The dirty part is the fission products, not the unspent uranium. You wouldn't want to launch a nuclear salt water rocket from the surface even if it achieved 10

The dirty part is the fission products, not the unspent uranium. You wouldn't want to launch a nuclear salt water rocket from the surface even if it achieved 100% fuel burnup. https://en.wikipedia.org/wiki/Nuclear_fission_product https://en.wikipedia.org/wiki/Nuclear_fission_product