Four different design concepts were also investigated in the early 1970s by the European GBR Association, taking the LMFBR fuel and core technology with a gas thermal reactor system. These designs were not developed commercially at the time, due to their cost, and the preference for LMFR and LWR systems, which were further developed. They are now being reconsidered in modular designs with more favourable economics and with more natural safety characteristics.

Three 1000 MWe systems were considered, GBR1 (helium (He) and fuel pins), GBR2 (He and coated particles), GBR3 (CO2 and coated particles) and one 1200 MWe system GBR4 (He and fuel pins). The latter was the favoured option at the time.

GBR4 had vented pins containing coated particle fuel. The coolant pressure was 90 bars, necessary to achieve the efficiency using fuel pins. Two independent shutdown systems were employed. There is an advantageous safety feature associated with a negative reactivity expansion coefficient. C&I systems were based on 1970s technology and therefore a new design concept today would require a more up-to-date approach.

The reactor pressure vessel enclosed an integrated system, incorporating the boilers in individual pods in the vessel in a design similar to that employed by the AGRs. An independent decay heat removal system was also included. The containment included an inner steel liner and outer concrete shell.

Additional safety features were incorporated to accommodate cooling in a depressurisation accident, un-tripped loss of flow, etc. At the time, it was concluded that further development was required in the plant safety concept, particularly in the field of severe accidents, core melt and containment.