Titanium-Water Heat Pipe Radiator for Spacecraft Fission Power

The proposed program will develop titanium/water heat pipes suitable for Spacecraft Fission Power. NASA is examining small fission power reactors for future space applications with the most recent being Kilopower, which provides roughly 1 kW of electric power. Kilopower uses titanium/water heat pipes to remove the waste heat from the cold end of the convertors. Previous water heat pipe designs for space fission power are not suitable for Kilopower as they are either for surface fission power and use thermosyphons, or are grooved heat pipe designs, which are not suitable for ground testing. ACT will develop heat pipes with two different designs that are suitable for Kilopower: Hybrid grooved/screen wick and Self-venting arterial wick. Hybrid wick heat pipes will satisfy the Kilopower requirements and ACT has already successfully tested similar hybrid wick heat pipes. The self-venting arterial wick has not previously been tested in a vertical orientation but will be investigated as a higher performance, lower mass alternative to hybrid grooved pipes. The overall technical objective of the Phase I and Phase II projects is to develop a titanium/water heat pipe radiator suitable for Spacecraft Fission Power, such as Kilopower. During Phase I, ACT will investigate both a hybrid wick system, utilizing a screened evaporator and grooved condenser design, and a self-venting arterial wick design. The heat pipe design will also include a small NCG charge, which allows the fluid in the heat pipe to freeze in a controlled fashion as the heat pipe is shut down, avoiding damage, and aids with start-up from a frozen condition. In addition to testing the heat pipes in different orientations, freeze/thaw tolerance will also be demonstrated.

The immediate NASA application is for space fission nuclear reactors that utilize Stirling converters or thermoelectrics for power conversion. Specifically, the 1kWe Fission Power System with a 15 year design life that could be available for a 2020 launch. The titanium-water CCHPs developed in this program would be capable of transporting the waste heat from the Stirling or thermoelectric convertors to the system radiator.

There is a commercial application for high temperature VCHP heat exchangers in fuel cell reformers. In a fuel cell reformer, steam, air and diesel fuel react in a High Temperature Shift (HTS) and a Low Temperature Shift (LTS) reactor to produce as much hydrogen as possible. Feed streams to and from the reactors must be maintained under tight temperature control, typically within 30�C despite a turndown ratio of 5:1 in reactant flow rate. ACT believes that VCHP heat exchangers can replace the current heat exchanger and control system with a passive system. The VCHP heat pipes passively adjust the heat removed, to maintain the output stream at a constant temperature. The direct bonding of the single facesheet GFRC to the titanium condenser would allow low cost and mass radiators for atmospheric high altitude applications like waste heat rejection from balloon payloads or from UAV electronics.