The unramified Brauer group of norm one tori

Let k be a number field and K/k Galois. We transform the construction of the unramified Brauer group of the norm one torus RK/k1(Gm) into the construction of a special abelian extension over K. If k=Q and K/Q biquadratic, we explicitly construct the unramified Brauer group of RK/Q1(Gm).