5.1 This practice supports the development of materials behavior models that can be used to predict alterations in materials over the very long time periods pertinent to the operation of a high-level nuclear waste repository; periods of time much longer than can be tested directly. Under the very extended service periods relevant to geological disposalmuch longer periods than those encountered in normal engineering practiceequilibrium or steady state conditions may be achieved and models for reaction kinetics may be replaced by models, if justified, describing equilibrium extents of alteration. This practice is intended for use for waste form materials and materials proposed for use in an EBS that is designed to contain radionuclides released from high-level nuclear waste forms as they degrade over tens of thousands of years and more. Various U.S. Government regulations pertinent to repository disposal in the United States are as follows:

5.2 The current governing regulations are 10 CFR 60 as applicable to generic requirements for a repository in the US and 10 CFR 63 as applicable to the proposed repository site at Yucca Mountain. Other site-specific regulations may be required in the development of any alternative or additional US geologic repository site (per 10 CFR 60 ).

5.3 This practice recognizes that technical information and test data regarding the actual behavior of waste forms and materials that are used in the EBS and exposed to repository conditions for such long periods of time will not be sufficient to develop fully validated models in the classical sense. Rather, the (necessarily) short-term test data acquisition, and use of the data in formulating reliable long-term predictive models, is to be used to support the design, performance assessment, and even the selection of waste package/EBS materials (e.g., low confidence in a degradation model may justify the selection of alternative EBS barrier materials).

5.4 This practice aids in defining acceptable methods for making useful predictions of longterm behavior of materials from such sources as test data, scientific theory, and analogs.

5.5 The EBS environment of interest is that defined by the natural conditions (for example, minerals, moisture, biota, and mechanical stresses) as modified by effects of time, repository construction and operations, and the consequences of radionuclide decay, for example radiation radiation damage, heating., and radiolytic effects. Environmental conditions associated with both anticipated and unanticipated scenarios should be considered.

1.1 This practice describes test methods and data analyses used to develop models for the prediction of the long-term behavior of materials, such as engineered barrier system (EBS) materials and waste forms, used in the geologic disposal of spent nuclear fuel (SNF) and other high-level nuclear waste in a geologic repository. The alteration behavior of waste form and EBS materials is important because it affects the retention of radionuclides by the disposal system. The waste form and EBS materials provide a barrier to release either directly (as in the case of waste forms in which the radionuclides are initially immobilized), or indirectly (as in the case of containment materials that restrict the ingress of groundwater or the egress of radionuclides that are released as the waste forms and EBS materials degrade).

1.2 The purpose of this practice is to provide methods for developing models that can be used for the prediction of materials behavior over the long periods of time pertinent to the service life of a geologic repository as part of the basis for performance assessment of the repository.

1.3 This practice also addresses uncertainties in materials behavior models and their impact on the confidence in the performance assessment.

1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory requirements prior to use.