Historical research guide · 0996 · 2022
Accessing Icy World Oceans Using Lattice Confinement Fusion Fast Fission
Theresa L. Benyo · Lawrence P. Forsley
In one minute
Under the ice of Europa, Enceladus, Ceres and Pluto there is probably liquid water, and probably the best place in the solar system to look for life that is not ours. The obstacle is the lid: up to forty kilometres of ice a robot has to melt or cut its way through, on power it carries with it. Theresa Benyo of NASA Glenn and Lawrence Forsley propose a power source that contains no plutonium-238 and no enriched uranium at all. Pack deuterium into a metal lattice, let the lattice electrons screen the positive charges so the nuclei fuse, and the neutrons that come out are energetic enough to split thorium or depleted uranium — materials that cannot sustain a chain reaction by themselves and so carry none of the launch-safety and security cost. This paper, given at the American Nuclear Society’s space-nuclear meeting in 2022, sets out the concept, the icy-world conditions it has to survive, and the three modelling tasks that would turn it into a design.
Connection
Chapter 12 treats lattice confinement fusion as the energy substrate of the whole programme, and this is the paper where NASA Glenn takes it out of the laboratory and puts it to work as the neutron source of a flight reactor. Chapter 8 gets the honest engineering frame: what a self-contained power source has to weigh, shield and survive before anything at all can be sent through forty kilometres of ice.
From the source
The proposed energy source uses neither highly enriched uranium nor plutonium-238. Neutrons from lattice confinement fusion fast-fission thorium-232 or depleted uranium-238, neither of which is fissile, so the reactor carries none of the fabrication, safety or launch costs that come with plutonium-238 or highly enriched uranium — and it can be throttled, which a radioisotope heat source with an 87.7-year half-life cannot.Section I.A, Innovation; and Section I, Introduction
Lattice confinement fusion holds deuterons in a metal lattice at about 10²³ ions per cubic centimetre indefinitely, against the 10¹⁴ of a magnetically confined plasma and the 10²⁶ a large laser can reach for about a nanosecond. The negative charge of the lattice electrons screens and partially neutralises the positive deuteron charge, which reduces the Coulomb barrier and lets the deuterons fuse. Triggered and controlled by bremsstrahlung or by phonon-nuclear coupling, the process reaches an equivalent local deuteron ion temperature of up to 2.1 kiloelectronvolts, about 24 million degrees Celsius, against the Sun’s 1.5 kiloelectronvolts.Section I.A.1, Lattice Confinement Fusion Technology
A fusion-fast-fission micro-reactor has already been demonstrated using natural thorium-232 and both natural and depleted uranium, at a previously demonstrated average neutron energy of 6.4 megaelectronvolts — enough to fast-fission all three. The hybrid fusion-fission experiment at the US Navy SPAWAR-Pacific laboratory produced a sustained neutron flux averaging a million neutrons per second, with the neutron energy read from neutron-germanium recoils in the detector and the fission and activation products read by liquid scintillator.Section I.A, Innovation, the paragraph on the hybrid fusion-fission reactor design
Ongoing work with the Naval Surface Warfare Centers has observed 90 to 200 watts-thermal per gram, without actinides, by a patented protocol — against the 0.54 watt-thermal per gram of the plutonium-238 in radioisotope thermal generators. Current estimates put the power density of a non-optimised lattice confinement fusion fast fission system at one third that of the highly-enriched-uranium Kilopower reactor.Section I.A.1, closing paragraph
The mission drives the reactor design rather than the other way round. An icy crust runs from vacuum to pressures possibly exceeding 10 kilobars and from cryogenic temperatures to 270 kelvin, so the probe passes through ice XI at the surface and then ice IX, II, III and XV before it meets pressurised liquid water, and each phase change alters the travel rate — which is why the power output has to be variable. The team also proposes ultrasonically assisted ice fracturing, borrowed from Navy work on supercavitating torpedoes, with the same transducer listening to its own echoes to measure ice density and possibly to communicate back to the surface.Section I.A.2, Addressing Icy World Conditions; Section I.A, Innovation
What has not yet been done is named plainly: although lattice confinement fusion can efficiently fast-fission depleted uranium and thorium, this kind of fission has not been done in a high-temperature, lithium-based molten salt. The three-task programme is to fix the mission requirements, model the fast-fission process in the Los Alamos Monte Carlo N-Particle code from first principles using the measured neutron spectra as input, and then model reactor materials and shielding — including molten lithium-salt thorium and depleted-uranium cores — to see whether the mass, volume and power requirements can be met.Section I.D, Technical Approach; Section II, Conclusions
The way in
https://ntrs.nasa.gov/citations/20220002596How to cite it
Theresa L. Benyo, Lawrence P. Forsley (2022) Accessing Icy World Oceans Using Lattice Confinement Fusion Fast Fission doi:10.13182/nets22-38638