Development of Probabilistic Risk Assessment Methodology Against Volcanic Eruption for Sodium-Cooled Fast Reactors

2017 ◽  
Vol 4 (3) ◽  
Author(s):  
Hidemasa Yamano ◽  
Hiroyuki Nishino ◽  
Kenichi Kurisaka ◽  
Takahiro Yamamoto
Keyword(s):  
Risk Assessment ◽  
Failure Probability ◽  
Volcanic Eruption ◽  
Heat Removal ◽  
Probabilistic Risk Assessment ◽  
Sensitivity Analyses ◽  
Fast Reactors ◽  
Decay Heat ◽  
Volcanic Tephra ◽  
Probabilistic Risk

The objective of this paper is to develop a probabilistic risk assessment (PRA) methodology against volcanic eruption for decay heat removal function of sodium-cooled fast reactors (SFRs). In the volcanic PRA methodology development, only the effect of volcanic tephra (pulverized magma) is taken into account, because there is a great distance between a plant site assumed in this study and volcanoes. The volcanic tephra (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. This study evaluated a volcanic hazard using a combination of tephra fragment size, layer thickness, and duration. In this paper, functional failure probability of each component is defined as a failure probability of filter replacement obtained by using a grace period to filter failure. Finally, based on an event tree, a core damage frequency has been estimated by multiplying discrete hazard frequencies by conditional decay heat removal failure probabilities. A dominant sequence has been identified as well. In addition, sensitivity analyses have investigated the effects of a tephra arrival reduction factor and prefilter covering.

Development of Probabilistic Risk Assessment Methodology of Decay Heat Removal Function Against Combination Hazard of Low Temperature and Snow for Sodium-Cooled Fast Reactors

2017 ◽  
Author(s):  
Hiroyuki Nishino ◽  
Hidemasa Yamano ◽  
Kenichi Kurisaka
Keyword(s):  
Risk Assessment ◽  
Low Temperature ◽  
Heat Removal ◽  
Ambient Air ◽  
Probabilistic Risk Assessment ◽  
Event Tree ◽  
Snow Removal ◽  
Decay Heat ◽  
Core Damage ◽  
Probabilistic Risk

A Probabilistic Risk Assessment (PRA) should be performed not only for earthquake and tsunami which are major natural events in Japan, but also for other natural external hazards. However, PRA methodologies for other external hazards and their combination have not been sufficiently developed. This study is intended to develop PRA methodology for a combination of low temperature and snow for a Sodium-cooled Fast Reactor (SFR) that uses the ambient air as its ultimate heat sink for decay heat removal under accident conditions. Annual excess probabilities of low temperature and of snow are statistically estimated based on the meteorological records of low temperature, snow depth and daily snowfall depth. To identify core damage sequence, an event tree was developed by considering the impact of low temperature and snow on decay heat removal systems (DHRSs), e.g., plugged intake and/or outtake for the DHRS and for the emergency diesel generator (EDG), unopenable door on the access routes due to accumulated snow, failure of the intake filters due to accumulated snow, possibility of freezing of the water in cooling circuits. Recovery actions (i.e., snow removal and filter replacement) to prevent loss of DHRS function were also considered in developing the event tree. Furthermore, considering that a dominant contributor to snow risk can be failure of snow removal around the intake and outtake induced by loss of the access routes, this study has investigated effects of electric heaters installed around the intake and outtake as an additional countermeasure. By using the annual excess probabilities and failure probabilities, the event tree was quantified. The result showed that a dominant core damage sequence is failure of the electric heaters and loss of the access routes for snow removal against the combination hazard at daily snowfall depth of 2 m/day, duration time (snow and low temperature) of 1 day.

Development of Risk Assessment Methodology of Decay Heat Removal Function Against Natural External Hazards for Sodium-Cooled Fast Reactors: Project Overview and Volcanic PRA Methodology

2016 ◽  
Author(s):  
Hidemasa Yamano ◽  
Hiroyuki Nishino ◽  
Kenichi Kurisaka ◽  
Yasushi Okano ◽  
Takaaki Sakai ◽  
...  
Keyword(s):  
Risk Assessment ◽  
Layer Thickness ◽  
Failure Probability ◽  
Volcanic Hazard ◽  
Heat Removal ◽  
Tephra Layer ◽  
Atmospheric Concentration ◽  
Decay Heat ◽  
Filter Clogging ◽  
Tephra Fallout

This paper describes mainly volcanic probabilistic risk assessment (PRA) methodology development for sodium-cooled fast reactors in addition to the project overview. In the volcanic PRA, only the effect of volcanic tephra (ash) was taken into account because there is a great distance between a plant site assumed in this study and volcanos. 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 3 × 10−6/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 10−2 kg/m3 of atmospheric concentration, 0.1 mm of tephra diameter, 50–75cm of deposited tephra layer thickness, and 1–10 hr of tephra fallout duration.

Sign in / Sign up

Export Citation Format

Share Document