[en] Based on a two-dimensional model, the representation of control element oscillations in the relative fluctuations of neutron flux density, considering the space dependence of the problem and heterogeneous loading, are studied at the WWER-440 during normal operation. In particular, the dependence of corresponding transfer functions on the burnup, on xenon poisoning, and boric acid concentration correlated with those two quantities, are considered. Calculation of the noise sources is performed by the two-group diffusion approximation, while the transfer of neutron flux density disturbances is described by a nodal method which is based on the effective one-group equation. To eliminate frequency dependence, the prompt response approximation is used. The results are compared with experimental data and discussed, and the reasons of occurring effects explained. (orig.)

[en] A new generalized model for bubble coalescence and breakup has been developed. It is based on physical considerations and takes into account various mechanisms that can lead to bubble coalescence and breakup. First, in a detailed literature review, the available models were compiled and analyzed. It turned out that many of them show a contradictory behaviour. None of these models allows the prediction of the evolution of bubble size distributions along a pipe flow for a wide range of combinations of flow rates of the gas and the liquid phase. The new model has been extensively studied in a simplified Test-Solver. Although this does not cover all details of a developing flow along the pipe, it allows - in contrast to a CFD code - to conduct a large number of variational calculations to investigate the influence of individual sizes and models. Coalescence and breakup cannot be considered separately from other phenomena and models that reflect these phenomena. There are close interactions with the turbulence of the liquid phase and the momentum exchange between phases. Since the dissipation rate of turbulent kinetic energy is a direct input parameter for the new model, the turbulence modelling has been studied very carefully. To validate the model, a special experimental series for air-water flows was used, conducted at the TOPFLOW facility in an 8-meter long DN200 pipe. The data are characterized by high quality and were produced within the TOPFLOW-II project. The test series aims to provide a basis for the work presented here. Predicting the evolution of the bubble size distribution along the pipe could be improved significantly in comparison to the previous standard models for bubble coalescence and breakup implemented in CFX. However some quantitative discrepancies remain. The full model equations as well as an implementation as ''User-FORTRAN'' in CFX are available and can be used for further work on the simulation of poly-disperse bubbly flows.

[en] Relative fluctuations of neutron flux density caused by control rod vibrations during regular operation are investigated on the basis of a two-dimensional model. The local dependence of the fluctuations and the heterogeneous arrangement of fuel elements are taken into account. Special consideration is given to the dependence of the transfer functions on burnup, xenon poisoning and boron concentration, the latter being correlated with the two other parameters. The results of the calculations are discussed and compared with experimental data. The causes of the observed effects are explained. (orig./HP)

[en] An LDRD (Laboratory Directed Research and Development) project is underway at the Idaho National Laboratory (INL) to apply the three-dimensional multi-group deterministic neutron transport code (Attila) to criticality, flux and depletion calculations of the Advanced Test Reactor (ATR). This paper discusses the development of Attila models for ATR, capabilities of Attila, the generation and use of different cross-section libraries, and comparisons to ATR data, MCNP, MCNPX and future applications

[en] 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 fur Anlagen- und Reaktorsicherheit (GRS). Up to now only point kinetics and 1-dimensional neutron kinetics have been included. The DYN3D code, developed in the Research Centre Rossendorf (RCR) for the improvement of the simulation of reactivity initiated accidents in nuclear reactors with hexagonal fuel elements comprises 3-dimensional neutron kinetics, models for the thermohydraulics of the core including heat transfer from the fuel to the coolant and a fuel rod behavior model. The reactor core model DYN3D was coupled with the ATHLET code on two basically different ways. The first way of coupling uses only the neutron kinetics part of the DYN3D code (internal coupling). This coupling along the core is very close and demands an high effort of programming due to the high number of coupling parameters. In the second way the whole core is cut out from the ATHLET plant model. The core is completely modeled by the DYN3D code (external coupling). In this case the interfaces are located at the bottom and at the top of the core. At this interfaces the pressures, mass flow rates, enthalpies and concentrations of boron acid have to be transferred. This way of coupling is easy to realize by interconnection of an interface routine. It is effectively supported by the General Control and Simulation Modul (GCSM) of the ATHLET code. Almost no changes of the single programs are necessary. Another advantage of this coupling is that the complete DYN3D model can be used. The disadvantage of this method is the splitting of the thermohydraulics

[en] The advanced thermohydraulic code ATHLET was coupled with the three-dimensional reactor core model DYN3D. The coupling was accomplished realizing two different coupling strategies. Operating tests of the coupled code system were performed. (orig./DG)

[en] The main goal of the project was the coupling of the 3D core model DYN3D for Russian VVER-type reactors, which has been developed in the RCR, with the thermohydraulic code ATHLET. The coupling has been realized on two basically different ways: - The implementation of only the neutron kinetics model of DYN3D into ATHLET (internal coupling), - the connection of the complete DYN3D core model including neutron kinetics, thermohydraulics and fuel rod model via data interfaces at the core top and bottom (external coupling). For the test of the coupling, comparative calculations between internal and external coupling versions have been carried out for a LOCA and a reactivity transient. Complementary goals of the project were: - The development of a DYN3D version for burn-up calculations, - the verification of DYN3D on benchmark tasks and experimental data on fuel rod behaviour, - a study on the extension of the neutron-physical data base. The project contributed to the development of advanced tools for the safety analysis of VVER-type reactors. Future work is aimed to the verification of the coupled code complex DYN3D-ATHLET. (orig.)

[en] A molecular beam mass spectrometer was used to obtain quantitative data on the species present in the core regions of nitrogen and air plasma jets. The dilution of the jet as ambient gas is entrained into the turbulent plume was measured by probing an argon jet discharging into a nitrogen atmosphere. The gas from the jet igniter cavity is diluted by more than a factor of ten in the first 4 mm downstream of the orifice, with a further dilution of 40% over the next 8 mm of jet penetration. Comparison of peak mole fractions of reactive species in the jet with the dilution results indicate that the plasma species are not decaying at a rate solely due to dilution. The nitrogen plasma jet was probed for metastable nitrogen (N/sub 2/*) and N atoms as a function of time at various distances along the centerline of the jet. Peak mole fractions of metastable nitrogen and N atoms plotted as a function of probe location demonstrate that N/sub 2/* persists at distances farther downstream than N atoms. Metastable nitrogen concentrations are relatively constant from 4 to 14 mm, indicating that metastable nitrogen is produced in the plume. Nitrogen atoms, oxygen atoms, and nitric oxide were measured in an air plasma. Nitric oxide persists significantly longer than the atoms, and has a peak mole fraction at the point where O and N decrease significantly. The O atom mole fraction of 0.02 is an order of magnitude higher than the N atom concentration, but the radical concentration in the air plasma is lower than that in pure N/sub 2/. No evidence of metastable nitrogen was found in this system. The chemistry with respect to these results is discussed, both in terms of the kinetics of the system and calculated equilibrium concentrations

[en] Gathering data and reporting on an accident such as at a nuclear power plant where a human performance problem is implicated is not a straightforward activity. A variety of factors can affect the level of detail reported about a particular incident. In this paper a classification system is proposed which can be used to assess whether a particular accident reporting scheme would collect accurate and detailed information on human performance problems and their root causes. In addition, a demonstration system of how assistance can be given in collecting and analysing relevant data is outlined. (author)

[en] Purpose: To validate dose calculation for a rigidly moving object with jaw motion and MLC shifts to compensate for the motion in a TomoTherapy™ treatment delivery. Methods: An off-line version of the TomoTherapy dose calculator was extended to perform dose calculations for rigidly moving objects. A variety of motion traces were added to treatment delivery plans, along with corresponding jaw compensation and MLC shift compensation profiles. Jaw compensation profiles were calculated by shifting the jaws such that the center of the treatment beam moved by an amount equal to the motion in the longitudinal direction. Similarly, MLC compensation profiles were calculated by shifting the MLC leaves by an amount that most closely matched the motion in the transverse direction. The same jaw and MLC compensation profiles were used during simulated treatment deliveries on a TomoTherapy system, and film measurements were obtained in a rigidly moving phantom. Results: The off-line TomoTherapy dose calculator accurately predicted dose profiles for a rigidly moving phantom along with jaw motion and MLC shifts to compensate for the motion. Calculations matched film measurements to within 2%/1 mm. Jaw and MLC compensation substantially reduced the discrepancy between the delivered dose distribution and the calculated dose with no motion. For axial motion, the compensated dose matched the no-motion dose within 2%/1mm. For transverse motion, the dose matched within 2%/3mm (approximately half the width of an MLC leaf). Conclusion: The off-line TomoTherapy dose calculator accurately computes dose delivered to a rigidly moving object, and accurately models the impact of moving the jaws and shifting the MLC leaf patterns to compensate for the motion. Jaw tracking and MLC leaf shifting can effectively compensate for the dosimetric impact of motion during a TomoTherapy treatment delivery.