Hans-Josef Allelein

A new Markov-chain-related statistical approach for modelling synthetic wind power time series

The integration of rising shares of volatile wind power in the generation mix is a major challenge for the future energy system. To address the uncertainties involved in wind power generation, models analysing and simulating the stochastic nature of this energy source are becoming increasingly important. One statistical approach that has been frequently used in the literature is the Markov chain approach. Recently, the method was identified as being of limited use for generating wind time series with time steps shorter than 15–40 min as it is not capable of reproducing the autocorrelation characteristics accurately. This paper presents a new Markov-chain-related statistical approach that is capable of solving this problem by introducing a variable second lag. Furthermore, additional features are presented that allow for the further adjustment of the generated synthetic time series. The influences of the model parameter settings are examined by meaningful parameter variations. The suitability of the approach is demonstrated by an application analysis with the example of the wind feed-in in Germany. It shows that—in contrast to conventional Markov chain approaches—the generated synthetic time series do not systematically underestimate the required storage capacity to balance wind power fluctuation.

Validation and Application of the REKO-DIREKT Code for the Simulation of Passive Autocatalytic Recombiner Operational Behavior

Abstract In order to reduce the accumulation of hydrogen and thus to mitigate the risk of combustion, many countries have installed passive autocatalytic recombiners (PARs) within light water reactor containments. The severe hydrogen combustion events of the recent Fukushima Daiichi accident are likely to incentivize an increased demand in upgrading nuclear power plants with PARs. Numerical simulation is an important tool for assessing PAR operation during a severe accident in terms of efficiency and proper installation. Advanced numerical PAR models are required for the challenging boundary conditions during a severe accident, for example, low oxygen amount, high steam amount, and presence of carbon monoxide. The REKO-DIREKT code has been developed in order to provide a PAR model capable of simulating complex PAR phenomena and at the same time being suitable for implementation in thermal-hydraulic codes. The development of REKO-DIREKT was supported by small-scale experiments performed at Forschungszentrum Juelich in the REKO facilities. These facilities allow the study of PAR-related single phenomena such as reaction kinetics under different conditions including variation of steam, oxygen, and carbon monoxide (REKO-3) and the chimney effect (REKO-4). Recently, the code has been validated against full-scale experiments performed in the Thermal-Hydraulics, Hydrogen, Aerosols, Iodine (THAI) facility at Eschborn, Germany, in the framework of the Organisation for Economic Co-operation and Development/Nuclear Energy Agency THAI project. By this, the code has proven its applicability for different PAR designs and for a broad range of boundary conditions (pressure of up to 3 bars, steam amount up to 60 vol %, low-oxygen conditions). REKO-DIREKT has been successfully implemented in the commercial computational fluid dynamics code ANSYS-CFX as well as in the LP code COCOSYS [Gesellschaft für Anlagen- und Reaktorsicherheit (GRS), Germany].

Modeling and evaluation of combined photovoltaic-battery systems in the decentralized german power generation

This paper presents the overall economic evaluation of a battery storage in a decentralized energy supply system by means of mathematical optimization methods. The energy supply system, consisting of a battery storage in combination with a photovoltaic system as well as a heat pump with a heat storage and a conventional gas boiler, will be evaluated with regard to technical and economic criteria. The aim of the optimization is to identify the optimal structural dimensioning and the optimal operating mode of each component in the system to cover the energy consumption with connection to the public electricity grid. Based on exemplary studies for a German state it can be seen that photovoltaic systems nowadays operate economically. However, the use of a battery storage system is not optimal in economic terms. The reduction of the investment costs shows that the use of a battery storage system can prove advantageous in the future. In connection with a time variable electricity tariff it can be seen that this has an increasing effect on the optimal capacity of the battery. On the whole the use of a combined photovoltaic-battery system can improve own benefit and consumption towards the use of a single photovoltaic system without a storage option.

Investigation of PAR Behavior in the REKO-4 Test Facility

Passive auto-catalytic recombiners (PARs) play a key role in the hydrogen mitigation strategy of European LWRs. In order to avoid possible threats related to hydrogen combustion, PARs are installed to remove hydrogen released during a loss-of-coolant accident. The possible impact of hydrogen explosions became evident during the reactor accident in Fukushima (Japan) in March 2011, where leaked hydrogen ignited and largely destroyed the upper part of the reactor building. The mitigation strategy is based and verified by computational accident assessments. Code validation against experimental data is vital in order to achieve reliable results.Copyright

COMBINED ANALYTICAL AND EXPERIMENTAL INVESTIGATIONS FOR LWR CONTAINMENT PHENOMENA

Main focus of the combined nuclear research activities at Aachen University (RWTH) and the Research Center Julich (JULICH) is the experimental and analytical investigation of containment phenomena and processes. We are deeply convinced that reliable simulations for operation, design basis and beyond-design basis accidents of nuclear power plants need the application of so-called lumped-parameter (LP) based codes as well as computational fluid dynamics (CFD) codes in an indispensable manner. The LP code being used at our institutions is the GRS code COCOSYS and the CFD tool is ANSYS CFX mostly used in German nuclear research. Both codes are applied for safety analyses especially of beyond design accidents. Focal point of the work is containment thermal-hydraulics, but source term relevant investigations for aerosol and iodine behavior are performed as well. To increase the capability of COCOSYS and CFX detailed models for specific features, e.g. recombiner behavior including chimney effect, building condenser, and wall condensation are developed and validated against facilities at different scales. The close connection between analytical and experimental activities is notable and identifying feature of the RWTH/JULICH activities.

CFD Simulation of Passive Auto-Catalytic Recombiner Operational Behaviour

In order to mitigate the impact of a possible hydrogen combustion and to avoid containment failure, passive auto-catalytic recombiners (PAR) are used for hydrogen removal in an increasing number of European nuclear power plants. Hydrogen and oxygen react exothermally even below conventional flammability limits on the catalytic surfaces inside a PAR generating steam and heat. Modelling of the operational behaviour of PAR is one part of development issues in order to achieve reliable predictions of local atmosphere conditions. International research activities, e.g. in the European FP-6/FP-7 Network of Excellence SARNET (Severe Accident Research NETwork), investigate these aspects. At Forschungszentrum Julich, two strategies are pursued. First, the detailed evaluation of the reaction kinetics and heat and mass transport phenomena on a single catalyst element is performed by a direct implementation of the transport and kinetic approaches in ANSYS CFX-11. Second, in order to model the interaction of PAR with the containment, REKO-DIREKT, a detailed user model of the relevant processes inside a PAR based on Fortran 90 will be implemented in CFX. At present, validation against the available experimental database is performed. The validity of the numerical models strongly depends on the experimental data available. For this purpose, detailed experiments are performed at Julich. In the small-scale test facility REKO-3, representing a recombiner section, detailed investigations of the reaction kinetics under well-defined and steady-state conditions have been performed. In co-operation with RWTH Aachen University, a new test vessel REKO-4 is currently under preparation for testing PAR behaviour under natural convection conditions. It will provide various possibilities for instrumentation to measure temperatures and gas compositions and in particular it will be equipped with particle image velocimetry (PIV) for measuring the flow field around the PAR.© 2009 ASME

A Review of the CFD Modeling Progress Triggered by ISP-47 on Containment Thermal Hydraulics

Abstract The Organisation for Economic Co-operation and Development (OECD)/Nuclear Energy Agency International Standard Problem 47 (ISP-47) was aimed at assessing the predictive capabilities of computational fluid dynamics (CFD) and lumped-parameter codes regarding hydrogen mixing under representative thermal-hydraulic conditions of a loss-of-coolant-accident. The benchmark consisted of two systematic steps. The first step was a fundamental model assessment based on quasi-steady-state separate-effects tests in the French TOSQAN facility (7 m3, IRSN, Saclay) and MISTRA facility (100 m3, CEA, Saclay) regarding steam condensation, buoyant turbulent flows, and mixed atmospheric conditions. The second step was based on a more realistic experimental transient in the multicompartmented German Thermal-hydraulics, Hydrogen, Aerosols and Iodine (THAI) facility (60 m3, Becker Technologies, Eschborn). At that time, the blind and open analysis revealed that CFD codes needed further improvement regarding modeling of turbulence in buoyant flows, steam condensation, temperature and species concentration, and stratification buildup as well as their dissolution. This result triggered a comprehensive experimental and analytical effort, e.g., within the German national THAI, the OECD-THAI, and the OECD-SETH-1 and OECD-SETH-2 projects. Now, 10 years later, this paper aims to benchmark the state-of-the-art containment CFD model, developed at Forschungszentrum Juelich and RWTH Aachen University, and to highlight the progress made and the remaining open issues.

A First Orienting Investigation of the Interaction of Cable Fire Products with Passive Autocatalytic Recombiners

Abstract Passive autocatalytic recombiners (PARs) have been installed inside light water reactor containments in many countries to remove hydrogen and, thus, to mitigate the combustion risk during a severe accident (SA). Due to the challenging SA boundary conditions, PARs are exposed to several deactivation risks during operation, which may cause a reduction of the hydrogen removal capacity. Such a deactivation may occur through different mechanisms and could in principle affect the start-up behavior up to the full loss of catalytic activity. To assess the interaction of PARs with the products of cable fires, a set of PAR catalyst samples has been introduced to the atmosphere of cable fire tests performed at Institut de Radioprotection et de Sûreté Nucléaire (IRSN), France. The subsequent surface analyses performed at Forschungszentrum Jülich (Germany) reveal a significant amount of carbon, chlorine (a constituent of polyvinyl chloride), zinc, and antimony (a flame retardant) on all catalyst samples compared to reference samples. The subsequent performance tests confirm that all catalyst sheets suffer a significant start-up delay of between 17 and 45 min compared to the reference samples. However, after burning off the soot deposition, the catalyst samples reach full conversion capacity and show immediate start-up behavior in a subsequent test. The present results clearly demonstrate the adverse effect of cable fire products on the efficiency of hydrogen conversion in a PAR. To further understand and quantify the impact of cable fire products and to assess their relevance for SA scenarios, further experimental as well as theoretical investigations are required. In particular, the combined influence of cable fire products and humidity, which has intentionally been omitted in the present study, should be investigated in the future due to the possible corrosive impact on the catalyst as well as the influence of humidity on the nature of the soot deposition.

High temperature reactors for cogeneration applications

Abstract There is a large potential for nuclear energy also in the non-electric heat market. Many industrial sectors have a high demand for process heat and steam at various levels of temperature and pressure to be provided for desalination of seawater, district heating, or chemical processes. The future generation of nuclear plants will be capable to enter the wide field of cogeneration of heat and power (CHP), to reduce waste heat and to increase efficiency. This requires an adjustment to multiple needs of the customers in terms of size and application. All Generation-IV concepts proposed are designed for coolant outlet temperatures above 500 °C, which allow applications in the low and medium temperature range. A VHTR would even be able to cover the whole temperature range up to approx. 1 000 °C.

Qualification of pebble fuel for HTGRs

Abstract The German HTGR fuel development program for the HTR-Modul concept has resulted in a reference design based on LEU UO2 TRISO coated particle fuel in a spherical fuel element. The coated particles consist of minute uranium particle kernels coated with layers of carbon and silicon carbide. Analyses on quality of as-manufactured fuel, its behavior under HTR-Modul relevant operating and accident conditions have demonstrated excellent performance. Coated particles can withstand high internal gas pressure without releasing their fission products to the environment. International efforts are on-going for further improvement of coated particle fuel to meet the needs of future generation-IV HTR concepts.