material is classified as high-level waste, it requires secure, long-term management—ideally in deep, stable geologic repositories—to ensure that any potential release of radioactive material into the environment remains minimal and slow.1423