Over the last three decades, public understanding of science (PUS) has been one of the foremost research topics in the Korean society where numerous social scientific conflicts have taken place. As a lead channel of risk communication, Twitter has been studied in experimental research designs or among target user groups, leaving the measurement of overall PUS still unknown. Hence, this study aimed to examine every message collected during the one-year research period by a Twitter backup system installed on an online server, regarding the two key words: "nuclear power" and "nuclear power plants." The results indicate that 20% of the entire posts contained promotional advertisements distributed by five state institutes related to nuclear power. Excluding these advertisements, there were 397,827 messages posted by 87,966 users forming a skewed distribution in the cumulative chart--a sled-shape model. Top 5% of the entire users posted half of the entire messages, while top 0.2% users entitled as active speakers posted 10% of the entire messages. As only 11% messages among the active users held balanced attitudes, it is likely that most of messages posted by the supporting state institutes and the opposing individuals would gain less credibility than they were intended, according to the theory of bidirectional risk communication.