A fence or wall or a similar impediment which provides penetration delay and complements access control (IAEA definition (INFCIRC/225/Rev.4).

The US Nuclear Regulatory Commision (NRC) further defines a physical barrier as:

• Fences constructed of No. 11 American wire gauge, or heavier wire fabric, topped by three strands or more of barbed wire or similar material on brackets angled inward or outward between 30 and 45 from the vertical, with an overall height of not less than eight feet, including the barbed topping;
• Building walls, ceilings and floors constructed of stone, brick, cinder block, concrete, steel or comparable materials (openings in which are secured by grates, doors, or covers of construction and fastening of sufficient strength such that the integrity of the wall is not lessened by any opening), or walls of similar construction, not part of a building, provided with a barbed topping described in paragraph (1) of this definition of a height of not less than 8 feet; or
• Any other physical obstruction constructed in a manner and of materials suitable for the purpose for which the obstruction is intended.