Catalyzes the oxidation of L-aspartate to iminoaspartate, the first step in the de novo biosynthesis of NAD(+). L-aspartate + O2 = H2O2 + iminosuccinate Binds 1 FAD per subunit. Cofactor biosynthesis; NAD(+) biosynthesis; iminoaspartate from L-aspartate (oxidase route): step 1/1. Belongs to the FAD-dependent oxidoreductase 2 family. NadB subfamily.