In the worst accident in Japan’s troubled history with nuclear power, workers in a fuel plant 87 miles northwest of Tokyo accidentally set off a chain reaction yesterday morning that spewed high levels of radiation into the air. More than 40 people, most of them plant employees, were being treated for radiation exposure, three people were seriously injured, and 300,000 nearby residents were ordered to stay indoors and keep their windows shut. The reaction was brought under control early this morning, but officials have said nothing about potential contamination of areas outside the plant nor the potential long-term impacts of the accident. Arjun Makhijani, president of the Institute for Energy and Environmental Research in Washington, D.C., said the Tokaimura plant has had other accidents in recent years and should be shut down for a thorough, independent investigation. A strong public backlash in Japan against the nuclear industry is expected.