Project Subjects
Seismic Margin Evaluation of Civil-engineered Structures at Nuclear Power Plants
Background and Objective
Since the Nuclear Safety Commission revised the Regulatory Guide for Reviewing Seismic Design ofNuclear Power Reactor Facilities in 2006, electric power companies have been required to reevaluate the seismic
safety of existing nuclear power plants (back-checks), seismic margin evaluation, residual risk evaluation, etc.
in response to public concerns about the seismic safety of nuclear power plants. The seismic margin evaluation
aims to facilitate public understanding with respect to seismic safety of nuclear power plants by the quantitativeevaluation of seismic margin.
The purposes of the project are to investigate the limit of strong-ground motion, the functional limit
strength of structures, and to show how to communicate the risks with citizens in order to establish seismic margin
evaluation method for civil-engineering structures at nuclear power plants.

Main results
1. Simultaneous rupture evaluation of active fault segments
It is important to estimate the simultaneous rupture of active fault segments for seismic design of nuclear
power plants. In order to detect important factors in the propagation of fault ruptures, we performed morphological, geological and geophysical surveys around the step-over between the two active faults which moved in the1891 Nobi earthquake (Fig. 1). As preliminary results, it may be inferred that (1) the fault structure in a step-overand step-over length, and (2) abrupt change of seismic velocity structure are important factors.
2. Evaluation of tensile strength and dynamic characteristics of rock mass
When an enormous earthquake occurs, the foundation ground under a reactor building and the surrounding slopes are subjected to tensile stress. The tensile strength has been estimated to be zero for various reasons, forinstance, the reliable tensile strength of the rock mass has not been able to be measured in the ﬁeld. We developed
a uniaxial tension test apparatus (Fig. 2) that can measure the tensile strength of rock mass in the ﬁeld for the ﬁrst
time in the world to show that the foundation ground and the surrounding slopes will withstand strong larger-thanexpected ground motions.
3. Flowchart of seismic stability assessment of rock slopes
A new seismic stability assessment ﬂow for surrounding rock slopes, which includes evaluation of theinﬂuence of sliding blocks, was proposed. The experiments concerned with the maximum rolling distance ofrocks were conducted [N09029]. The way to separate a sliding surface which is able to fall through the weight ofsliding blocks from a sliding surface which only moves during an earthquake was also included [N09030].
4. Development of methods for seismic margin evaluation and risk communication
It is required that we clarify the relationship between seismic margin and risk for structures because
the government of Japan may quantitatively prescribe safety targets based on probabilistic risk. Therefore, we
propose a diagram which relates to structural seismic margin (the difference between limit and design value) toseismic risk (residual risk). This diagram makes it possible to estimate the seismic risk level based on the deterministic seismic margin.

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Nuclear Technology

The loading plate is pulled
by mechanical jacks.

center guidelinear guidemechanical jacks（Cyclicloading test is possible.)load cellloading plate

The loading plate isglued onto a top endof the rock specimen.

φ60cm rock specimen(hollow cylindrical)
tensile f ailure

Rock mass

Fig. 1 Possible factors in the propagation ofearthquake ruptures around the step-overbetween the Nukumi and Neodani faults in
the 1891 Nobi earthquake fault system

The center guide is f ixedon the center hall in therock（as a reaction f orce).

Fig. 2 Uniaxial tension test apparatus
The rock specimen is pulled along the center guideusing mechanical jacks.

slidingsurface

Fig. 3 Flowchart of seismic stability assessment of rock slopes

Fig. 4 Structural capacity (limit acceleration) vs. seismic risk
We can relate seismic margin which is deﬁnes as
structural capacity (i.e. limit acceleration) to seismic
risk (annual failure probability) based on the seismic
hazard curve for a site and structural fragility curve.

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