On NTRs (and space reactors in general) you only need a "shadow shield" that casts a radiation shadow in the direction of thing to protect (crew, sensitive syst

On NTRs (and space reactors in general) you only need a "shadow shield" that casts a radiation shadow in the direction of thing to protect (crew, sensitive systems). This makes the necessary shielding much less than a full sphere, reducing the mass to a mere fraction. Thing get a bit hairy though when you need to dock with something or even when running multiple engines (the neutron balance would apparently be totally wrong).