Dimming Sun is a joint US/UK nuclear weapons accident exercise that was supposed to have taken place in June, 1999, on the Stanford Battle Ground in Norfolk, England. The exercise was to involve personnel from local authorities, government departments, and staff from all 3 UK military services. The cost was around £10 million, and was to employ around 1000 people.
The scenario is briefly described as follows: "A US military transport plane taking off from USAF Lakenheath and loaded with nuclear weapons being returned to America for servicing. There is a mid-air collision with a local light plane over an open area, and the US plane and its four B-61 nuclear weapons
crashes and is destroyed on impact. All crew on both planes are killed along with a number of local residents".
The exercise was cancelled due to the NATO commitments in Kosovo during Operation Allied Force. It is now scheduled to take place in the summer of 2003 on the same location.
Source: UK Campaign for the Accountability of American Bases. http://www.gn.apc.org/cndyorks/caab/.