Clean Energy from First Principles

Radioisotope Thermoelectric Generator

Converts heat from radioactive decay (primarily Pu-238) directly into electricity via the Seebeck effect, providing decades of reliable power with no moving parts.

Fundamental Principle

Radioactive Decay as Heat Source

Radioisotope thermoelectric generators convert heat from the natural decay of radioactive isotopes directly into electricity. Unlike nuclear fission reactors, RTGs involve no chain reactions, no critical mass considerations, and no possibility of meltdown. They are essentially nuclear batteries that produce power continuously for decades with no moving parts.

The decay process:

Unstable atomic nuclei spontaneously emit particles and energy to reach more stable configurations. For RTGs, alpha-emitting isotopes are preferred because alpha particles (helium nuclei) deposit their energy over very short distances within the fuel itself, efficiently converting nuclear decay energy into heat.

Plutonium-238 decay (primary RTG fuel):

94238Pu→92234U+24He+5.6 MeV^{238}_{94}Pu \rightarrow ^{234}_{92}U + ^{4}_{2}He + 5.6 \text{ MeV}

Plutonium-238 emits an alpha particle (5.5 MeV kinetic energy) and gamma rays, decaying into uranium-234. The alpha particle travels only micrometres before being absorbed, heating the surrounding fuel matrix.

Key properties of Pu-238:

The long half-life means power output decreases slowly and predictably. After one half-life (87.7 years), thermal power drops to 50%. After 14 years (a typical mission design life), power remains at ~90% of initial output.

The Seebeck Effect

RTGs convert heat to electricity using the thermoelectric (Seebeck) effect, discovered by Thomas Johann Seebeck in 1821.

Physical principle:

When two dissimilar electrically conductive materials are joined and their junctions are held at different temperatures, a voltage difference develops across the junction. This voltage drives an electric current through an external circuit.

V=S⋅ΔTV = S \cdot \Delta T

Where:

Thermocouple construction:

A thermocouple consists of two different semiconductor materials (n-type and p-type) connected electrically in series but thermally in parallel. Hundreds or thousands of thermocouples are connected in series to generate useful voltage and power.

RTG configuration:

Efficiency Limits

Thermoelectric conversion is inherently limited by thermodynamics.

Carnot efficiency (theoretical maximum):

ηCarnot=1−TcTh\eta_{Carnot} = 1 - \frac{T_c}{T_h}

For typical RTG operating temperatures (T_h = 1273 K, T_c = 573 K):

$$\eta_{Carnot} = 1 - \frac{573}{1273} = 55%$$

Practical thermoelectric efficiency:

Real thermoelectric materials achieve only a fraction of Carnot efficiency due to:

The thermoelectric figure of merit (ZT) characterizes material performance:

ZT=S2σTκZT = \frac{S^2 \sigma T}{\kappa}

Where:

Higher ZT requires high electrical conductivity, high Seebeck coefficient, and low thermal conductivity. These properties are coupled, making optimization difficult. The best thermoelectric materials achieve ZT ≈ 1-2.

Actual RTG efficiencies:

Despite low efficiency, RTGs are valuable because the "fuel" produces heat continuously whether or not it is converted to electricity, and waste heat can warm spacecraft systems.

RTG Components and Design

Heat Source

General Purpose Heat Source (GPHS) module:

The GPHS is the standardized building block for modern US RTGs:

Fuel configuration (per GPHS module):

  1. Fuel pellet: Plutonium dioxide (PuO₂) ceramic, ~150 g each
  2. Iridium cladding: Surrounds each pellet, provides chemical and mechanical containment
  3. Graphite impact shell (GIS): Protects clad during potential reentry
  4. Carbon-bonded carbon fiber (CBCF) sleeve: Thermal insulation
  5. Fine-weave pierced fabric (FWPF) aeroshell: Outer housing, ablation protection

The multi-layer design ensures fuel containment even in severe accidents (rocket explosion, atmospheric reentry, Earth impact).

Thermoelectric Converter

Materials evolution:

Generation Materials Hot/Cold Junction Temps Notes
SNAP-19 PbTe/TAGS 811K/483K Viking, Pioneer
MHW-RTG SiGe 1273K/573K Voyager
GPHS-RTG SiGe 1273K/573K Cassini, New Horizons
MMRTG PbTe/TAGS 811K/483K Curiosity, Perseverance

PbTe/TAGS thermoelectrics:

SiGe thermoelectrics:

Heat Rejection

Heat not converted to electricity must be radiated to space (or conducted away on planetary surfaces).

Radiator fins:

Thermal balance:

$$P_{thermal} = P_{electrical} + P_{radiated}$$

For a typical MMRTG:

The "waste" heat is often used productively to warm spacecraft electronics and instruments.

RTG Types and Specifications

Historical RTG Generations

SNAP-3 (1961):

SNAP-19 (1968-1975):

SNAP-27 (1969-1972):

MHW-RTG (Multi-Hundred Watt, 1976-1977):

GPHS-RTG (1989-2006):

Current RTG: MMRTG

The Multi-Mission Radioisotope Thermoelectric Generator is the current US standard.

Specifications:

Key features:

Missions:

Cost:

Next Generation RTG

NASA is developing a higher-power successor to the MMRTG.

Specifications (target):

The Next Gen RTG would provide more than twice the power of an MMRTG, enabling more capable missions.

Radioisotope Fuels

Plutonium-238 (Primary Choice)

Pu-238 has been the fuel of choice for US space RTGs since the 1960s.

Advantages:

Disadvantages:

Pu-238 is NOT weapons-grade plutonium:

Pu-238 Supply Crisis

Historical production:

- 1960s-1988: Produced at Savannah River Site (South Carolina) as byproduct of weapons production - 1988: US domestic production ceased with Cold War drawdown - 1992-2010: US purchased Pu-238 from Russia (~16 kg total) - 2010: Russian exports ceased

Supply status (as of 2023-2024):

This inventory supports only a few more missions without new production.

Production restart:

In 2015, the Department of Energy began restarting Pu-238 production at Oak Ridge National Laboratory:

  1. Neptunium-237 targets shipped from Idaho National Laboratory
  2. Targets irradiated in High Flux Isotope Reactor (HFIR)
  3. Neutron capture converts Np-237 to Np-238
  4. Np-238 beta-decays (half-life 2.1 days) to Pu-238
  5. Chemical processing separates Pu-238
  6. Shipped to Los Alamos for fuel pellet fabrication
  7. Pellets sent to Idaho for RTG assembly

Production targets:

Production is scaling up but remains challenging and expensive.

Alternative Isotopes

Given Pu-238 scarcity, several alternatives have been studied.

Strontium-90:

Property Value
Half-life 28.8 years
Specific power 0.46 W/g (as SrTiO₃)
Radiation Beta (requires more shielding)
Source Nuclear waste (abundant)

Americium-241:

Property Value
Half-life 432 years
Specific power 0.11 W/g
Radiation Alpha + gamma
Source Nuclear waste (aged plutonium)

Other isotopes considered:

Isotope Comparison

Isotope Half-life Power Density Shielding Availability Best Use
Pu-238 87.7 yr 0.56 W/g Minimal Very limited Space missions
Am-241 432 yr 0.11 W/g Moderate Moderate Long-duration
Sr-90 28.8 yr 0.46 W/g Heavy Abundant Terrestrial
Cm-244 18.1 yr 2.8 W/g Moderate Limited Short missions
Po-210 138 days 140 W/g Minimal Limited Very short missions

Applications

Space Exploration

RTGs have enabled exploration of the outer solar system, where sunlight is too weak for solar panels.

Major RTG-powered missions:

Mission Launch RTG Type Power Status
Transit 4A 1961 SNAP-3 2.7 W First RTG in space
Apollo 12-17 1969-72 SNAP-27 73 W Lunar surface
Pioneer 10/11 1972-73 SNAP-19 155 W Jupiter, Saturn
Viking 1/2 1975 SNAP-19 70 W Mars landers
Voyager 1/2 1977 MHW-RTG 470 W Outer planets, interstellar
Galileo 1989 GPHS-RTG 570 W Jupiter orbiter
Ulysses 1990 GPHS-RTG 285 W Solar polar mission
Cassini 1997 GPHS-RTG 870 W Saturn orbiter
New Horizons 2006 GPHS-RTG 245 W Pluto flyby
Curiosity 2011 MMRTG 110 W Mars rover
Perseverance 2020 MMRTG 110 W Mars rover
Dragonfly 2028 MMRTG 110 W Titan rotorcraft

Voyager longevity:

The Voyager spacecraft demonstrate RTG durability:

Power decline comes from both Pu-238 decay (~0.8%/year) and thermocouple degradation. The spacecraft progressively shut down instruments to match declining power.

Radioisotope Heater Units (RHUs)

Small devices that provide thermal energy without electricity generation.

Specifications:

Applications:

Missions using RHUs:

RHUs allow solar-powered missions to operate in cold environments without dedicating electrical power to heating.

Terrestrial Applications

Soviet/Russian RTGs:

The Soviet Union deployed over 1,000 Beta-M and similar RTGs for:

These used strontium-90, which is abundant but requires heavy shielding and has a shorter half-life.

Post-Soviet problems:

US terrestrial RTGs:

Limited use compared to Soviet Union:

Medical applications (historical):

Nuclear-powered cardiac pacemakers:

Safety

Design Philosophy

RTGs are designed for "defense in depth" with multiple containment barriers to prevent fuel release under all foreseeable accident conditions.

GPHS module safety features:

  1. Ceramic fuel form: PuO₂ is chemically stable, high melting point, low solubility
  2. Iridium cladding: Corrosion resistant, high melting point, ductile
  3. Graphite impact shell: Absorbs mechanical shock
  4. CBCF insulation: Thermal protection during reentry
  5. FWPF aeroshell: Survives atmospheric reentry intact

Safety testing: GPHS modules have been tested under conditions far exceeding credible accidents:

Modules survived intact in nearly all test conditions.

Accident History

Apollo 13 (1970):

Nimbus B-1 (1968):

Transit 5BN-3 (1964):

Cosmos 954 (1978):

Since the GPHS design was implemented (1980s), no RTG has released fuel despite several mission anomalies.

Radiation Exposure

Alpha particle shielding: Pu-238's alpha emissions are stopped by:

Alpha radiation cannot penetrate intact skin. The primary hazard is inhalation or ingestion of particles.

Gamma and neutron shielding: Pu-238 produces minimal gamma and neutron radiation. The MMRTG surface dose rate is low enough for workers to handle the unit during integration with minimal protective equipment.

Launch safety:

Before any RTG launch, NASA conducts extensive safety analysis:

The probability of fuel release in a launch accident is extremely low (<1 in 10,000 for most scenarios), and the maximum credible release would produce doses far below harmful levels.

Performance and Reliability

Long-Term Performance

RTGs are among the most reliable power systems ever built. Every NASA RTG mission has exceeded its design life.

Power degradation sources:

  1. Fuel decay: Pu-238 decays at 0.8%/year (~8% per decade)
  2. Thermocouple degradation: Material changes reduce conversion efficiency
  3. Thermal contact degradation: Interfaces between components may change

Typical total degradation: 1-2% per year (fuel + thermocouples combined)

Voyager performance (example):

Year Years in Flight Power (W) % of Initial
1977 0 470 100%
2000 23 315 67%
2024 47 220 47%

The faster-than-expected decline results from thermocouple degradation (SiGe germanium migration), not fuel decay alone.

Reliability Comparison

RTGs vs. solar panels:

Factor RTGs Solar Panels
Moving parts None Possibly (tracking)
Power variability None Day/night, seasons
Distance from Sun Unlimited Limited (~5 AU practical)
Dust accumulation N/A Major issue (Mars)
Design life 14+ years 5-15 years
Mass (for equivalent power in outer solar system) Lower Much higher

RTGs vs. batteries:

Factor RTGs Batteries
Energy storage Continuous generation Finite capacity
Duration Decades Days to months
Recharging N/A Required
Mass for long missions Much lower Impractical

Why Not Always Use RTGs?

Despite their advantages, RTGs are used selectively:

Limitations:

When solar power is preferred:

When RTGs are essential:

Future Developments

Dynamic Radioisotope Power Systems

Alternative conversion technologies could significantly improve efficiency.

Stirling Radioisotope Generator (SRG):

Advanced Stirling Radioisotope Generator (ASRG):

Thermophotovoltaics:

Americium-Based Systems

ESA and UK are developing Am-241 RTGs as alternative to Pu-238 dependency.

Advantages:

Challenges:

Timeline:

Mission Concepts Enabled by RTGs

Dragonfly (2028):

Europa/Enceladus landers:

Interstellar probes:

Summary

Key Specifications

Parameter MMRTG (Current) GPHS-RTG MHW-RTG (Voyager)
Electrical power (BOL) 110-125 W 285-300 W 157 W
Thermal power 2000 W 4400 W 2400 W
Efficiency ~6% ~6.5% ~6.5%
Fuel mass (PuO₂) 4.8 kg 7.8 kg 4.5 kg
Total mass 45 kg 56 kg 37.7 kg
GPHS modules 8 18 N/A
Design life 14 years 10+ years 5 years
Atmosphere operation Yes No No

Strengths and Limitations

Strengths:

Limitations:

Role in Energy Landscape

RTGs occupy a unique niche: reliable, long-duration power for remote, unmaintained applications where no other technology is viable. They are not a general energy solution but rather an enabling technology for specific high-value applications.

Terrestrial relevance: Minimal. RTGs produce too little power for most applications and the fuel is too scarce and expensive. Solar, wind, batteries, and even small nuclear reactors are more practical for Earth-based power.

Space exploration relevance: Essential. RTGs have enabled humanity's exploration of the outer solar system and continue to power missions where solar panels cannot function. Without RTGs, missions to Jupiter, Saturn, Pluto, and beyond would be impossible with current technology.

The future of RTGs depends on:

  1. Sustained Pu-238 production
  2. Development of alternative isotopes (Am-241)
  3. Improved conversion technologies (dynamic systems)
  4. Continued investment in deep space exploration

As humanity's ambitions extend to ocean worlds, interstellar space, and permanent presence on the Moon and Mars, RTGs and their successors will remain essential enabling technologies.