Development of ARKADIA-Safety for severe accident evaluation of sodium-cooled fast reactors

Aoyagi, Mitsuhiro; Sonehara, Masateru; Ishida, Shinya; Uchibori, Akihiro; Kawada, Kenichi; Okano, Yasushi; Takata, Takashi

Proceedings of Technical Meeting on State-of-the-art Thermal Hydraulics of Fast Reactors (Internet), 3 Pages, 2022/09

Development of dynamic PRA methodology for external hazards (Application of CMMC method to severe accident analysis code)

Li, C.; Watanabe, Akira*; Uchibori, Akihiro; Okano, Yasushi

Dai-26-Kai Doryoku, Enerugi Gijutsu Shimpojiumu Koen Rombunshu (Internet), 4 Pages, 2022/07

https://doi.org/10.1299/jsmepes.2022.26.A234

Identifying accident scenarios that could lead to severe accidents and evaluating their frequency of occurrence are essential issues. This study aims to establish the methodology of the dynamic Probabilistic Risk Assessment (PRA) for sodium-cooled fast reactors that can consider the time dependency and the interdependence of each event. Specifically, the Continuous Markov chain Monte Carlo (CMMC) method is newly applied to the SPECTRA code, which analyzes the severe accident conditions of nuclear reactors, to develop an evaluation methodology for typical external hazards. Currently, a fault-tree model of air coolers of decay heat removal system is implemented as the CMMC method, and a series of preliminary analysis of the plant's transient characteristics under the scenario of volcanic ashfall has been conducted.

A Study of probabilistic risk assessment methodology of external hazard combinations; Identification of hazard combination impacts on air-cooling decay heat removal system

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.

Level 1 PRA for external vessel storage tank of Japan sodium-cooled fast reactor in whole core refueling

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.

Study on combination hazard curve of forest fire with lightning and strong wind

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.

Hazard curve evaluation for forest fire smoke effects on air-cooling decay heat removal systems

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 110 per year for 3.5 kg/m/(m/s).

The Safety design criteria development and summary of its update for the Generation-IV SFR systems

Sofu, T.*; Okano, Yasushi

Proceedings of International Conference on Fast Reactors and Related Fuel Cycles; Next Generation Nuclear Systems for Sustainable Development (FR-17) (USB Flash Drive), 8 Pages, 2017/06

The Generation-IV International Forum (GIF) completed development of Safety Design Criteria (SDC) for the Generation-IV SFR systems in May, 2013. SDC reflects the high-level GIF safety and reliability goals and follows GIF basic safety approach. It aims to establish reference criteria for safety design of structures, systems and components and achieve harmonization of safety approaches among GIF member states. Following its public release, SDC report was distributed to international organizations and national regulatory bodies for review and feedback. Based on comments received during the following two-year period, SDC report underwent a revision reflecting feedback received from IAEA, NRC (USA), IRSN (France), and NNSA (China). This paper provides an overview of SDC development effort, and summarizes its revisions based the comments/suggestions received from the international review.

Development of risk assessment methodology against natural external hazards for sodium-cooled fast reactors; Project overview and margin assessment methodology against volcanic eruption

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.

Numerical simulations of forest fire propagation and smoke transport as an external hazard assessment methodology development for a nuclear power plant

Okano, Yasushi; Yamano, Hidemasa

Mechanical Engineering Journal (Internet), 3(3), p.15-00592_1 - 15-00592_11, 2016/06

http://doi.org/10.1299/mej.15-00592

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.

Sensitivity study on forest fire breakout and propagation conditions for forest fire hazard curve evaluations

Okano, Yasushi; Yamano, Hidemasa

Proceedings of 24th International Conference on Nuclear Engineering (ICONE-24) (DVD-ROM), 10 Pages, 2016/06

https://doi.org/10.1115/ICONE24-60047

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.