What is the primary requirement for a material to be classified as Special Form?

A material is classified as "Special Form" primarily because it must be a sealed capsule or a single solid piece. This classification is crucial in the context of transporting materials that contain radioactive substances because it ensures a high level of safety and containment. The design of Special Form materials allows for a well-defined manner of protecting the material from accidental release, reducing risks of exposure or environmental contamination during handling, storage, or transport.

This classification serves to establish regulations under which these materials can be safely transported as they are deemed to provide a low probability of dispersal of radioactive content in the event of an accident. The design characteristics required for Special Form reflect the need for robust containment, significantly improving safety measures compared to other forms that may not be as secure or stable.