A new engineering method for prediction of the fracture toughness temperature dependence for RPV steels

In order to predict the temperature dependence of fracture toughness for RPV steels with various degrees of embrittlement, up to extremely high levels, an engineering method is proposed which is similar to the Master Curve concept and named the Unified Curve concept. Wide verification of the Unified Curve concept is performed and comparison of the Master and Unified Curve concepts is carried out. It is shown that the Master Curve concept is a partial case of the Unified Curve concept.     

Analysis of fracture tests on large bend beams containing an embedded flaw

As part of a collaborative project to investigate the transferability of Master Curve technology to shallow flaws in reactor pressure vessel applications, a series of fracture tests were performed on large scale bend beams, which were fabricated from a reactor pressure vessel steel and contained simulated sub-surface defects. A 2-dimensional finite element model was used to simulate the behaviour of the test pieces and to study the variation in crack tip constraint at both the near surface and deep crack tips with increasing load. Wallin's two-parameter model, which uses the Master Curve representation of the fracture toughness transition curve together with the elastic crack tip T_stress parameter to estimate the shift in the T₀ reference temperature due to constraint loss, has been applied to arrive at estimates of fracture initiation probability. These are found to be consistent with the experimental results.     

Fracture toughness predictions for a reactor pressure vessel steel in the initial and highly embrittled states with the Master Curve approach and a probabilistic model

Predictions of the temperature dependence of cleavage fracture toughness are performed on the basis of the Master Curve approach and a probabilistic model for a 2Cr–Ni–Mo–V reactor pressure vessel steel in the initial (as-produced) state and in the highly embrittled state. Calculations of the K_IC(T) curve are carried out with both approaches on the basis of fracture toughness test results from pre-cracked Charpy specimens at some (one) temperature. The K_IC(T) curves for the initial state calculated with the Master Curve approach and the probabilistic model are compared and show good agreement. Cleavage fracture toughness values for the embrittled 2Cr–Ni–Mo–V steel are obtained by testing 2T-CT specimens over a wide temperature range. The calculated K_IC(T) curves of the embrittled steel are compared with the test results from the 2T-CT specimens. It is shown that, for the embrittled steel, the K_IC(T) curve predicted with the Master Curve approach does not fit adequately into the experimental data, whereas the agreement of the test results and the K_IC(T) curve calculated with the probabilistic model is good.     

An updated probabilistic fracture mechanics methodology for application to pressurized thermal shock

The US Nuclear Regulatory Commission (NRC) is, in concert with the US nuclear industry, currently revisiting its rule and analysis requirements for pressurized thermal shock (PTS) scenarios. This paper provides an overview of an updated probabilistic risk analysis (PRA) methodology that is continuing to evolve as part of that effort. The evolution process includes a careful assessment of recent advancements that have been made in the various parts of the computational methodologies. The process also involves interactions between experts in relevant disciplines (thermal hydraulics, PRA, materials, fracture mechanics, and non-destructive and destructive examination). Representatives include staff members from the USNRC staff, research laboratories, and the nuclear industry. The updated methodology is being integrated into the FAVOR (Fracture Analysis of Vessels: Oak Ridge) computer code for application to re-examine the adequacy of the current regulations and to determine if the updates provide sufficient technical bases for revisions. This paper also discusses recent modifications to the probabilistic fracture mechanics (PFM) methodology that is central to FAVOR.     

Effects of geometry and flow properties on the fracture toughness of a C–Mn reactor pressure vessel steel in the lower shelf region

To estimate the fracture toughness of thick section nuclear reactor pressure vessel (RPV) steel in the irradiated condition, it is necessary to apply a size effect correction to the test results obtained on small-scale surveillance specimens. This correction is usually derived using toughness data obtained on different sizes of fracture mechanics testpieces made of non-irradiated material, for which the flow properties are quite different from those of material in the irradiated state. This paper describes the results of a fracture toughness test programme carried out on a C–Mn steel plate for two different specimen geometries (10 mm thickness precracked Charpy and 25 mm thickness compact tension) in the lower shelf region of the temperature/fracture–toughness curve. A comparison of the fracture behaviour and failure micromechanisms has been made for the material in the ‘start of life’ condition and after the application of cold prestraining, which was used to simulate the effects of neutron irradiation on flow properties during service. Although the Master Curve methodology predicts no size effects on the lower shelf, size effects were observed.     

Pressurized thermal shock screening criteria re-evaluation effort — US industry activities

A joint project is under way between the US nuclear power industry and the US Nuclear Regulatory Commission (NRC) to re-evaluate the pressurized thermal shock (PTS) screening criteria as presently defined in title 10 of the Code of Federal Regulations, 10 CFR 50.61. Advances in probabilistic risk assessment (PRA), probabilistic fracture mechanics (PFM), thermal hydraulics (TH), and overall plant risk assessment are being incorporated into a comprehensive program to establish a technical basis for revising the present screening criteria. US industry activities are being coordinated through the Reactor Pressure Vessel Integrity Issue Task Group (RPV Integrity ITG) of the EPRI Materials Reliability Project (MRP). The EPRI MRP was formed in 1998 to identify and address issues that could affect operability of major components in pressurized water reactor (PWR) plants. Major activities are coordinated with the nuclear steam supply system (NSSS) vendors, the vendor owner's groups, The Nuclear Energy Institute (NEI), and the NRC. The MRP provides for a unified industry approach to resolution of technical and regulatory issues related to PWR materials degradation. A major task under the Reactor Pressure Vessel Integrity Issue Task Group (RPV Integrity ITG) is to support industry activities associated with the pressurized PTS re-evaluation effort. This paper provides a brief overview of the EPRI MRP program, the RPV Integrity ITG, and industry activities associated with the PTS re-evaluation effort.     

Fracture toughness characterisation in the ductile-to-brittle transition and upper shelf regimes using pre-cracked Charpy single-edge bend specimens

Fracture toughness data of pre-cracked Charpy single-edge bend, SE(B), specimens are compared with those of standard compact, C(T), specimens in the upper shelf and ductile-to-brittle transition regimes. Charpy sized SE(B) specimens provide ductile fracture toughness data, which are compatible with those of standard C(T) specimens. Statistical methods such as the exponential curve fitting method (ECF), the engineering lower bound toughness method (ELB), and the Master Curve method (MC) are used to provide meaningful lower bound cleavage fracture toughness estimates from the toughness scatter of the Charpy sized SE(B) specimens in the ductile-to-brittle transition regime. In this regime, according to the ELB and MC methods, SE(B) specimens provide cleavage toughness data, which tend to be non-conservative compared to those of standard C(T) specimens. However, analyses based on the exponential curve fitting method show good agreement between the fracture toughness estimates for the C(T) and Charpy size SE(B) specimens. At the lower bound toughness level (5% cleavage failure probability), corresponding to J=100 N/mm, the ductile-to-brittle transition curves of SE(B) specimens are reduced by 5–8 °C compared to those of standard C(T) specimens according to the MC-method. A constraint correction function for SE(B) specimens is presented that can be used to make cleavage toughness data of SE(B) specimens compatible with those of standard C(T) specimens.     

R&D advancement,technology diffusion,and impact on evaluation of public R&D

In a 2001 report titled Energy Research at DOE: Was It Worth It? a National Research Council (NRC) committee defined a set of simplifying rules to estimate the net economic benefits from technologies supported by the Department of Energy (DOE). We evaluate the efficacy of the NRC rules compared to published literature on acceleration of technology introduction into markets, technology diffusion, and infrastructure change. We also offer considerations for revisions of the rules that call for the use of technology and sector-specific data, advanced forecasting techniques, and sensitivity analysis to test the robustness of the methodology.     

Integrated seismic and magnetotelluric exploration of the Skierniewice,Poland, geothermal test site

The purpose of the research activities at the Skierniewice geothermal test site is to develop and apply an exploration methodology for low-enthalpy systems in sedimentary formations. Work included seismic and magnetotelluric surveys carried out close to well Kompina-2 to create a detailed structural–geologic model and characterize the anisotropic fracture system around the borehole. The study included the reprocessing of archival data from selected boreholes and 2D seismic lines. The collected data were used to identify formations with high fracture permeability and the presumed flow path of geothermal (∼110 °C) brine in high productivity zones, and determining rock porosities and salinity distribution in the subsurface. The next stage of the investigations will focus on siting a second borehole and studying the possibility of installing a plant for electrical generation or direct geothermal heat applications.     

Fracture assessment of ferritic steel components under dynamic loading

This paper summarizes recent studies on application of the Weibull stress model to predict cleavage fracture of structural components under dynamic loading. Two pressure vessel steels, the strongly rate-sensitive A515-70 steel and the moderately rate-sensitive Euro material (22NiMoCr37), are considered in the investigation. The results, based on independent calibrations at different loading rates, demonstrate that the Weibull modulus (m) is invariant of loading rate for both materials. While m remains a constant for each material, σ_u decreases and σ_w–min increases with higher loading rates. The studies also show that dynamic loading reduces constraint loss, i.e., it drives the response towards the small-scale yielding configuration, and this rate effect tends to saturate at higher loading rate. The demonstrated loading rate invariance of m, when combined with the Master Curve for dynamic loading, can provide a practical approach which simplifies the process to estimate σ_u as a function of loading rate.     

A dynamic fracture-toughness-based reference temperature characterization for the weld metals of modified 9Cr–1Mo steel in two different weld positions

The ductile–brittle transition temperatures (DBTT) of modified 9Cr–1Mo steel welds from two different weld positions, namely down hand (1G) and overhead (4G), have been evaluated and compared using the ASTM E 1921-05 based reference temperature (T₀) approach, but under dynamic-loading conditions. The reference temperatures thus obtained, termed as T₀^dy to signify the dynamic condition, have been found to be higher for the 4G position than the 1G position. A scanning electron microscopic study of the fracture surfaces close to the fatigue crack front reveals that while lath boundary fracture is the dominant mechanism for brittle crack initiation in both the welds, the higher T₀^dy value is linked to the higher concentration of probable crack initiation sites in the 4G position. The experimentally obtained Weibull slope in both the welds has been found to be different (7.526 and 7.205 for the 1G and 4G positions, respectively) from the ‘fixed slope of 4’ assumption, used in ASTM E 1921-05. However, in the present instance, the ‘fixed Weibull slope of 4’ concept yields more conservative T₀^dy values compared to those obtained using the experimentally determined Weibull slope. For these welds, the RT_NDT-based ASME K_IR curve proved to be ultra-conservative compared to the realistic dynamic fracture toughness variation described by the Master Curve indexed with T₀^dy.     

Leak Before Break procedure: Recent modification of RCC-MR A16 appendix and proposed improvements

This paper presents recent modifications of the Leak Before Break procedure of the A16 appendix from French RCC-MR nuclear code and the improvements that will be proposed. The first part of the study deals with the prediction of the crack size at penetration. In the case of small initial defect the validity of the Master Curve used in the A16 appendix is confirmed for pipes. For large initial defects, a correction to the A16 procedure is proposed to take into account the effect of large initial crack length on its size at penetration.     

Application of total process energy-integration in retrofitting an ammonia plant

A case study of total process energy-integration in a commercial ammonia plant was performed using a modified pinch analysis. Several tools, including Grid Diagram, Composite Curve (CC), Grand Composite Curve (GCC), Balanced Grand Composite Curve (BGCC) and Splitting Grand Composite Curve (SGCC), were adapted in the diagnosis of process energy-utilizing. It is shown that the utility loads calculated by operating pinch calculation (OPC) were very close to those in the existing process, and that the method could be used to describe the heat-flow profile of the plant. Some inefficient energy use in the plant was observed in the calculation. The design pinch calculation (DPC) revealed that 1150 kg/h of fuel gas (natural gas) and 1322 t/h of cooling water could be saved through optimizing the temperature difference contribution value (ΔT_c) of heat transfer in each stream for an existing ammonia plant. And a proposal for energy saving is presented. The energy-integration technology showed great promise in the retrofit of large-scale complex process. The successful application of this technology in more than 10 Chinese plants has generated a profit of about 80 million RMB per year.     

Switchgrass selection as a “model” bioenergy crop: A history of the process

A review of several publications of the Oak Ridge National Laboratory's Biofuels Feedstock Development Program and final reports from the herbaceous crop screening trials show that technology, environmental, and funding issues influenced the decision to focus on a single herbaceous “model” crop species. Screening trials funded by the U.S. Department of Energy in the late 1980s to early 1990s assessed thirty-four herbaceous species on a wide range of soil types at thirty-one different sites spread over seven states in crop producing regions of the U.S. Several species, including sorghums, reed canarygrass, wheatgrasses, and other crops, were identified as having merit for further development. Six of the seven institutions performing the screening included switchgrass among the species recommended for further development in their region and all recommended that perennial grasses be given high research priority. Reasons for the selection of switchgrass included demonstration of relatively high, reliable productivity across a wide geographical range, suitability for marginal quality land, low water and nutrient requirements, and other positive environmental attributes. Crop screening results, economic and environmental assessments by the Biofuels Feedstock Development Program staff, and Department of Energy funding limitations all contributed to the decision to further develop only switchgrass as a “model” or “prototype” species in 1991. The following ten year focus on development of switchgrass as a bioenergy crop proved the value of focusing on a single “model” herbaceous crop. The advancements and attention gained were sufficient to give government leaders, policymakers, farmers, and biofuel industry developers the confidence that lignocellulosic crops could support an economically viable and environmentally sustainable biofuel industry in the U.S.     

Environmental Kuznets Curve for carbon emissions in Pakistan: An empirical investigation

This study investigates the relationship between carbon emissions, income, energy consumption, and foreign trade in Pakistan for the period 1972–2008. By employing the Johansen method of cointegration, the study finds that there is a quadratic long-run relationship between carbon emissions and income, confirming the existence of Environmental Kuznets Curve for Pakistan. Moreover, both energy consumption and foreign trade are found to have positive effects on emissions. The short-run results have, however, denied the existence of the Environmental Kuznets Curve. The short-run results are unique to the existing literature in the sense that none of the long-run determinants of emissions is significant. The contradictory results of short- and long-run give policy makers the opportunity to formulate different types of growth policies for the two terms taking environmental issues into consideration. In addition, the uni-directional causality from growth to energy consumption suggests that the policy makers should not only focus on forecasting future demand for energy with different growth scenarios but also on obtaining the least cost energy. Furthermore, the absence of causality from emissions to growth suggests that Pakistan can curb its carbon emissions without disturbing its economic growth.     

Recent trends in PEM fuel cell-powered hybrid systems: Investigation of application areas,design architectures and energy management approaches

Recently, the renewable energy issue is becoming significant due to increasing power demand, instability of the rising oil prices and environmental problems. Among the various renewable energy sources, fuel cell (FC) technology has received considerable attention as an alternative to the conventional power units due to its higher efficiency, clean operation and cost-effective supply of power demanded by the consumers. Particularly, proton exchange membrane (PEM) FC technology plays a leading role for many applications when comparing with other competitive types of FCs. PEMFCs have recently passed the test or demonstration phase and have partially reached the commercialization stage due to the impressive worldwide research effort. Besides, providing a hybrid system by integration of PEMFC with an auxiliary power source may provide better results considering the issues of performance and component durability. This paper presents a comprehensive review of the recent trends in PEMFC powered hybrid systems including a detailed explanation of application areas and design architectures with different power electronics interfaces as well as the energy management methods utilized in the daily life and taking part in the literature.     

Improved safety margins for Belgian nuclear power plants by the application of the Master Curve approach to RPV surveillance materials

In the context of existing regulatory codes, the integrity assessment of the pressure vessel of a nuclear power plant (NPP) is based on the empirical assumption that the fracture toughness of the surveillance materials, expressed in terms of a lower bound curve indexed by a reference temperature RT_NDT, undergoes a shift under irradiation by an amount equal to the increment of the T_41 J index temperature measured from surveillance Charpy tests. Nowadays, an alternative route exists, based on: reconstitution of previously tested specimens; execution of fracture toughness tests in the irradiated condition; Master Curve analysis of the results obtained and finally determination of an alternative toughness-based reference temperature (_RTT0)$({\mathrm{RT}}_{T_0})$, which can be used to index the lower bound K_Ic curve. As we demonstrate in this paper for several surveillance materials extracted from Belgian power plants, this “advanced” approach can provide NPP owners and plant engineers an additional safety margin with respect to the operating limits of the reactor and the pressurized thermal shock (PTS) screening criteria. These additional safety margins have been found particularly significant for older plants, for which the actual fracture toughness in the unirradiated condition is often underestimated by the approach based on RT_NDT.     

Morphological features (defects) in fuel cell membrane electrode assemblies

Reliability and durability issues in fuel cells are becoming more important as the technology and the industry matures. Although research in this area has increased, systematic failure analysis, such as a failure modes and effects analysis (FMEA), are very limited in the literature. This paper presents a categorization scheme of causes, modes, and effects related to fuel cell degradation and failure, with particular focus on the role of component quality, that can be used in FMEAs for polymer electrolyte membrane (PEM) fuel cells. The work also identifies component defects imparted on catalyst-coated membranes (CCM) by manufacturing and proposes mechanisms by which they can influence overall degradation and reliability. Six major defects have been identified on fresh CCM materials, i.e., cracks, orientation, delamination, electrolyte clusters, platinum clusters, and thickness variations.