Why do radioactive waste remain hot for thousands of years?

Radioactive waste remains hot for thousands of years primarily due to the decay of radioactive isotopes it contains. As these isotopes decay, they release energy in the form of heat and radiation. Some isotopes have long half-lives, meaning they take thousands to millions of years to decay to safe levels, thus prolonging the heat generation. This process creates a significant challenge for safe storage and management of nuclear waste over extended time periods.