Enemy RVs would strike this cloud of dust at approximately 7 kilometres per second (4.3 mi/s) which would rapidly abrade the RV's heat shield causing the warhead to fail due to reentry heating or for the warhead to be knocked off course.
[1] The effectiveness of the system depended on the hardness of enemy RVs to abrasion and the yield of the weapon used.
Approximately 1⁄3 million tonnes (330×10^6 kg) of dust would be produced per megaton of warhead yield.
Therefore, a dust defense system to prevent the need for launch on warning would be safer as in a false alarm you have only slightly irradiated some of your nation instead of facing the full brunt of a retaliatory attack.
However, the stationary nature of the system means that weight and size is not an issue, so very low fission fraction technologies developed for Project Plowshare could be used to reduce the effects of fallout.