6.29. The application of the concept of defence in depth to disposal facilities is explained in Ref. [2], which states in Requirement 7:@@ “The host environment shall be selected, the engineered barriers of the disposal facility shall be designed…to ensure that safety is provided by means of multiple safety functions. Containment and isolation of the waste shall be provided by means of a number of physical barriers of the disposal system. The performance of these physical barriers shall be achieved by means of diverse physical and chemical processes.... The capability of the individual barriers...shall be demonstrated. The overall performance of the disposal system shall not be unduly dependent on a single safety function.”@@Reference [2] (in para. 1.16) also states: “In accordance with the graded approach…the ability of the chosen disposal system to provide containment [and isolation] of the waste…will be commensurate with the hazard potential of the waste.” As a consequence, the number and extent of barriers necessary to meet the requirements will depend on the type of waste to be disposed of. The required assessment of defence in depth should comprise an evaluation of the levels of defence provided by barriers of the disposal facility (see para. 3.9 and Section 4) [3].