6.34. The complementary fulfillment of the different safety functions should be evaluated over different time periods. Each safety function should be independent of the others, to the extent possible, to ensure that they are complementary and that barriers are unlikely to fail through a single failure mode. In the safety case, the functions provided by each barrier should be explained and justified and the time periods over which they are expected to perform their various safety functions should be identified, together with alternative or additional safety functions that operate if a barrier does not fully perform (e.g. Fig. 5). Figure 5 illustrates the periods during which the various safety functions are fulfilled for one concept for the disposal of spent fuel and high level waste in ‘Boom clay’11. In this example, the engineered barriers are expected to provide complete containment of the waste throughout the period that the disposal facility experiences elevated temperatures, a period of some thousands of years. Thereafter, the Boom clay is expected to retard the migration of radionuclides. More details on this particular example are provided in Ref. [11].@@11) A moderately swelling clay found in the region of the town of Boom, Belgium