4.29. Reference [IAEA SSR-5 (2011)] requires that multiple safety functions be provided, such that safety does not depend unduly on any single safety function and to ensure that if one safety function does not perform as intended, there are further safety functions to compensate. For example, if waste packaging is assigned a containment function and degrades more quickly than anticipated, the surrounding backfill material can provide a further element of physical containment to retard the migration of radionuclides by adsorption; or the host geological environment can provide for dispersion of radionuclides. The safety strategy should identify the intended safety functions, the time frames over which they will be available and how degraded performance of one barrier will be compensated by another mechanism or by components of the disposal system. The safety strategy should also address how the adequacy of the various safety functions will be demonstrated, e.g. by assessment, analogy, testing, etc. The strategy should indicate how an adequate degree of defence in depth will be ensured by the various safety functions. The adequacy of the defence in depth may be expressed in quantitative and in qualitative terms.