T. M. AHN, US Nuclear Regulatory Commission, USA

DOI: 10.1533/9780857097446.1.273

Abstract: Examples of analytical approaches and methodologies for modelling the behaviour of waste forms and waste package metals in long-term management of spent nuclear fuel (SNF) and high level waste (HLW) are presented. Two cases, long-term geological disposal and interim extended dry storage, are considered. The integrity of the waste package (or canister) that serves as a barrier is dependent upon the performance of construction metals. Corrosion degradation modes of the construction metals are evaluated. The waste behaviour during SNF degradation is also evaluated. In each mode of corrosion or degradation, the associated risk insights are discussed in the system performance of disposal or storage.

Key words: assessment and modelling, nuclear waste form, nuclear waste package, storage, disposal.

7.1 Introduction

This chapter presents example analytic approaches and methodologies for modelling the behaviour of waste forms and different metals used in pack­aging spent nuclear fuel (SNF) and high level waste (HLW). The long-term behaviour of waste forms and different metals used for packaging SNF and HLW are important attributes in assessing safety and security associated with nuclear waste management. This behaviour is a core component in determining radionuclide source-term and/or criticality control, used in assessing radionuclide release to the human environment. The assessments and modelling of the long-term behaviour of the waste form and different metals are further complicated by a variety of environmental conditions, including natural and human-induced external hazards. This is especially true when the purported waste management time is very long, e. g., several thousand years or beyond.

Disclaimer: The NRC staff views expressed herein are preliminary and do not constitute a final judgment or determination of the matters addressed or of the acceptability of any licens­ing action that may be under consideration at the NRC.

The approaches and methodologies presented in this chapter also cover model uncertainties that affect assessment of public health and safety. The content of this work is considered by the US Nuclear Regulatory Commis­sion (NRC) which assesses cases for management of SNF and HLW in the US. The NRC prepares risk and performance insights. Information that the NRC obtained from the past activities in the management of SNF and HLW in the US, along with relevant information from different international programs, is included.

In the US, the long-term management of SNF and HLW is considered for geological disposal for thousands of years and beyond, and interim extended dry storage of SNF for up to 300 years. In both management cases, materials performance issues related to waste form and corrosion of differ­ent construction metals are considered, i. e., container metal in waste pack­ages used for geological disposal, and canister construction metal for extended dry storage. The following four topics related to the waste-form dissolution and corrosion are addressed in this chapter: long-term integrity of passive film, slow general corrosion, and localized corrosion of different metals; stress corrosion cracking (SCC) of carbon steel and stainless steel; SNF degradation; and cladding performance. The discussion of each topic addresses how it applies to the two management cases, as appropriate. Broader performance issues of waste form and different metals under the two management cases are also discussed. Finally, risk insights are addressed with respect to performance (or risk) assessment for the disposal or storage system. Both management cases incorporate laboratory data, analytical models, archaeological (for disposal) and/or industrial (for extended dry storage and disposal). Some similar classes of metals are used in both cases for different purposes. For example, stainless steels are primarily used for extended storage, but may also be considered for disposal. On the other hand, carbon steel is mainly applied in disposal.