To what do you refer, that is not a "huge complicated nuclear reactor" but also is not an RTG? There are and have been fission reactors in space already. RTGs

To what do you refer, that is not a "huge complicated nuclear reactor" but also is not an RTG? There are and have been fission reactors in space already. RTGs seem to typically be in the hundreds of watts, while the reactors are more in the kilowatt range. The reactors are not RTGs, and they are definitely a lot more complex.