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 disposal—much longer periods than those encountered in normal engineering practice—equilibrium 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:

5.1.1 Public Law 97–425, the Nuclear Waste Policy Act of 1982, provides for the deep geologic disposal of high-level radioactive waste through a system of multiple barriers. The radiation release limits are to be set by the U.S. Environmental Protection Agency (EPA) (40 CFR 191). Licensing of such disposal will be done by the U.S. Nuclear Regulatory Commission (NRC).

5.1.2 The analyses described in this Standard Guide can be used to support the demonstration of compliance of the EBS components and design to the applicable requirements of 10 CFR 60 (pertaining to any HLW repository in the U.S.) and 10 CFR 63 (pertaining to the planned HLW repository at Yucca Mountain, NV).

5.1.2.1  10 CFR 60.135 and 60.113 require that the waste form be a material that is solid, non-particulate, non-pyrophoric, and non-chemically reactive, and that the waste package contain no liquid, particulates, chemically reactive or combustible materials and that the materials/components of the EBS be designed to provide – assuming anticipated processes and events - substantially complete containment of the HLW for the NRC-designated regulatory period.

5.1.2.2 10 CFR 63.113 provides that the EBS be designedsuch that, working in combination with the natural barriers, the performance assessment of the EBS demonstrates conformance to the annual reasonably expected individual dose protection standard of 10 CFR 63.311 and the reasonably maximally exposed individual standard of 10 CFR 63.312 , and shall not exceed EPA dose limits for protection of groundwater of 10 CFR 63.331 during the NRC-designated regulatory compliance period after permanent closure.

5.1.3 The regulations of the U.S. Environmental Protection Agency (EPA) in Part 191 of Title 40 of the CFR provide that cumulative releases of radionuclides from the disposal system—this refers to the total system performance not just the EBS performance—for the regulatory compliance period after disposal shall have a likelihood of less than one chance in ten of exceeding the values stated for each radionuclide in the regulation. These environmental standards relate to the overall system performance of a geologic repository and they are referred to in NRC requirements of 10 CFR 60.112 and 63.111. Analyses of overall repository system performance may include anticipated and unanticipated events.

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 long–term 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).

1.1.1 Steps involved in making such predictions include problem definition, testing, modeling, and model confirmation.

1.1.2 The predictions are based on models derived from theoretical considerations, expert judgment, interpretation of data obtained from tests, and appropriate analogs. 1.1.3 For the purpose of this practice, tests

1.1.3 For the purpose of this practice, tests are categorized according to the information they provide and how it is used for model development and use. These tests may include but are not limited to the following:

1.1.3.1 Attribute tests to measure intrinsic materials properties,

1.1.3.2 Characterization tests to measure the effects of material and environmental variables on behavior,

1.1.3.3 Accelerated tests to accelerate alteration and determine important mechanisms and processes that can affect the performance of waste form and EBS materials,

1.1.3.4 Service condition tests to confirm the appropriateness of the model and variables for anticipated disposal conditions,

1.1.3.5 Confirmation tests to verify the predictive capacity of the model, and

1.1.3.6 Tests or analyses performed with analog materials to identify important mechanisms, verify the appropriateness of an accelerated test method, and to confirm long-term model predictions.

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.