A physical obstruction that prevents or inhibits the movement of people, radionuclides or some other phenomenon (e.g. fire), or provides shielding against radiation. See: also Cladding (Material), Containment, Defence In Depth. Intrusion Barrier: Components of a repository designed to prevent inadvertent access to the waste by humans, animals or plants. Multiple Barriers: Two or more natural or engineered barriers used to isolate radioactive waste in, and prevent migration of radionuclides from, a repository. The term chemical barrier is sometimes used in the context of waste disposal, to describe the chemical effect of a material that enhances the extent to which radionuclides react chemically with the material or with the host rock, thus inhibiting the migration of the radionuclides. As defined above, this is not strictly a barrier (unless the material also constitutes a physical barrier), but the effect may be equivalent to that of a barrier, and it may therefore be convenient to regard it as such