The term resource has both a geological and an economic component. It must exist in sufficient concentration to be useful and there should be indications that its extraction could be technically and economically feasible.

The helium-3 was one of the first elements proposed back in in the 20th century as an useful resource to be extracted to the Moon because its potential as nuclear fusion fuel. Surprinsingly, the nuclear physics community do not show the same excitement about helium-3 than that shown by the space community. In this post, I explain briefly the physics and prospects of the nuclear fusion using helium and the quantities of the element that can be found on the Moon.

Nuclear Fusion

During the nuclear fusion two light atomic nuclei collide to produce a single heavier one releasing energy in the process. This energy is generated because the mass of the product is less than the sum of the mass of the original particles, and because the mass-energy equivalence principle, part of extra mass is transformed into energy. This is the reaction that occurs in the Sun and all the stars. Current research is focusing on industrializing this process to have a sustainable energy source, avoiding pollution and contributions to the climate change.

The fusion reaction (Figure 1) happens in a vessel called tokomak. There, hydrogen isotopes deuterium (D) and tritium (T) are confined and heated in a plasma. To confine the plasma and maintain its stability a powerful magnetic field is used. The DT reaction produces helium-4, a neutron and energy. The energy produced is absorbed as heat within the reactor’s walls producing steam that through turbines and generators creating electricity.

The problem with DT fusion reaction is the neutron emission. Neutrons are considered radioactive contamination as they are life-threatening and very damaging for the surrounding materials. Because their lack of electrical charge, neutrons are very difficult to contain by the tokamak’s electromagnetic field and efforts on research for shielding materials are in development.

There are other fusion reactions that could generate energy without the dangerous neutron emission. In this reaction the helium-3 reacts with deuterium and emits helium-4, a proton and energy (figure 2). Protons emitted during this type of fusion are easier to contain by electromagnetic fields. Furthermore, the momentum energy of the protons interacting with the electromagnetic field in theory will result in direct net electricity generation making unnecessary to heat water to move any turbine.

And this is why the helium-3 is so interesting. It is clean and perfect, isn’t it?

Well, unfortunately, it is not that easy.

The DHe-3 fusion requires the two fuelling components to be mixed and despite the theory, in the practice some deuterium-deuterium reactions will happen. This will form some tritium and protons. Because deuterium reacts 100 times faster with tritium than with helium-3, the tritium will interact with the some of the existing deuterium, generating helium-4 and neutrons. So, this reaction is not as clean as claimed and still shielding for the reactor are required to avoid radioactive contamination.

But the main technical challenge for helium-3 fusion is the temperature. These temperatures need to be up to 4 times larger than those (already extremely high) needed for the DT reaction (figure 3). Furthermore, at those temperatures the plasma tends to lose lot of energy by two types of radiation: bremsstrahlung created when the plasma’s particles are slowed down, and synchrotron radiation created by the motion of the particles within the magnetic field. These two types of radiation are affected differently by temperature so the temperature in the reactor should be high enough but not too much, to reduce the bremsstrahlung and the synchrotron radiations. But even in that case, unlike the DT fusion, some radiation losses will happen.

Research on nuclear fusion is already favoring DT reactions over helium-3 for several reasons including isotopic reactivity and technical feasibility. This trend does not seem to change any time soon so, prospects on industrial nuclear fusion of helium-3 appears very far. Radioactive contamination is no doubt an important problem to solve, but it seems that many investigations on promising shielding materials are already working on it.

Fusion fuels

On the Earth, deuterium occurs naturally on the oceans with a deuterium to hydrogen ratio of 1.6 × 10^-4. For industrial and military purposes, the cheapest way to produce deuterium in bulk is separating the heavy water fraction contained in ordinary water using distillation, Girdler sulphide process, or other methods.

Tritium is a radioactive and rare isotope of hydrogen that virtually does not occur naturally on the Earth’s surface and only small traces can be found on the atmosphere. It is used to enhance the efficiency and yield of fission and thermonuclear bombs. As it does not occur naturally, it needs to be manufactured, being the most common method the neutron activation of Lithium-6 within a breeder blanket. For proposed nuclear fusion applications lithium-bearing ceramics pebbles are being developed for T breeding within a ⁴He-cooled pebble bed.

Helium-3 is a stable isotope existing in its current form since the solar system was formed. It exists both on the Earth’s atmosphere and on the mantle although their abundances are very low. On the atmosphere abundances are about 7.2 parts per trillion (ppt).  On the terrestrial mantle releases from deep-source hotspot volcanoes, mid-ocean ridges and around subduction zones suggest an approximately content ranging between 0.1 to 1 megaton.

Helium-3 can be created by decay of tritium. Actually to date, the principal source of helium for human applications is formed in nuclear weapons reservoirs. When the tritium used in the stored warheads decays into helium-3, reduces their efficienty and is constantly removed from the weapons reservoirs and marketed for other applications. International treaties such as the Treaty of Non-Proliferation of Nuclear Weapons (NPT) aiming to reduce the ready-to-use storage of nuclear warheads as well as the increasing demand of helium-3 for neutron radiation detectors and medical diagnostic technologies, has caused the diminishment of the already small stockpiles.

To summarize, on the Earth helium-3 in the atmosphere is tiny, the mantle is inaccessible, and humans produce very little to supply an increasing demand.

Lunar helium-3

My starting point is that it is not foreseen a large demand of helium-3 in the near or mid term for nuclear fusion. But I would like to write this section nonetheless for you to visualize the quantities of helium-3 present of the lunar regolith, and what would imply to extract it.

The amount of helium-3 in the lunar regolith is governed by 3 factors: solar wind implantation, maturity of the regolith, and ilmenite content. Because the lack of atmosphere or magnetic field, the elements carried by the solar wind, including He, can get implanted in the first µm of the regolith layer and incorporated into the existing mineral phases. The longer the regolith is exposed to the solar wind (maturity), the higher the amount of helium-3. The mineral ilmenite (FeTiO₃) present in the high-Ti basalts on the lunar maria appears to be remarkably good at retaining it. Once implanted, those ³He-bearing particles may have migrated to lower levels by regolith reworking thanks to impact processes. The higher lunar helium-3 abundances are expected to be found in mature regolith with high ilmenite contents i.e., high-Ti mare basalts.

Work done by Fa and Jin (2007 and 2010) using remote sensing and solar wind flux models, have estimated helium-3 lunar abundances to reach up to 20 parts per billion (ppb) in some maria (figure 4). However, this content was estimated using remote sensing techniques and models that includes many optimistic assumptions on ilmenite retention, outgassing and vertical mineral transportation. Measurements on Apollo samples from high-Ti regions have given much lower content (<10 ppb). In any case, Fa and Jin estimate a total content of 6.6 × 10⁸ kg in the entire Moon.

But

Helium-3 is not homogeneously distributed within the Moon and the most promising deposits because abundances and extension, would be the high-Ti basalts at Mare Tranquillitatis and Oceanus Procellarum (figure 4 delineated in white). Those two deposits combined would yield a mass of 2 × 10⁸ kg (20 ppb and 3 m depth; Crawford, 2015). 

There is no doubt that there are more helium-3 on the Moon than on the Earth, but it is far from being abundant. Direct deposit evaluations could be done in the entire regolith layer to understand its distribution with depth and sample measurements taken, but it is very unlikely that they will give us much higher concentrations that those estimated.  

The exercise I propose here is to estimate the rate of helium-3 extraction that will be required, to feed a nuclear fusion plant to provide energy to Germany.

To obtain 10 tonnes, it will be necessary to process an area of 221 km² of bulk regolith from Tranquillitatis or Procellarum to a depth of 3 m. I want to highlight that this is a very optimistic scenario. It is likely that the helium-3 is confined to the first cms of the regolith, in this case the area to be processed will increase significantly.

Operationally, this means it will be necessary to process 75 thousands of m³ of regolith per hour (!!) to get to 10 tonnes per year (undercalculated, probably would be more). This mass flow is probably impossible to achieve even in the most advanced mining processing plant.

Some would argue that still quantities required for other applications are much lower, so maybe there is still hope for the lunar helium-3 economy. However, because its scarcity, researchers have already started to investigate alternatives. For example, neutron detectors based on ¹⁰B and BF₃ and particle detectors like Medipix (Jakubek et al. 2006) are commercially available. In medical MRI devices, nitrogen can be used for cooling the scanner magnets and many gas chromatography laboratories are changing from helium to hydrogen because its lower cost.

Theres is only one case I coud imagine that maybe has some interest. During the processing of regolith for other resources, helium-3 can be released as by-product. In that case, maybe that tiny amount still can be used on the Moon or marketed somehow.

Bibliography

Crawford I. A. (2015). Lunar Resources: A Review. Progress in Physical Geography: 39(2) 137–167

Fa W and Jin Y-Q (2007). Quantitative estimation of helium-3 spatial distribution in the lunar regolith layer. Icarus 190: 15–23.

Fa W and Jin Y-Q (2010). Chinese Science Bulletin: 55, 4005-4009.

Jakubek J., Holy T., Lehmann E., Pospisil P., Uher J., Vacik J., and Vavrik D. (2006). Neutron imaging with Medipix-2 chip and a coated sensor. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment: 560 (1), 143-147.

Schmitt HH (2006) Return to the Moon. New York: Copernicus Books.

https://scipython.com/blog/plotting-nuclear-fusion-cross-sections/