Claiming that it needed to test monitoring equipment that would ultimately combat the Soviet Nuclear Threat, Hanford released 8,000curies of uranium into the air, The test was kept secret, so homeowners and others were kept from knowing about the extremely radioactive release in the air above them. The notorious accident at Three Mile Island in 1979 only leaked between 15 and 24curies of radiation into the environment. Although still much more than is safe, it was nowhere near the amount that Hanford released. It wasn't until 1986, when the freedom of information laws made hundreds of Hanford documents open to the public, that the enormous radioactive release was discovered. Downwind citizens of Washington, Idaho and Oregon could have been exposed to levels over twenty times the safe level of radiation from this event.

The Green Run release was uranium that had not been cooled for the appropriate length of time before being moved at all. Usually, uranium was cooled for 90 days, but for Green Run it was only cooled for 16 days. Common practice is to cool uranium for 180 days, more than double what the people in Richland, Spokane and other cities were exposed to during Operation Green Run.

References

Release Estimates

What was Green Run?