Yasushi Okano

A Feasibility Study on a Small Sodium Cooled Reactor as a Diversified Power Source

A conceptual design study of a small sized sodium cooled reactor with 165 MWe output with a metallic fuel, which aimed at the application for the diversified power supply has been carried out. A metal fuel core has been developed with 550°C core outlet temperature and 20 years core life time by utilizing the three zone core having different Zr contents in U-Pu-Zr of metal fuel. Major components in the nuclear steam supply system has been design and safety analyses has been performed to evaluate economical and safety potential of the concept. ATWS analyses show the passive safety feature of this concept adopting control rod driver-line (CRD) expansion reactivity and prolongation of electromagnetic pump (EMP) coastdown. Enhancing passive safety features, a improved upper inner structure enhancing CRD expansion and a reliable power source for EMP has been proposed. Though construction cost in first of a kind (FOAK) does not satisfy the economical goal, the present concept has potential for achieving the economical goal in Nth of a kind (NOAK) considering learning effect.

Hazard curve evaluation method development for a forest fire as an external hazard on nuclear power plants

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.” Condition parameters of the logic boxes are static if stable during a forest fire or not sensitive to a forest fire intensity, and non-static parameters are variables whose frequency/probability is given based on existing databases or evaluations. Response surfaces of a reaction intensity and a fireline intensity were prepared by interpolating outputs from a number of forest fire propagation simulations by fire area simulator (FARSITE). The Monte Carlo simulation was performed where one sample represented a set of variable parameters of the logic boxes and a corresponding intensity was evaluated from the response surface. The hazard curve, i.e. an annual exceedance frequency of the intensity, was therefore calculated from the histogram of the Monte Carlo simulation outputs. 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.

Development of Risk Assessment Methodology Against External Hazards for Sodium-Cooled Fast Reactors

In this study, hazard evaluation methodologies were developed for the decay heat removal of a typical sodium-cooled fast reactor in Japan against snow, tornado, wind, volcanic eruption, and forest fire. In addition, probabilistic risk assessment and margin assessment methodologies against snow were developed as well. Snow hazard curves were developed based on the Gumbel and Weibull distributions using historical records of the annual maximum values of snow depth and daily snowfall depth. Wind hazard curves were also evaluated using the maximum wind speed and instantaneous speed. The tornado hazard was evaluated by an excess probability for the wind speed based on the Weibull distribution multiplied by an annual probability of the tornado strike at a target plant. The volcanic eruption hazard was evaluated using geological data and tephra diffusion simulation which indicated tephra layer thickness and tephra diameter. The forest fire hazard was evaluated based on numerical simulation which contributed to creating a response surface of frontal fire intensity and Monte Carlo simulation for excess probability calculation. After developing an event tree and failure probabilities, the snow PRA showed the order of 10−7/year of core damage frequency. Event sequence assessment methodology was also developed based on plant dynamics analysis coupled with continuous Markov chain Monte Carlo method in order to apply to the event sequence against snow. Furthermore, this study developed the snow margin assessment methodology that the margin was regarded as the snowfall duration to the decay heat removal failure which was defined as when the snow removal speed was smaller than the snowfall speed.