In this paper passive reactivity control devices for a Generation IV gas-cooled fast reactor (GCFR) are discussed. The proposed devices use liquid 6Li as absorber. The device is triggered by a freeze seal, and upon activation the 6Li is irreversibly introduced into the core region by pressure differences. The device is dubbed the lithium injection module (LIM). Transient thermohydraulic calculations were done using the CATHARE2 code on a simplified thermohydraulic model of GFR600, a 600-MW(thermal) GCFR investigated in the scope of the European GCFR-STREP. The thermohydraulic model uses an accurate model of the ceramic fuel plates and includes natural convection decay heat removal circuits. To properly account for power production during the transient, a synthetic decay power curve was made based on the ANSI/ANS-5.1-1994 law. Loss-of-flow and control rod withdrawal/ejection transients are presented. Neutronic calculations show that the LIMs have a low reactivity worth between -2.1 and -1.5 $. In spite of their low worth, the LIMs are capable of keeping the reactor power bounded during all calculated transients. Shutdown is not always achieved, depending on the kind of transient under consideration. For pressurized loss of flow, recriticality due to Doppler feedback may become problematic in the natural-circulation phase. For rapid control rod ejections, the resulting very fast power transients cause concern for material degradation. One LIM would be enough to control reactor power, but redundancy may call for more than one LIM in the core.