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The aim of this work is to compare the operation experiences of large grid connected PV systems in the Megawatt class. Two systems with a total power of more than 2000 kWP are located on the roofs of the new Munich Trade Fair Centre, the two others are situated in West Saxonia near Leipzig (5000 kWp ) and Meerane (1000 kWp). In November 1997 the 1016 kWP solar roof on the new Munich Trade Fair was connected to the grid. Since then, until the end of December 2006, more than 8.900 MWh of solar electricity on the AC-side have been fed into the local 20-kV-medium-voltage-grid. The photovoltaic generator consists of 7812 modules, each one showing a nominal power of 130 WP. System voltages up to 600 V DC are applied. The cables on the DC-side show cross-sections up to 600 mm². One central inverter - three 330-kVA-units in master-slave-operation - transforms the DC energy into AC. The neighbour system, which was installed 5 years after the first PV system, shows a nominal power of 1056 kWP. Although equipped with key components such as modules and inverters from the same manufacturers as the first system, it shows some differences coming from the lessons learned from the first system. After some initial problems (over temperature in the inverter rooms, grid fluctuations causing inverter shut downs and damages, faulty isolation measurements, poor designed string fuses) the systems now operates highly reliable and at superior performance. The Saxon systems were put into operation in 2004. In both plants central inverters are used in master-slave operation. The generators of both systems are mounted on ground of former waste sites. It is the purpose of this work to compare the operation of these PV plants. First, a detailed comparison of the yields will be done. Where do the differences come from? Which are the influences of irradiation, modules and their tilt angle, inverters and their operation mode, maintenance etc? In the next step the operation experiences will be compared. Have there been any severe damages? Are there any lessons learned from initial operation? What about the long term behavior of modules, cables, boxes, inverters? As a conclusion recommendations will be given to improve the long term behavior of large PV systems.

The workshop ¨Portierbare numerische Simulationen auf parallelen Architekturen¨ (¨Portable numerical simulations on parallel architectures¨) was organized by the Fac- ulty of Informatics/Professorship Computer Architecture at 18 April 1996 and held in the framework of the Sonderforschungsbereich (Joint Research Initiative) ¨Numerische Simulationen auf massiv parallelen Rechnern¨ (SFB 393) (¨Numerical simulations on massiv parallel computers¨) ( http://www.tu-chemnitz.de/~pester/sfb/sfb393.html ) The SFB 393 is funded by the German National Science Foundation (DFG). The purpose of the workshop was to bring together scientists using parallel computing to provide integrated discussions on portability issues, requirements and future devel- opments in implementing parallel software efficiently as well as portable on Clusters of Symmetric Multiprocessorsystems. I hope that the present paper gives the reader some helpful hints for further discussions in this field.

The purpose of this study is to investigate transient pressure loads from hydrogen combustion. Specially, the study relates to the analysis of the pressure timehistoryof detonating hydrogen-air-mixtures from balloon experiments. The study has shown that measured pressure time-functions can be reproduced by numerical simulations with the detonation code DET. Furthermore, it was shown that edges and corners focus detonation waves in real enclosures with obstacles which significantly increase pressure loads from normally reflected detonation waves.

1. SCOPE AND OBJECTIVES The aim of investigations an the HTR-100 accident behaviour was to verify the safety concept of the plant for balance and to quantify the radiological risk to be expected in operating an HTR-100 double unit systeme Moreover, aspeets of the investment risk were considered. The spectrum of initiating events ranged from so-called transients to leaks in the primary circuit and steam generator andeven included earthquakes. Sosse of the event trees derived were highly complex and extensive due to the situation of the steamgenerator above the core and with regard to the double unit plant concept with increased possibilities of accident control, but also with respect to potential accident propagation. Correspondingly sophisticated analyses were required to identify riskerelevant event sequences, but ultimately only a few courses of events were concerned. 2. PLANT RISKAn HTR-100 plant produces 250 MW(th) per reactor. The limitation to this comparatively low output, a low power density and a core geometry with a relatively lange surface permit the omission of active core cooling without significantly affecting fission product retention in the fuel elements. The fission product decay heat is in this case transferred to a two-line natural circulation cooling water system via the surface of the reactor steel pressgare vessel so that the reactor concrete cell is cooled. Unlike larger reactors, the radiological risk is therefore not determined by active core cooling failure which, due to natural convection in the primary circuit of a pressurized reactor, is only possible in the event of a failure of the steam generator feed system, whereas blower failure is an additional cause in a depressurized primary loop. Although local temperatures of 1620°C are reached in a depressurized reactor, which ultimately also lead to the release of radiologically relevant fission products from the fuel elements in a small section of the core, cesium and strontium are retained praetically completely in the core region due to redeposition an colder graphite surfaces and only iodine and silver are reieased from the core as radioiogieaily relevant fission products. However, this only occurs at a time when the average gas temperatures in the primary circuit decrease again due to natural convection which also takes place in a pressureless reactor. The associated coolant contraction prevents fission product transport from the reactor pressure vessel. Another essential feature reducing the risk of cooling accidents is the fact that the reactor pressure vessel safety valves blow off into the gas store. Active core cooling failure in the pressurized reactor therefore does not lead to gas release into the environment, although the pressure increases to a level where one of the safety valves responds. Primary circuit depressurization with corresponding reactor core heatup can only develop if several engineered safeguards are assumed to fall in addition to active core cooling. A failure of active core cooling after accidenteinitiating leakage of the primary circuit is also highly improbable because the afterheat removal system is designed to cope with this event and possesses very high availability for reasons of plant availability and Investment protection. The radiological risk of the HTR-100 is essentially determined by accident sequenees initiated by heating tube leakage at the steam generator and causing release of pari of the fission products deposited an the steam generator surface during operation. The highest releases of noble gases and iodine are caused by a sequence of events due to failure of the steam generator relief system comprising the two pressure reducing stations so that the steam generator pressure increases due to continued natural convection heating until the main steam safety valves respond. If the valve in the defective leg then fails in the open position, there will be a direct connection between the primary loop and the environment causing slow depressurization of the primary system through the leak and open safety valve. Such depressurization temporarily leads to condensation at the steam generator due to rapid cooling of the primary circuit through the intact steam generator leg causing about 6 % of the metallic fission products deposited there to be detached. The radiologically relevant nuclides released near ground level comprise 78 GBq of cesium-137, 1.2 GBq of strontium-90 and 880 GBq ofiodine-131. More than 90 % of this iodine quantity is assumed to originate from "hydrolyzed:' fuel particles with defective [...]