Could a nuclear accident like the 2011 meltdown that crippled the Fukushima Daiichi plant in Japan happen here?

David Lochbaum, a former nuclear engineer, director of the Nuclear Safety Program for the Union of Concerned Scientists and one of the authors of the new book-length account “Fukushima: The Story of a Nuclear Disaster,” thinks it’s more than possible. The safety preparations at the plant before the accident, he says, weren’t that different from the precautions taken at U.S. plants.

“It’s not that Japan was behind the standards of the rest of the world, or that the Japanese regulators or [Fukushima Daiichi operator] TEPCO was especially inept,” Lochbaum says. “They’re on par with everyone else.”

U.S. regulators have already been warning operators about the possibility of Fukushima-type disasters happening in the U.S. for years.

One of the most likely scenarios that could cause a meltdown is a flood. Nuclear reactors require a lot of water to carry away their waste heat, so they’re generally built next to oceans, lakes or rivers. Plants near lakes and rivers are often located downstream of a dam. In that case, if a dam bursts, the plant could be flooded and lose power, similarly to what happened at the Fukushima plant when the tsunami hit. In 2009, U.S. Nuclear Regulatory Commission staff identified about 35 reactors in the U.S. (out of the 100 currently operating) that were vulnerable to dam failures, according to Lochbaum.

In June 2010, about nine months before Fukushima’s three reactors melted down, the NRC issued a letter to Duke Energy, the owner and operator of the Oconee Nuclear Station near Seneca, South Carolina (view the letter here on Scribd). The letter – initially not released to the public, but unearthed by a reporter from The Cascadia Times in Oregon, through a Freedom of Information Act Request -- lists various actions Duke Energy is supposed to take to mitigate the risk of flood damage.

This letter came after NRC risk analysts concluded that the failure of the Jocassee Dam had a 100 percent chance of causing Oconee’s three reactors to melt down, according to Lochbaum. (Duke’s own reports disagree.) The main reason for concern? The plant’s flood wall was five feet high; the flood waters caused by a dam breakage were estimated to rise about 14 feet. Fukushima’s seawall was also easily breached by a 50-foot tsunami wave.

“In other words, both Oconee and Fukushima were protected by flood walls that worked just fine, unless there was a flood,” Lochbaum says.

Another risk to U.S. nuclear plants is fire. Like floods, flames can disable safety systems and their backups.

“At Fukushima, workers had literally dozens of pumps that could put water into the reactor vessel,” Lochbaum says. “But the flooding disabled all forms of electricity such that all these pumps literally stood by powerlessly. Fire can have this same consequence.”

In fact, a U.S. plant came close to a fire-related meltdown in 1975. A worker at the Browns Ferry nuclear plant in Alabama had accidentally caused a fire while using a candle to check for air leaks in a room directly below the control room for two reactors. The fire burned for almost seven hours, and damaged the electrical cables in the room such that all of the emergency cooling systems for one reactor shut down, along with most of the emergency systems for the other reactor.

"Only heroic operator actions prevented two meltdowns that day," Lochbaum says.

The NRC adopted fire protection regulations to prevent another Browns Ferry incident in 1980, and updated them in 2004.

But “today, about half of the reactors operating in the US do not comply with either the 1980 or the 2004 regulations,” Lochbaum says. “Sadly, the three reactors at the Browns Ferry nuclear plant are among those that operate in violation of fire protection regulations. As [are] the three reactors at Oconee.”

So, what has been done, post-Fukushima? The NRC issued recommendations and orders for upgrades shortly after the accident to try and apply lessons from the incident to domestic plants. By December 2016, U.S. nuclear plant owners must make various upgrades to help guard against extended blackouts, in order to be able to keep spent nuclear fuel cool and avoid meltdowns. Plant owners were ordered to invest in more portable power equipment in plants and nearby sites, improve instruments that measure the levels of water inside spent fuel pools and expand post-9/11 protections against terrorist attacks from single reactors to multiple reactors, among other actions. Plant operators think the first wave of these post-Fukushima upgrades are thought to cost around $3.6 billion over the next few years, according to a Platts survey.

With all of this potential danger for catastrophe, how can nuclear power still be a viable option? Lochbaum, for his part, says he’s not pro-nuclear or anti-nuclear -- just pro-safety.

“There’s no totally good or totally bad way to generate electricity,” Lochbaum says. “Nuclear has some certain advantages -- we basically recognize that it’s better than fossil fuel in terms of [contributing to] global warming. The key is to extract as many benefits from the technology that we can, while minimizing opportunities for bad things to happen.”