A 500 kg mass solid nuclear reactor concept with 20 wt.% uranium nitride fuel was proposed in the 2009 ICAPP conference for the application to space probe. Critical mass reduction of this solid reactor is studied here in order to obtain reference data for possible power/mass ratio improvement. By considering some possibilities on structural material and fuel material, mass reduction effects of some options are summarized. By replacing stainless steel with low neutron absorbing material, about 100 kg mass reduction is possible. Uranium-233 can reduce the reactor mass to less than half. Despite the small temperature coefficient and the delayed neutron fraction, minor actinide 241Am also showed a good mass reduction effect as a fuel in the fast neutron spectrum system. Highly enriched U thermal neutron spectrum system may be an reasonable option, provided nuclear non-proliferation restriction is cleared.