Phillip Lipscy is an assistant professor of political science and a fellow at the Shorenstein Asia-Pacific Research Center at Stanford University. Kenji Kushida is a research associate in Japanese studies at the Shorenstein Center, where Trevor Incerti is a researcher.

As superstorm Sandy bore down on the East Coast, three nuclear power plants were shut down and an alert was issued for the Oyster Creek plant in New Jersey. This is an important reminder that the United States has several low-lying nuclear plants on the Eastern seaboard, with minimal protection against inundation. Particularly with climate change increasing the likelihood of extreme weather, this hidden threat to public safety should be remedied.

The disaster at Japan’s Fukushima Daiichi plant on March 11, 2011, revealed how much damage a tsunami can inflict on a nuclear power facility. To assess the vulnerability of such plants around the world, we collected information on plant height, sea wall height and the location of emergency power generators for 89 nuclear plants that lie next to water. We compared this to historical information on high waves triggered by various sources such as earthquakes, landslides and hurricanes.

Our findings were striking. Several nuclear plants in Japan had inadequate protection — the average height of a historic tsunami exceeded the height of the sea wall, the plant itself and on-site emergency power generators. Fukushima Daiichi was not even the most vulnerable plant in Japan. We found that plants operated by the largest power companies — Tokyo Electric, Kansai Electric and Chubu Electric — were particularly unsafe.

Equally striking, our data also suggested that several U.S. nuclear power plants are unprepared for high waves. In our database, the United States came in second, behind Japan, as the country with the largest number of inadequately protected nuclear power plants. The 1938 New England hurricane triggered a storm surge as high as 25 to 30 feet, considerably higher than waves generated this week by Sandy. A wave that tall would easily overtake many nuclear plants on the East Coast, which on average lie about 20 feet above sea level, with minimal sea wall protection.

According to our data, the U.S. plants most vulnerable to inundation are the Salem and Hope Creek plants on the New Jersey/Delaware border; the Millstone plant in Connecticut; and the Seabrook plant in New Hampshire. All are close to large cities: The Salem and Hope Creek plants are about 90 miles from Washington and about 35 miles from Philadelphia. The Millstone plant is about 40 miles from Hartford, Conn., and 100 miles from New York City. The Seabrook plant is about 35 miles from Boston. As points of reference, consider that the U.S. government recommended a 50-mile evacuation radius during the Fukushima disaster, and Tokyo is about 140 miles away from the Fukushima Daiichi site.

The threat posed by extreme weather is not hypothetical. In 1999, waves caused by high tide and a storm surge breached the sea wall at the Blayais plant in France, cutting off external power and knocking out several pieces of equipment. That incident did not result in a major accident, but the outcome could have been much worse. Blayais is situated on a river adjacent to the ocean, and it was protected by a 17-foot sea wall at the time of the accident. The Salem and Hope Creek nuclear plants similarly lie slightly inland on the Delaware River, but the plants would be threatened with inundation if wave heights exceed 11 feet.

To the credit of U.S. regulators, nuclear safety was enhanced dramatically after the Sept. 11, 2001, attacks. Backup energy sources in the United States, which provide electricity when external power is unavailable, have generally been made watertight, while those in Japan were left vulnerable — and the loss of on-site backup power was the final straw that led to the Fukushima catastrophe. In addition, surges caused by hurricanes and storms usually offer more time for preparation than do tsunamis triggered by earthquakes.

There are, however, other reasons to question the safety of U.S. nuclear power plants. Our research found that the risk to plants in this country is probably understated. Historical data regarding tsunamis are available going back about 2,000 years for East Asia and only about 350 years for the United States. Most information on historical waves comes from written records, and records are scarce for the Americas before European settlement. The 2011 tsunami in Japan is often compared to the 869 Jogan earthquake and tsunami. Japanese regulators were widely criticized for failing to prepare for a “one-thousand-year” wave. In the United States, we don’t even know what a once-in-a-thousand-years wave looks like.

This uncertainty means we should do much more to protect U.S. nuclear power plants against potential threats. The Fukushima task force recommendations, which said that operators should reevaluate their preparations, are a good starting point but do not go far enough. More sea walls should be constructed, and existing walls should be raised to minimize the danger of inundation. Back-up power generators should be located well above sea level and within watertight structures. Waterproofing should be inspected regularly and aggressively. Risk assessments should be conducted by independent observers. With so many nuclear plants perilously close to major population centers, reasonable precautions should not await the next disaster.