EDF Energy North America

Abstract Austenitic stainless steels used in light water reactor coolant environments can be susceptible to environmentally assisted fatigue due to non-monotonic loading conditions, primarily associated with load-follow, thermal transients, or intermittent plant shutdowns and start-ups. One of the main goals of a five-year INCEFA-SCALE (INcreasing safety in NPPs by Covering gaps in Environmental Fatigue Assessment - focusing on gaps between laboratory data and component SCALE) project, kicked off in October 2020 and supported by the European Commission HORIZON2020 program, is to develop an improved mechanistic understanding behind the effect of a pressurized water reactor (PWR) primary environment on the fatigue behavior of austenitic stainless steels through coordinated extensive characterization of tested specimens at partner organizations. Post-mortem characterization of fatigue specimens tested in air and high-temperature PWR water environments with different loading parameters was performed. In this program, the effects of a PWR environment containing hydrogen and cyclic loading parameters on the low cycle fatigue (LCF) behavior of 316L stainless steel were investigated by comprehensive striation spacing evaluation and microscopic characterizations. The exposure to a PWR environment results in a decreased LCF lifetime, an enhanced fatigue crack initiation and an accelerated fatigue crack growth rate of 316L austenitic stainless steel. The interaction of hydrogen and localized deformation contributes to the accelerated fatigue crack growth rate in a PWR environment. The effect of waveform (periodic underload PUL, periodic overload POL and constant amplitude sawtooth CA) was evaluated. PUL and POL reduce the low cycle fatigue lifetime, accelerate the fatigue crack growth rate and advance the cycle where initiation of fatigue crack occurs compared to CA loading. LCF waveform strongly influences the shear band formation, localization of plastic deformation and stress state.

Abstract INCEFA-SCALE is a five-year project within the European Commission HORIZON 2020 programme starting in October 2020. Nuclear plant operators have generally observed that the number of failure events attributable to Environmentally Assisted Fatigue (EAF) are fewer than predicted by current assessment methodologies. It is recognised that a contributor to this discrepancy is the transferability of data from laboratory-scale tests to real nuclear components. This is the main knowledge gap addressed by INCEFA-SCALE. This is being achieved through the combination of testing, analysis, and materials characterisation to provide laboratory testing data, and the required meta data to understand the resulting behaviour, generated under conditions that are more representative of those experienced by plant components. The EAF assessment methodology described in NUREG/CR-6909 Rev 1 designed for use with the ASME BPVC Section III fatigue design curves (using factors of 12 on life and 2 on stress/strain) provides an international benchmark and technical justification. This benchmark has been the subject of over a decade of research defining the levels of conservatism provided and proposing alternative approaches to clearly define appropriate margins in EAF assessment methods. One such approach, initiated by the Japanese ‘Subcommittee on Design Fatigue Curve’ and further developed by Rolls-Royce, offers modifications to the design curve approach through the use of ultimate tensile strength dependent mean curves, justification of a new approach to transference factors, the use of the Smith-Watson-Topper mean stress correction approach and explicitly accounting for variable amplitude loading in the high cycle regime of fatigue. Substantial research has been conducted and improvements proposed for the low-cycle regime of the design curves (where the factor on life applies for the ASME approach). However, research data in relevant high temperature water environments relating to the transference factors influencing the factor of 2 on stress/strain is limited. Part of this value is defined by loading history and understanding how to account for this phenomenon in a fatigue assessment requires the production of research data that enables the behaviour of materials subjected to variable loading to be understood. Variable loading is known to affect hardening and mean stress behaviour. Additionally, failure modes where a crack can initiate under larger loading amplitudes and grown to failure by sub-endurance limit loading may be important. This means laboratory testing under relevant environments and variable strain amplitudes is required to advance this area and define new benchmark methods that can adequately account for such behaviours while providing an appropriate level of conservatism. This paper focuses on the presentation and analysis of test data produced by the INCEFA-SCALE project. The data investigates the effects of loading history, environment, and surface finish, through the application of single and variable amplitude waveforms to 316L stainless steel uniaxial fatigue specimens in both air and simulated Pressurised Water Reactor primary coolant environments. By altering the above variables, the aim of this paper is to provide an understanding of how the method outlined in NUREG/CR-6909 and the alternative method proposed by the Subcommittee on Design Fatigue Curve accounts for these effects. It is shown that at time of writing the data would provide support for reducing the NUREG transference factor on stress to 1.4, however a recommendation on reducing the factor on cycles requires more analysis. The reduction in the stress factor is consistent with the approach of the Japanese ‘Subcommittee on Design Fatigue Curve’. There is however inconsistency between the use of elastic pseudo stress amplitudes used in fully reversed design life predictions, and the yield stress value used to govern the boundaries of stress-based mean stress corrections which should be considered in any new design curve approaches.

The development of electric vehicles (EV) is a major lever towards low carbon transportation. It comes with increasing numbers of charging infrastructures which can be smartly managed to control the CO2 cost of EV electricity consumption or used as flexible assets for grid management. To achieve that, an efficient day-ahead forecast of charging behaviours is required at different spatial resolutions (e.g., household and public stations). We propose an extensive benchmark of 14 models for both load and occupancy day-ahead forecasts, covering 8 open charging session datasets of different types (residential, workplace and public stations). Two modelling approaches are compared: direct and bottom-up. The direct approach forecasts the aggregated load (resp. occupation) directly of an area/station whereas the bottom-up approach models each individual EV charging session before aggregating them. This second approach is key to the effective implementation of smart charging strategies. We consider both machine learning models (Random Forests and Gated Recurrent Units) and statistical models (Generalised Additive Models, Poisson Regression, Mixture Regression, Auto-Regressive) in order to maximise the spectrum of our benchmark. We finally propose an adaptive aggregation strategy to assess the variety of forecasts at hand. Overall, we demonstrate that direct approaches reach better performances than bottom-up approaches across all datasets considered. We further show that the different approaches used can lead to an improved performance of direct approaches when using an adaptive aggregation strategy. In fact, our best model produces a forecast which is more than 5 times better relative to the persistence on residential data.

We study Peak-Time-Rebates (PTR) contracts in day-ahead electricity markets.Such contracts reward customers for reducing their consumption when wholesale prices are high. We start by pointing out that these market designs create arbitrage opportunities which, under asymmetric information, incentivize strategic consumers to inflate their baseline. We then show that an incentive compatible PTR design is equivalent to a variable Critical-Peak-Pricing design (vCPP), in which customers have to purchase their peak consumption at the spot price. Under asymmetric information, a relevant question is thus to design vCPP contracts optimally in order to achieve high enrollment rates under voluntary opt-in. This problem has different solutions depending on whether policy-makers choose to maintain existing cross-subsidies or not.

The major change in the last decade concerning distribution network is related to the penetration of distributed generation (DG). The current practice in France concerning the connection of DG was to fit the network to the worse case of use of DG to avoid any problem. This approach will reach his limits and many developments will be necessary to optimize the network operation, taking the presence of DG into account. In this document, we introduce some research topics going on in our team as voltage control, management of load and generation, stability analysis of networks with DG, optimization of use of fault indicator, choice of optimal configuration and automatic post fault restoration, distribution system availability assessment. We also stress on the problem of interdependency between power and ICT infrastructures.

Ce papier presente une simulation numerique 3D avec le code de dynamique rapide EUROPLEXUS de la depressurisation rapide du reacteur HDR (accident de type APRP) ou le phenomene de l'interaction fluide-structure est modelise dans toute sa generalite. Les differents modeles de couplage fluide-structure disponibles dans le code sont rappeles brievement. La modelisation EUROPLEXUS de l'essai V32 de la campagne HDR est presentee ainsi que les resultats numeriques que l'on compare avec les mesures experimentales.

This equipment features a radio-nuclide (barium 137) generator directly connected to a pneumatic injector, and two similar detectors on the outside of the pipe connected to an electronic counter giving a direct reading of the values from which the rate of flow is derived. In developing these measurement methods, use was made mainly of such medium-period radio-nuclides as iodine 131, bromine 82 and sodium 24, With the following main purposes in view : (i) To find applications for short pipes (i.e. up to 80 diameters) ; (ii) Experimental confirmation of mathematical approaches ; (iii) Design and development of equipment for simple use. A barium 137 generator was developed, which has added considerable interest to the measurement of pipe flows by of radio-active salts. It relies on the natural caesium → barium 137 relationship, with the parent caesium 137 being fixed to a highly-selective ion- exchange resin. Washing this resin in water carries the barium 137 bred by the disintegrating caesium away, but without moving the latter. As barium 137 has a period of about 2.5 minutes, much higher radio-activity can now be fed into the pipe, for 100 minutes later finds it reduced to only 2-40 times its original value. This even makes repetitive closed-circuit tests feasible. In view of these promising results, the author's team developed an injector fitting the generator jointly with Societe Saint-Gobain-Techniques Nouvelles-who had supplied the generator-in order to climinate any need to manipulate the radioactive solution between preparing it and feeding it into the pipe. A brief description of the method now follows, in order to seriate the principal questions considered in the study : In order to determine the rate of flow in a pipe at least 50 diameters long and whose cross-sections are known, a radio-active solution is injected and its times of passing two specific cross-sections A and 13 are detected from outside the pipe. The most important points considered in describing the method are the following : (i) Injection ; (ii) Distance of point of from first detection section ; (iii) Distance between the two detection sections ; (iv) Detection. The mathematical approaches are specifically dealt with in a three-part Appendix, as follows: The tirst part shows that, knowing the abscissa of the temporal centre of gravity of the curve corresponding to the passage of the radio-active solution, the rate of flow in the pipe can be asceriained by outside observation. The second part discusses deformation due to the detector filters. The third part leads to the estimation of discharge mor where interpretation is based on one particular geometrical point only. The heading injection ((i) above) covers the injector and its operation. Heading (ii) refers to a study of turbulent diffusion phenomena and the efficient mixing length where detection is outside the pipe, and especially the determination of I∞ from observed I2 and I2 values. The distance between detection sections (heading (iii)) does not enter into the method error calculations. It determines the accuracy of radio-active solution passage time measurement at the two sections. Heading (iv) comprises the analysis of the two following data abstraction methods : (a) Plotting of curves and determination of a point with about the same characteristics as the centre of gravity as regards measurement. (b) Bringing the centre of gravity of the radio-active cloud into evidence by direct processing of observed data. The discussion of (a) shows the difficulties of using a single geometrical point and defines one of them, and Table 1 lists the results obtained from 110 injections in successive groups of 10. Under (b), the calculator associated with the detectors is described, sub-groups are listed, and the development tests involved are reviewed. Finally, the advantages, applications and potential development scope of the method are discussed.

Because large increases in electrical usage in the downtown area of San Diego, Calif., began overtaxing the primary radial feeders of the San Diego Gas and Electric Company, it was decided to establish an entirely new substation to feed the area, taking advantage of existing 12-kv tie lines near by. The primary radial feeders were feeding a portion of the area from already heavily loaded substations and a new substation seemed to be a better solution to the problem than to increase facilities in the existing substations, where space was limited, and recopper lengthy radial feeders or install new ones.