Which one of the following is a characteristic of high level nuclear wastes?

High-level nuclear wastes are primarily characterized by their origin from spent nuclear reactor fuel. When nuclear fuel is used in a reactor, it undergoes significant changes and becomes highly radioactive due to the presence of various isotopes produced during the fission process. This spent fuel contains a range of radioactive materials, including cesium, strontium, and plutonium, which retain high levels of radioactivity and decay heat.

Due to these characteristics, high-level nuclear waste requires stringent handling and management protocols to ensure safety. The other aspects related to shielding, temperature, and handling methods underscore the inherent dangers associated with high-level nuclear waste, making it crucial to understand its origins and the implications for storage and disposal.