Complex sentence: In nuclear weapon development, the main problem is securing sufficient "weapons grade" material, in particular the fissile isotopes of either uranium-235 (235 U) or 239 Pu.
Split into simpler sentences: In nuclear weapon development, the main problem is securing sufficient "weapons grade" material. In particular, it is difficult to acquire either fissile isotopes of uranium-235 (235 U) or of 239 Pu.