Antonio Hurtado

Biotechnological hydrogen production by photosynthesis

Microbiological photosynthesis is a promising tool for producing hydrogen in an ecologically friendly and economically efficient way. Certain microorganisms (e.g. algae and bacteria) can produce hydrogen using hydrogenase and/or nitrogenase enzymes. However, their natural capacity to produce hydrogen is relatively low. Thus, there is a need to optimize their core photosynthetic processes as well as their cultivation, for more efficient hydrogen production. This review aims to provide a holistic overview of the recent technological and research developments relating to photobiological hydrogen production and downstream processing. First we cover photobiological hydrogen synthesis within cells and the enzymes that catalyze the hydrogen production. This is followed by strategies for enhancing bacterial hydrogen production by genetic engineering, technological development, and innovation in bioreactor design. The remaining sections focus on hydrogen as a product, that is, quantification via (in‐process) gas analysis, recent developments in gas separation technology. Finally, a discussion of the sociological (market) barriers to future hydrogen usage is provided as well as an overview of methods for life cycle assessment that can be used to calculate the environmental consequences of hydrogen production.

A NOVEL RESULT IN THE FIELD OF NONLINEAR STABILITY ANALYSIS OF BOILING WATER REACTORS

The nonlinear stability analysis of boiling water nuclear reactors (BWRs) is conducted with the aid of so-called advanced, well validated, system codes and an advanced reduced order model to build a detailed mathematical understanding of the BWR behavior in the practical relevant parameter space. In the last years, the existence of Hopf-bifurcation points was confirmed by some researchers. In the framework of this paper, a parameter region was analyzed in which the coexistence of different stability states is realized. As a novel result, we found a parameter region in which stable fixed points, unstable limit cycles and stable limit cycles coexist. This system behavior can be explained by a saddle-node bifurcation of cycles (turning point). The existence of this solution type in a BWR system indicates the possibility of large amplitude limit cycle oscillations in the linear stable region.

Investigation on primary side oriented accident management measures in a hypothetical station blackout scenario for a VVER-1000 pressurized water reactor

Abstract As a consequence of a total loss of AC power supply (station blackout) at a VVER-1000 leading to unavailability of major active safety systems, the safety criteria ensuring the safe operation of the nuclear power plant would be violated and core heat-up with possible core degradation could occur. A dedicated accident management measure (primary side depressurization) can be applied to reduce the primary pressure and to activate the injection from the passive emergency core cooling systems (accumulators). The analyses presented in this paper are aiming at both a detailed investigation of the accident sequence, taking into account the depressurization of the primary circuit, and the possibilities to prevent or at least to mitigate a damage of the reactor core so as to gain additional time for taking necessary countermeasures. The analyses are performed using the codes ASTEC and ATHLET developed by IRSN (Institut de Radioprotection et de Sûreté Nucléaire) and GRS (Gesellschaft für Anlagen- und Reaktorsicherheit mbH).

Remarks on boiling water reactor stability analysis – part 2: stability monitoring

Abstract In part 1 of this article we explained the partly relative complex solution manifold of the differential equations describing the stability behaviour of a BWR, in particular the coexistence of different types of solutions, such as the coexistence of unstable limit cycles and stable fixed points are of interest from the operational safety point of view. The part 2 is devoted to the surveillance of the stability behaviour. We summarize some stability monitoring methods and suggest to support stability tests by RAM-ROM analyses in order to reveal in advance the stability “landscape” of the BWR in a parameter region high sensitive for appearing of linear unstable states. The analysis of an especial stability test, performed at NPP Leibstadt (KKL), makes it clear that the measurement results can only be interpreted by application of bifurcation analysis.

Component exposure in hypothetical accidents with very fast depressurization in a HTR module reactor

The starting event of the massive air ingress into the core of the HTR module reactor, classified as hypothetical incident, is the very fast depressurization of the primary circuit. Provided that the integrity of the reactor pressure vessel is not in question, a rupture of the connecting pressure vessel between reactor pressure vessel and steam generator vessel is the maximum possible leak cross-section. In this work it is investigated whether the components of the reactor pressure vessel are exposed by the depressurization process to mechanical loads which exceed the load limits. These loads are caused by two different events, the strong momentum change of the fluid and the local pressure differences, respectively. Due to the momentum change the bottom reflector receives the maximum load, whereby only 2% of the compressive strength of the graphite quality used there are reached. However, the load by local pressure differences is between passed volumes and in normal operation, not-passed volumes lead to high load values. A maximum pressure difference of 44.5 bar was calculated at the thermal top shield.

Remarks on boiling water reactor stability analysis - part 1: theory and application of bifurcation analysis

Abstract Modern theoretical methods for analysing the stability behaviour of Boiling Water Reactors (BWRs) are relatively reliable. The analysis is performed by comprehensive validated system codes comprising 3D core models and one-dimensional thermal-hydraulic parallel channel models in the frequency (linearized models) or time domain. Nevertheless the spontaneous emergence of stable or unstable periodic orbits as solutions of the coupled nonlinear differential equations determining the stability properties of the coupled thermal-hydraulic and neutron kinetic (highly) nonlinear BWR system is a surprising phenomenon, and it is worth thinking about the mathematical background controlling such behaviour. In particular the coexistence of different types of solutions, such as the coexistence of unstable limit cycles and stable fixed points, are states of stability, not all nuclear engineers are familiar with. Hence the part I of this paper is devoted to the mathematical background of linear and nonlinear stability analysis and introduces a novel efficient approach to treat the nonlinear BWR stability behaviour with both system codes and so-called (advanced) reduced order models (ROMs). The efficiency of this approach, called the RAM-ROM method, will be demonstrated by some results of stability analyses for different power plants.

The Release of Radionuclides in the Laser Decontamination Process

The decommissioning of nuclear installations requires the decontamination of radioactively contaminated concrete surfaces in order to minimize the amount of radioactive waste to be disposed of as well as the exposure time of the staff during this works. The rapid progress in the development of laser technology has yielded high-performance diode lasers whose radiation can be guided over a long distance by means of glass-fibre optical units. This opens up the possibility of implementing unconventional laser-based decontamination processes. The aim of the method presented here is to combine melting and contactless ablation of a radioactively contaminated concrete surface by means of a laser beam with waste product conditioning. It is intended to design the process in such a way that a maximum of the radioactivity present at the surface is incorporated in the glass melt (= conditioning of waste products). The glassy granulate obtained is very well suited for direct final storage due to its physical and chemical properties. The portion of radioactive isotopes that are released in the process, but not incorporated during the ablation process is selectively deposited in a cooled electro-filter. To prove the effectiveness of the method, research was focused on decontamination experiments conducted on concrete samples contaminated with 137 Cs, 60 Co and 85 Sr. Furthermore, the chemical composition of the concrete samples was varied (quartzitic, quartzitic-calcitic) to take account of the different release conditions in real concrete structures. The experiments showed that 85 Sr and 60 Co are highly soluble in the glass melt. Their release rate is very low as they have a relatively high boiling point. 137 Cs also exhibits a great affinity to the glass melt, but is more easily released again in the high temperature range due to its low boiling point of approx. 700 °C. The released portion of 137 Cs is then deposited in the upstream electro-filter. The overall assessment is that the intended decontamination process with simultaneous conditioning of waste products is basically feasible using today’s laser technology. The special advantage can be seen in the great versatility and easy control of the laser unit that is equipped with a fibre-optical system. Furthermore, laser ablation can be set up as a low-dust process, which minimizes problematic secondary contamination.Copyright

NON-OXIDE CERAMICS - CHANCES FOR APPLICATION IN NUCLEAR HYDROGEN PRODUCTION

Research and development are increasingly focusing on the provision and utilization of heat in the high-temperature range above 900 °C, in particular under the aspect of resource-saving energy technologies. On the one hand, the exploitation of the high-temperature range helps to improve the efficiency of energy conversion processes; on the other hand, the provision of high-temperature heat makes it possible to utilize innovative thermochemical processes, which in turn represent environmentally compatible processes. An example to be quoted here is the thermally induced production of hydrogen by the iodine-sulfur process. The high temperatures alone place extremely high requirements on the materials to be used so that metallic materials soon reach their limits of application. If additionally chemically aggressive process media are used, as in the iodine-sulfur process, basically only ceramic materials can be considered as construction materials. In this application, notably silicon carbide (SiC) is favored owing to its excellent high-temperature properties. The possible technical fields of application of such high-performance ceramics can be broadly extended provided that suitable, highly efficient joining methods are available for these ceramics. In addition to its use as a constructional ceramic, SiC can principally also be used as a functional ceramic. For this purpose, the basic ceramic is modified with different additives, providing it with electrical properties that permit its application as a full ceramic heat conductor or sensor. In this case, it also holds true that a suitable joining method for making electrically conductive joints will extend the fields of application considerably. Laser-based joining technologies are being developed for both applications at the Dresden University of Technology. The research work presented here notably focuses on laser joining of electrically conductive SiC ceramics. In addition to a CO2 laser, a diode laser has been used. Basically, electrical connection has been made in two ways. In the first variants, graphite pins are inserted into the joining zone as electrically conductive bridges. In an alternative concept, the oxidic glass filler itself is made electrically conductive with additives. Like that a full ceramic heating conductor joined by means of laser radiation has been tested. The temperature resistance and functionality of the laser-joined heating conductor could be fully demonstrated.Copyright

Laser Joining of Ceramics: A Contribution to High Temperature Range Application of Ceramic Components

Non-oxide ceramics, such as silicon carbide (SiC) and silicon nitride (Si3 N4 ), have excellent properties that make the materials interesting for application also in the nuclear sector. Due to their exceptional resistance to high temperatures, aggressive and abrasive media as well as nuclear radiation, the materials seem to be particularly suitable for developments in such fields as high-temperature reactors ((V)HTR) and peripheral systems (e.g. for hydrogen production). To simplify and thus to enable the technical application of these high-tech ceramics, the Dresden University of Technology has developed a laser beam joining process. This opens up many possibilities, e.g., to encase HTR fuel elements (as well as spheres and composites) in SiC, to encapsulate highly radioactive waste in SiC or to build a highly efficient heat transformer using high-temperature energy from VHT reactors. The progress made in laser beam technology in the last few years is a major element that has contributed to the developments achieved to date. Research has been focused mainly on the following three areas: (1) optimization of the laser parameters in combination with the advancement of oxide brazing fillers, (2) transfer of the basic technology to other high-tech ceramics like oxide ceramics, and (3) application of the laser process to develop electrically conductive joints. The possibility to laser join also Al2 O3 and ZrO2 ceramics has created the opportunity to produce full ceramic sensors for (V)HTR specific applications at low cost. This requires adaptation of laser technology to the special properties of oxide ceramics. These are markedly less resistant to thermally induced stress than non-oxide ceramics, placing high requirements on laser process control. Another peculiarity is the property of oxide ceramics to be partly transparent to the laser wavelengths emitted by diode lasers (808 nm and 940 nm), with the result that the ceramic material is not heated primarily at the surface but inside its volume. This produces joint seams inside ceramic components even without any excessive thermal stress. The R&D work has made it possible to produce novel sensors for the high-temperature range that are also highly resistant to aggressive media. It is considered a further advantage that this joining technology has no special requirements regarding the process atmosphere such as vacuum or inert gas, which ensures that the process lends itself well to automation.Copyright

State of the Art of Helium Heat Exchanger Development for Future HTR-Projects

In Germany two HTR nuclear power plants had been built and operated, the AVR-15 and the THTR-300. Also various projects for different purposes in a large power range had been developed. The AVR-15, an experimental reactor with a power output of 15 MWel was operated for more than 20 years with excellent results. The THTR-300 was designed as a prototype demonstration plant with 300 MWel and should be the technological basis for the entire future reactor line. The THTR-300 was prematurely shut down and decommissioned because of political reasons. But because of the accompanying comprehensive RD this was not a requirement for the previous THTR design. Methodologies for in-service inspections already had been developed, but they are not sufficient for today’s tube lengths and have to be adapted. Another example, based on operating experience, is using reheaters to increase the efficiency is not recommended today. Using supercritical steam conditions to increase the efficiency should be investigated instead. In general, the economic benefit has to be balanced against the additional costs resulting from better material and more complex manufacturing.Copyright