State of the art of an MMRTG

The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) (Figure 2.1) is a type of Radioisotope Thermoelectric Generator (RTG) developed and provided to NASA for civil space applications by the U.S. Department of Energy (DOE). The first MMRTG was mounted on the Curiosity Mars rover, which was launched in the Mars Science Laboratory (MSL) mission on 26 November, 2011, and landed on the surface of Mars on August 6, 2012 [6]. An MMRTG produces almost 2000 W of power at the beginning of life. Still it is not very

Figure 2.1: Multi-Mission Radioisotope Thermoelectric Generator [7].

efficient in converting heat into electricity: the most used combination of materials, such as plutonium-238 with Si–Ge TE cells, give about 7% conversion efficiency of the total generated power, about 110 W. The Heat Rejection System (HRS) uses some of the remainders to keep electronics warm to maintain temperatures in the operating range. Some of the waste heat is dissipated back to space or planet’s surface via a radiator. An MMRTG is made up of different parts, each of which performs several functions:

• GPHS modules;

• Cooling tube;

• Heat Source liner;

• Heat distribution block;

• Loading cover;

• Mounting end cover;

• Insulation;

• Fins;

• Thermoelectric modules.

GPHS module The General Purpose Heat Source (GPHS) module is the fundamental build-ing block for the MMRTG, and it is designed to survive launch accidents because these modules contain and keep safe fuel, such as Plutonium-238. In Figure 2.2 are shown all constituent parts of a GPHS module:

Figure 2.2: Illustration of a GPHS module [7].

 Fuel Pellet: the fuel is produced into ceramic pellets³of Plutonium-238 dioxide,²³⁸PuO2, encapsulated in a protective casing of a ductile, high-temperature iridium-based alloy, which blocks the alpha particles emitted in the decay, known as Fueled Clad. Besides, thanks to the ceramic compound, even if the pellets undergo high forces to break them, it will break into large blocks rather than dust that could be inhaled;

 Graphite Impact Shell: fuel clads, in turn, are encased and protected by graphite sleeves;

3Increase the melting point (Section 2.3.1) in case the cooling system fails.

Chapter 2 2.2. State of the art of an MMRTG

 Aeroshell: the mentioned above elements are, in turn, enclosed and protected within nested layers of high stiffness and high chemical resistance carbon-fiber material, known as Aeroshell, which forms one of the eight GPHS modules that power an MMRTG.

Thermoelectric Modules These solid-state devices convert heat flux into usable electricity, when any two dissimilar materials are maintained at different temperatures, through the Seebeck effect (Section 1.2.3). A pair of conductive materials joined in this way is called Thermocouple.

A typical MMRTG has 16 thermoelectric modules connected in electrical series, and each mod-ule contains 48 thermocouples in an electrical series-parallel configuration for fault tolerance, increasing reliability. A thermocouple has a "hot shoe" and a "cold shoe", which is, in turn, separated into two parts: N-Leg and P-Leg (Figure 2.3a). The thermocouple’s hot shoes is pressed by a spring loading system, mechanically integrated with the cold shoes (Figure 2.3b), to the heat distribution block typically made of AXF-5Q Graphite, while cold shoes touching the outer shell of the MMRTG and its convecting-radiating fins. The N-Leg Cold Shoe con-sists of PbTe (Lead Telluride) alloy named TE1010, while the P-Leg Cold Shoe is made up of PbSnTe⁴/TAGS⁵. Moreover, the P and N-Leg elements are suitably doped in order to increase and optimize the electrical conductivity. Besides, P-Leg is divided into two parts optimized to stay in contact with the hot and cold parts respectively, which typically operate at the temperat-ure of 520 °C, for hot shoes, and 75 °C for the cold shoes [8].

(a) Illustrative diagram of a thermocouple, with details on the different materials that make up the "hot shoe" and a "cold shoe".

(b) Illustration of the assembly of thermoelec-tric modules.

Figure 2.3: Thermoelectric modules of an MMRTG [9].

Cooling tube A fluid heat exchanger is used to dissipate the waste heat. Aluminium tubes (Al 6063) are used in MMRTGs that are designed to work in both vacuum and atmospheric conditions. The temperature can not fall below −269 °C (4 K) to not create strong temperature gradients in the materials.

4TE2003 element.

5The alloys (GeTe)x(AgSbTe₂)1−x, typically known as TAGS-x, are among the best performing p-type thermo-electric materials; the optimal composition value commonly used in MMRTG thermocouples is x = 85 (TAGS-85).

Insulation The thermocouples are surrounded and protected by insulating materials that min-imize waste heat to the outside and reduce the sublimation rate of the thermoelectric materi-als. Different solutions have been proposed, such as using a Sol-Gel coating⁶ of aluminium oxide (Al2O3), titanium oxide (TiO2), or silicon (SiO2): these materials strongly reduce the sublimation rate, but the coating is non-uniform and does not reach the required density to stem sublimation of the thermocouples material and to ensure good insulation. An Auger Electron Spectroscopy⁷ (AES) indicates that Al2O3provides the most favourable results of all the coat-ings and that Ge thermocouple component is the migrating species.

The problems of Sol-Gel process can be solved using the Atomic Layer Deposition (ALD) tech-nique, based on a gas phase coating process which allows angstrom level precision, ensures very thin films and uniform coating; this process must be repeated for either 200 or 400 ALD cycle, to produce a thickness of 30/40 nm of Al2O3. Therefore, the ALD-Al2O3 coating is typically used, because it leads to a significant reduction of the diffusion of Germanium (Ge), Tellurium (Te), and Silver (Ag) from the TAGS surface [10].

Fins As mentioned above, the MMRTG produces 110 W to power the electronic systems, the remaining part is used to create the correct thermal operating environment, and some of the waste heat is dissipated to the outside. The only way to increase the heat flow is to increase the exchange surface area and to do so it is essential to install finned walls on the surface to be cooled, or if necessary to be heated. In Figure 2.1 are shown the technical specifications of an MMRTG.

2.2.1 Advantages and disadvantages of MMRTGs

As mentioned in the Table 2.1, over a period of time the available power supplied by the MM-RTG decreases exponentially according to the law:

P_t= P₀exp −0.693 τ_1/2 t



(2.1)

where P_tis the power at time t after the initial time t₀ [11].

For a deep-space mission, two fundamental requirements: half-life of the isotope and its power density; for these reasons, often, depending on the duration of the mission, one isotope is chosen

6The Sol-Gel process involves the transition of the system from a liquid phase (Sol) to a solid phase (Gel) employing chemical reactions of hydrolysis and condensation of the metal precursors. Thanks to this technology, ceramic materials based on inorganic oxides are obtained, such as Al₂O₃, TiO₂, SiO₂, ZrO₂.

7It is a surface-sensitive analytical technique that utilizes a high-energy electron beam as an excitation source and when the previously excited atoms can relax, they release quantum energy, leading to the emission of Auger electrons(Appendix A), so with the kinetic energies of the ejected Auger electrons, it can be identified surface elements within the top 10 nm of the piece.

Chapter 2 2.2. State of the art of an MMRTG

Table 2.1: Technical specifications of an MMRTG.

Number of GPHS modules 8

Thermoelectric materials (T AGS)

N-Leg Lead telluride (PbTe, TE1010)

P-Leg TAGS

PbSnTe (TE20003)

Number of thermocouples 768

Beginning of life (BOL) power [W] 110

Estimate EOL power at 14 years[W] 98.48

BOL system efficiency 6%

Load voltage[V] 30

Fin-root temperature[°C] 157

rather than another (Table 2.2). Thus, for many missions Plutonium is used exclusively.

Table 2.2: Radioisotopes that can be used in an MMRTG. Polonium-210 provides a high power density (due to its high decay rate), emits almost only alpha α-particles (gamma emissions are insignificant), but is not widely used due to its very short half-life. Strontium-90 has a lower power density than plutonium, which translates into a lower temperature and, in turn, lower RTG efficiency, and also decays by β-emission (with low gamma emission), but the advantage is that it is highly available and cheap, being a high-yield waste product of nuclear fission.

Plutonium-238 and Americium-241 are explained in detail in the next section.

Material Shielding Power density (W/g) Half-life (years)

238Pu Low 0.57 87.7

90Sr High 0.46 28.8

210Po Low 140 0.378 (138 days)

241Am Medium 0.104 432.7

Advantages The advantages of MMRTGs compared to other systems concern:

• Power production does not depend on the spacecraft orientation and shadowing: for this reason, they are suitable for missions with long eclipse periods;

• They guarantee the independence of distance from the Sun;

• They provide low power levels, but are not susceptible to radiation damage for an exten-ded period.

Disadvantages In addition to the advantages, disadvantages must also be considered:

• They affect the radiation environment of the spacecraft;

• The installation of the radioactive source requires careful handling procedures due to the risk of radiation;

• The high thermal gradient is essential for better energy conversion, however, this affects the thermal environment and vehicle configuration;

• They are an interference source for plasma diagnostic equipment.