Scientific Bases for Cladding Credit as a Barrier to Radionuclide Release at the Proposed Yucca Mountain Repository
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T. M. AHN*, G. A. CRAGNOLINO**, K. S. CHAN***, AND N. SRIDHAR** *U.S. Nuclear Regulatory Commission, Washington, DC 20555, USA "**Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, 6220 Culebra Rd., San Antonio, TX 78238, USA ***Southwest Research Institute, 6220 Culebra Rd., San Antonio, TX 78238, USA 'Disclaimer.The contents of this paper are the sole opinions of the authors, and do not represent the position of the U.S. Nuclear Regulatory Commission. ABSTRACT
The performance of Zircaloy nuclear fuel cladding in the environment of the proposed Yucca Mountain (YM) high-level waste (HLW) repository is evaluated. Because the uniform aqueous corrosion/oxidation rate is extremely slow, this evaluation focuses on mechanical failure, localized corrosion, stress corrosion cracking (SCC), hydrogen embrittlement, and initial failure. Mechanical failure is expected to result from (1) disruptive events such as rockfalls from seismicity, faulting, and igneous activities, (2) creep, and (3) splitting by oxidation of the spent fuel (SF) matrix. Effects of chloride ions and radiolysis are evaluated in localized corrosion and SCC. Embrittlement can be caused by delayed-hydride cracking (DHC) and hydride reorientation. Among these cladding failure modes, rockfalls, other disruptive events and initial defects can be important to performance prior to the container failure by corrosion. Confirmatory tests are required to evaluate the susceptibility to splitting by secondary mineral formation, localized corrosion, SCC, and hydride reorientation. Reliable temperature calculations with backfilling are also required. After breach, cladding may still assure slow release of radionuclides through perforations because localized failures may limit the exposure of the SF matrix and may provide high mass-transfer resistance. Failures prior to the repository emplacement arising from reactor operation, pool storage, dry storage, and transportation are also considered. INTRODUCTION
The U.S. Nuclear Regulatory Commission (NRC) is currently updating its Issue Resolution Status Reports (IRSRs) and upgrading its Total system Performance Assessment (TPA) code. These two capabilities are being developed for the NRC's independent evaluation of the U.S. Department of Energy (DOE)'s planned license application for a repository at Yucca Mountain, Nevada. As part of these activities, the NRC staff is resolving subissues associated with the source term in the Container Life and Source Term (CLST) IRSR [1,2] and improving the TPA source-term model [3], In this paper, the effectiveness of cladding protection against SF dissolution and subsequent radionuclide release is evaluated since the DOE has induced the role of cladding in the Total System Performance Assessment - ViabilityAssessment (TSPAVA) 14,51. Cladding protection can be effective if: (i) cladding remains essentially intact: or (ii) radionuclide release through limited perforations in failed cladding is slow. Literature data partly support these two hypotheses [6]. Zircaloy
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