Given that the radiation of nuclear waste has frequency way higher than UV, why can’t it be used to feed a photoelectric generator?

You're probably using one of these right now (albeit indirectly)! They're called Photovoltaic nuclear batteries and they're critical to modern encryption. They ensure that encryption keys, which are stored in highly volatile memory (memory where if power is ever lost the contents are immediately erased), never lose power unless the memory modules are physically disconnected.

The reason they're not used more extensively is that they just don't produce very much power - the high-energy electromagnetic radiations are very difficult to harness constructively (things like gamma and X-rays) and as a result we have to do some weird physics stuff to convert them. PVN batteries convert particle radiation, beta radiation from tritium decay specifically, into usable photons via a thin coating of phosphorus on the glass, instead of them being captured directly.

(this is a wild oversimplification just to be clear)