Following a massive earthquake and tsunami in Japan, hydrogen explosions rocked three reactors at the Fukushima Daiichi nuclear power plant. Radioactive spent fuel stored in pools was also affected, especially at one reactor -- the plant has a total of six -- where multiple fires erupted. Evacuation orders were issued, potassium iodine tablets distributed, and plant employees used seawater and external electrical power to cool the stricken reactors, three of which had a partial core meltdown.

Having followedJapan'snuclear crisis, ProPublica began to look at questions about nuclear safety in the United States. Our investigation found that the Nuclear Regulatory Commission is routinely waiving fire rule violations at nearly half of the nation's 104 commercial reactors, even though fire presents one of the chief hazards at nuclear plants. Since 1995, there have been at least 153 fires at U.S. plants, according to NRC records. Those fires have damaged essential equipment and forced emergency shutdowns, reports reviewed by ProPublica show.

The NRC also began inspecting U.S. plants in March in response to Japan's nuclear crisis. While the NRC said of the inspection results that "out of 65 operating reactor sites, 12 had issues with one or more of the requirements during the inspections," a ProPublica review of the reports found that 60 plant sites had deficiencies that ranged from broken machinery, missing equipment and poor training to things like blocked drains or a lack of preventive maintenance. In July, a special task force of NRC experts -- the Japan Task Force -- called for potentially sweeping and costly changes to address those deficiencies and to protect plants against catastrophic events like earthquakes and long-term blackouts.

Although the NRC polices the nuclear industry, its Office of the Inspector General has traditionally filled a key oversight role for the commission, conducting investigations that have changed how the agency regulates nuclear waste, fire protection and security. But two former OIG employees told ProPublica the inspector general's office buried a critical report and dropped an investigation into whether the NRC is relying on outdated methods to predict damage from an aircraft crashing into a plant.