Tornado-resisting design speeds of nuclear power plants in China

More and more nuclear power plants are being constructed in China, while the nationwide intensity regions with tornado-resisting design speeds are not specified. With the development of tornado damage investigations, more records of tornado events have been accumulated, which makes it possible to establish the probability distribution model of tornado speed and predict the design wind speeds of nuclear power plants. This paper summarizes the derivation of probability distribution of tornado speed based on the parameters of mean occurrence rate, intensity rating, path length and width. The tornado events in China are introduced, while the geographical distribution of tornadoes is described. The mean occurrence rates for each local region with square of 1 degrees longitude and latitude are obtained by moving average method. The records of tornado events, which experienced post-disaster investigations and scattered in research articles, dissertations and yearbooks, are collected and revaluated. Using the accepted tornado records, both the probability distribution of path area and the joint probability distribution between tornado speed and path area are established. The equivalent speeds corresponding to different annual exceedance probabilities are calculated. The suggestions of intensity regions for tornado-resisting design of NPPs in China are proposed. The uncertainties of design wind speeds are simulated by Monte Carlo simulation method. It is indicated that the most significant impact on the uncertainties of design speeds originates from the variation of equivalent speed.