A device for increasing the thermal expansion effect of control rod drive lines on negative reactivity feedback in fast reactors has been developed. The enhanced thermal expansion of this device can be utilized for both passive rod drop and forced insertion of absorbers in unprotected transients, e.g., unprotected loss of flow (ULOF). In this way, the reactor is automatically brought into a permanently subcritical state, and temperatures are kept well below the boiling point of the coolant. A prototype of such a device called ATHENa (German: Shutdown by THermal Expansion of Na) has been manufactured and will be tested. The principle, design features, and thermal properties of ATHENa are presented, as well as results of reactor dynamics calculations of ULOF accidents for the European Fast Reactor with enhanced thermal expansion control rod drive lines.