文里有写

In nuclear-weapon development, the main problem is securing sufficient weapons-grade material; it is particularly difficult to acquire fissile isotopes of uranium-235 (²³⁵U) or ²³⁹Pu. Weapons-grade uranium requires mining, extracting and enriching natural ore. Plutonium can be "bred" in reactors fueled by unenriched uranium, which requires chemical separation of the ²³⁹Pu produced.

Heavy water has been demonstrated as an effective moderator for ²³⁹Pu production, and may be separated from ordinary water by electrolysis..... 

是的, 德国离制造还远, 基本材料都收集不齐