Swedish BWRs Research Articles

Wavelet techniques for the determination of the decay ratio in boiling water reactors

Abstract The usefulness of wavelet transform and wavelet filtering techniques for the improvement of the estimation of the decay ratio (DR), characterising the stability of BWRs, is discussed. There are two distinct areas investigated. The first concerns the improvement of the quality of the traditionally used auto-correlation function (ACF) for the estimation of the DR, by trend elimination and denoising. The subsequent estimation of the DR itself is made by traditional methods such as the peak-to-peak method or curve fitting. The second area is the estimation of the DR by the use of continuous wavelet transform. The possibility of estimating two different DRs in case of dual oscillations, and in particular the finding of the higher DR, is also investigated. It was found that wavelet pre-processing does not always improve the estimation of the ACF of a non-ideal signal, compared to other methods; but for signals containing various trends and data scatter in the ACF, it brings a noticeable improvement. As an extension of the discrete wavelet methods, the continuous wavelet transform appears to be a promising candidate to determine the critical DR even in the case of two oscillations being co-existent with different stability properties. The methods investigated or developed here were also tested on measured data from Swedish BWRs.

Development of Core Diagnostic Methods and their Application at Swedish BWRs and PWRs

Some results of the noise diagnostic research, performed at the Department of Reactor Physics of Chalmers University of Technology are presented. This work has been made in co-operation with the Swedish safety authority and the power plants. The subjects discussed in the paper are: (a) Localization of a channel instability in a BWR; (b) Determination of the axial elevation of a control rod with neural networks in a PWR; (c) Diagnostics of core-barrel vibrations in PWRs. New methods were developed or known methods improved in order to handle these cases and the methods were tested through analysis of measurements. This paper gives a review of this work and its results.

Development of Core Diagnostic Methods and their Application at Swedish BWRs and PWRs.

Some results of the noise diagnostic research, performed at the Department of Reactor Physics of Chalmers University of Technology are presented. This work has been made in co-operation with the Swedish safety authority and the power plants. The subjects discussed in the paper are: (a) Localization of a channel instability in a BWR; (b) Determination of the axial elevation of a control rod with neural networks in a PWR; (c) Diagnostics of core-barrel vibrations in PWRs. New methods were developed or known methods improved in order to handle these cases and the methods were tested through analysis of measurements. This paper gives a review of this work and its results.

Repair and replacement of reactor internals for plant life extension

Recent experience from early Swedish BWRs corroborate that all components in a nuclear power plant can be repaired or replaced with new ones. Oskarshamn 1 has gone through a thorough refurbishment project. A number of internals were repaired or replaced including the core shroud support which was welded to the bottom of the reactor pressure vessel. The project verifies that it is fully possible to carry out complicated inspection and repair work inside a nuclear pressure vessel which has been in operation for more than 20 years. Along with increased capacity factor, operating nuclear power plants get the financial conditions needed for extensive repair and modernization projects. Large power output leads to short pay-back times for the investments. The FENIX project at Oskarshamn 1 is such a project. There are utilities whose policy is to keep their plants in as-new condition for an unlimited length of time.

Ex-vessel melt-coolant interactions in deep water pool: studies and accident management for Swedish BWRs

In Swedish BWRs having an annular suppression pool, the lower drywell beneath the reactor vessel is flooded with water to mitigate against the effects of melt release into the drywell during a severe accident. The Thirmal-1 code has been used to analyze the effectiveness of the water pool to protect lower drywell penetrations by fragmenting and quenching the melt as it relocates downward through the water. Experiments have also been performed to investigate the benefits of adding surfactants to the water to reduce the likelihood of fine-scale debris formation from steam explosions. This paper presents an overview of the accident management approach and surfactant investigations together with results from the Thirmal-1 analyses. A description of the modeling incorporated in Thirmal-1 is also provided.

BWR instrument tube vibrations: Interpretation of measurements and simulation

In 1986 and 1987, neutron noise measurements were made in two Swedish BWRs on a number of detector strings, in order to detect and quantify detector guide tube vibrations, and possibly impacting. Vibrations of various amplitude were detected in several strings, but quantification of the strength of vibrations, including impacting, was not trivial. For quantification of the vibrations, there appeared a need for spectral descriptors other than just the amplitude of the vibration peak in the detector auto power spectral densities (APSDs). To find such descriptors, and investigate their sensitivity, numerical simulation of the signal of a vibrating detector both with and without impacting was performed. Two possible descriptors of impacting, namely the widening of the vibration peak and the distortion of the amplitude probability distribution (APD) function were investigated. The first of these two was found to be a relatively reliable indicator of impacting, in accordance with the experimental findings. The use of the APD is much less effective, and reasons for this are discussed in the paper.