Current status of development in the 3D seismic isolation applied to SFRs

Yamamoto, Tomohiko; Watakabe, Tomoyoshi; Miyazaki, Masashi; Okamura, Shigeki; Miyagawa, Takayuki; Yokoi, Shinobu*; Fukasawa, Tsuyoshi*; Fujita, Satoshi*

Mechanical Engineering Journal (Internet), 11(2), p.23-00393_1 - 23-00393_21, 2024/04

https://doi.org/10.1299/mej.23-00393

Application of quasi-Monte Carlo and importance sampling to Monte Carlo-based fault tree quantification for seismic probabilistic risk assessment of nuclear power plants

Kubo, Kotaro; Tanaka, Yoichi; Hakuta, Yuto*; Arake, Daisuke*; Uchiyama, Tomoaki*; Muramatsu, Ken

Mechanical Engineering Journal (Internet), 10(4), p.23-00051_1 - 23-00051_17, 2023/08

https://doi.org/10.1299/mej.23-00051

The significance of probabilistic risk assessments (PRAs) of nuclear power plants against external events was re-recognized after the Fukushima Daiichi Nuclear Power Plant accident. Regarding the seismic PRA, handling correlated failures of systems, components, and structures (SSCs) is very important because this type of failure negatively affects the redundancy of accident mitigation systems. The Japan Atomic Energy Research Institute initially developed a fault tree quantification methodology named the direct quantification of fault tree using Monte Carlo simulation (DQFM) to handle SSCs' correlated failures in detail and realistically. This methodology allows quantifying the top event occurrence probability by considering correlated uncertainties related to seismic responses and capacities with Monte Carlo sampling. The usefulness of DQFM has already been demonstrated. However, improving its computational efficiency would allow risk analysts to perform several analyses. Therefore, we applied quasi-Monte Carlo and importance sampling to the DQFM calculation of simplified seismic PRA and examined their effects. Specifically, the conditional core damage probability of a hypothetical pressurized water reactor was analyzed with some assumptions. Applying the quasi-Monte Carlo sampling accelerates the convergence of results at intermediate and high ground motion levels by an order of magnitude over Monte Carlo sampling. The application of importance sampling allows us to obtain a statistically significant result at a low ground motion level, which cannot be obtained through Monte Carlo and quasi-Monte Carlo sampling. These results indicate that these applications provide a notable acceleration of computation and raise the potential for the practical use of DQFM in risk-informed decision-making.

Experiment of floating seismic isolation system for SMR

Mori, Takashi*; Shimada, Takahiro*; Kai, Satoru*; Otani, Akihito*; Yamamoto, Tomohiko; Yan, X.

Proceedings of 30th International Conference on Nuclear Engineering (ICONE30) (Internet), 8 Pages, 2023/05

https://doi.org/10.1299/jsmeicone.2023.30.1553

A Scoping study on the use of direct quantification of fault tree using Monte Carlo simulation in seismic probabilistic risk assessments

Kubo, Kotaro; Fujiwara, Keita*; Tanaka, Yoichi; Hakuta, Yuto*; Arake, Daisuke*; Uchiyama, Tomoaki*; Muramatsu, Ken*

Proceedings of 29th International Conference on Nuclear Engineering (ICONE 29) (Internet), 8 Pages, 2022/08

https://doi.org/10.1115/ICONE29-88773

After the Fukushima Daiichi Nuclear Power Plant accident, the importance of conducting probabilistic risk assessments (PRAs) of external events, especially seismic activities and tsunamis, was recognized. The Japan Atomic Energy Agency has been developing a computational methodology for seismic PRA, called the direct quantification of fault tree using Monte Carlo simulation (DQFM). When appropriate correlation matrices are available for seismic responses and capacities of components, the DQFM makes it possible to consider the effect of correlated failures of components connected through AND and/or OR gates in fault trees, which is practically difficult when methods using analytical solutions or multidimensional numerical integrations are used to obtain minimal cut set probabilities. The usefulness of DQFM has already been demonstrated. Nevertheless, a reduction of the computational time of DQFM would allow the large number of analyses required in PRAs conducted by regulators and/or operators. We; therefore, performed scoping calculations using three different approaches, namely quasi-Monte Carlo sampling, importance sampling, and parallel computing, to improve calculation efficiency. Quasi-Monte Carlo sampling, importance sampling, and parallel computing were applied when calculating the conditional core damage probability of a simplified PRA model of a pressurized water reactor, using the DQFM method. The results indicated that the quasi-Monte Carlo sampling works well at assumed medium and high ground motion levels, importance sampling is suitable for assumed low ground motion level, and that parallel computing enables practical uncertainty and importance analysis. The combined implementation of these improvements in a PRA code is expected to provide a significant acceleration of computation and offers the prospect of practical use of DQFM in risk-informed decision-making.

Response reduction effect of seismic isolation system considering uncertainty parameters for seismic margin assessment

Yamano, Hidemasa; Okamura, Shigeki*

Transactions of the 26th International Conference on Structural Mechanics in Reactor Technology (SMiRT-26) (Internet), 9 Pages, 2022/07

Seismic response analyses were conducted for the pipe with and without the seismic isolation system based on the response waveforms. This study performed a fragility analysis by setting uncertainty parameters on the basis of existing studies. The comparison results showed that the seismic isolation technology is effective for the pipe to prevent cliff-edge effects. In other words, the seismic margin for the seismically isolated plant is 1.2 times larger than that of the non-isolated plant. To evaluate the response reduction effect, this study focused on response coefficients of components as uncertainty parameters, which were specified within a physically possible range. Even if the uncertainty is considered, the HCLPF for the isolated plant is nearly twice as high as the non-isolated plant, namely the response reduction effect is still significant for the isolated plant. Therefore, the isolation technology is effective to avoid cliff-edge effects.

Applicability of equivalent linear analysis to reinforced concrete shear walls; 3D FEM simulation of experiment results of seismic wall ultimate behavior

Ichihara, Yoshitaka*; Nakamura, Naohiro*; Moritani, Hiroshi*; Horiguchi, Tomohiro*; Choi, B.

Nihon Genshiryoku Gakkai Wabun Rombunshi, 21(1), p.1 - 14, 2022/03

AA2020-0911.pdf:8.99MB

https://doi.org/10.3327/taesj.J20.038

In this study, we aim to approximately evaluate the effect of nonlinearity of reinforced concrete structures through seismic response analysis using the equivalent linear analysis method. A simulation analysis was performed for the ultimate response test of the shear wall of the reactor building used in an international competition by OECD/NEA in 1996. The equivalent stiffness and damping of the shear wall were obtained from the trilinear skeleton curves proposed by the Japan Electric Association and the hysteresis curves proposed by Cheng et al. The dominant frequency, maximum acceleration response, maximum displacement response, inertia force-displacement relationship, and acceleration response spectra of the top slab could be simulated well up to a shear strain of approximately =2.010. The equivalent linear analysis used herein underestimates the maximum displacement response at the time of ultimate fracture of approximately =4.010. Moreover, the maximum shear strain of the shear wall could not capture the locally occurring shear strain compared with that of the nonlinear analysis. Therefore, when employing this method to evaluate the maximum shear strain and test results, including those during the sudden increase in displacement immediately before the fracture, sufficient attention must be paid to its applicability.

Characterizing the reactivation mechanisms of coseismic surface ruptures associated with the 2011 Mw 6.7 Fukushima-ken Hamadori earthquake in Japan through borehole hydromechanical testing

Guglielmi, Y.*; Aoki, Kazuhiro; Cook, P.*; Soom, F.*; Cappa, F.*; Tanaka, Yukumo

Tectonophysics, 819, p.229084_1 - 229084_22, 2021/11

https://doi.org/10.1016/j.tecto.2021.229084

The reactivation mechanisms of coseismic surface ruptures associated with the 2011 Mw 6.7 Fukushima-ken Hamadori earthquake in Japan are investigated using in-situ controlled hydraulic injections in subsurface boreholes. Two fault segments were selected for reactivation studies, one across a coseismic rupture, the Shionohira site, and one across a non-coseismically ruptured segment, the Minakami-kita site. A series of water injections in sealed sections of boreholes set across the fault progressively bring the fault to rupture by a step-by-step decrease of the effective normal stress clamping the fault. While the fault is rupturing during these hydraulic stimulations, borehole displacements, fluid pressure and injection flowrate are continuously monitored. Then, the tests were analyzed using fully coupled hydromechanical modeling. The model was calibrated on field data, and a parametric study was conducted to examine the modes of fault reactivation. Coseismic surface rupture of the Shionohira fault showed a pure dilatant slip response to hydraulic tests, while the tectonically un-activated Itozawa fault (South) indicated a complex hybrid response to tests related to both a higher frictional and cohesive strengths of the fault. The analysis of the induced Shionohira slip event showed that it is reasonably modeled as a Coulomb rupture with an eventual dependency of friction on slip velocity, in good accordance with laboratory-derived rate-and-state friction data on the Shinohira gouge samples. In contrast, the Itozawa fault reactivation mechanism appears dominated by tensile failure with limited Coulomb shear failure. Thus, the applied protocol proves to be able to isolate significant differences in fault physical properties and rupture mechanisms between two segments of the same fault system, opening perspectives to better assess near-surface rupture effects, and therefore the safety of structures subject to large earthquakes.

A Conceptual design study of pool-type sodium-cooled fast reactor with enhanced anti-seismic capability

Kubo, Shigenobu; Chikazawa, Yoshitaka; Ohshima, Hiroyuki; Uchita, Masato*; Miyagawa, Takayuki*; Eto, Masao*; Suzuno, Tetsuji*; Matoba, Ichiyo*; Endo, Junji*; Watanabe, Osamu*; et al.

Mechanical Engineering Journal (Internet), 7(3), p.19-00489_1 - 19-00489_16, 2020/06

https://doi.org/10.1299/mej.19-00489

The authors are developing the design concept of pool-type sodium-cooled fast reactor (SFR) that addresses Japan's specific siting conditions such as earthquakes and meets safety design criteria (SDC) and safety design guidelines (SDGs) for Generation IV SFRs. The development of this concept will broaden not only options for reactor types in Japan but also the range and depth of international cooperation. A design concept of 1,500 MWt (650 MWe) class pool-type SFR was thought up by applying design technology obtained from the design of advanced loop-type SFR, named JSFR, equipped with safety measures that reflect results from the feasibility study on commercialized fast reactor cycle systems and fast reactor cycle technology development, improved maintainability and repairability, and lessons learned from the Fukushima Daiichi Nuclear Power Plants accident.

Fracture characterization and rock mass behavior induced by blasting and mechanical excavation of shafts in Horonobe Underground Research Laboratory

Aoyagi, Kazuhei; Tokiwa, Tetsuya*; Sato, Toshinori; Hayano, Akira

Proceedings of 2019 Rock Dynamics Summit in Okinawa (USB Flash Drive), p.682 - 687, 2019/05

In high-level radioactive disposal projects, it is important to investigate the extent of the excavation damaged zone (EDZ) for safety assessment because EDZ can provide a migration pathway for radionuclides from the facility. To investigate the quantitative differences between EDZs formed because of blasting and mechanical excavation, we studied the characteristics of fractures induced by excavation based on fracture mapping performed during shaft sinking (V- and E-Shafts). As a result, it was found that blasting excavation can lead to the formation of a large number of newly created fractures (EDZ fractures) compared with mechanical excavation. In addition, the seismic velocity (P-wave velocity) measured during blasting excavation (E-Shaft) was lower than that measured during mechanical excavation (V-Shaft). Furthermore, we found that the support pattern that reinforces forward rocks to be appropriate for limiting damage to the shaft wall.

Failure behavior analyses of piping system under dynamic seismic loading

Udagawa, Makoto; Li, Y.; Nishida, Akemi; Nakamura, Izumi*

International Journal of Pressure Vessels and Piping, 167, p.2 - 10, 2018/11

https://doi.org/10.1016/j.ijpvp.2018.10.002

It is important to assure the structural Integrity of piping systems under severe earthquakes because those systems comprise the pressure boundary for coolant with high pressure and temperature. In this study, we examine the seismic safety capacity of piping systems under severe dynamic seismic loading using a series of dynamic-elastic-plastic analyses focusing on dynamic excitation experiments of 3D piping systems which was tested by NIED. Analytical results were consistent with experimental data in terms of natural frequency, natural vibration mode, response accelerations, elbow opening-closing displacements, strain histories, failure position, and low-cycle fatigue failure lives. Based on these results, we concluded that the analytical model used in the study can be applied to failure behavior evaluation for piping systems under severe dynamic seismic loading.