6.35. A detailed analysis of how the various safety functions are fulfilled by the elements of the disposal facility may be performed by relating the safety functions of the disposal system to measurable or calculable quantities. For example, if a particular barrier performs a safety function relating to limiting water flow, then the hydraulic conductivity of the barrier may be a suitable quantity to use in order to evaluate the extent to which the safety function is fulfilled. In such a case, the hydraulic conductivity of the barrier is said to be the ‘safety function indicator’ for that safety function. A safety function indicator is, thus, a measurable or calculable quantity through which a safety function can be quantitatively evaluated. In order to determine whether a safety function will continue to be fulfilled, quantitative criteria should be determined against which the safety function indicator can be evaluated over the time period covered by safety assessment. The definition of an a priori quantitative value for the safety function indicator may help to initiate the optimization process, but this should not be considered a target in itself (at least at the first stages of development of the safety case), since the satisfactory fulfillment of a safety function may rely on a combination of several processes and components whose characteristics may be fixed or can be changed by subsequent design or operational changes.