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Ishida, Shinya; Tagami, Hirotaka; Tobita, Yoshiharu; Okano, Yasushi; Yamano, Hidemasa; Kubo, Shigenobu
Dai-27-Kai Doryoku, Enerugi Gijutsu Shimpojiumu Koen Rombunshu (Internet), 5 Pages, 2023/09
no abstracts in English
Ohshima, Hiroyuki; Morishita, Masaki*; Aizawa, Kosuke; Ando, Masanori; Ashida, Takashi; Chikazawa, Yoshitaka; Doda, Norihiro; Enuma, Yasuhiro; Ezure, Toshiki; Fukano, Yoshitaka; et al.
Sodium-cooled Fast Reactors; JSME Series in Thermal and Nuclear Power Generation, Vol.3, 631 Pages, 2022/07
This book is a collection of the past experience of design, construction, and operation of two reactors, the latest knowledge and technology for SFR designs, and the future prospects of SFR development in Japan. It is intended to provide the perspective and the relevant knowledge to enable readers to become more familiar with SFR technology.
Okano, Yasushi; Nishino, Hiroyuki; Yamano, Hidemasa; Kurisaka, Kenichi
Proceedings of International Topical Meeting on Probabilistic Safety Assessment and Analysis (PSA 2019), p.274 - 281, 2019/04
A sodium-cooled fast reactor uses the ambient air as an ultimate heat sink to remove decay heat, thus meteorological phenomena can potentially pose risks to the reactor. If a rare and intense external hazard occurs concurrently with another external hazard, it would affect the systems (i.e. air cooler of decay heat removal system). In this study, a new scheme of screening of the external hazard combinations was proposed. The authors classified simultaneous or sequential combinational hazards, and identified associated potential effects in terms of hazard duration and sequential order. As a result, this study identified scenarios of the external hazard combinations of preceding rare and intense external hazard with an following additional external hazard.
Yamano, Hidemasa; Kurisaka, Kenichi; Nishino, Hiroyuki; Okano, Yasushi; Naruto, Kenichi*
Proceedings of 12th International Topical Meeting on Nuclear Reactor Thermal-Hydraulics, Operation and Safety (NUTHOS-12) (USB Flash Drive), 15 Pages, 2018/10
Spent fuels are transferred from a reactor core to a spent fuel pool through an external vessel storage tank (EVST) filled with sodium in sodium-cooled fast reactors in Japan. This paper describes identification of dominant accident sequences leading to fuel failure, which was achieved through probabilistic risk assessment for the EVST designed for a next sodium-cooled fast reactor plant system in Japan to improve the EVST design. The safety strategy for the EVST involves whole core refueling (early transfer of all core fuel assemblies into the EVST) assuming a severe situation that results in sodium level reduction leading finally to the top of the reactor core fuel assemblies in a long time. This study introduces the success criteria mitigation along the decay heat decrease over time. Based on the design information, this study has carried out identification of initiating events, event and fault tree analyses, a probability analysis for human error, and quantification of accident sequences. The fuel damage frequency of the EVST was evaluated to be approx. 10
/year. The dominant accident sequence resulted from the static failure and human error for the switching from the stand-by to operation mode in the three stand-by cooling circuits after loss of one circuit for refueling heat removal operation as an initiating phase.
Yamano, Hidemasa; Naruto, Kenichi*; Kurisaka, Kenichi; Nishino, Hiroyuki; Okano, Yasushi
Proceedings of 26th International Conference on Nuclear Engineering (ICONE-26) (Internet), 9 Pages, 2018/07
Spent fuels are transferred from a reactor core to a spent fuel pool through an external vessel storage tank (EVST) filled with sodium in sodium-cooled fast reactors in Japan. This paper describes identification of dominant accident sequences leading to fuel failure by conducting probabilistic risk assessment for EVST designed for a next sodium-cooled fast reactor plant system in Japan to improve the EVST design. Based on the design information, this study has carried out identification of initiating events, event and fault tree analyses, human error probability analysis, and quantification of accident sequences. Fuel damage frequency of the EVST was evaluated approx. 10
/year in this paper. By considering the secondary sodium freezing, the fuel damage frequency was twice increased. The dominant accident sequence resulted from the common cause failure of the damper opening and/or the human error for the switching from the stand-by to the operation mode in the three stand-by cooling circuits. The importance analyses have indicated high risk contributions.
Okano, Yasushi; Yamano, Hidemasa
Proceedings of Asian Symposium on Risk Assessment and Management 2017 (ASRAM 2017) (USB Flash Drive), 3 Pages, 2017/11
Forest fire hazard assessment methodologies using a logic tree have been applied for the evaluation of combination hazard curves of a forest fire with lightning as an initiator of a forest fire and with a strong wind being independent from a forest fire. The complex shape of the combinational hazard curve of forest fire and lighting is due to that both lightning and high velocity wind tend to appear under unstable weather conditions, and there is correlation between two hazards. The evaluated combinational hazard curve of forest fire and strong wind for the instantaneous wind velocity over 80 m/s has extremely small frequency in the range below 10
/year.
Yamano, Hidemasa; Naruto, Kenichi*; Kurisaka, Kenichi; Nishino, Hiroyuki; Okano, Yasushi
Proceedings of Asian Symposium on Risk Assessment and Management 2017 (ASRAM 2017) (USB Flash Drive), 3 Pages, 2017/11
Spent fuels are transferred from a reactor core to a spent fuel pool through an external vessel storage tank (EVST) filled with sodium in sodium-cooled fast reactors in Japan (JSFR). The objective of this study is to identify dominant accident sequences leading to fuel failure by conducting PRA for EVST. The EVST heat removal system in JSFR consists of four independent loops with for primary and secondary ones. Based on the JSFR design information, this study has identified initiating events, event and /fault tree analyses, human reliability analysis, and quantification of accident sequences. Fuel damage frequency of the EVST was evaluated approx. 10 /year in this paper. The main contributor of the fuel damage frequency is the loss of heat removal function of the cooling system. The dominant initiating event was the loss of one circuit of normal heat removal operation.
Okano, Yasushi; Yamano, Hidemasa
Proceedings of International Topical Meeting on Probabilistic Safety Assessment and Analysis (PSA 2017) (USB Flash Drive), p.1334 - 1342, 2017/09
This study evaluates a hazard curve of smoke effects generated by a forest fire by applying a new method using a logic tree which consists of variable parameters on a forest fire, weather conditions, types of vegetation and topography, and simulation conditions. A response surface of the smoke spatial density was evaluated using two simulation codes: FARSITE for forest fire propagation and ALOFT-FT for smoke transportation. It is followed by a Monte Carlo simulation to evaluate the hazard curve representing the annual exceedance frequency of the total amount of the smoke captured on air filters. The evaluated hazard curve is about 1
10
per year for 3.5 kg/m
/(m/s).
Okano, Yasushi; Yamano, Hidemasa
Mechanical Engineering Journal (Internet), 4(3), p.16-00517_1 - 16-00517_10, 2017/06
A sensitivity study on forest fire hazard curves was performed. The probability fluctuation on forest fire breakout time affects the reaction intensity and the fireline intensity around 4% and 14% respectively. The probability fluctuation on forest fire breakout points affects the hazard curve frequency around +70% to -40%. The probability fluctuation due to forest firefighting operation only affects the frequency of the hazard curves, but not the intensity. The hazard curves without the effect of firefighting remarkably increase around 40 to 80 times in frequency in comparison with those with considering the forest firefighting operation effect outside the plant. This study indicated that the most significant factor in the forest fire hazard risk is whether the forest firefighting operation outside the plant is expected before the forest fire arrival at the plant.
Okano, Yasushi; Yamano, Hidemasa
Proceedings of 10th Japan-Korea Symposium on Nuclear Thermal Hydraulics and Safety (NTHAS-10) (USB Flash Drive), 9 Pages, 2016/11
This paper deals with an event sequence by forest fire heat effect on a decay heat removal function of a sodium-cooled fast reactor. Related to the potential vulnerability, an event scenario was developed using conservative assumptions. An event tree was developed with an initiating event of the loss of off-site power, and the headings are related to "external diesel fuel tanks", "emergency diesel generator and its auxiliary system", "alternative cooling system and its power source", and "decay heat air cooler". A failure probability on each heading was given from a fragility curve as a function of reaction intensity or by assumptions based on conservative models. A core damage frequency, under the conditional of the loss of off-site power, was conservatively evaluated around 10
/year. A key heading in the event tree with large effect on the frequency is the intactness of the external diesel fuel tanks.
Yamano, Hidemasa; Nishino, Hiroyuki; Kurisaka, Kenichi; Okano, Yasushi; Sakai, Takaaki; Yamamoto, Takahiro*; Ishizuka, Yoshihiro*; Geshi, Nobuo*; Furukawa, Ryuta*; Nanayama, Futoshi*; et al.
Proceedings of 11th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, Operation and Safety (NUTHOS-11) (USB Flash Drive), 12 Pages, 2016/10
This paper describes mainly volcanic margin assessment methodology development in addition to the project overview. The volcanic tephra could potentially clog filters of air-intakes that need the decay heat removal. The filter clogging can be calculated by atmospheric concentration and fallout duration of the volcanic tephra and also suction flow rate of each component. In this paper, the margin was defined as a grace period to a filter failure limit. Consideration is needed only when the grace period is shorter than the fallout duration. The margin by component was calculated using the filter failure limit and the suction flow rate of each component. The margin by sequence was evaluated based on an event tree and the margin by component. An accident management strategy was also suggested to extend the margin; for instance, manual trip of the forced circulation operation, sequential operation of three air coolers, and covering with pre-filter.
Okano, Yasushi; Yamano, Hidemasa
Journal of Nuclear Science and Technology, 53(8), p.1224 - 1234, 2016/08
Times Cited Count：4 Percentile：36.53(Nuclear Science & Technology)A method to obtain a hazard curve of a forest fire was developed. The method has four steps: a logic tree formulation, a response surface evaluation, a Monte Carlo simulation, and an annual exceedance frequency calculation. The logic tree consists domains of forest fire breakout and spread conditions, weather conditions, vegetation conditions, and forest fire simulation conditions. The new method was applied to evaluate hazard curves of a reaction intensity and a fireline intensity for a typical location around a sodium-cooled fast reactor in Japan.
Okano, Yasushi; Yamano, Hidemasa
Mechanical Engineering Journal (Internet), 3(3), p.15-00592_1 - 15-00592_11, 2016/06
A new method has been developed to assess potential challenges by forest fire smoke on a cooling function of a decay heat removal system (DHRS) of a sodium-cooled fast reactor. Combinational numerical simulations of a forest fire propagation and a smoke transport were performed by FARSITE and ALOFT-FT to evaluate a cumulative amount of smoke captured on air filters of the DHRS. The evaluated time-dependent changes of spatial particle matter (PM) density were utilized to calculate a cumulative amount of PM captured on the air filters of the DHRS. Sensitivity analysis was performed on prevailing wind speed to which both the fireline intensity and the smoke transport behavior are sensitive. The total amount of PM on the air filters was conservatively estimated around several hundred grams per m which is well below the utilization limit.
Yamano, Hidemasa; Nishino, Hiroyuki; Kurisaka, Kenichi; Okano, Yasushi; Sakai, Takaaki; Yamamoto, Takahiro*; Ishizuka, Yoshihiro*; Geshi, Nobuo*; Furukawa, Ryuta*; Nanayama, Futoshi*; et al.
Proceedings of 24th International Conference on Nuclear Engineering (ICONE-24) (DVD-ROM), 10 Pages, 2016/06
This paper describes mainly volcanic probabilistic risk assessment (PRA) methodology development for sodium-cooled fast reactors in addition to the project overview. The volcanic ash could potentially clog air filters of air-intakes that are essential for the decay heat removal. The degree of filter clogging can be calculated by atmospheric concentration of ash and tephra fallout duration and also suction flow rate of each component. The atmospheric concentration can be calculated by deposited tephra layer thickness, tephra fallout duration and fallout speed. This study evaluated a volcanic hazard using a combination of tephra fragment size, layer thickness and duration. In this paper, each component functional failure probability was defined as a failure probability of filter replacement obtained by using a grace period to a filter failure limit. Finally, based on an event tree, a core damage frequency was estimated about 310/year in total by multiplying discrete hazard probabilities by conditional decay heat removal failure probabilities. A dominant sequence was led by the loss of decay heat removal system due to the filter clogging after the loss of emergency power supply. A dominant volcanic hazard was 10 kg/m
of atmospheric concentration, 0.1 mm of tephra diameter, 50-75 cm of deposited tephra layer thickness, and 1-10 hr of tephra fallout duration.
Okano, Yasushi; Yamano, Hidemasa
Proceedings of 24th International Conference on Nuclear Engineering (ICONE-24) (DVD-ROM), 10 Pages, 2016/06
This paper presents a sensitivity study of the hazard curves on condition parameters where frequency and probability variables in the logic tree vary within respective fluctuation ranges. With regard to "fluctuation of breakout time of a forest fire", the hazard curves on the reaction intensity and the fireline intensity increased around 4% and 14% respectively on intensity. As for "probability distribution fluctuation of breakout point", the reaction intensity and the fireline intensity vary within around +70% to -40% on frequency. "Firefighting effect on a probability of forest fire arrival at an nuclear power plant" remarkably increase the hazard curves around 40 to 80 times. It only affects the frequency of the hazard curves. This study indicated that the most significant factor in the forest fire hazard curve is whether the firefighting action outside an nuclear power plant is expected before the arrival.
Nishikiori, Ryo; Kojima, Atsushi; Hanada, Masaya; Kashiwagi, Mieko; Watanabe, Kazuhiro; Umeda, Naotaka; Tobari, Hiroyuki; Yoshida, Masafumi; Ichikawa, Masahiro; Hiratsuka, Junichi; et al.
Plasma and Fusion Research (Internet), 11, p.2401014_1 - 2401014_4, 2016/03
One of critical issues for high-energy high-current beam acceleration in ITER and JT-60SA is the high voltage holding which is dominated by vacuum discharges. The past results suggest that vacuum discharge occurs beyond the threshold of the dark current. The dark current can be derived from F-N theory where electric field enhancement factor beta is included. Though, beta could only be evaluated from the experiment previously. Therefore, the method to decide beta without experiment is required. This time dark currents were measured at three different areas to compare beta in different electric field. As a result, the effective electric field 
E, where E is average electric field, were found to be almost constant for different areas although the beta is largely different. By applying E, beta can be evaluated analytically, leading to the analytical prediction of the dark current and voltage holding capability without the measurements.
Kojima, Atsushi; Hanada, Masaya; Tobari, Hiroyuki; Nishikiori, Ryo; Hiratsuka, Junichi; Kashiwagi, Mieko; Umeda, Naotaka; Yoshida, Masafumi; Ichikawa, Masahiro; Watanabe, Kazuhiro; et al.
Review of Scientific Instruments, 87(2), p.02B304_1 - 02B304_5, 2016/02
Times Cited Count：11 Percentile：49.05(Instruments & Instrumentation)Optimization techniques of the vacuum insulation design have been developed in order to realize a reliable voltage holding capability of Multi-Aperture Multi-Grid accelerators for giant negative ion sources for nuclear fusion. In this method, the nested multilayer configuration of each acceleration stage in the MAMuG accelerator can be uniquely designed to satisfy the target voltage within given boundary conditions. The evaluation of the voltage holding capabilities of each acceleration stages were based on the past experimental results of the area effect and the multi-aperture effect on the voltage holding capability. Moreover, total voltage holding capability of multi-stage was estimated by taking the multi-stage effect into account, which was experimentally obtained in this time. In this experiment, the multi-stage effect appeared as the superposition of breakdown probabilities in each acceleration stage, which suggested that multi-stage effect can be considered as the voltage holding capability of the single acceleration gap having the total area and aperture. The analysis on the MAMuG accelerator for JT-60SA agreed with the past gap-scan experiments with an accuracy of less than 10% variation.
Hanada, Masaya; Kojima, Atsushi; Tobari, Hiroyuki; Nishikiori, Ryo; Hiratsuka, Junichi; Kashiwagi, Mieko; Umeda, Naotaka; Yoshida, Masafumi; Ichikawa, Masahiro; Watanabe, Kazuhiro; et al.
Review of Scientific Instruments, 87(2), p.02B322_1 - 02B322_4, 2016/02
Times Cited Count：11 Percentile：49.05(Instruments & Instrumentation)In International Thermo-nuclear Experimental Reactor (ITER) and JT-60 Super Advanced (JT-60 SA), the D
ion beams of 1 MeV, 40 A and 0.5 MeV, 22 A are required to produce 3600 s and 100 s for the neutral beam injection, respectively. In order to realize such as powerful D ion beams for long duration time, Japan Atomic Energy Agency (JAEA) has energetically developed cesium (Cs)-seeded negative ion sources (CsNIS) and electro-static multi-aperture and multi-stage accelerators (MAMuG accelerator) which are chosen as the reference design of ITER and JT-60 SA. In the development of the CsNIS, a 100s production of the H ion beam has been demonstrated with a beam current of 15 A by modifying the JT-60 negative ion source. At the higher current, the long pulse production of the negative ions has been tried by the mitigation of the arcing in the plasma inside the ion source. As for the long pulse acceleration of the negative ions in the MAMuG accelerator, the beam steering angle has been controlled to reduce the power loading of the acceleration grids A pulse duration time has been significantly extended from 0.4 s to 60 s at reasonable beam power for ITER requirement. The achieved pulse duration time is limited by the capacity of the power supplies in the test stand. In the range of 
60 s, there are no degradations of beam optics and voltage holding capability in the accelerator. It leads to the further extension of the pulse duration time at higher power density. This paper reports the latest results of development on the negative ion source and accelerator at JAEA.
Yamano, Hidemasa; Nishino, Hiroyuki; Okano, Yasushi; Yamamoto, Takahiro*; Takata, Takashi*
Earthquakes, Tsunamis and Nuclear Risks, p.111 - 121, 2016/01
The present study is developing risk assessment methodologies that include probabilistic risk assessment (PRA) and margin assessment methodologies against snow, tornado, strong wind, rain, volcanic eruption and forest fire mainly for a sodium-cooled fast reactor. The present paper describes briefly the project overview and then mainly the development of PRA and margin assessment methodologies against strong wind. In the strong wind PRA, the hazard curve was estimated using the Gumbel distributions based on weather data. Next, failure probabilities for components were calculated and event trees were developed. Using them, the strong wind PRA methodology was developed to quantify a core damage frequency. The present study also developed the wind margin assessment methodology that the margin was regarded as wind speed leading to the decay heat removal failure.
Okano, Yasushi; Yamano, Hidemasa
Case Studies in Fire Safety, 4, p.1 - 10, 2015/10
As a part of a development of risk assessment methodology on a forest fire as an external hazard, potential challenges to safety function and structural integrity of a nuclear power plant are investigated. Forest fire propagation simulations are performed by using FARSITE simulator to evaluate intensity and key parameters related to "heat" and "flame" effects. Forest fire propagation simulations were performed "with/without prevailing wind" and "high/low of ambient temperature and relative humidity" The study indicates that fireline intensity and reaction intensity are sensitive to prevailing wind and relative humidity, and not to ambient temperature. The maximum reaching height of a flame on a canopy top is close to the range of power line height and a loss of offsite power is recognized as a possible subsequent event during the forest fire.
