Supersymmetry-breaking compactifications on Riemann-flat manifolds

We consider compactifications of higher-dimensional supergravities on Riemann-flat manifolds of dimension d ($3 \le d \le 7$) that fully break supersymmetry at the classical level on a resulting D-dimensional Minkowski space. We systematically discuss consistency conditions, the Kaluza-Klein (KK) spectrum and harmonics, and the resulting one-loop effective potential $V_1$, focusing for illustration on maximal supergravity and d=3, in particular on the $T^3/Z_3$ and on the Hantzsche-Wendt manifolds. We show how the KK spectrum is organized in multiplets of the broken supersymmetry, derive new universal supertrace mass relations valid at each KK level and obtain an analytic finite expression for $V_1$ after resumming the contributions of all KK levels. In all examples $V_1$ is negative definite and scales with inverse powers of some internal radii. We extensively comment, when applicable, on the relation with the Scherk-Schwarz mechanism and with supersymmetry-breaking string compactifications on freely acting symmetric orbifolds. We also finally clarify the assumptions and constraints for Scherk-Schwarz reductions to correspond to twisted tori compactifications.