The 235u isotope = amu 235 undergo fussion when bombarded with neutrons. natural abundance is 0.72% to seperate it from the more abundant u238 isotope (238), uranium is first converted to UF6, which is easily vaperized RT. The mixtture of 235UF6 and 238 UF6 gases is then subjected to many stages of effusion. Calculate how much more 235UF6 effuse than 238UF6. 235UF6 to UF6 238. 4 decimal.