Preliminary conceptual design of an electronuclear system for space applications

The purpose of this paper is to present a preliminary conceptual design of a nuclear power system for space applications. This design was produced within the framework of a collaboration between CNES and CEA. The targeted electrical power of the system is 10 kWe for a period of at least 10 years. The scalability towards larger (30 kWe) and lower (about 1 kWe) power has been investigated. The system must be technologically "affordable" and robust over a 10-year horizon, which de facto eliminates the choice of too innovative and unqualified technologies in conditions representative of the missions; the system must be able to be launched in Ariane 6 while respecting the launcher mass and size capacities. Given the intermediate power level targeted and the potential use of the system on the Moon or Mars, the CEA favored design options associated with a compact and monolithic radiator (allowing it to be landed) rather than minimizing the system mass. Taking into account the desired characteristics for the nuclear power system, the CEA proposes main conceptual design features which includes: a thermal spectrum core whose fuel is UO2 enriched to 19.75% in U235, zirconium fuel cladding, a zirconium hydride (ZrH2) moderator, and a beryllium reflector. The proposed shield is made of tungsten and lithium hydride (LiH). A core cooling system made up of 150 steel heat pipes operating with a sodium-potassium alloy (NaK) has been pre-designed and a thermoelectric energy converter made up of SiGe thermo-elements (hot temperature of 950 K and cold temperature of 700 K) generates electrical power. A "hot" carbon composite radiator lined with titanium NaK heat pipes rejects the heat of the system. This system has intrinsic redundancies conferred by the heat pipes and the large number of thermo-elements of the converter as well as high passivity, in particular in its ability to adapt its power to electricity demand. In its version limited to 10 kWe, the system whose height is about 5 m, enables to keep a height of 13 m for a payload to be launched in the Ariane 6 fairing and its mass is estimated at about 2 tons. To access a version of the system capable of producing 30 kWe, an additional mass estimated at around 1 ton is to be expected as well as a lower useful height in the fairing under the system, which would then be 10 m.