Tatsuya Itoi

Comparison of correlated Gumbel probability models for directional maximum wind speeds

Abstract In this paper we present a comparison of two probabilistic models for directional annual maximum wind speeds to clarify the characteristics of directional maximum wind speeds. First, two models for directional annual maximum wind speeds are summarized. These models are applied to recorded 10-min average wind speeds at several meteorological stations, and the consistency of models and effects of correlation coefficients between directional annual maximum wind speeds on the relation between the CDF for direction-free annual maximum wind speed and those for individual directional wind speeds are presented. The effects of correlation coefficients are clearly seen at some stations, and the usefulness of the proposed models is demonstrated for these sites.

In search of extendable conditions for cable environmental qualification in nuclear power plants

ABSTRACT The environmental qualification (EQ) for cable insulators in nuclear power plants (NPPs) has been developed on the basis of the design basis accident (DBA) to prevent reactor core damage. However, the latest safety principles require extending the design concept to prepare the utilized equipment for scenarios after core damage. Thus, we propose a modification to the EQ for cables connecting utilized equipment at design extension conditions. This paper surveys all electrical components for accident management in boiling water reactor-4 (BWR-4), and identifies their connecting cables’ functional category as low-voltage power, instrumentation, and control cables. The EQ temperature profile of these cables during the incident phase was addressed for extension. This required postulating maximum temperature environments according to accident scenarios, knowledge of cable integrity degradation, and their current evaluation by the EQ. To evaluate whether these environments are suitable stressors, heat testing was conducted on flame-retardant ethylene propylene rubber (FR-EPR)-insulated cables. On the basis of those results, we suggest a maximum primary peak temperature of the EQ temperature profile of 250 °C. We also suggest increasing the primary peak period of the EQ temperature profile to 48 h without experiment, on the basis of inherent excessive margin for mechanical integrity during the ageing phase.

Load combination of aftershocks and tsunami for tsunami-resistant design

Occurrence of huge tsunami and numerous aftershocks are expected after a gigantic subduction earthquake occurs. Therefore, the important coastal structures (tsunami refuge buildings, seawalls and nuclear power plants etc.) must be designed against tsunami as well as ground shaking. In tsunami-resistant design, it is needed to consider that tsunami may arrive at the structure in a short time after the mainshock from the experience of 2011 Tohoku earthquake. When the action effects both from aftershocks and tsunami to the structure occur simultaneously, practically reasonable assessment of load combination from aftershocks and tsunami is needed. In order to treat the load combination problem reasonably, stochastic load combination technique can be used, which requires stochastic modeling of action effects from aftershocks and tsunami. Once the combined action effect is estimated reasonably, the reliability analysis follows, where load and resistance factors can be obtained under the condition that the conditional target reliability for a limit state function is given. Load combination method of aftershocks and tsunami on the tsunami-resistant design is demonstrated at some sites in Japan. Finally, load and resistance factor design format for the tsunami-resistant design is proposed.

Seismic Probabilistic Risk Assessment

This chapter presents an overview of the seismic probabilistic risk assessment (PRA) for nuclear power plants. First, the basic concept of seismic PRA and its application to continuous improvement of seismic safety of nuclear power plants are explained. Then, the methodology of seismic PRA for nuclear power plants is explained. Seismic PRA process can be divided into probabilistic seismic hazard analysis, fragility analysis of structures/components, and accident sequence analysis. As for the probabilistic seismic hazard analysis, modeling of earthquake occurrence, a method for earthquake ground motion prediction, and logic tree model are introduced. Then, the fragility analysis of structures/components is described. This includes an explanation of the selection of failure modes, and the realistic strength and realistic response of buildings/components.

International Standards and National Regulation on Seismic Safety Assessment

Many nuclear power plants have been or will be constructed around the world, including countries and regions with middle to high seismicity. Safety standards published by the International Atomic Energy Agency (IAEA) reflect an international consensus. Each nation, e.g., the United State or Japan, develops its own codes and standards on its own responsibility. IAEA safety standards are bases for codes and standards for each nation. This chapter outlines the standardized practice with respect to seismic safety including earthquake-resistant design. IAEA safety standards and US nuclear safety regulations are introduced mainly focusing on the structures and hierarchy, as well as methods and techniques related to seismic safety assessment.

Challenges for Nuclear Safety from the Viewpoint of Natural Hazard Risk Management

Lessons learned from the Fukushima Daiichi NPP accident and challenges for enhancement of the concept of nuclear safety are summarized from the viewpoint of risk management as well as the concept of defense in depth, for the protection against natural hazards, i.e., design against natural hazards and emergency response combined with regional disaster prevention and mitigation. The concept of resilience is also discussed, as a means for refining the fundamental concept of nuclear safety.

A Framework for Seismic Design of Items in Safety-Critical Facilities for Implementing a Risk-Informed Defense-in-Depth-Based Concept

Recently, especially after the 2011 off the Pacific coast of Tohoku Earthquake and the Fukushima Daiichi Nuclear Power Plant accident, the need for treating residual risks and cliff edge effects in safety-critical facilities has been widely recognized as an extremely important issue. In this paper, the sophistication of seismic designs in safety-critical facilities is discussed from the viewpoint of mitigating the consequences of accidents, such as the avoidance of cliff edge effects. For that purpose, the implementation of a risk-informed defence-in-depth-based framework is proposed in this study. A basic framework that utilises diversity in the dynamic characteristics of items and also provides additional seismic margin to items important for safety when needed is proposed to prevent common cause failure and to avoid cliff edge effects as far as practicable. The proposed method is demonstrated to be effective using an example calculation.

Development of stochastic heterogeneous slip distribution model for simulation of earthquake ground motion

Ground motion simulation is an important tool for analyzing seismic risk in engineering systems. Recently, ground motion simulations using fault model are being widely applied. Characterized fault models are conveniently used to model the heterogeneous slip distribution on fault plane, which divides the fault into two areas, i.e., asperity area and background area. The model, however, is too simplified to model the complex characteristics of actual slip. In this paper, a model to simulate the slip distribution of the fault plane is proposed.