Radiator Demonstration Unit for Space Nuclear Power Systems

Under Project Prometheus, NASA examined space nuclear power systems for several missions. One such mission was the Jupiter Icy Moons Orbital (JIMO) Mission. A conceptual design of the orbiter is shown in Fig. 1

Consistent with all large space nuclear systems, the radiator for the cold end (to reject the waste heat) was a substantial portion of the overall system mass. In the JIMO spacecraft, the radiator was planar, and designed to deploy once the craft is in space. The radiator is roughly triangular, to keep it within the cone of the radiation shielding. One design examined during the Prometheus used a Brayton cycle to generate electricity so the radiator could operate in the “intermediate” temperature range of 450 to 750K, where there were no established working fluids for advanced heat-spreading devices such as heat pipes and loop heat pipes.

Figure 1. JIMO conceptual design with a system of deployable radiator panels for heat rejection.

Under a Radiator Demonstration Unit (RDU) program with NASA Glenn Research Center, ACT developed low-mass, high-temperature titanium/water heat pipe radiators to support the JIMO mission. The program produced three full-size panels, each 1 m by 0.5 m, and each containing three titanium water heat pipes as shown in Fig. 2. These panels were designed to operate at temperatures from 350 to 530 K using water as the working fluid, and were tested in vacuum at NASA Glenn Research Center (GRC).

Figure 2. Three RDU panels with Graphite Fiber Reinforced Composite (GFRC) face-sheets and titanium heat pipes were delivered to NASA Glenn for testing. Each panel was 1m by 0.5m, had three heat pipes, and was designed to operate at temperatures up to 550K.

Radiator Demonstration Unit Program Achievements

Development of an analytical model for optimizing the spacecraft radiators in this temperature range
Heat pipe life tests to verify the compatibility of titanium and titanium alloys with water at temperatures up to 550 K.
Heat pipe wicks for water made of titanium.
Panel materials development and property measurements.
Subscale panel fabrication and demonstration of the panels withstanding thermal cycling.
Fabrication of three full size panels, each 1 m by 0.5 m, each with three titanium water heat pipes.

life tests

The first step in developing the radiator heat pipes was to select a working fluid. Water has the best merit number of the known heat pipe fluids up to about 550K. Copper/water heat pipes are only suitable up to about 425K, since the copper envelope must be very thick to withstand the saturated water pressure at higher temperatures. Titanium, titanium alloys, Monel 400, and Monel K500 have higher yield strength and lower density than copper, and so are suitable for higher-temperature water heat pipes. When using a new working fluid/heat pipe envelope material combination, life tests are required to verify that the materials and working fluids are compatible. ACT performed life tests for various titanium and Monel alloys with water as working fluid as well as for the so called “intermediate temperature range” (450 to 750K) fluids.

Life Tests for Radiator Demonstration Unit (Titanium Water)

The main reasons for considering water as the working fluid for the Radiator Demonstration Unit (RDU) are that it possesses good heat transfer properties in the 350-550K temperature range and its compatibility with titanium that is a high strength and low-density material. During the RDU program, ACT started a large test matrix of life tests to verify that titanium and titanium alloy heat pipes were compatible with water.

The tested titanium/water heat pipes include:

Ti CP-2 Heat Pipe, with CP Titanium Screen
Ti Grade 5 Cylinder (6% Aluminum, 4% Vanadium), with CP Titanium Screen
Ti Grade 7 Cylinder (0.2% Pd), with CP Titanium Screen
Ti CP-2 Cylinder, with 21S foil and CP Titanium Screen
Ti Grade 9 cylinder (3% Aluminum, 2.5% Vanadium) with CP Titanium Screen
Ti CP-2 Heat Pipe, with Sintered Cylindrical Wick
Monel K500 Heat Pipe, with Monel 400 Screen

Table 1 below shows the different life test pipes on test. Monel 400 is a solid solution alloy with roughly 63% nickel and 30% copper. It is a single-phase alloy, since the copper and nickel are mutually soluble in all proportions. It can only be hardened by cold working. Monel K500 is a similar nickel-copper alloy, with the addition of small amounts of aluminum and titanium that give greater strength and hardness. The system is age-hardened by heating so that small particles of Ni₃(Ti, Al) are precipitated throughout the matrix, increasing the strength of the material. The advantage of Monel K-500 is that the strength can be partially recovered after a wick is sintered inside.

Several of the heat pipes were selected by destructive analysis. To examine the cross-sections to determine the type and amount of corrosion in the wicks and heat pipes, the heat pipes were cut in half, pressure infiltrated with epoxy and sectioned at a location approximately one-third of the way above the bottom of the heat pipe. The sections were polished through 0.05-micrometer silica and examined using optical and scanning electron microscopes (SEM).

Titanium/water and Monel/water heat pipes are compatible at temperatures up to 550 K, based on the life tests that ran for up to 72,000 hours (8.2 years). Analysis of titanium/water heat pipe cross-sections using optical and electron microscopy revealed little if any corrosion even when observed at high magnifications. When any evidence of corrosion was observed, the layer was typically around 1 micrometer thick. Copper depleted zones, as well as copper surface nodules formed on the Monel 400 screen wick. This was not observed on the Monel K500 envelopes. An analysis of the water working fluids showed minimal pickup of metals.

Titanium-Water and Titanium-Monel Pipes Life Test

Initial Quantity	Wall Material	Wick	Operating Temperature	Operating Hours
4	Monel K 500	200×200 Monel 400 Screen 0.064 mm wire	550 & 500 K	72,192 hours
4	CP-2 Ti	150x150CP-Ti Screen 0.069 mm wire	550 & 500 K	72,192 hours
2	CP-2 Ti	Sintered Titanium -35+60 Mesh CP-2	550 K	60,672 hours
2	CP-2 Ti	100 x100 CP-Ti Screen 0.05 mm wire	550 K	61,064 hours
1	CP-2 Ti	Integral Grooves	550 K	41,345 hours
2	CP-2 Ti 21 S Foil Inside	100 x100 CP-Ti Screen 0.05 mm wire	550 K	62,622 hours
2	Grade 5 Ti	100 x100 CP-Ti Screen 0.05 mm wire	550 K	69,845 hours
2	Grade 7 Ti	100 x100 CP-Ti Screen 0.05 mm wire	550 K	60,672 hours
2	Grade 9 Ti	100 x100 CP-Ti Screen 0.05 mm wire	550 K	60,672 hours
2	Monel 400	120×120 Monel 400 Screen 0.05 mm wire	550 K	60,168 hours
2	Monel K 500	120×120 Monel 400 Screen 0.05 mm wire	550 K	67,536 hours
2	Monel 400	-100+170 Mesh Monel 400 Powder	550 K	58,824 hours
2	Monel K 500	-100+170 Mesh Monel 400 Powder	550 K	57,792 hours

Table 1. Titanium-Water and Titanium-Monel Pipes Life Test – Final Operating Hours.

Life Tests for the Intermediate Temperature Range (450-750K)

Several applications could benefit from heat pipes in this intermediate temperature range (that is generally defined as the range between 450 and 750 K), and one of these applications is space nuclear power system radiators. Despite intense efforts by the community to qualify or develop fluids for this temperature range, there is still no commonly accepted working fluid over the entire intermediate temperature range. At temperatures above 700-725 K, alkali metal (cesium) heat pipes start to become effective. Below about 725 K, the vapor density for cesium is so low that the vapor sonic velocity limits the heat transfer. At the lower side of this temperature range, water was historically used at temperatures up to about 425 K. This case study has shown that water can be used with titanium or Monel envelopes at temperatures up to 550 K.

Potential working fluids in the intermediate temperature range include elemental working fluids such as sulfur, organic compounds, and halides. ACT performed, under various NASA programs, investigation and life tests for these potential fluids. 30 different intermediate temperature working fluids, and over 60 different working fluid/envelope combinations have been life tested.

Elemental Working Fluids

Three elemental working fluids: sulfur, sulfur-iodine mixtures, and mercury were among the tested fluids. However, other fluids offer benefits over these three elemental ones in this temperature range. Mercury is toxic, has a high density, and problems have been observed with getting the mercury to wet the heat pipe wick. Sulfur and Sulfur/Iodine have high viscosities, low thermal conductivities, and are chemically aggressive.

Organic Working Fluids

Life tests have been conducted with 19 different organic working fluids. As the temperature is increased, all of the organics start to decompose. Typically, they generate non-condensable gas (NCG), and often the viscosity increases. At high enough temperatures, carbon deposits can be generated. The maximum operating temperature is a function of how much NCG can be tolerated, and the heat pipe operating lifetime. Three sets of organic fluids stand out as good intermediate temperature fluids:

Diphenyl, Diphenyl Oxide, and Eutectic Diphenyl/Diphenyl Oxide (Dowtherm A, Therminol VP, Diphyl)
Naphthalene
Toluene

Non-Organic Working Fluids

A non-organic working fluid is desirable for nuclear fission space power and other applications where radioactivity can generate gas with organic working fluids. Long term life tests show that Superalloys/TiCl4₄ at 573 K (300°C), and Superalloys/AlBr₃ at 673K (400°C) are compatible. AlBr₃ and TiCl₄ tests have over 59,000 hours (6.7 years) of testing. Hastelloy C-2000 underwent little corrosion when used with TiCl₄ working fluid, with the formation of only a 1-2 micrometer thick corrosion layer. Hastelloy C-22 exhibited a 5-10 micrometer thick dual corrosion layer when tested with AlBr₃ working fluid. The working fluids of these two heat pipes exhibited total metal contents between 300 and 350 ppm. The results indicate that the tested envelope materials and working fluids can form viable material/working fluid combinations.

Compatibility Prediction

Life tests were conducted at NASA Glenn with three halides (AlBr₃, SbBr₃, and TiCl₄) and water in three different envelopes: two aluminum alloys (Al-5052, Al-6061) and CP-2 titanium. The AlBr₃ attacked the grain boundaries in the aluminum envelopes, and formed TiAl compounds in the titanium. The SbBr₃ was incompatible with the only pipe that it was tested with, Al-6061. Finally, TiCl₄ and water were both compatible with CP2-titanium. A theoretical model based on electromotive force differences has been developed to predict the compatibility of halide working fluids with envelope materials. The envelope material halide should have a lower decomposition potential than the working fluid halide. AlCl₃ and TiCl₄ have a high decomposition potential, so should be good working fluids. Molybdenum and iron have a low decomposition potential, so should be good envelope materials. The method almost always predicted the compatibility of halide life tests. For example, it successfully predicted that TiCl₄ was incompatible with aluminum and was compatible with mild steel. The only two cases without full agreement were (1) AlBr₃ and aluminum, where the AlBr₃ attacked the alloying materials at the grain boundaries, and (2) AlBr₃ and titanium, where the method predicted incompatibility, but not that TiAl compounds would form.

titanium heat pipe wick development

ACT developed several titanium heat pipes with various wicks on the Radiator Demonstration Unit program. Representative samples are shown in Fig. 3. This set of wicks represented the selection for further relevant/suitable applications.

Figure 3. Heat pipe wicks developed on the RDU program. Counter-clockwise from top right: (a) solid groove wick, (b) screen wick for life test pipes, (c) sintered annular wick for life test pipes, (d) sintered vapor-groove wick, (e) slab wick, and (f) sintered liquid groove wick.

Panel development and panel property management

The radiator panels were developed with three different fin materials: K13D2U fibers with 5250-4, EX1551, and HPFE resin. The fin material is 0.38 mm thick and uses a layup pattern that gives a coefficient of thermal expansion (CTE) along the heat pipe that matches the CTE of titanium. ACT has measured the mechanical and thermal properties of these panels after thermal cycling. This information was used to eventually design the actual radiator.

prometheus project – final results

Prior to ACT’s Radiator Demonstration Unit program with NASA, the baseline heat rejection system was aluminum heat pipes, with ammonia working fluid, and aluminum honeycomb panels with aluminum fins, suitable for rejecting heat at temperatures up to 60°C (333K).

To minimize the radiator size for Space Nuclear Fission, it is necessary to increase the heat rejection to 280 C (550K). Higher temperature titanium-water and Monel-water heat pipes were developed and qualified.

Ultimately, a high temperature radiator with titanium-water heat pipes, GFRC fins, and aluminum honeycomb was developed and demonstrated. Restart of the heat pipes from a frozen state in the horizontal position was also demonstrated. In addition, ACT was also able to demonstrate restart of the heat pipes from a frozen state in the horizontal position.

The Prometheus-funded Radiator Demonstration Unit program was one of the precursors for ACT’s involvement in the NASA Kilopower, Fission Surface Power and Space Nuclear Propulsion projects.