LWRSP FY09 testing and analysis of reactor metal degradation
- ORNL
- University of California, Santa Barbara
- University of Michigan
Current regulations require RPV steels to maintain conservative margins of fracture toughness so that postulated flaws do not threaten the integrity of the RPV during either normal operation and maintenance cycles or under accident transients, like pressurized thermal shock. Neutron irradiation degrades fracture toughness, in some cases severely. Thermal aging, while not generally considered a significant issue for a 40-y operating life, must be an additional consideration for operation to 60 or 80 years. Regulations, codified in the ASME Boiler and Pressure Vessel Code, Regulatory Guide 1.99 Rev 2, etc., recognize that embrittlement has a potential for reducing toughness below acceptable levels. The last few decades have seen remarkable progress in developing a mechanistic understanding of irradiation embrittlement. This understanding has been exploited in formulating robust, physically-based and statistically-calibrated models of CVN-indexed transition-temperature shifts (TTS). These semi-empirical models account for key embrittlement variables and variable interactions, including the effects of copper (Cu), nickel (Ni), phosphorous (P), fluence ({phi}t), flux ({phi}), and irradiation temperature (T{sub i}). However, these models and our present understanding of radiation damage are not fully quantitative, and do not treat all potentially significant variables and issues. Over the past three decades, developments in fracture mechanics have led to a number of consensus standards and codes for determining the fracture toughness parameters needed for development of databases that are useful for statistical analysis and establishment of uncertainties. The CVN toughness, however, is a qualitative measure, which must be correlated with the fracture toughness and crack-arrest toughness properties, K{sub Ic} and K{sub Ia}, necessary for structural integrity evaluations. Where practicable, direct measurements of the fracture toughness properties are desirable to reduce the uncertainties associated with correlations. Moreover, fracture-toughness data have been obtained in sufficient quantity to permit probabilistic application. The progress notwithstanding, however, there are still significant technical issues that need to be addressed to reduce the uncertainties in regulatory application. The major issues regarding irradiation effects are summarized in [1,2]. Of the many significant issues discussed, those deemed to have the most impact on the current regulatory process are: (1) high fluence, long irradiation times, and flux effects, (2) material variability and surrogate materials, (3) high-nickel materials, (4) the fracture toughness master curve, (5) the bias in reference toughness derived from precracked Charpy specimens, (6) attenuation, (7) modeling and microstructural analysis, (8) thermal annealing and reirradiation, and (9) thermal aging. Material variability and surrogate materials are an overarching issue. A more complete understanding of the other issues is needed in order to reduce the uncertainties associated with material variability. Moreover, the combination of irradiation experiments with modeling and microstructural studies provides an essential element in aging evaluations of RPVs. It is clear that embrittlement of RPV steels is a critical issue that may limit LWR plant life extension. The primary objective of the LWRSP RPV task is to develop robust predictions of transition temperature shifts (TTS) at high fluence ({phi}t) to at least 1020 n/cm2 (>1 MeV) pertinent to plant operation for 80 full power years. New and existing databases will be combined to support developing physically based models of TTS for high fluence-low flux ({phi} < 10 11/n/cm2-s) conditions, beyond the existing surveillance database. A summary of progress on the RPV task of the LWRSP Materials Pathway is presented here.
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
- Sponsoring Organization:
- USDOE Office of Nuclear Energy (NE)
- DOE Contract Number:
- DE-AC05-00OR22725
- OSTI ID:
- 1014220
- Report Number(s):
- ORNL/TM-2009/243; AF3690100; NEAF265; TRN: US201111%%409
- Country of Publication:
- United States
- Language:
- English
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