Accessing Icy Worlds Using Lattice Confinement Fusion (LCF) Fast Fission
Theresa L. Benyo
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In February 2023 Theresa Benyo and Lawrence Forsley took a NASA Glenn NIAC project to JPL’s Cryobot Workshop with a direct pitch: give an ice-melting probe its own small nuclear engine. NASA wants to reach the oceans under Europa, Enceladus, Ceres and Pluto, which means boring through as much as forty kilometres of ice, at pressures running to ten kilobar, on a power source that is small, long-lived and throttleable. Their answer is lattice confinement fusion — the reaction NASA Glenn published in Physical Review C in 2020, in which deuterium packed into a metal lattice reaches close to solid fuel density and fuses because the metal’s own electrons screen the charge, so only a few atoms need heating at a time. Benyo and Forsley pair it with fast fission: fusion neutrons of up to 14.1 MeV are energetic enough to split thorium-232 and natural uranium-238, so no enriched uranium is needed. Waste heat melts the ice, the electricity runs the probe, and the same system could drive nuclear electric propulsion on the way out.
Why it matters hereThis is chapter 12’s programme stated as a mission: the NASA Glenn lattice fusion result turned into a power plant small enough to ride inside a probe, with the enriched-uranium problem engineered out.
What it claims
01Lattice confinement fusion sidesteps the pressure, temperature and containment problem of ordinary fusion. Deuterium loaded into a metal lattice sits at close to solid fuel density, and the lattice’s electron cloud screens the deuteron’s charge so that only a few atoms need to be heated at a time.slide 8, How LCF Works; Pines et al., Phys. Rev. C 101, 044609 (2020)
Published and peer-reviewed02The fusion reactions supply neutrons energetic enough to fission material that is not fissile on its own — about 2 MeV is the requirement, and D(t,n)α delivers 14.1 MeV — so thorium-232 and natural uranium-238 can carry the fission half of the cycle and no enriched uranium is needed.slide 9, Hybrid Fusion-Fast Fission, reaction tables
Designed, not yet built03The result is a variable-output power source smaller than existing fissile reactors, free of the plutonium-238 supply problem and of high-enriched or high-assay low-enriched uranium, with the reactor’s thermal waste heat used directly to melt or ultrasonically vibrate a path through the ice.slide 10, Potential Impact; slide 11, Takeaways
Designed, not yet built04Icy-world conditions set the requirement and they are severe: crusts up to 40 km thick, pressure from vacuum to over 10 kbar, temperature from cryogenic to above 270 K, and several ice phases that change how fast a probe can travel — which is why the power source has to be throttleable.slides 3 and 5, citing Journaux et al., PNAS (2023) and Culbert et al., Nature Communications 13:2007 (2022)
Published and peer-reviewed05The same LCF-driven fast-fission unit could serve as high specific-impulse nuclear electric propulsion, shortening the cruise to the outer planets. What to watch: a thrust-and-power figure from a built unit rather than a design estimate.slide 4, Mission Context; slide 7, Innovation
What to watch06Scaling is the open item, and the team names it as their own next step: raising the technology readiness level of LCF fast fission. What to watch: a demonstrated throttleable unit meeting the cryobot reference numbers of 8–12 kW thermal at better than 1 W per cubic centimetre.slide 3, Cryobot reference; slides 11 and 12
What to watch
Read it
Cryobot Workshop, JPL. February 21–23, 2023.
Theresa L. Benyo, Ph.D., Analytical Physicist, Principal Investigator, NASA GRC. Lawrence P. Forsley, Experimental Physicist, Deputy PI, NASA GRC; GEC LLC; UT Austin, NETL.
Overview
- Introduction
- Mission Context
- Robotic Probe Specifications/Options
- Innovation — How Lattice Confinement Fusion (LCF) Works; Hybrid Fusion Fast Fission
- Potential Impact
- Takeaways
Introduction
- Ocean Worlds Exploration Program
- Search for Extraterrestrial Life
- Ceres, Europa, Enceladus, Pluto
- Challenges: extreme operating environmental conditions; break through up to 40 km thick ice
- Robotic Probe
- Small, robust, long-lived electrical energy and heat source
- Traditional nuclear power systems require significant radioactive shielding
- Enriched actinide-based systems: significant fabrication, safety, launch costs
- Cryobot reference
- Power Density > 1 W/cc
- Total (thermal) power: 8 – 12 kW
- Lifetime: 2-6 years, operating at full power
- Maturity: TRL 6, flight ready in ~10 years
Reference: B. Hockman, et al., “PRIME: Probe using Radioisotopes for Icy Moons Exploration — A Comprehensive Cryobot Architecture for Accessing Europa’s Ocean”.
Mission Context
- Icy World Exploration
- Proposed probe capable of powering the probe and a drilling mechanism with enough Watt-electric and Watt-thermal to accomplish its mission
- Heated and/or (ultra) sonic drilling mechanism will enable the probe to travel through icy crusts
- LCF-driven Fast Fission can provide Nuclear Electric Propulsion for shorter journey
- Ceres, Europa, Enceladus and Pluto are icy world candidates
Slide figures: Europa cutaway; Enceladus cutaway; GRC Tunnelbot.
- Addressing Icy World Conditions
- Icy crust likely exist over a pressure range from vacuum to possibly over 10 kbar
- Temperature range from cryogenic to > 270 ºK
- Various ice phases impact probe travel rate and pressure
- Sub-surface lakes likely
- With these conditions, variable power output is required
References: B. Journaux, et al., “On the identification of hyperhydrated sodium chloride hydrates, stable at icy moon conditions”, PNAS (21 Feb 2023). R. Culbert, et al., “Double ridge formation over shallow water sills on Jupiter’s moon Europa”, Nature Communications 13:2007 (2022).
Robotic Probe Specifications/Options
- Cryobot
- Europa Tunnelbot
References: S. Oleson, et al., “Compass Final Report: Europa Tunnelbot”, NASA/TP-2019-220054. B. Hockman, et al., “PRIME: Probe using Radioisotopes for Icy Moons Exploration — A Comprehensive Cryobot Architecture for Accessing Europa’s Ocean”.
Innovation
- Lattice Confinement Fusion (LCF) Technology
- Develop a non-fissile, compact, scalable nuclear energy source sufficient to power and provide heat for melting and boring through icy shelves with untethered, autonomous probes.
- Possible high Isp (specific impulse) Nuclear Electric Propulsion (NEP)
- Future development could go beyond the icy-moon mission to a lightweight power source for human and robotic missions.
Slide figure: depiction of the ocean underneath Europa’s icy layer.
How LCF Works
- Traditional fusion: heats plasma 10x hotter than center of sun – hard to control
- LCF addresses the pressure, temperature, and containment challenges with fusion
- Heats very few atoms at a time
- Approaches solid fuel density
- Lattice provides containment
Technical details simplified:
- Part A: Electron Screening (increases fusion probability) — lattice electron screening, i.e. a cloud of electrons makes the D look like a neutral particle and no repulsion exists
- Part B: High Fuel Density (billion times more dense than traditional fusion)
- A + B + Trigger = Viable Fusion
Slide figures: lattice of atoms; inside the lattice; cold D, hot d, n*, hot He-3.
Hybrid Fusion-Fast Fission
- Takes advantage of both processes
- Fusion reactions provide the neutrons to fission non-fissile material
- Require ~2 MeV neutrons to fission Th and natural U
- Fusion reactions can provide up to 14.1 MeV neutrons
| Fusion reaction | MeV | Occurrence | Useful particle energy (MeV) | |---|---|---|---| | D(d,n)³He | 4.00 | primary ≈ 50% | n = 2.45 | | D(d,p)T | 3.25 | primary ≈ 50% | p = 3.00 | | D(³He,p)α | 18.30 | secondary | p = 15.00 | | D(t,n)α | 17.60 | secondary | n = 14.10 | | T(t,α)2n | 11.30 | low probability | n = 1 to 9 | | ³He(³He,α)2p | 12.86 | low probability | p = 1 to 10 |
| Fission reaction | MeV | Occurrence | Useful particle energy (MeV) | |---|---|---|---| | ²³²Th(n,γ)f | 200 | high probability | n = 1 to 9 | | ²³²Th(p,γ)f | 200 | some probability | p = 1 to 10 | | ²³⁸U(n,γ)f | 200 | high probability | n = 1 to 9 | | ²³⁸U(p,γ)f | 200 | some probability | p = 1 to 10 |
Potential Impact
- Probes for icy moons require unacceptable amounts of ²³⁸Pu isotope.
- A small, low-mass, variable power source is needed.
- New hybrid approach yields a variable output power source smaller than existing fissile reactors.
- Non-fissile alternative to high-enriched uranium (HEU) or high-assay, low-enriched uranium (HALEU) core saves uranium enrichment, security and launch safety costs.
- Efficient operation with reactor thermal waste heat allows probe to melt and/or vibrate through ice shelf.
Takeaways
- Hybrid Fusion-Fast Fission Power system
- No HEU or HALEU necessary
- Built on NASA GRC and US Navy research published in Phys Rev C and elsewhere
- With scaling, suitable for ice crust penetration and power
- Variable output power possible so probe is throttleable
- Compact system supports small size of the probe
- Recognition of Icy World ice-phase temperature and pressure changes
- Requires power/penetration flexibility
- Possible near-surface ice pools
- Combined ice melting/ultrasonic penetration
- Takes advantage of skin layer adjacent to probe
References: Pines, et al., “Nuclear Fusion Reactions in Deuterated Metals”, Phys. Rev. C 101, 044609 (2020). Mosier-Boss, et al., “Investigation of Nano-Nuclear Reactions in Condensed Matter”, Defense Threat Reduction Agency (2016). R. Culbert, et al., “Double ridge formation over shallow water sills on Jupiter’s moon Europa”, Nature Communications 13:2007 (2022).
Acknowledgments
- Thanks to the Cryobot Workshop Organizers for inviting us!
- We’re looking forward to learning more from you as to the changing requirements
- While looking forward to increasing the TRL of LCF Fast-Fission
The way in
https://ntrs.nasa.gov/citations/20230002343
How to cite it
Theresa L. Benyo (2023) Accessing Icy Worlds Using Lattice Confinement Fusion (LCF) Fast Fission. https://ntrs.nasa.gov/citations/20230002343
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