Design and R&D of a megawatt lithium-cooled space nuclear reactor was the subject of a recent detailed article in the scientific journal SCIENTIA SINICA Technologica (v.54, Issue 3, pp. 365-376) co-sponsored by the Chinese Academy of Sciences and the National Natural Science Foundation of China.

According to the abstract of the paper: “Space nuclear reactor power, with the advantages of high energy density, high output power, long duration, and minimal influence from the external environment, is the preferred route for energy supply for future high-power long-life space missions and deep space exploration missions. Based on the developmental requirements and characteristics of different design options for megawatt-class space nuclear reactors, a technical scheme for a megawatt-class small lithium-cooled space reactor is designed. This scheme uses a lithium-cooled reactor coupled with a Brayton power conversion system that is lightweight and durable.”

Chinese team selected liquid metal as the most suitable cooling option. Inert gas was excluded due to the need to have a large active zone and heavy protection, and heat pipes. Various options for liquid metal coolants (sodium, potassium, etc) were investigated before lithium was selected.

Lithium has large specific heat capacity and thermal conductivity, which makes possible a compact active reactor. Lithium also has a high boiling point (over 1600 degrees Kelvin), which ensures a single phase of the heat carrier flow in a wide temperature range. The inlet and outlet temperatures of the coolant are 1400 and 1550 K respectively. The neutron spectrum in the reactor design is fast and the fuel is uranium nitride with a uranium enrichment of 93%. The power of the reactor is about 6 MWt or 1.5 MWe.

The reactor's active zone is assembled from honeycomb uranium-nitride fuel cells with lithium pumping channels. The channel diameters are different, which makes it possible to reduce the radial coefficient of uneven power output. The design life of the reactor is 10 years.

The control system has a boron carbide absorber below which a compressed spring is located. The output of the system is sealed with a sealing material, which melts when heated, after which the absorber is pushed by a spring into the active zone.

The article discusses experiments using a prototype stand. It includes the first lithium circuit and the second circuit for the Brayton helium-xenon cycle. The first circuit uses electric heating elements with a capacity of more than 100 kW, which can provide a maximum lithium temperature of about 1500 degrees Kelvin. Lithium pumping is carried out by an electromagnetic pump. Heat exchange between the circuits occurs through a plate heat exchanger. The second circuit has a turbine for generating electricity. Excess heat is discharged using a water cooling radiator.

In addition to checking the proposed technological solutions using the stand, a set of experiments was performed to study the corrosion of molybdenum alloy in contact with casting heated to high temperature.

The authors noted a number of issues related to the level of technology readiness. With respect to nitride fuel, they recognised that in China the technology of its manufacture is in its infancy. Significant progress has been made in Russia, but additional experimental checks of the behaviour of such fuel under radiation will be required.

It is proposed to use molybdenum and rhenium alloy as the structural material of the active zone. It is refractory, with high heat resistance, but at the same time very expensive. High prices for Mo-Re alloy seriously limit the volume of experimental studies of its properties and behaviour in various environments. It is also necessary to solve the problem of impurities in the alloy and choose the right welding methods.

Lithium hydride is selected for protection. However, this material can decompose during irradiation and swell at low temperatures, and it must be combined with boron carbide and temperature control systems. The authors mentioned other technological issues needing solution. In China now there are pumps operating with lithium but only at relatively low temperatures.

Another important question is the creation of a reliable system for separating helium from lithium (helium will be formed under radiation) in the absence of gravity. The volume of tests available in China on this topic is insufficient. Also, there is no solution on how to compensate for the expansion of lithium when the temperature changes. The authors also touched upon other problems on other elements of the reactor installation, and also noted that it is necessary to carefully analyse safety issues in emergency situations – for example, in accidents at the start of the launch vehicle.

Image: Space reactor core (courtesy of Rosatom)