Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

Barium carbonate is a widely accepted stable chemical form for the safe disposal of carbon-14. This requires, however, oxidation of carbon-14-containing organic waste to carbon dioxide. We habe developed a method for the mineralization of organic compounds by anodic oxidation in silver-sulfate-containing sulfuric-chromic acid. This process can also be used for complete oxidation of "difficult" organic wastes, such as hydrophobic substances like hydrocarbons, plastics, activated carbon, etc. The generation of additional chromium-containing hazardous wastes are avoided in this process by electrolytically regenerating chromium(III) to chromium(VI). The degradation of C-14 labeled organic compounds during the process is monitored by on-line measurement the β-radiation of the produced ¹⁴CO₂

The investigation of the effects of humic substances (humic and fulvic acids) on the migration of radionuclides is important to assess their impact on the long-term safety of both radioactive waste repository sites (e.g. Gorleben, Germany) and abandoned uranium mines (Saxony and Thuringia, Germany).
We extracted humic substances from bog water of the 'Hochmoor Kleiner Kranichsee' which is in the vicinity of the uranium mining sites at Johanngeorgenstadt (Saxony). The humic material was separated into humic and fulvic acid fractions and characterized for its elemental composition, functional properties including proton exchange capacity, charge/size distribution ratios and spectroscopic characteristics. The size and size distribution of humic colloids were studied by photon correlation spectroscopy. Furthermore, the complexation of site specific humic substances with uranyl ions was studied. The results were compared with data of a commercial humic acid from Aldrich.

The influence of humic substances on the speciation of actinide ions and their migration behavior in natural aquifer
systems is of great interest within environmental research. Specified knowledge about this is required, for instance, to
develop strategies of risk assessment for long-time safety in the regions of the former uranium mining in Saxony and
Thuringia. Due to the complex and heterogeneous nature of humic substances a thermodynamic founded description of their
complexation behavior with metal ions is difficult. Numerous uncertainties exist in the description of the complexation
process, e.g., the kind and number of complexing functional groups is uncertain. Therefore, it is necessary to
investigate the complexation process with well defined model substances.
We synthesize and characterize humic acid functionality models from reducing sugars and  -amino acids, with functional
properties comparable to those of natural humic acids with respect to type and number of functional groups. Former
measurements have shown that this synthetic humic acids mimic the interaction humic acid - metal ion very well.
This work deals with the possibilities to synthesize modified humic acids, e.g., humic acids with blocked phenolic OH
groups. With this model substances we investigate the influence of phenolic OH groups on the complexation behavior
of humic acids with uranyl ions by time-resolved laser-induced fluorescence spectroscopy. We expect to get a better
understanding of the humic acid - metal ion interaction process and to gather information regarding geochemical
modeling in the presence of humic acids.

Natural humic acids are instable and chemically not well defined chemical systems. It is difficult to accurately describe their physical-chemical properties. Therefor, studies of the interaction between humic acids and heavy metals, including uranium and other actinide elements, are often limited by this fact. For systematic studies of the interaction processes, stable and wellcharacterized humic acid model compounds have been suggested.
We have separated humic-acid-like fractions with good chemical stability from synthetic melanoidins. It is possible to design the functionality of these melanoidins by varying their precursor substances and the conditions of the synthesis.
Furthermore, we have compared different melanoidins and their interaction with UO₂ ²⁺ with the behavior of a commercial natural humic acid (Aldrich) jby infrared spectroscopy (FTIR). The investigation reveals many functional similarities between the melanoidins and the natural humic acid. Comparable symmetric deformation vibrations of uranium in the far infrared range (FIR) indicate similar uranium coordination. In the middle infrared range (MIR), the antisymmetric stretching vibrations of the bound uranium are nearly identical.
In connection with other investigations we have shown, that the melanoidins are excellent humic acid model substances for fundamental radioexological research.

Capillary electrophoresis was applied to investigate pK_a values of ^99mTc radiotracers used in nuclear medicine. Therefore, the protonation equilibria of the carboxyl groups of ^99mTc- mercaptoacetylglycylglycylglycine (^99mTc-MAG₃) and ^99mTc-ethylenecysteine dimer (^99mTc-EC) were studied by pH-dependent determination of electrophoretical velocities. ^99methylenecysteine dimer diethyl ester (^99mTc-ECD) was used as a non-protonable standard. The capillary electrophoresis system was equipped with a radioactivity detector. Measurements were performed using a pressure-driven capillary zone electrophoresis which allowed runs even in the low pH range. For the determination of pK_a values, the electrophoretical velocities of the analytes were referred to the electrophoretical velocities of tetraphenyle arsonium chloride as a positively charged marker. Calculation of pK_a values was accomplished by non-linear curve fitting of both structure-based equilibria equations and sigmoidal decay functions to the experimental data. ^99mTc-MAG₃ was shown to have a carboxyl group pK_a value of 4.22. the value for the carboxyl groups of ^99mTc-EC is 2.90 (determined by structure-based equilibria equations), which represents a common value for both carboxyl groups. By the use of sigmoidal functions, similar values were elucidated. As expected, ^99mTc-ECD shows no protonation step.

The paper presents an electrode-mesh tomograph for the high-speed visualisation of transient gas fraction distributions in two-phase flows in pipes. It is based on the measurement of the local instantaneous conductivity of the two-phase mixture. The time resolution of the device is 1024 frames per second. The sensor consists of two electrode grids with 16 electrodes each. This results in 16 x 16 sensitive points, which are equally distributed over the cross section. The sensor is available in two designs: (1) wire-mesh sensor for lab applications and (2) sensor with enforced electrode rods for high mechanical loads. The device was recently tested in a vertical and a horizontal air-water flow in a pipe of 51.2 mm diameter.

The impressive development of focused ion beam (FIB) systems from the laboratory level to high performance industrial machines during the last twenty years is briefly reported. The design and the functional principle of a liquid metal ion source as well as a FIB column are described. Main application fields of the FIB technology are stoichiometric writing implantation or ion milling which are dominated by the sputtering effect. The FIB is a very suitable tool for sputtering of well defined holes which can easily be analysed by surface profiling. By applying this volume loss method the sputtering yields and milling rates of crystalline, amorphous, and poly-silicon, as well as SiO2, CVD- and high pressure (HP) - diamond and 6H:SiC were investigated for 35 and 70 keV Co, Ga, Ge, Nd and Au ions. For crystalline silicon and 6H:SiC targets, the sputtering yield was determined as a function of the incident angle of the ions and the substrate temperature. In addition, the influence of the pixel dwell time on the erosion process in the case of high dose cobalt implantation was investigated. The experimental obtained yield data are compared with calculated values using different known models.

In the frame of the so-called BWR-CA concerted action of the European Union, one has to analyse some experiments performed on the NOKO facility using the CATHARE2 (CEA) and the ATHLET (FZR) thermalhydraulic codes. These calculations have shown that both CATHARE and ATHLET are able to predict the global performance of the NOKO emergency condenser. In reference calculations, the unrelevance of the Shah correlation used in CATHARE was not seen apart from the BLIND case. Taking into account the sensitivity calculations, it is thought that this problem is due to an overestimation of the liquid film velocity. This latter can be considered as a consequence of a wrong estimation of the film entrainment onset or as a limitation of the two-phase six-equation model (only one average liquid velocity). Further verification and qualification of computer codes require the analysis of tests with a better instrumentation. Measurements of local parameters like the inside tube temperature or the distribution of pressure losses, are really necessary to undertake this work. It is why it has been proposed, in the frame of a new concerted action, to use the NOKO single tube experiments already performed by FZJ and which are much better instrumented in comparison of the just considered bundle tests.

The SWR1000 is a new innovative boiling water reactor concept,
which is developed by Siemens AG. This concept is characterized
in particular by passive safety systems (e.g., four emergency
condensers, four building condensers, eight passive pressure
pulse transmitters, six gravitydriven core flooding lines).
Passive safety systems need no external energy sources. The
function is directly based on the physical phenomena:
gravity, natural convection and/or evaporation. Therefore the
effectiveness is independent of operator actions and smaller
failure rates are assigned to passive safety systems.
For the experimental investigation of the operation mode and
the effectiveness of these passive safety systems the multipurpose
thermohydraulic test facility NOKO (NOKO is an abbreviation for
the German translation of emergency condenser) was constructed
at the Forschungszentrum Jülich (FZJ). The facility has a maximum
power of 4 MW for steam production and a maximum operating pressure
of 10 MPa.

In the frame of an European Union programme (EU BWR R&D Cluster),
six test series with an emergency condenser test bundle were
performed in 1996. Within the Physics and Thermalhydraulics
Complementary Actions (BWR/CA) to the EU BWR R&D Cluster the
German Forschungszentrum Rossendorf (FZR) e.V. and the French
Commisariat a'l Energie Atomique (CEA/DRN) have calculated 9 tests
and an additional blind test of the NOKO bundle experiments. These
posttest calculations were carried out using ATHLET (FZR) and
CATHARE2 (CEA/DRN). The results of these calculations are presented
in this paper.

Detailed comparisons of ATHLET and CATHARE results show that despite
the good prediction of global parameters for both codes, the variations
of local parameters calculated (e.g., film velocity and temperature)
are not in such a good agreement. This can be explained by some
compensating errors in computer codes (e.g., an overestimation of
the condensing film velocity can hide an error in the condensation
correlation) and the significant effect of the tube wall conduction
in controlling the heat transfer. The emergency condenser test bundle
of the available and considered experiments were not instrumented.
Therefore a deeper analysis (including a detailed comparison of local
parameters) is not possible. New test series released in the frame of
a new European union action will be analyzed in 1998.

In 1995 at the integral test facility ISB-VVER in Elektrogorsk near Moscow natural circulation experiments were performed, which were scientifically accompanied by the Forschungszentrum Rossendorf. These experiments were the first of this kind at a test facility, which models VVER-1000 thermalhydraulics. Using the code ATHLET which is being developed by "Gesellschaft für Anlagen- und Reaktorsicherheit", pre- and posttest calculations were done to determine the thermalhydraulic events to be expected and to define and tune the boundary conditions of the test. The conditions found for natural circulation instabilities and cold leg loop seal clearing could be confirmed by the tests. Besides the thermalhydraulic standard measuring system, the facility was equipped with needle shaped conductivity probes for measuring the local void fractions.

Abstract Contaminated sites represent a serious environmental problem in Germany. The decision that remediation technology is optimal for a given site is complicated by the existence of multiple objectives to be optimized simultaneously, significant uncertainties about the remediation results, and the involvement of several decision-makers with conflicting interests. Decision analysis is a methodology to deal with problems of this kind. The application of decision analysis at a test site demonstrated that remediation decisions can greatly benefit from the structural guide, sound methodological approaches, and manifold results that can be deduced from decision analytic models. The careful preparation of the decision helps to prevent momentous wrong decisions, especially due to the sophisticated support, that decision analysis offers for risky decisions. Because remediation decisions can be regarded as prototypical for many decisions in the public sector, the results of this study may also impact other fields like waste management, water resource administration, traffic planning, or siting of hazardous industrial facilities.

Es ergab sich die Notwendigkeit, die Vermischungseffekte mit einem 1:5 skalierten Modell nachzuvollziehen. In dieser Arbeit wurden Skalierungseffekte hervorgehoben und ein Vergleich der Strömungen im Originalreaktor und 1:5 Plexiglasmodell mit Hilfe eines numerischen Strömungsberechnungsprogrammes vollzogen. Dabei wurde das Modell und der Originalreaktor möglichtst originalgetreu abgebildet und mit den kalten Strängen zusammen modelliert.Die Vergleichsrechnungen belegen, daß es ausreichend ist, die Vermischungsvorgänge in einem mindestens 1:6.6 skalierten Modell eines DWR zu untersuchen. Die Parameter (Druck, Temperatur, Geschwindigkeit) erlauben den Aufbau als Plexiglasmodell, das eine optische Beobachtung der Vermischung ermöglicht. Das Forschungszentrum Rossendorf hat mit dem Aufbau eines 1:5 Modells 1997 begonnen.

The Siemens AG is developing the innovative boiling water reactor concept SWR1000. New features are the passive safety systems (e.g. emergency condensers, building condensers, passive pressure pulse transmitters).

For the experimental investigation of the emergency condenser effectiveness, the NOKO test facility has been constructed at the Forschungszentrum Jülich. The facility has an operating pressure of 10 MPa and a maxi-mum power of 4 MW. The emergency condenser bundle consists of eight tubes and is fabricated with planned geometry and material of the SWR1000. In more than 200 experiments, the emergency condenser capacity was determined as a function of pressure, water level and concentration of noncondensables in the pressure vessel as well of pressure, water level and temperature in the condenser.

For the evaluation of the NOKO experiments, the program system CASH-Graphics (Computergestützte Auswertung und Unsicherheitsanalyse) was developed. This evaluation is the basis for the determination of the operation conditions of the emergency condenser.

Post test calculations of NOKO experiments were per-formed with an improved version of ATHLET. To calculate the heat transfer coefficients during condensation in horizontal tubes the module KONWAR has been developed and implemented in ATHLET. KONWAR is based on the flow regime map of Tandon and includes several semi-empirical correlations for the determination of the heat transfer coefficients. The comparison between calculations and experiments shows a good agreement.

Die zu Beginn der neunziger Jahre in der Ukraine praktizierte KKW-Überwachung ermöglichte der Aufsichtsbehörde nur einen unzureichenden Zugang zu Informationen über den jeweils aktuellen betrieblichen Sicherheitszustand. Im Zusammenhang mit den Ergebnissen von Sicherheitsanalysen für Kernkraftwerke war es daher erforderlich, die Möglichkeiten der betrieblichen Überwachung dieser KKW durch die ukrainische Behörde zu verbessern. Für das KKW Saporoshje wurde vom Forschungszentrum Rossendorf im Verbund mit dem TÜV Rheinland ein modernes Überwachungssystem als Pilotprojekt konzipiert, eingerichtet und Ende 1995 in den Probebetrieb überführt. Es ergänzt die vorhandenen betrieblichen Kontroll- und Überwachungseinrichtungen durch Einbeziehung moderner Informations- und Rechentechnik. Das System ermöglicht eine kontinuierliche Beobachtung des Zustandes der Anlage im Normalbetrieb und bei Betriebsstörungen bzw. Störfällen, so daß bei erkennbaren Abweichungen vom bestimmungsgemäßen Betrieb frühzeitig durch Anfrage und Anordnung darauf reagiert werden kann.

The considered test was a break on the upper plenum with different modes of emergency core cooling. The reference case was the non-availability of emergency cooling. Injecting the emergency coolant into the cold leg, no increasing of rod cladding temperatures was observed, but natural circulation instabilities occurred. Injecting the cooling into the hot leg, the cooling situation was getting worse. Due to the injected cold emergency coolant, the fluid density in the discharge volume was enhanced and the break mass flow increased. The observed events in the test were reproduced by the code with good agreement

The ISB-VVER facility of the Elektrogorsk Research and Engineering Center is currently the only operating integral test facility for the thermal hydraulic behaviour of the Russian pressurized water reactor VVER-1000. In 1993 it was decided, to perform a test for a small break at the upper plenum with locked rotor of all circulating pumps as the 1st Russian Standard Problem. The experimental results were made available to the Research Center Rossendorf, where post test calculations were carried out using the GRS-code ATHLET. The main results of these calculations are presented in this paper.

The continuing exponential increase in computer power together with the recent developments of very efficient numerical procedures allow nowadays to perform predictive atomic-scale computer simulations for material science. This holds especially for advanced microelectronic devices where functional units consists more and more often of 10^6 atoms or even less. In this situation the design of new materials and devices is more and more frequently supported by atomic-scale computer simulations.

A kinetic 3D Monte-Carlo code based on stochastic probabilistic two-center cellular automaton using a double bookkeeping technique, one in the particle vector and the other in the lattice space was originally coded in PASCAL and tested on an INTEL PC, later in C and on HP workstations. This implementation should be speeded up considerably in order to undertake real scientific simulations. The only way to get the needed speedup is parallelisation.

We started with a look at the given sequential code, translated it into C and run it on our S-Class Server. While translating we had to understand a lot of things, e.g. the double booking technique, so that we could do first steps of scalar optimisation during this process.

The most simple idea of a parallel approach is distributing the lattice across the processors. But because of the physical problem modelling the growth of clusters of implanted particles in the lattice such an approach will lead to load imbalances and destroy the effect of parallelisation.

With an another approach we tried to distribute the work done in the particle space instead of the lattice space. We developed a graph-theoretical approach based on skeletons to find out a work distribution across 2^n processors that assure s a good load-balance. The disadvantage of this approach is the work needed to compute the distribution which is of order O(n^3) compared to the simulation which is of order O(n).

At this point it turned out the given model was not suitable for parallelising the algorithm. So we had feed-back with the physicist and together we developed a model of the physical process where the algorithm which solves the problem could be formulated in parallel. The new model allowed us to divide the solution into three main steps, where two of them can be done parallel and will therefor improve the speed of the algorithm.

We implemented this algorithm in a mixed language modularised program using FORTRAN for the computational part and C for the I/O part. The program runs on our S-Class server using maximum 8 processors.

In our presentation the methodology of creating an optimised parallel code for the Monte-Carlo simulation of Ostwald Ripening, the schema of the parallel code and first real physical simulation results will be shown.

In Ion Beam Synthesis (IBS) of nanoclusters the postimplantation
annealing step causes major redistribution of the implanted impurity atoms
by Ostwald ripening (OR). The diffusion-reaction equations describing OR
are highly nonlinear. Therefore, analytical studies are restricted to special
cases like the asymptotic behaviour, the diffusion or reaction controlled
limit, low cluster concentration, etc. An alternative to analytical studies
of OR is the numerical integration of the diffusion-reaction equations.
Using this method we take into account the diffusion and reaction control
of OR, i.e. we present for the first time an unified treatment for the
whole range of and in between the diffusion and reaction controlled limits.
Based on a local mean field theory, our model starts with a multipole expansion
of the concentration field, which satisfies the stationary diffusion equation.
Using appropriate boundary conditions on the precipitates we derive self-consistently
the governing equations for the evolution of the clusters. Alternatively
to Ewald's summation method we apply a Yukawa-like screening of precipitates.
The dependence of the particle radius distribution (PRD) the critical radius
and the density of nanoclusters on process parameters will be discussed.

The goal of this work was to evaluate the potential of small YAP:Ce crystals, especially designed for a high-resolution PET system. We directly compared the scintillator properties of 3x3x20 mm³ crystals YAP with those of BGO. The light output, energy resolution, detection efficiency and timing properties for the irradiation using ¹³⁷Cs and ²²Na sources were investigated. Special consideration was given to the influence of the reflector coating on the light output as well as on the overall performance of the quality of the studied crystals.

Abstract
Relying on the Luttinger-Ward theorem we derive a thermodynamically selfconsistent and scale independent approximation of the thermodynamic potential for the scalar Φ⁴ theory in the tadpole approximation. The resulting thermodynamic potential as a function of the temperature is similar to the one of the recently proposed screened perturbation theory.

Studies on the measurement of turbulent dispersion of a radiotracer in an experimental setup with natural convection liquid-gaseous flow were carried out at the Forschungszentrum Rossendorf. Liquid-gaseous bubbly flow was generated in a narrow tank by injection of pressurized air into water or by catalytic disintegration of H₂O₂. A small amount of a positron emitting radiotracer liquid ( ¹⁸F in an aqueous NaF solution) was injected instantaneously at a certain point of the tank. The Positron Emission Tomography (PET) technique was used to observe the spreading of the tracer liquid in the bubbly flow. A double head gamma detector array was used for measuring the positron annihilation rate which is proportional to the tracer concentration. The dispersion coefficient D for the tracer liquid was calculated from the experimental data assuming an isotropic spreading of the concentration profile after separation of the linear displacement of the maximum concentration point. Calculations of the two-phase flow with spreading tracer were carried out for selected experiments using the computational fluid dynamics code CFX-4. The Euler-Euler continuum approach including a homogeneous low Reynolds number K,? -model was applied for the two-phase flow. Turbulent Prandtl numbers for gas and tracer dispersion were varied. In case of higher gas injection rates into water (superficial gas velocities j_gas of 5 - 15 mm/s), a reasonable agreement was achieved between calculated and measured values of the tracer transport velocity and the dispersion coefficient. A nearly linear correlation between j_gas and D was found in agreement with other authors. However, it was necessary to take into account the significant deviation of the tracer distribution from the ideal diffusion equation solution in some cases, in particular if the tracer has been spread over upwards and downwards regions of the natural circulation velocity field. Further investigations are necessary to improve the agreement between measured and calculated values of the dispersion coefficient and tracer transport velocity in the case of H₂O₂ disintegration and low gas superficial velocities.

Das Schaumverhalten eines Stoffsystems bei Druckentlastungsvorgängen hat einen wesentlichen Einfluß auf die Druckabfallgeschwindigkeit und die ausgetragene Masse. Es wurden kleinmaßstäbliche Druckentlastungsversuche mit einem 1,9 l Gefäß durchgeführt. Das Abblasen erfolgte über eine kopfseitige Abblaseleitung mit einer Blende. Unter sonst gleichen Bedingungen wurde reines Wasser und Wasser mit Zusatz von Isobutanol abgeblasen. Gemessen wurden der Behälterdruck, die ausgetragene Masse, sowie Temperaturen und Dampfgehalte an jeweils 2 verschiedenen Höhenpositionen. Es wurden Experimente mit einem Blendendurchmesser von 2 mm und 3 mm sowie Isobutanolzusätzen von 2, 4 und 6 Gew.% durchgeführt. Für beide Abblasequerschnitte zeigte sich eine deutliche Zunahme der ausgetragenen Masse mit der Isobutanolkonzentration. Auf Grund der Schaumbildung und des dadurch vergrößerten Flüssigkeitsaustrags wird der Druckabbau zu Beginn des Druckentlastungsvorgangs behindert. Löst sich schließlich die Oberkante der Schaumschicht vom Deckel, erfolgt der Druckabbau auf Grund der dann geringeren Masse im Behälter schneller.

Die Einrichtung zur Druckentlastung (Sicherheitsventil, Berstscheibe) ist in der Praxis über eine mehr oder weniger lange Abblaseleitung mit dem Reaktor verbunden. Bei der Auslegung der Sicherheitseinrichtung wird jedoch lediglich der notwendige Ausströmquerschnitt ermittelt. Berücksichtigt wird dabei gegebenenfalls der Druckverlust in der Leitung. Die Auswirkungen auf die Dynamik des Druckentlastungsvorgangs während des zweiphasigen Ausströmens bleiben bisher weitgehend unbeachtet. Aus diesem Grund wurden verschiedene Modelle für die Simulation des gekoppelten Systems bestehend aus Druckbehälter, Abblaseleitung und Entlastungsarmatur aufgestellt. Dabei wurden mehrere Varianten von vereinfachten Leitungsmodellen untersucht. Zur Überprüfung der Modellgüte wurden dabei Ergebnisse von Experimenten genutzt, die an der Druckhalterversuchsanlage DHVA der Hochschule für Technik, Wirtschaft und Sozialwesen Zittau/Görlitz durchgeführt wurden und bei denen Massenstrompulsationen bei Abblasen auftraten. Die Schwingungen hängen sehr empfindlich von einzelnen Parametern des Schaummodells, des Modells für die Kopplung zwischen Behälter und Abblaseleitung und des Modells für die Abblaseleitung ab. Experimentelle Daten zu diesen Schwingungen stellen daher eine ausgezeichnete Grundlage für die Validierung von Codes zur dynamischen Simulation eines Blow-Down Systems dar.

Im Rahmen der Entwicklung eines Gesamtmodells für die dynamische Simulation komplexer Druckentlastungssysteme bestehend aus Reaktor, Abblaseleitung,
Entlastungsarmatur und Auffangeinrichtungen wurde ein 1-D-Behältermodell
aufgestellt. Die Transportvorgänge im Behälter werden auf der Grundlage einer
neu entwickelten Partikelmethode gelöst. Dadurch wird numerische Diffusion
vermieden, was insbesondere bei der Berücksichtigung von Diskontinuitäten, wie
z. B. dem Gemischspiegel, von Vorteil ist. Die implizite Wiedergabe der aktuellen Position des Gemischspiegels sowie ein spezielles Interface ermöglichen die
Beachtung der Entwicklung einer Schaumkrone am Übergang zwischen dem
Zweiphasengemisch und dem Gasraum. Die weitgehende Entkopplung der einzelnen
Phänomene erlaubt eine modulare Codestruktur, bei der Modelle für
Einzelphänomene leicht ausgetauscht werden können.

The EUROBALL Cluster detector is composed of seven encapsulated Ge detectors
in a common cryostat with a total volume of 2000 ccm of HP Germanium. The
development and the performance of the Cluster detector is summarized.
Up to six Cluster detectors were used in pre-EUROBALL experiments at the
S-DALINAC Darmstadt, at the tandem-postaccelerator facility of the MPI-K
Heidelberg and the UNILAC at GSI. Examples of these experiments - the excitation
of dipole modes with (gamma, gamma')-reactions and the first coincidence
spectroscopy of the N=Z nucleus ⁶⁸Se with a CLUSTER Cube - are
discussed. The development of segmented encapsulated Ge detectors for a
MINIBALL at the radioactive beam facility REX-ISOLDE has been launched.
The status of the project is presented.

Fe/Al nm-period multilayers deposited by pulsed laser deposition were investigated by conversion electron Mößbauer spectroscopy and transmission electron microscopy. No Fe-Al intermetallic phases were found in the
intermixed transition range with an Al-concentration of 20-60 at.%. A qualitative depth distribution of the impurity concentration in the transition layers between subsequent Fe and Al layers is suggested.

The safety concept of the new innovative boiling water reactor
SWR1000, which is developed by Siemens AG, is aiming at the
increase of safety margins and of the grace period for operator
actions during accidents by increasing the water inventory
inside the pressure vessel, by the arranging large water pools
in- (core flooding pool) and outside of the containment (dryer-
separator storage pool), simplifying the (emergency) cooling
systems and further decreasing the failure probabilities of
safety systems. Therefore, active safety systems are substituted
by passive ones or combined with passive safety systems in cases
where a replacement is not possible. The function of passive
systems is directly based on the physical phenomena: gravity,
natural convection and/or evaporation. Their effectiveness is in
dependent of operator actions. Therefore, smaller failure rates
are assigned to passive safety systems. The probability of
severe accidents of the new SWR is reduced significantly.

For the experimental investigation of the operation mode and the
effectiveness of these passive safety systems the multipurpose
thermohydraulic test facility NOKO was constructed at the
Forschungszentrum Jülich (FZJ) within a German collaboration
sponsored by the German Federal Ministry of Education, Sciences,
Research and Technology (BMBF) and German Utilities (EVU). The
facility has a maximum power of 4 MW for steam production and a
maximum operating pressure of 10 MPa.

In the EU BWR R&D Cluster six test series with an emergency
condenser test bundle and four test series with a single tube
were performed in 1996. Within the Physics and Thermalhydraulics
Complementary Actions to the BWR Cluster (BWR/CA) the Forschungs
zentrum Rossendorf (FZR) e.V. has performed 10 post test calcula
tions and an additional blind test calculation of NOKO bundle
experiments. The tests were selected by FZJ. The results of
these calculations are presented in this paper.

Post test calculations of NOKO experiments were performed with
an improved version of ATHLET. To calculate the heat transfer
coefficients during condensation in horizontal tubes the module
KONWAR has been developed and implemented in ATHLET. KONWAR is
based on the flow regime map of Tandon and includes several semi-
empirical correlations for the determination of the heat trans
fer coefficients. The post test calculations presented here
prove qualitatively and quantitatively the expected operation
mode and the capacity of the emergency condenser. Additionally
the blind calculation proves, that the improved version of
ATHLET (ATHLET with the extension KONWAR) is able to perform
proper calculations.

Control elements of VVER-440 type reactors consist of absorber and fuel parts. They can approximately be considered as spatial double pendulums. The length of both parts is equal to the height of the core. In the present paper the main interest is directed to the neutron noise due to random vibrations of the fuel parts (lower parts of the pendulums). It is assumed, that the fuel parts of all control elements are completely inserted into the core. A homogeneous cylindrical reactor model is used to describe the neutron flux. It is also assumed, that the control elements can be excited to flow induced random 3D transverse oscillations and pendulum motions. The transfer functions for the considered two kinds of control element motions were calculated by means of the Prompt-Response-Approximation for the Modified-One-Group-Diffusion-Equation. Using numerical results, studies have been made to clear up the contribution of the random pendulum motions to the neutron noise.

A finite-element-model describing the mechanical vibrations of the whole
WWER-440 primary circuit was established to support the early detection of mechanical component faults. A special fluid-structure module was developed to consider the reaction forces of the fluid in the downcomer upon the moving core barrel and the rector pressure vessel. This fluid-structure interaction module is based on an approximated analytical 2D-solution of the coupled system of 3D fluid equations and the structural equations of motions. By means of the vibration model all eigenfrequencies up to 30 Hz and the corresponding mode shapes were calculated. It is shown that the fluid-structure interaction strongly influences those modes that lead to a relative displacement between reactor pressure vessel and core barrel. Moreover, by means of the model the shift of eigenfrequencies due to the degradation or to the failure of internal clamping and spring elements was investigated.Comparing the frequency spectra of the normal and the faulty structure, it could be proved that a
recognition of such degradations and failures even inside the reactor pressure vessel is possible by pure excore vibration measurements.

During a hypothetical accident the reactor pressure vessel internals of boiling water reactors can be exposed to considerable loads resulting from temperature gradients and pressure waves. The finite element (FE) analysis is an efficient tool to evaluate the consequences of those loads by computing the maximum mechanical stresses in the components. 3 dimensional FE models were developed for the core shroud, the upper and the lower core supporting structure, the steam separator pipes and the feed water distributor. The models of core shroud, upper core support structure and lower core support structure were coupled by means of the substructure technique. All FE models can be used for thermal and for structural mechanical analyses. As an example the FE analysis for the case of a station black-out scenario (loss of power supply for the main circulating pumps) with subsequent emergency core cooling is demonstrated. The transient temperature distributions within the core shroud and within the steam dryer pipes as well were calculated based on the fluid temperatures and the heat transfer coefficients provided by thermo-hydraulic codes. At the maximum temperature gradients in the core shroud, the mechanical stress distribution was computed in a static analysis with the actual temperature field being the load. It could be shown that the maximum resulting material stresses do not exceed the permissible thresholds fixed in the appropriate regulations. Another scenario which was investigated is the break of a feed water line leading to a non-symmetric subpressure wave within the reactor pressure vessel. The dynamic structural response of the core shroud was assessed in a tranisient analysis. Even for this load case the maximum resulting stresses remain within the allowed limits at any time.

The coupling of advanced thermohydraulic codes with 3-dimensional neutron kinetic codes corresponds to the effort to replace conservative estimations by best estimate calculations. ATHLET is an advanced thermohydraulic code, developed by the German "Gesellschaft für Anlagen- und Reaktorsicherheit" (GRS). The DYN3D code, developed at Forschungszentrum Rossendorf (FZR) for the simulation of reactivity initiated accidents in nuclear reactors with hexagonal and Cartesian fuel element cross section geometry comprises 3-dimensional neutron kinetics, models for the thermohydraulics of the core and the thermomechanical fuel rod behaviour.

Both versions of DYN3D for hexagonal and Cartesian fuel element geometry were coupled with ATHLET according to two basically different strategies. The first way of coupling uses only the neutron kinetics part of DYN3D (internal coupling). In the second way, the whole core is cut out from the ATHLET plant model and is completely described by DYN3D (external coupling). In this case the values of pressure, mass flow rate, enthalpy and boron acid concentration at the bottom and at the top of the core have to be transferred between the codes. This way of coupling is efficiently supported by the General Control and Simulation Module (GCSM) of ATHLET.

The coupled code DYN3D/ATHLET was used to investigate the possibility of recriticality during an asymmetrical overcooling of the reactor core of a NPP with VVER-440 after a main steam line break (MSLB). This MSLB analysis was performed for hot zero power and end of fuel cycle conditions. Different coolant mixing conditions in the lower plenum of the reactor were simulated. The results show the importance of these conditions. In case of a realistic mixing model and without consideration of mixing, i.e. where each loop is connected to a particular 1/6 sector of the core, a recriticality after reactor scram was predicted. For ideal mixing only, recriticality can be avoided.

In the case of external events (e.g. earthquake or explosion pressure wave) the reactor pressure vessel (RPV) and its internal components are excited to mechanical vibrations. In such cases it is important to know the dynamic component stress for the assessment of the mechanical integrity of the system.
For this reason a theoretical global vibration model based on the finite element method (FEM) was developed for a German BWR. The model comprises the RPV including the main coolant pumps (MCP), the control element drives, the core flux measuring pipes and the RPV internals.

In 1997 a "BWR Physics and Thermohydraulic Complementary Action (BWR-CA)" was established to further assist and broaden the objectives of the "European BWR R&D Cluster for Innovative Passive Safety Systems". The activities are divided into 4 work packages (WP). The work presented in this paper is performed in WP2: Passive decay heat removal from the core region, where two different decay heat removal systems were investigated. These are isolation condensers (e.g. as operated in Gundremmingen A, Dodewaard or in the PANDA facility) or emergency condensers (e.g. as foreseen on the SWR1000 and tested in the NOKO facility).

Within the BWR Cluster the Forschungszentrum Jülich had performed 10 NO-KO tests. Six of these tests were carried out with a four tube bundle, four tests with a single tube. Several post test calculations of NOKO tests were performed in the BWR-CA by CEA with CATHARE V1.3U and FZR with an improved version of ATHLET Mod. 1.1 Cycle C. Additionally FZJ had specified data for a blind calculation. The blind NOKO calculation - as well as the other NOKO post test calculations - proves, that the modified version of CATHARE as well as the improved version of ATHLET (ATHLET coupled with KONWAR) are able to predict the global parameters in the range of the fluctuations of the measured values. In the next complementary action it is planned to perform additional post test calculations of the single tube experiments, experiments with gas/non-condensable mixtures and pretest calculations of a second optimized emergency condenser test bundle. The optimization suggest is to increase the heat transfer, which allows to decrease the number of heat exchanger tubes and the emergency condenser dimensions. This is desirable because it reduces the surface of the pressurized BWR circuit and the possibilities for leakages, which is a safety relevant task.

Loss of vacuum accidents (LOVA) in fusion reactors will
lead to an air ingress into the vacuum vessel, oxidation
of the hot armour material and a partial mobilization of
the sorbed tritium. Also burnable carbon monoxide will
be formed during these accidents. In continuation to the
testing of carbon candidate materials for the First Wall
under oxidizing conditions in oxygen and steam in
collaboration with NET the carbon compound Aerolor AO5
of Carbone Lorraine has been examined in detail in the
experiments INDEX2 and SPALEX of the Institute of Safety
Research and Reactor Technology of Forschungszentrum
Jülich. The chosen parameters for these oxidation
experiments with oxygen in the inpore diffusion
controlled regime are temperatures between 973 K and
1173 K and partial pressures of oxygen between 1140 Pa
and 21000 Pa at system pressures between and; that means
reaction gas con centrations between 1 vol.-% and 20
vol.% oxygen in argon. From these results a semi-
empirical kinetic equation for the heterogeneous primary
reaction was developed. The equation considers the
dependence of the reaction rate on temperature, partial
pressure and burn-off of the material.
For calculations of LOVA the code RALOC Mod 4.0 AD -
developed by the Gesellschaft für Anlagen- und
Reaktorsicherheit (GRS) mbH has been extended by the
model COX (Carbon Oxidation) to consider carbon
oxidation kinetics. This kinetic model is based on the
above mentioned experimental results. First calculations
have been done for leak areas of 0.01 m², 0.1 m² and 1.0
m² and for different numbers of nodes in the reaction
chamber. The comparison of these results with earlier
calculations of JRC Ispra, GRS and Technicatome showed
that the new model COX and the nodalisation scheme lead
to more reliable statements concerning the spatial
dependencies especially for the burn-off of the armour
material and for the masses of the formed gases CO and
CO2.

Es wird ein Ultraschall-Meßverfahren für Zweiphasenströmungen vorgestellt, das die simultane Messung der Volumenstromdichte der Gas- und der Flüssigphase in einer Rohrleitung ermöglicht. Ein Prototyp wurde in Zusammenarbeit mit der Universität Nishny Novgorod (Rußland) aufgebaut. Es realisiert einen gepulsten Transmissions- und Reflexionsbetrieb mit Wellenleitersensoren, die auch an heiße Rohrleitungen bis zu 350°C direkt angekoppelt werden können. Kernstück der Meßmethode ist ein Mustererkennungsverfahren, das in bestimmten Volumenstrombereichen nach einer umfangreichen Kalibrierung (Trainingsprozeß) einen Meßfehler von kleiner 10 % besitzt, wenn die thermodynamischen und geometrischen Randbedingungen nahezu konstant bleiben. Durch die Erweiterung der Trainingsmatrizen um einen, die thermodynamischen Eigenschaften determinierenden Parameter (z. B. Temperatur, Druck) kann das Klassifizierungsverfahren auch auf Fälle mit veränderlichen Stoffwerten angewandt werden. Das Mustererkennungsverfahren und die Ultraschalltechnik wurden experimentell optimiert und erprobt. Die Ultraschallsignale wurden außerdem mit Hilfe der Methode des unüberwachten Lernens klassifiziert. Die gefundenen Gruppen von Signalmustern weisen eine deutliche Ähnlichkeit zu bekannten Strömungskarten auf. Es wird gezeigt, daß das Verfahren des unüberwachten Lernens sich für die objektive Aufstellung von Strömungskarten eignet. In einem speziellen Kapitel werden die Ergebnisse von Tests zur Füllstandsmessung in einem Segment der Hauptumwälzleitung von Druckwasserreaktoren vorgestellt.

Abstract:

We study the reaction NN → NNΦ slightly above the threshold within an extended one-boson exchange model which also accounts for uud knock-out. It is shown that olarization observables, like the beam-target asymmetry, are sensible quantities for identifying a ss admixture in the nucleon wave function on the few per cent level.

New examples of characterization of vacancy-type defects
in ion implanted and annealed SiC by the established technique of slow
positron implantation spectroscopy are presented. In particular, the estimation
of the depths of damaged regions and their change (a) after post-irradiation
annealing, or (b) due to variation of substrate temperature during implantation,
is addressed.

Im Rahmen des Forschungsvorhabens 15 NU 09485 "Berechnung des passiven Notkondensators eines mit Naturumlauf arbeitenden innovativen Siedewasserreaktors (SWR600) mit ATHLET" wurden vom Forschungszentrum Rossendorf e.V. Auswertungen und Interpretationen von Messungen zu Strömungsformen bei Kondensationsvorgängen in einem umfangreich instrumentierten Einzelrohr durchgeführt. Ziel der Experimente war, die Aussagesicherheit der NOKO-Bündelexperimente zu erhöhen sowie experimentelle Daten für die Validierung von Thermohydraulikprogrammsystemen (u. a. den ATHLET-Code) bereitzustellen.
Die theoretische Modellierung des Wärmeübergangs an der Innenseite der Notkondensatorrohre erfordert die Identifizierung der im aktuell betrachteten Rohrquerschnitt vorliegenden Strömungsform sowie die Auswahl einer geeigneten (d.h. strömungsformabhängigen) Korrelation zur Berechnung des Wärmeübergangskoeffizienten. Sensitivitätsanalysen haben gezeigt, daß sich im Betrieb der Notkondensatoren in Abhängigkeit von der Wirkung der Dampfschubspannung auf den Kondensatfilm sowie dessen Schwerkrafteinfluß zahlreiche Strömungsformen (z. B. eine Ring-, Schichten, Blasen-, Schwall- und Pfropfenströmung) einstellen. Da der Flüssigkeitsanteil längs des Kondensatorrohres zunimmt, können die verschiedenen Strömungsfor-men nacheinander auftreten bzw. ineinander übergehen.

Plasmagestütztes Randaufsticken ist eine erfolgversprechende Technologie zur Verbesserung der Verschleiß- und der Korrosionseigenschaften von Bauteilen aus nichtrostendem Stahl. Bei moderaten Temperaturen (ca. 400°C) kann eine Randschicht mit einem hohem Gehalt an gelöstem Stickstoff (ca. 20at.%) erzeugt werden. Diese Randschicht weist eine deutlich erhöhte Härte und Verschleißbeständigkeit und eine hervorragende Korrosionsbeständigkeit auf. Die Prozesse, die zur Bildung dieser Randschicht führen, sind weitgehend unverstanden. Um den Einfluß der passivierenden Oxidschicht auf die Bildung der stickstoffreichen Randschicht zu untersuchen, ist ein in-situ Experiment mit einer gekreuzten Strahlgeometrie aufgebaut worden. Bei kontrollierten Restgaszusammensetzungen wird während des plasmagestützten Randaufstickens zeit- und tiefenaufgelöste Elementanalyse betrieben. Dabei wird zum Randaufsticken eine Hochstrom-Ionenquelle und zur tiefenaufgelöste Elementanalyse „elastic recoil detection analysis“ (ERDA) verwendet. ERDA ist sensitiv auf die leichten Elemente und wurde hier hinsichtlich schneller Datenaufnahme optimiert. Quantitative zeitaufgelöste Messungen der Elementverteilungen werden für verschiedene Sauerstoffpartialdrücke vorgestellt. Die Bildung der stickstoffreichen Randschicht ist stark mit der Reduzierung der Oxidschicht korreliert. Hohe Sauerstoffpartialdrücke stabilisieren die Oxidschicht. Dieses Ergebnis stimmt gut überein mit einem einfachen Transportmodell, das die Sauerstoffadsorptionsrate und die Zerstäubungsrate berücksichtigt.

The moderator temperature coefficient (MTC) of a 1400 MW pressurized water reactor (PWR) has been measured from neutron and temperature fluctuations in different reactor positions. In order to explain the deviations observed between the estimations, numerical evaluations of the MTC were performed using the 3D two energy groups DYN3D code for a prototype PWR. The differences between the experimental MTC estimations can be partially explained considering that the numerical MTC evaluations show a definite devitation of the neutronics from point kinetics.

The effect of chromium implantation and subsequent annealing
on the hardness and wear behaviour of a silicon nitride-based ceramic is
investigated. Implantation energies were 200, 400, 1000 and 2000 keV, the
fluence was 1017 Cr+ ions cm-2 in all cases. The annealing temperatures
were 800 C, 1000 C and 1200 C. The phase composition was determined with
X-ray diffraction. Annealing at 800 C does not affect the crystal structure.
At 1000 C the -Si3N4-phase is formed for 1 and 2 MeV implantation energy.
At 1200 C, Cristobalite and Keiviite are formed at the lower implantation
energies; at higher energies -Si3N4 is formed. We discuss the phase formation
for the different implantation energies and annealing temperatures and
the relationship with the observed hardness and wear.

Surfaces of SiO2, Si3N4, Ta2O5 and glass were implanted
with Ag+ ions. Studies using X-ray diffraction (XRD) and atomic force microscopy
(AFM) show that the surface morphology is dominated by metallic silver
features consisting of regularly shaped, mostly spherical, Ag particles
spanning a size range from some nanometers to some 100 nm in diameter.
The particle size metrics, distribution, shape, and their density are shown
to be controlled by the process paramters ion dose, dose rate, and ion
energy. Adjusting the energy of the incident ions results in various degrees
of submerging into the substrate including exposed Ag features anchored
onto the surface or buried Ag particles overcoated by a thin layer of the
matrix material. The substrate material differs by its ability of stabilizing
the dispersion and blocking the Ag movement. Ta2O5 and glass substrates
provide more homogeneously dispersed Ag particles, much smaller in size
and regularly shaped compared to SiO2 and Si3N4 substrate systems. Potential
application refers to surface enhanced Raman scattering.