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This paper shows an application of the CSAU methodology to an ATWS in a BWR reactor, when the temperature of the suppression pool is taken as the critical safety parameter. The method combines CSAU methodology with recent techniques of information theory. In this paper we use auxiliary tools to help in the evaluation and improvement of the parameters distribution that enter in the elements II and III of CSAU based methodologies. These tools have been implemented in two FORTRAN programs: GEDIPA (Generation of the Parameter Distribution) and UNTHERCO (Uncertainty in Thermal Hydraulic Codes). The first one analyzes the information data available on a given parameter or parameters with the goal to know all the information about the probability distribution function of these parameters. The second apply information theory methods, as the maximum entropy principle (MEP) and the maximum relative entropy Principle (MREP), in order to build conservative distribution functions for the parameters from the available data. Also, the distribution function of a given parameter can be updated using the MREP principle when new information is provided. UNTHERCO performs the MONTECARLO sampling for a given set of parameters when the distribution function of these parameters is previously known. If the distribution of a parameter is unknown, then, the MEP is applied to deduce the distribution function for this parameter. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this paper is to show a validation method of a stability monitor using a BWR model with multiple Wiener noise sources, of additive and multiplicative nature. This model is solved using the modern methods to integrate stochastic differential equation systems, that are based on the stochastic Îto-Taylor expansion, and developed by Kloeden and Platen (1995), Kloeden et al. (1994). The synthetic signals generated with this BWR reduced order model with multiple Wiener processes are then used to obtain what are the optimal ways of filtering the signals for the different methods to estimate the decay ration (DR) and the natural frequency (ω) of the system. Also, for each DR estimation method, we study what is the optimal combination of algorithms to obtain the order and coefficients of the AR model that yields the best prediction of the reactor stability parameters for a broad range of DR values. [Copyright &y& Elsevier]

In this paper, we present an application of the empirical mode decomposition method [Proc. R. Soc. Lond. A 454 (1998) 903], to the stability analysis of BWR. The methodology developed in this paper decomposes the original time series data in intrinsic oscillation modes or IMFs. Then we compute for each IMF, its Hilbert amplitude spectrum and its Hilbert marginal spectrum. From the intrinsic mode related to BWR stability we have obtained by ordinary autoregressive methods the decay ratio value and the oscillation frequency. Also we have proven that the original signal can be reconstructed with seven IMFs and that this modes are mutually orthogonals. [Copyright &y& Elsevier]

In this paper we study the boundary conditions during out of phase oscillations, in a system formed by two parallel channels coupled to multimodal neutron kinetics. The fact that the pressure drop can change with time, but remains the same in all the parallel channels, leads us to analytical integration of the time derivative term of the channel momentum equation, in order to get a dynamic equation for the inlet mass flux to each channel. From these inlet mass flux dynamic equations plus the appropriate boundary conditions, we have obtained the expression of the time dependent common pressure drop to all the channels. Finally the oscillations in the pressure drop and in the inlet mass flux to the channels have been investigated when out of phase oscillations take place. [Copyright &y& Elsevier]

This paper deals with the instabilities in advanced heavy water reactors driven by natural circulation at high pressure and power conditions. This paper also study which are the design and operating parameters that have more influence on the appearance of type II instabilities. Some of the results are compared with usual BWR reactors to stablish which are the differences between both types of reactors in the development of limit cycle oscillations and the maximum amplitude values of these oscillations when some of the influential parameters on the stability are changed. This paper also explains the main characteristics of the modelling used to simulate the fuel channels, the risers or outlet feeders, the recirculation loop of this reactor, and the kinetic and heat transfer models. In addition, we compare the closure relations used in this model with the experimental correlations for the AHWR, to know the applicability of the correlations for the AHWR under natural circulation conditions. Finally, we study in this paper the influence on the decay ratio (DR) and the limit cycle amplitude of a set of design and operational parameters. Some of these parameters are: the amount of coolant direct heating by neutrons and gamma-rays, the form loss coefficient at the channel inlet, the form loss coefficient in the spacers, the phase change number N p c h , the characteristic natural circulation time and the effective inertia of the recirculation loop. [ABSTRACT FROM AUTHOR]

Within the 5th Euratom framework programme the NACUSP project focuses on natural-circulation and stability characteristics of Boiling Water Reactors (BWRs). This paper gives an overview of the research to be performed. Moreover, it shows the first results obtained by one of the four experimental facilities involved. Stability boundaries are given for the low-power low-pressure operating range, measured in the CIRCUS facility. The experiments are meant to serve as a future validation database for thermohydraulic system codes to be applied for the design and operation of BWRs. [Copyright &y& Elsevier]