Assessing Subsurface Resilience at Tokyo Electric Power Company Nuclear Plants

Thesis Abstract

The subsurface conditions surrounding nuclear facilities pose critical challenges to structural integrity, resilience during seismic and hydrogeological events, and long-term waste management, yet empirical understanding of how site-specific geotechnical variability governs operational reliability remains limited. This study addresses the problem of assessing and enhancing subsurface resilience at Tokyo Electric Power Company (TEPCO) nuclear plants by integrating geotechnical characterization, geophysical imaging, and resilience-based risk assessment to inform decision-making for siting, design, and retrofit strategies. The aim is to quantify subsurface resilience indicators and to identify site-specific vulnerabilities across TEPCO’s nuclear portfolio, with objectives to (i) develop a high-fidelity geotechnical-hydrogeological model of representative plant sites, (ii) evaluate seismic velocity, lithology, pore pressure, and groundwater flow patterns under saturated and partially saturated conditions, (iii) test the applicability of a resilience framework incorporating probabilistic risk assessment and performance-based design concepts, and (iv) propose actionable mitigation measures and monitoring protocols. The methodological approach combines a mixed-methods design anchored in both quantitative and qualitative data streams. The population comprises subsurface data from three TEPCO reactors with complementary in-situ measurements, borehole logs, and monitoring records spanning 15–20 years, supplemented by publicly available seismological catalogs. A stratified random sampling of 120 boreholes and 30 cone penetration tests across plant sites will be used, with 60 pore pressure transducers installed for dynamic hydrogeological monitoring. Data collection instruments include borehole geophysics (P- and S-wave tomography, crosshole seismics), in-situ tests (Standard Penetration Tests, cone penetration tests, in-situ vane shear), multi-parameter pore pressure and groundwater monitoring, remotely sensed subsurface imagery, and expert elicitation interviews with TEPCO engineers. Validity and reliability will be ensured through calibration against standard reference sites, cross-validation of geophysical inversions, and test-retest procedures for instrumentation; content validity for interview protocols will be established via pilot testing and triangulation with technical reports. Analytical methods comprise (i) Bayesian hierarchical modeling to integrate geotechnical measurements with seismic and hydrogeological data for site-specific resilience indices; (ii) regressions and ANOVA to quantify relationships among lithology, shear wave velocity, groundwater conditions, and resilience outcomes; (iii) stochastic groundwater flow and poroelastic simulations to assess pore pressure evolution under seismic loading; (iv) NPV-based risk assessment and Monte Carlo simulations to quantify probabilistic failure probabilities; (v) sensitivity analyses to identify dominant parameters controlling resilience; and (vi) thematic analysis of expert inputs to contextualize quantitative results within TEPCO’s operational risk framework. The study anticipates finding that subsurface resilience correlates strongly with a combined metric of soil-structure interaction velocity contrasts, pore pressure response, and heterogeneity length scales, with specific thresholds indicating elevated risk during beyond-design-basis events. Expected contributions include (a) a transferable, site-specific subsurface resilience framework for nuclear installations that couples geotechnical characterization with probabilistic risk assessment, (b) a geophysical-geotechnical dataset and a calibrated model repository for TEPCO plants that supports performance-based design and retrofit prioritization, and (c) policy-relevant guidance on monitoring regimes and early-warning indicators for subsurface degradation. The research is expected to inform enhancements to TEPCO’s seismic and hydrogeological safety strategies, including prioritized ground improvement measures, adaptive monitoring networks, and decision-support tools for emergency response planning. In conclusion, the study will demonstrate that an integrated, data-driven assessment of subsurface resilience can reduce uncertainties in seismic risk and groundwater-induced instability at nuclear sites, and recommend adopting resilience-based criteria in design codes,Site Safety Reviews, and long-term stewardship plans for TEPCO facilities.