Framatome (Germany)

Since 2021, there is an ongoing cooperative PhD work on how the energy transition and mostly the large-scale development of renewables will impact the grid and more specifically, the operation of traditionally baseload nuclear power plants in Hungary. The general aim of the research was to collect the possible new requirements from the grid side and the possibilities for improving the flexibility of the nuclear power plant (NPP) units. We have investigated the grid side effects by hourly dispatch simulations using PLEXOS software and have compared different flexibility properties of the nuclear power plant units in Hungary. The need for nuclear load reductions has increased substantially in Hungary in the last years. Another important objective of the work was to prepare the implementation of a technology that can safely increase the maneuverability of the power plants as well. The target technology has been chosen after thorough assessment of different solutions and further development of the chosen technology is ongoing. This paper gives an overview of the formulation and evolution of the problem, the studies conducted and the project in a broader perspective.

Abstract The effects of carbon macro-segregation on the mechanical properties of low alloy steel forgings have been extensively studied and reported in several papers in the past. The present paper focuses on a forged ring made with a hollow ingot of the grade RCCM 18 MND5 (similar to ASME A508 Gr3), in the so called “end of solidification zone”. This zone at the top end of the ring and close to mid-thickness, has a limited macro-segregation (%C∼ 0.23) and contains “band type” segregations at a millimetric scale. These segregated bands are inherited from the equiaxed solidification structure of the original ingot in which inter-dendritic segregations are embedded. The non-homogeneity of the structure at the millimetric scale is considered to be the cause of an important scatter of Charpy notched bar impact tests. The segregated bands were shown to be prone to inter-granular cracking because of a relatively high amount of Phosphorus concentrated in the segregation. In the present paper, fracture toughness tests are reported with 1TCT specimens taken in the same zone of the forging containing these local segregations. The Reference Temperatures T0 are determined in this position, and in the conventional ¼ Thickness position. Fractographic examinations are performed in order to verify if the same fracture mechanisms as those observed in Charpy specimens, can also be observed in CT specimens. The scatter of the KJC data in the segregated position is analyzed in terms of the ASTM E1921 homogeneity criteria.

Large forgings fabricated from heavy ingot can involve homogeneity challenges for the mechanical properties. The 2.25Cr-1Mo steel has been used for a long time for very thick and heavy forgings for the pressure vessels. Since it is only lightly sensitive to the macro segregation and exhibits good bainitic hardenability due to its chemical composition. However, for the quenched and tempered thick forgings, very few data on mechanical properties and microstructure have been reported for this grade of steel. In this study, different quenching and tempering conditions have been applied on forging blocks to simulate the actual heat treatment conditions at different depths in the thick forgings (thickness up to 700 mm at the stage of quenching). Low quenching rates, down to 150 °C/h, have been chosen to investigate the mechanical properties and the microstructure at the mid-thickness of thick forgings. The influence of the carbon content has also been analyzed for low quenching rate condition. For each metallurgical state, tensile tests and Charpy impact tests have been conducted. The microstructures have been characterized by optical and electron microscopies and by EBSD analysis. The micro-hardness has been measured. The results show that even with a quenching rate as low as 150 °C/h, a considerable proportion of bainite can still be formed for a 2.25Cr-1Mo steel with relatively low carbon. Its bainitic microstructure contains highly disoriented bainite blocks which is in favor of a good fracture toughness. At 0 °C, the upper shelf of the transition curve is basically reached with energies above 150J.