Requirement 7 of SSR-5 [4]: Multiple safety functions
“The host environment shall be selected, the engineered barriers of the disposal facility shall be designed and the facility shall be operated 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 together with various operational controls. The capability of the individual barriers and controls together with that of the overall disposal system to perform as assumed in the safety case shall be demonstrated. The overall performance of the disposal system shall not be unduly dependent on a single safety function.”
4.35.
The establishment of multiple safety functions, which should be fulfilled by various barriers and system features, provides for defence in depth. Defence in depth implies that safety is not unduly dependent: on a single element of the disposal system, such as the waste package; or on a control measure, such as verification of the inventory of waste packages; or on the fulfilment of a single safety function, such as containment of radionuclides or retardation of their migration: or on an administrative process such as procedures for controlling access to the site or for maintenance of the facility. Adequate defence in depth should be ensured by demonstrating that long term safety is provided by means of multiple safety functions.
4.36.
In accordance with the application of a graded approach, the ability of a disposal system to provide containment and isolation of the waste is required to be commensurate with the hazard potential of the waste [3]. As a consequence, the type and number of barriers and features necessary to meet the requirements for containment and isolation will depend on the type of waste to be disposed of and on the hazard posed by the waste, which will change with time through radioactive decay. The required assessment of defence in depth should comprise an evaluation of the performance of the disposal system and its components and features in terms of their ability to perform the safety functions over time, both in situations that are expected to occur (such as the normal degradation of barriers) and in situations where disturbance of the system might occur.
4.37.
In the safety case, all elements of the system design and all features of the disposal site that are important for demonstrating defence in depth of the planned disposal facility should be identified and assessed in a structured manner. Assessments should be performed to verify that a defect in one safety related characteristic, or a decrease of performance with time of one or more components, is compensated for by the performance of the safety functions or by a decrease of the hazard through radioactive decay. A systematic safety assessment of the various scenarios of evolution of the system and its components should be conducted to demonstrate that adequate defence in depth will be maintained. This assessment should also address how the increase in uncertainty with respect to the performance of components and the system for longer time frames is considered in the demonstration of safety and is taken into account in the design of the disposal facility.
4.38.
The capability of the system as a whole to provide adequate containment and isolation is of prime importance, and the facility should be designed in a manner that is sufficiently flexible to take due account of the features of the site that can be considered less favourable. All relevant site features that could determine or influence the design of the facility should be systematically identified.
4.39.
A safety function (e.g. the confinement of radionuclides to prevent or control their release) may be performed by means of a physical entity or a chemical property or process that contributes to containment of the radionuclides and/or isolation of the waste within the disposal system, such as impermeability or low permeability of a barrier to water, limited corrosion rate, and low solubility and high sorption capacity of radionuclides, which result in low leach rates.
4.40.
A barrier means a physical entity, such as the waste, waste packaging, the backfill or liner and cap of the facility, the characteristics of which restrict (or, for a limited time, prevent) the migration of radionuclides or render direct access to the waste more difficult. A particular barrier may perform a number of safety functions, while a single safety function may be provided by a number of barriers. The use of a number of barriers and safety functions enhances both safety and confidence in safety and will ensure that the overall performance of the disposal system is not unduly dependent on a single barrier or safety function. Hence, even if a barrier or safety function does not perform fully as expected (e.g. owing to an unexpected process or an unlikely event), it should be demonstrated that the disposal facility is still safe.
4.41.
The performance of a near surface disposal system is dependent on different barriers having different safety functions, the importance of which may vary over different time periods. The safety case should explain and justify the confidence attached to the safety functions provided by each barrier and should indicate the time periods over which they are expected to perform. The safety case should set out the scientific and technical arguments that support these claims and should also identify the complementary safety functions that will be effective if a barrier does not fully perform.
4.42.
The disposal system should use a combination of natural and engineered characteristics to support efficient containment and isolation by maintaining integrity of the waste packages, by, for example, limiting the solubility of radionuclides and by minimizing the ingress of meteoric water (i.e. derived from precipitation). The importance of the contribution of the natural barriers and the engineered barriers to the containment and isolation of the waste will depend to a large extent on the type of near surface disposal facility (i.e. surface or subsurface) and on the characteristics of the site where the facility is to be located. In the long term, progressive degradation of the engineered barrier system cannot be ruled out, and consequently, radionuclides may be released to the geosphere or biosphere, depending on the type of near surface disposal facility. While radioactive decay is an important factor for the short lived radionuclides in limiting the magnitude of a potential release from a near surface disposal facility over time, other considerations are also important and should be considered. Factors limiting the ingress of water (which in turn may contribute to prolonging the integrity of waste packages) that should be considered include the use of durable barriers with low water permeability and design of the system so that it maintains low hydraulic gradients for the required period of time. The potential for a release of radionuclides to the biosphere should be further reduced by maintaining low flow rates as well as by means of the retardation and precipitation capability of the engineered barriers and the host environment. Materials used for the backfill or elsewhere in the engineered system should have properties that do not contribute to degrading unduly the safety functions of the other barriers.