WASHINGTON -- Two years after the nuclear crisis in Japan, the top U.S. regulator says American nuclear power plants are safer than ever, though not trouble-free. A watchdog group calls that assessment overly rosy."The performance is quite good," Nuclear Regulatory Commission Chairwoman Allison Macfarlane said in an interview with The Associated Press.All but five of the nation's 104 nuclear reactors were performing at acceptable safety levels at the end of 2012, Macfarlane said, citing a recent report from the agency. "You can't engage that many reactors and not have a few that are going to have difficulty," she said.But the watchdog group, the Union of Concerned Scientists, has issued a scathing report saying nearly 1 in 6 U.S. nuclear reactors experienced safety breaches last year, in part because of weak oversight. The group accused the agency of "tolerating the intolerable."Using the agency's own data, the scientists group said 14 serious incidents, ranging from broken or impaired safety equipment to a cooling water leak, were reported last year. Over the past three years, 40 of the 104 U.S. reactors experienced one or more serious safety-related incidents that required additional agency action, the report said."The NRC has repeatedly failed to enforce essential safety regulations," wrote David Lochbaum, director of the group's Nuclear Safety Project and author of the study. "Failing to enforce existing safety regulations is literally a gamble that places lives at stake."Today is the two-year anniversary of the 2011 earthquake and tsunami that crippled Japan's Fukushima Dai-ichi nuclear plant.U.S. regulators, safety advocates and the industry are now debating whether safety changes imposed after the disaster have made the nation's 65 nuclear plants safer.New rules imposed by the commission require plant operators to install or improve venting systems to limit core damage in a serious accident and set up sophisticated equipment to monitor water levels in pools of spent nuclear fuel.The plants must also improve protection of safety equipment installed after 9-11 and make sure they can handle damage to multiple reactors at the same time.Macfarlane, who took over as the commission's chairwoman in July, said U.S. plants are operating safely and are making progress on the new rules, which impose a deadline for completion of 2016, five years after the Fukushima disaster. "So far, industry seems to be cooperating," she said.The agency has been working closely with plant operators "to make sure they understand what we are requiring and that we understand about their situation as well," Macfarlane said.Even so, the U.S. industry faces a range of difficulties.Problem-plagued plants in Florida and Wisconsin are slated for closure, and four other reactors remain offline because of safety concerns.Shut-down reactors include two at the beleaguered San Onofre nuclear power plant in Southern California, which hasn't produced electricity since January 2012, when a tiny radiation leak led to the discovery of damage to hundreds of tubes that carry radioactive water.Macfarlane said the agency won't let the San Onofre plant reopen until regulators are certain it can operate safely, which may take several months.Joseph Pollock, vice president of the Nuclear Energy Institute trade association, said plant operators are "working aggressively" to meet the 2016 timeline set by the commission and have spent more than $40 million on safety.