What does the term 'spent nuclear fuel' refer to?

Spent nuclear fuel refers to nuclear fuel that has undergone fission in a reactor and is no longer efficient for sustaining a nuclear reaction. Over time, as the fuel is subjected to neutron bombardment during its use in the reactor, it becomes less effective due to the accumulation of fission products and transuranic elements that are formed as a result of the fission process.

When the fuel is first placed in a reactor, it has a high degree of fissile material, which allows for efficient energy production. After a period of use, typically several years, the fuel is removed from the reactor, at which point it is termed "spent." At this stage, it cannot effectively continue the chain reaction needed for further energy production and must be managed and stored carefully due to its radioactivity.

The other options pertain to different states of nuclear fuel but do not accurately capture the meaning of "spent nuclear fuel." For example, fuel that is still efficient for reactions has not reached the spent stage, and fuel that is being recycled or inspected is either in a different process or state, and does not reflect the characteristics of spent nuclear fuel.