Am I understanding this right, that I have to use nuclear reactors to make waste to make raffinate to make space science packs? If so, that's a huge bottleneck. I mostly run my base on solar, so nuclear reactors are a backup. They only get used in high-power situations (large attacks, constructions, etc).

Is there another way to generate space science packs other than raffinate?

Yes you're understanding this right. If you don't want to use nuclear reactors for electricity generation, then you can let them void the heat as they eat through nuclear fuel assemblies and pretend that they're big crafting machines.

Ugh, that feels terrible, wasting that heat :(

Is there another way to generate raffinate?

No