Within the European Integrated Project NURESIM, the simulation of PTS is investigated. Some accident scenarios for Pressurized Water Reactors may cause Emergency Core Coolant injection into the cold leg leading to PTS situations. They imply the formation of temperature gradients in the thick vessel walls with consequent localized stresses and the potential for propagation of possible flaws present in the material. This paper focuses on two-phase conditions that are potentially at the origin of PTS. It summarizes recent advances in the understanding of the two-phase phenomena occurring within the geometric region of the nuclear reactor,that is, the cold leg and the downcomer, where the “PTS fluid-dynamics" is relevant. Available experimental data for validation of two-phase CFD simulation tools are reviewed and the capabilities of such tools to capture each basic phenomenon are discussed. Key conclusions show that several two-phase flow subphenomena are involved and can individually be simulated at least at a qualitative level, but the capability to simulate their interaction and the overall system performance is still limited. In the near term, one may envisage a simplified treatment of two-phase PTS transients by neglecting some effects which are not yet well controlled, leading to slightly conservative predictions.
ARM processors are well known for their energy efficiency and are consequently widely used in embedded platforms. Like other processor architectures, they are built with different levels of parallelism, from Instruction Level Parallelism (out-oforder and superscalar capabilities) to Thread Level Parallelism (multicore), to increase their performance levels. These processors are now also targeting the HPC domain and will equip the Fujitsu Post-K supercomputer. Some ARM processors from the Cortex-A series, which equip smartphones and tablets, also provide Data Level Parallelism through SIMD units called NEON. These units are able to process 128-bit of data at a time, for example four 32bit floating point values. Taking advantage of these units requires code vectorization which may be performed automatically by the compiler or explicitly by using NEON intrinsics. Exploiting all these levels of parallelism may lead to better performance as well as a higher energy consumption. This is not an issue in the HPC domain where application development is driven by finding the best performance. However, developing for embedded applications is driven by finding the best trade-off between energy consumption and performance. In this paper, we propose to study the impact of vectorization and multithreading on both performance and energy consumption on some Nvidia Jetson boards. Results show that depending on the algorithm and on its implementation, vectorization may bring a similar speedup as an OpenMP scalar implementation but with a lower energy consumption. However, combining vectorization and multithreading may lead close to both the best performance level and the lowest energy consumption but not when running cores at their maximum frequencies.
The European Platform for NUclear REactor SIMulations, (NURESIM project 2005 – 2008) addressed the creation of a Common European Standard Software Platform for modelling, recording, and recovering computer data for nuclear reactors simulations. One work package of the project was dedicated to the analysis and improvement of CFD capabilities for the simulation of two-phase PTS problems. Some SB-LOCA scenarios lead to a situation in which the cold leg is partially or totally uncovered when the Emergency Core Cooling injection is activated. The resulting complex two phase flow can be divided in characteristic flow regions: the jet flow with a free surface between steam and water, the zone of jet impingement, the horizontal two-phase flow and the flow in the downcomer. Many phenomena have to be reflected in a simulation of each separate region, but also when the simulations are coupled reflecting the integral process which is required to predict the thermal loads at the RPV wall. After analyzing the experimental database available for CFD model development and validation and identifying shortcomings of the models different activities were dedicated to the simulation of single flow regions as well as the integral flow. Based on these experiences recommendations for the CFD-simulation of the two-phase PTS problem were obtained.
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