WASHINGTON - Two years after Japan's nuclear crisis, the top U.S. regulator says American nuclear power plants are safer than ever, but not trouble-free.

Alison Macfarlane chairs the Nuclear Regulatory Commission. She says performance of U.S. plants are "quite good" and that all but five of the nation's 104 reactors were performing at acceptable safety levels at the end of last year.

However, a watchdog group calls her assessment overly rosy. The Union of Concerned Scientists wrote in a report that nearly one in six U.S. reactors had safety breaches last year, due in part to weak oversight.

Federal officials say none of the incidents hurt workers or the public.

Monday marks the two-year anniversary of the earthquake and tsunami that crippled Japan's Fukushima Dai-ichi nuclear plant.