The report is likely to reinforce the conviction of American companies that operate reactors of the design used at Fukushima that venting from the containment vessels around reactors early in an accident is better than waiting, even though radioactive material will be released. The delays in Japan appear to have contributed to explosions that damaged the vessels and ultimately led to larger releases of contaminants.

It has been clear for months that Fukushima operators delayed venting for hours, even after the government ordered that the action be taken. The chronology, however, suggests for the first time that some delays were because plant executives believed that they were required to wait for evacuation of surrounding areas.

Because the chronology is based mainly on accounts by Tepco and its workers and company data, it is by nature limited. It does not, for example, relate that there was tension between Tepco and the government over when to vent, as the news media have reported.

The report is also likely to incite more debate about how emergency equipment and material are stored and what types of contingency plans need to be made to ensure equipment can reach reactors in a disaster. Nuclear critics in the United States have long complained that American emergency rules do not take into account that a natural phenomenon could cause an accident at a plant and make it hard to get help from outside.

For example, although the plant had three fire engines that could have pumped in vital cooling water, one was damaged in the tsunami and another was blocked by earthquake damage to roads. Inspections at some American reactors after the Japanese quake and tsunami found that they were storing emergency gear in a way that made it vulnerable to the emergency it was intended for.

The report was perhaps most vivid when it was describing workers’ often unsuccessful efforts to salvage the situation. In one case, plant workers are said to have broken through a security fence to take a fire truck to unit 1 so it could pump water to cool the reactor. (The plant’s cooling system by that time was unusable, and without it, reactors and fuel pools can overheat and cause meltdowns.)