[en] It can be shown by modelling with the SSYST code and publicated experimental results cladding behaviour may essentially influence the course of a LOCA, especially the oxidation, creeping and bursting of the fuel rod cladding. For that reason, the safety analysis of WWER type reactors based on Zircaloy data may be usefull only for parametric studies, but that will not do on principle. The publicated data of the material behaviour of ZrNb 1 in the high temperature range enables the basic material phenomena to be described. But the modelling of all processes playing an essential part in cladding behaviour under LOCA conditions still requires further experiments to be carried out. (author)

[en] The (p,V_m,T) behaviour of (octane + 1-chloropentane) was measured at temperatures (298.15, 308.15, 318.15, and 328.15) K in the pressure range (0.1 to 40) MPa with an accuracy in density of ±1·10^-4 g·cm^-3. An apparatus to measure the (p,V_m,T) of liquids and liquid mixtures, whose main part is a high-pressure vibrating-tube densimeter working in a static mode, was used. Excess molar volumes were calculated from the data and fit to the Redlich-Kister equation

[en] Changes of fuel element design and modifications of the operational conditions have to be tested in experiments and pilot projects for nuclear safety. Experimental design is an useful statistical method minimizing costs and risks for this procedure. The main problem of our work was to investigate the connection between failure rate of fuel elements, sample size, confidence interval, and error probability. Using the statistic model of the binomial distribution appropriate relations were derived and discussed. A stepwise procedure based on a modified sequential analysis according to Wald was developed as a strategy of introduction for modifications of the fuel element design and of the operational conditions. (author)

[en] The SSYST code is a modular computer code developed at Karlsruhe Nuclear Research Centre (KFK) for modelling light water reactor fuel rods. This paper describes the experiences in modelling WWER type fuel rods with SSYST at the Institute of Nuclear Research Rossendorf. Problems of coupling SSYST code with STOFFEL-1, a fuel performance modelling code, are also discussed. (author). 4 refs, 4 figs, 1 tab

[en] The densities of (benzene + isopropylbenzene, or 1,2,4-trimethylbenzene, or 1,3,5-trimethylbenzene) were measured at temperatures (298.15, 308.15, 318.15, and 328.15) K by means of a vibrating-tube densimeter. The excess molar volumes V_m^E calculated from the density data provide the temperature dependence of V_m^E in the temperature range of (298 to 328) K. The V_m^E results were correlated using the fourth-order Redlich-Kister equation, with the maximum likelihood principle being applied for the determination of the adjustable parameters. It was found that the deviations from ideal behaviour in the systems studied (all being positive) decrease with increasing temperature

[en] So far fuel rods with annular pellets have been modelled by the SSYST code neglecting the central hole. Direct modelling of the central hole is not possible by using SSYST. However, two possibilities of the SSYST code have been found to take the central hole into account. The SSYST model of a central hole is described. It is shown on two examples that an approach neglecting the central hole leads to a non-conservative approximation of the solution in the field of safety analysis. (author)

[en] The proof that changes in the fuel element concept must not result in an increased fuel element failure rate can only be furnished in respect of operating power reactors. Suitable strategies of respective investigations can be derived from methods of mathematical experiment planning. The model of binomial distribution represents an appropriate basis. Data examples are given for estimates concerning confidence limits and the extent of necessary random samples. A sequential stepwise procedure has proved to be advantageous. A modification of the sequential analysis by Wals provides a simple statistical approximation to this effect. (orig.)

[en] MULTRAN is a module of the SSYST code for high temperature oxidation modelling of Zircaloy cladding under LOCA conditions. It bases on a diffusion model. Isothermal high temperature oxidation experiments with ZrNb1 tubes wre used for developing an input data set and its verification. Using this input data the MULTRAN results could be accepted. The sensitivity of the MULTRAN model was investigated by variations of the input data. It was shown, that the oxide properties influence the oxidation strongly. Numerical problems occurred at modelling the cooling phase of a transient experiment. (orig.)

[en] The oxidation behaviour of ZrNb was investigated in the temperature range of 700-1200deg C in water steam atmosphere. Some experiments was carried out with Zircaloy test specimens for comparing. The kinetics of oxidation and weight gain are similar for both materials. There are evident differences in the structure of the oxide scale and of the O-stabilized alpha layer. Also the oxygen distribution within the metal and the solubility of oxygen from each other differ. This leads to a rapid reduction of room-temperature ductility of ZrNb material. A statistical analysis demonstrates, that the experimental results can be described by the kinetic equations developed by Vrtilkova et al. These equations are also applicable to transient oxidation conditions. (orig.)

[en] Programs SSYST and FRAS were set up for simulating the behavior of light water reactor fuel elements in accident states. They were tested on two cases of thermomechanical calculations for WWER type reactor fuel elements under LOCA conditions. The first hypothetic case was chosen so as to attain the maximum permissible cladding temperature; the boundary conditions for the time-dependent coefficients of cladding-to-coolant heat transfer and for the coolant pressure and temperature were chosen accordingly. Diverse boundary conditions were chosen in the other case; they included the coolant flow rate, pressure and enthalpy in the upper and lower sections of the reactor vessel. The comparative results are given in tables and diagrams. A good agreement was achieved between the two programs. The program SSYST proved to suit well for simulating the behavior of fuel elements possessing the WWER geometry. (Z.M.). 14 figs., 3 tabs., 8 refs