 This eliminated fast fission by the fusion-stage neutrons so that approximately 97% of the total yield resulted from thermonuclear fusion alone (as such, it was one of the "cleanest" nuclear bombs ever created, generating a very low amount of fallout relative to its yield).There was a strong incentive for this modification since most of the fallout from a test of the bomb would likely have descended on populated Soviet territory.The first studies on "Topic 242" began immediately after Igor Kurchatov talked with Andrei Tupolev (then held in Autumn 1954).