Notice of RetractionRisk analysis of shield tunnel segment failure based on Fuzzy Fault Tree method
Through analyzing and summarizing the types of shield tunnel segment failure and the relevant causes of the accident, Fuzzy Fault Tree Analysis (FTA) was employed to study the failure risk of segment and the Fuzzy Fault Tree system model of segment and segment-related systems were established. The potential failure modes of risk accidents and their influencing factors were discussed through qualitative and quantitative analysis. With its application on practical project, some effective control measures were proposed. This research could be valuable reference for the design, construction and control of shield tunnel segments.
Tunnelling through a frequently changing and mixed ground : a case history in Singapore
The Kranji tunnel is part of the Deep Tunnel Sewerage System in Singapore. It is approximately 12.6 km in length. Along the tunnel alignment, all the ground is composed of granite with different weathering grades (from fresh rock to residual soil). The changing ground from hard rock to mixed face and soft ground (and vice versa) at the tunnel level was anticipated. The tunnel depth along the route is between 15 m and 50 m. Two EPB TBMs were deployed at this tunnel with a bored diameter 4.90 m. These machines were designed so that both hard rock and soft ground could be excavated. The cutter head was equipped with a combination of both rippers and disc cutters. During the excavation, it was found that the frequency of the ground change between hard rock and residual soil is much higher than that expected. Due to the frequently changing ground, correspondingly the tunnel boring machine (TBM) operation mode had to be transferred frequently from hard rock tunnelling to transition mode and to earth pressure balance (EPB) close mode. It resulted in great difficulties for the TBM in an optimized operation condition. These difficulties included high cutter wear and flat cutters, tunnel face instability, water inflow at weathering interface, and time delays. In order to overcome these problems and speed up the tunnelling progress, the TBM used in the north drive was modified to attempt the frequently changing ground. The performance of the modified TBM was highly improved. However, the highly abrasive and frequently changing mixed face ground still caused high cutter wear, especially flat cutter wear. These posed many challenges to the equipment and the tunnel crew.
Numerical tools applied to power reactor noise analysis
In order to be able to calculate the space- and frequency-dependent neutron noise in real inhomogeneous systems in two-group theory, a code was developed for the calculation of the Green's function (dynamic transfer function) of such systems. This paper reports on the development as well as the test and application of the numerical tools employed. The code that was developed yields the space-dependence of the fluctuations of the neutron flux induced by fluctuating properties of the medium in the two-group diffusion approximation and in a two-dimensional representation of heterogeneous systems, for both critical systems and non-critical systems with an external source. Some applications of these tools to power reactor noise analysis are then described, including the unfolding of the parameters of the noise source from the induced neutron noise, measured at a few discrete locations throughout the core. Other concrete applications concern the study of the space-dependence of the Decay Ratio in Boiling Water Reactors, the noise-based estimation of the Moderator Temperature Coefficient of reactivity in Pressurized Water Reactors, the modeling of the beam- and shell-mode core-barrel vibrations in Pressurized Water Reactors, and the investigation of the validity of the point-kinetic approximation in subcritical systems driven by an external source. In most of these applications, calculations performed using the code are compared with at-power plant measurements. Power reactor noise analysis applications of the above type, i.e. core monitoring without disturbing plant operation, is of particular interest in the framework of the extensive program of power uprates worldwide.