WASHINGTON — As Japan struggles with radioactive contamination from one of the world's worst nuclear accidents, American nuclear experts are watching for clues on how to make U.S. nuclear power plants more resistant to the forces of nature or hostile attacks.
U.S. nuclear regulators continue to assure Americans that U.S. plants are safe. They cite a stringent regulatory program, redundant security systems and improvements since the 1979 Three Mile Island accident and the 2001 terrorist attacks.
Yet the problems at Fukushima in the wake of last month's tsunami have reopened questions about the risks of U.S. nuclear reactors, and especially the pools that store spent fuel. Nuclear-safety and environmental groups have called for an independent investigation and a moratorium on relicensing and approvals of new designs.
"The irrefutable bottom line is that we have utterly failed to properly manage the risk from irradiated fuel stored at our nation's nuclear power plants," said David Lochbaum of the Union of Concerned Scientists, a nuclear engineer who worked on three U.S. nuclear plants that are similar to the one in Japan.
Spent-fuel pools don't have as much backup cooling as reactor cores do, and they aren't covered by the heavy concrete containment structures that surround core reactors, which protect the public from accidental releases of radioactivity, Lochbaum said last month in testimony to Congress.
The Obama administration says nuclear energy is clean and safe. Energy Secretary Steven Chu said he hoped that nuclear would be part of the energy mix until technological improvements allowed wind and solar power to displace gas and coal economically.
Germany responded to Japan's nuclear crisis by shutting down its seven oldest reactors for three months for safety checks. The U.S. Nuclear Regulatory Commission and the industry say there's no need to do that here.
"The next generation will be a safer generation of plants, but I want to emphasize this generation of plants has an extremely high performance rating over the last 30 years or so, well over any industry I can think of, and the plants have been operated safely," said Stewart Minahan, the executive director of operations for the Nuclear Energy Institute, an industry group.
Minahan cited many safety improvements: better instruments to monitor the buildup of explosive hydrogen, improvements to containment buildings, the installation of diesel generators and, after 9/11, additional equipment to help keep reactors and spent fuel cool during power outages.
Some of those upgrades addressed problems that Japan experienced.
Japanese officials say hydrogen explosions damaged four reactor buildings, with severe damage at three of them. Power outages prevented cooling in the reactors and the spent-fuel pools. Officials say that while it's not fully known what happened inside the plant, it's likely that spent fuel was damaged and was the source of some of the radioactive material that was released.
"It is the case that the U.S. undertook significant measures after 9/11 both to improve security and to enhance the capacity for U.S. plants to cope with extreme events of all kinds. Those measures would have helped in dealing with an event like those at the Fukushima plants," Richard Meserve, a former NRC chairman who's now the chairman of an international nuclear safety group, said in an email.
The Japanese plant's design dated to the late 1960s, as do many in the U.S., but Minahan said U.S. plants were upgraded routinely. Large components get replaced, and safety equipment is checked monthly.
"Our plants today are in better material condition than 10 or 20 years ago," he said.
Many U.S. improvements were made after the Three Mile Island accident in 1979 near Harrisburg, Pa., when an equipment failure caused fuel to melt. Even then, the containment structure prevented uncontrolled releases of radioactive materials.
Robert Youngblood, a senior risk consultant at the Idaho National Laboratory, the Department of Energy's lead nuclear research and development facility, said it was too early to say what Fukushima might change.
"The information we have about what really happened over there is really sketchy," he said. "One thing you learn in risk analysis is details matter."
What's known is that the reactors and spent-fuel pools in Japan lost cooling water after a power outage.
Nuclear fuel is consumed in nuclear fission, which creates heat to boil water, which produces steam that powers turbines, producing electricity. The spent fuel is removed from the reactor and placed in a pool to cool.
Even spent fuel is highly radioactive and hot. It must be kept under about 20 feet of water for about five years.
After that time, the fuel can be stored in steel and concrete casks. In the U.S., however, plants tend to keep the pools as full of spent fuel as possible, moving it to dry casks only when more space is needed.
"Right now, we believe this material can be stored safety and securely either in pools or in dry casks," Nuclear Regulatory Commission Chairman Gregory Jaczko told Congress on March 30.
But in 2008, before he became the NRC chairman, Jaczko said in a speech that new regulations were needed that would require moving spent fuel to dry casks. The casks had "inherent safety benefits," he said.
The NRC says spent fuel can be stored safely in either pools or casks for 100 years, but that neither is a long-term solution. Highly radioactive waste will need to be stored in a deep repository until it becomes harmless through decay after hundreds of thousands of years.
The pools originally were intended as temporary storage, because plans called for fuel reprocessing. In 1977, however, the government stopped the reprocessing of power plant fuel because plutonium, its byproduct, can be used to make nuclear weapons. As spent fuel accumulated, the pools had to be packed more densely.
Spent fuel pools measure 40 by 60 feet and are 40 feet deep. Their walls are thick steel-reinforced concrete around stainless-steel liners. If cooling is lost, the metal tubes that contain the fuel pellets heat up and can rupture and release radioactive gases. At Fukushima and at U.S. plants, the spent-fuel pools are outside the reactor-containment structures.
The NRC says the pools are designed to withstand the same level of natural disasters as the reactors.
After 9/11, investigators studied whether the pools could withstand attacks. A 2002 industry study, based on computer modeling, found that if a jet hit a pool, the concrete wall would be crushed and cracked but the stainless steel lining would keep the water in.
Fukushima, however, showed how little is known about anticipating and managing nuclear power's risks, Frances Beinecke, the president of Natural Resources Defense Council, an environmental group, said in a letter March 25 to President Barack Obama. She called for a new independent investigation into those risks.
A report published in the journal Science & Global Security in 2003 suggested that as much spent fuel as possible should be removed from the pools and put in dry casks to reduce the risk of fire and the release of radioactive materials.
As radioactive materials decay, they release particles that can damage the body and lead to cancer, particularly cesium-137 and iodine-131. In the 1986 nuclear accident at Chernobyl, releases of cesium-137 contaminated land. Some communities were abandoned permanently. Thousands of people who drank milk contaminated with radioactive iodine developed thyroid cancer.
A fire at a spent-fuel pool could release cesium-137. "The long-term land-contamination consequences of such an event could be significantly worse than those from Chernobyl," the 2003 report said.
The author of the 2003 report, Robert Alvarez, a former Department of Energy official who oversaw nuclear issues, said dry casks would provide safer storage until a permanent nuclear repository was built and loaded, a process that would take decades.
Germany shifted to dry-cask storage inside reinforced concrete buildings about 25 years ago because of dangers from terrorist attacks and accidental plane crashes, Alvarez said.
William Levis, the president of New Jersey's PSEG Power, which operates nuclear plants, said it was worth discussing how to handle waste fuel. The industry isn't reluctant to use dry casks, but it wants to limit how many times it must handle used fuel until there's a national plan for a repository, he told a congressional panel late last month.
The future of U.S. nuclear power will depend on how the federal government and investors evaluate its costs and risks.
Chu said new designs had passive safety systems and were far safer. A North Carolina alliance of public interest groups, however, says that one of these designs, the Westinghouse AP1000, has flaws. The alliance filed a motion last Wednesday that calls on the NRC to suspend the approval process for it until lessons from Fukushima are learned.
Among other things, the groups cited an NRC engineer's report that says the concrete in the containment structure wouldn't be strong enough. It also cited a warning by some scientists that filters in the passive cooling system might clog, increasing the risk of a meltdown.