Information from:
http://www.aero.org/publications/crosslink/summer2003/01.html

As with many other programs in the early years of the corporation, space radiation studies received a boost by a startling development in the arms race between the United States and the Soviet Union. Beginning in 1958, the United States had exploded a series of low-yield nuclear devices at high altitudes, but the scale and scope of nuclear testing in space escalated dramatically in the summer of 1962, leading to unforeseen consequences. The United States detonated a high-yield (1.4 megaton) nuclear device, code-named "Starfish," on July 9 above Johnston Island west of Hawaii at an altitude of 400 kilometers. The enormous explosion created a new radiation belt and produced an aurora that lasted about seven minutes. In the aftermath, the intensity of radiation in space increased a thousandfold. Several spacecraft were damaged or destroyed. The need to understand the characteristics of space radiation now acquired greater urgency, as it was clear that a nuclear detonation in space could conceivably disable military satellites. The sense of urgency was heightened when the Soviets began their own series of high-altitude nuclear detonations later that year.

Sure it happend some time ago, but then how long do the radioactive particles stay there? And don't quess, give me figures that are reliable.

Do look into
this web site from NASA. It describes a situation where there are air molecules escaping from the Earth's thermo/ion sphere into space.