Nuclear Power from Lunar ISRU - Juniper publishers
Journal of Insights in Mining Science & Technology
Abstract
Thorium on the lunar surface can be transmuted into fissile
uranium suitable for a controlled chain reaction to provide heat. Thorium is
fertile, requiring bombardment by neutrons to become a suitable nuclear fuel.
Oxides of thorium are dense and can be concentrated and beneficiated from
comminuted regolith via inertial or thermal means. A neutron flux can be provided
by encasing thoria within a beryllium and graphite vessel, which emits neutrons
upon exposure to gamma rays or galactic cosmic rays. After a brief period at
protactinium the transmuted material becomes U-233, a desirable fuel because
decay product half-lives are below 100 years. When compressed into fuel pellets
the uranium oxide is configured into a reactor through which a working fluid
can extract thermal power. With regolith tailings as shielding such a reactor
can operate safely for 30 years. A century later, the site can be harvested for
specialty elements and then made available for other uses. The advent of
launch-safe nuclear rockets in space greatly expands the potential for in situ
resource utilization, a space-based economy, and profitable exploitation of the
asteroid belt.
Introduction
Lunar missions involving in situ resource utilization (ISRU)
require ample supplies of thermal and electrical energy. Solar power is a poor
choice, being diffuse, and intermittent. Operations in permanently shadowed
regions (PSR) get no sunlight, inviting complex reflection or conversion and
beaming schemes to power operations on the floors of ultra-cold craters.
Nuclear power is an oft-cited option, as technology is readily available for
the reactor to only go “hot” after being installed on the moon. Even for such
reactor designs, the uranium needed is already part of the payload, and this
material is radioactive, albeit at a low level. Many people, often unaware of
radioactive uranium in their own granite countertops, have a deep-seated fear
of anything radioactive inside a rocket, worried it might explode and spread
the contamination across the environment. It is possible, therefore, that even
“nearly safe” nuclear reactors launched from earth will experience significant
public protest, resistance, Congressional pressure, and lawsuits. Another
solution exists and is explored in this paper.
The US nuclear power development, starting with the
Manhattan Project, had two primary purposes: electric power generation, and
radioisotope generation for bombs made of uranium, plutonium, and hydrogen.
Because of this dual focus, the enrichment of the U-235 fraction of uranium was
the pathway used for nearly all applications. A modest effort in breeder
reactors was made, which pursued the transmutation of thorium via the neutron
flux from U-235 reactors to make U-233. This
lighter isotope of uranium is fissile, as is U-235 (most
uranium is U-238, which is stable), and makes a suitable fuel, but less
well-suited to bomb-making. Such work is all but extinct in 2019, and is almost
forgotten, except by a few passionate advocates. The moon has very little
uranium but amounts of thorium which are at or above typical abundances on
earth’s crust - from which the moon is believed to have been formed. If this
resource can be concentrated and transmuted, it will be possible to fuel
nuclear reactors which can be launched completely free of radioactive material.
Furthermore, the high-density nuclear fuel comes from local resources on the
moon, helping to reduce launch mass of each such reactor.
Materials and Methods
Concentrating Thorium
Thorium is found across a large expanse of the moon’s Near
Side with concentrations of 10-20 parts per million (ppm) [1]. Impact
fracturing from meteoric bombardment has embedded or aggregated thorium dioxide
(ThO2, or “thoria”) into other minerals, classified as agglutinates [2]. Thoria
is an especially dense mineral with a specific gravity 10 times that of water.
A straightforward concentration method is to comminute thorium-rich surface
dust (average diameter 70 microns [3,4]) by grinding or milling to
nanometer-sized particulates and then separate them in the lunar gravity using
standard sorting techniques. Lunar soil, called regolith, includes free particles
of iron-nickel metal left by impacts of stony iron type meteorites.
Native alloys of these metals will have densities of
approximately 8 and may not segregate effectively from thoria-rich
agglutinates.Being magnetic, the iron-nickel fraction can be extracted by electromagnet,
perhaps driven by electric current delivered by arrays of solar cells. This is
a simple and low-energy method of concentrating thorium, likely to be used
first.
To achieve greater purity and higher concentration of thorium
another method is to exploit the exceptionally high melting point of thoria mineral,
at around 3500 K. In addition to its value as a nuclear fuel, this refractory
property of thoria has great value in other areas of lunar ISRU, including the extraction
of oxygen and silicon from regolith [5,6]. At the extreme temperature needed to
refine thoria, crucible materials are a challenge. The skull crucible method7
was developed to produce crystals of refractory zirconia and uses radio
frequency (rf) inductive heating to melt the interior of a bolus or “gob” of mineral.
The rf coils and the exterior of the bolus are cooled with a working fluid in
communication with a radiative heat exchanger (convection is absent on the
moon, and conduction through regolith is impractically slow), such that the
interior of the charge is liquefied while the exterior remains solid. The gentle
gravity of the moon (1/6 that of earth) will cause thoria to settle to the
bottom of the melt. The supernatant magma is then poured off, perhaps into
forms to create bricks for paving and building, and the remainder will be
concentrated thoria. A fortuitous side benefit of the skull crucible method is
the evolution of abundant amounts of oxygen, released from common lunar
minerals such as quartz, olivine, anorthosite, pyroxene, and ilmenite [7-10].
The skull crucible method is obviously energy intensive, and is therefore a
second generation thoria concentration method, used once the first U-233
fission reactor is operating on the moon.
Thorium Transmutation
Neutron bombardment of thorium (element 90, mass 232) results
in its transmutation to protactinium (Pa, element 91). The extra neutron, being
composed of a proton plus an electron, decays by electron emission (beta
radiation) to leave behind an additional proton and thereby change the chemical
identification of this isotope. The Pa-233 intermediate product has a half-life
of 27 days and decays by beta radiation to become U-233. This isotope of
uranium is fissile and suitable as a nuclear fuel.
Although
space is filled with radiation, very little is neutrons. Outside of an atomic
nucleus, wild neutrons have a half-life of 10.3 minutes, only slightly longer
than the transit time of light from the sun to the earth’s orbit. Energetic
solar flares produce neutrons in the sun’s corona, which, at relativistic
speeds, have a retarded decay time relative to the moon, and can reach the surface.
However, such neutrons fluxes are infrequent. Further, because the moon rotates
relative to the sun with a synodic period of 27.3 days, only a few locations
see the sun more than 50 percent of the time, and these are generally remote
from thorium ore bodies. A more reliable source of neutrons for transmutation
can be obtained by exposing beryllium (Be) to gamma rays. Gamma rays are
abundant in space, making them a health risk for humans, and are generated by
galactic cores, solar flares, supernovae, and even lightning on earth. There is
a gamma ray background pervading the universe for which no source has been
conclusively identified and is called the gamma ray “fog”. Gamma rays are more
energetic than X-rays, with each photon having energies above 100,000 electron
volts (eV). Data from the Fermi Gamma Ray Space Telescope indicates a flux of
approximately 0.5 photons per second through each square centimeter of space
[11]. When gamma rays impinge on beryllium metal, neutrons are generated
[12]. A Be vessel containing Th and exposed to gamma rays will therefore
transmute or “breed” U-233 fuel (Figure 1).
Gamma
ray energies from space span seven orders of magnitude in eV, so some neutrons
generated will be fast, and thus more likely to pass through without capture.
Graphite acts as a neutron moderator to slow down fast neutrons to become
thermal (slower) neutrons with a higher capture cross section by the thorium
nucleus. To enhance neutron flux within the Th it is advantageous to use a
neutron mirror material, which deflects wild neutrons back into the material to
be transmuted. Beryllium is an excellent neutron mirror, as is graphite. Therefore,
a vessel wall with exterior made of Be and interior made of graphite is a good
design choice. The vessel must protrude proud from the lunar surface, ideally
on a hilltop, and be sized to balance neutron capture probability with neutron
flux intensity. Figure 1 illustrates one design configuration of a thorium
transmutation vessel.
Fuel Considerations
Nuclear fission reactors often use pellets or spheres containing oxides
of the fissile species. The U-233 urania derived from thoria will already be in
the oxide form UO2. The enrichment of U-233 in the mineral charge of the
transmutation vessel will depend on the concentration of thoria and the time of
exposure. Once removed from the vessel, this fuel can be compressed into
pellets using mechanical or hydraulic compression. Cylindrical pellets are
loaded into fuel rods and inserted into the reactor core. If lunar-sourced
U-233 fuel pellets are used, this allows for a nuclear reactor to be launched
from earth having no radioactive materials in the payload manifest. Although
the lunar surface is depleted of uranium relative to the earth’s crust, it is
not negligible3. There may be some U-232 contamination of the thoria, and this
complicates the thorium fuel cycle. The skull crucible method is one means for
separation of thorium from uranium prior to transmutation, if required
depending on system considerations. Chemical methods for Th-U separation have
been developed as early as 1949 [13].
Thorium fuel cycle
byproducts generally decay within a century, greatly reducing the well-known
problems of disposing spent nuclear fuel. Some of these byproducts emit gamma
rays. It may be possible to accelerate the breeding of U-233 using the hot
waste, once the first fuel cycle of a lunar nuclear reactor is completed. The
lunar surface is bombarded by radiation constantly: alpha, beta, gamma,
energetic protons, and more. Relative to earth, which is protected by an
atmosphere and a magnetosphere, humans on the moon must make protection from radiation
second only to protection from the vacuum of outer space. It is a hostile
environment and requires much shielding. A commonly envisioned method of
radiation protection for humans is a layer of regolith some two meters thick.
Applying this logic to spent fuel rods, the expedient of a shallow grave, with
markers, signs, and beacons, should be sufficient to prevent harm to humans for
a hot century [14-16].
Results
The thorium atom
has a radius of 240 picometers, while oxygen is 60, so that thorium forms 2/3
of the area of the thoria molecule. Assuming that 50 percent of the impinging
gamma rays produce a neutron from the Be outer casing, and assuming that 50
percent of these neutrons are absorbed by a thorium atom (including reflection
within the vessel), the number of transmutations with 0.5 gamma ray particles
arriving per second, per square cm, is 1.6E13. Assuming a critical mass of 15kg
of nuclear fuel to start the reactor, and a time to fuel of three years,
accounting for two Pa half-lives in this duration, a cylindrical transmutation
vessel of 2 meters height requires an interior radius of 21cm.
One transmutation vessel holds 0.28 cubic meters of thoria. At 12 ppm
concentration of thoria, each charge, sufficient for one nuclear reactor with a
30-year supply, requires considerable excavation and concentration operations.
For a compacted regolith density of 2200 kg/cu.m this calls for 51,000 metric
tons to be processed. Given the lighter gravity on the moon, this requires the
same energy as processing 8,600 MT on earth. As a point of comparison, for a
large, mature mining operation on earth, this much mass is moved every 20
minutes (Figure 2).
Discussion
The infrastructure needed to process lunar thorium into nuclear
reactor fuel is non-trivial. First generation equipment includes: excavation
equipment; ball mills; gravity sorters on a shaker table; electromagnet and
solar panels; transmutation vessel; and a fuel pellet compaction device. Second
generation equipment includes skull crucible apparatuses driven by rf
generators, and a liquid cooling system with extensive radiative heat
exchangers. Three years after the first transmutation vessel is filled, the
first lunar nuclear reactor can be started up. Start-up generally requires an
extra source of neutrons to initiate the fission chain reaction, but as can be
seen above, these neutrons can be provided by a layer of beryllium exposed to
cosmic rays. Once the first nuclear reactor is operational, second generation
refining techniques can be brought online, greatly improving the throughput and
purity of the thoria concentration operations. Assuming that mining equipment
is operated by battery or fuel cell, the electric power from the nuclear
reactor can significantly enhance the rate of ore excavation and concentration.
A third generation becomes possible when the first reactor has its fuel rods
removed, the gamma rays for which can increase the breeding rate of Th to
U-233. It can be seen that the infrastructure investment has an accelerating
payback in capability of lunar operations. These favorable economics are
further augmented by the additional value which can be provided.
With more power to excavate, the mining operations can provide
fuel pellets as an export commodity. Beachhead customers will be other
settlements and mining operations on the moon, but also on Mars, and possibly
at ore-rich asteroids. To reach these more distant destinations will require
fast rockets. The obvious solution is a nuclear thermal rocket (NTR) which
superheats hydrogen gas and passes it through a de Laval nozzle to generate
thrust. The NTR is powered by U-233, and the hydrogen can be obtained by water
ice mining in permanently shadowed regions of the moon. Extraction of water ice
from ultra-cold craters can be facilitated by the warmth from spent fuel rods,
and a nearby nuclear power station can provide the electrical current needed to
electrolyze the water and produce hydrogen. If the NTR-powered spacecraft
configure their hydrogen fuel tanks to surround the crew capsule the capture
cross section for ionizing space radiation of the fuel significantly reduces
exposure for the humans inside.
Conclusion
Shown here for the first time is a means for safely fueling
nuclear reactors in space whereby no radioactive material is exposed to the
risks of a rocket launch. Mining operations which are modest by earth
standards, are needed to jump-start an expanding economy of ISRU capabilities
which greatly multiply the power, speed, and safety of human operations in
space. Nuclear reactors fueled by transmuted thorium native to the moon will
drive extraction of hydrogen, oxygen, silicon, aluminum, titanium, and iron.
Fast rocket ships powered by NTR open up the entire solar system to extraction
of resources useful on earth, such as platinum-rich asteroids which can greatly
reduce the cost of fuel cell systems on earth, making possible a hydrogen
economy with no atmospheric carbon involved. Nuclear reactors burning U-233 are
far safer than reactors currently in use on earth and produce generally
short-lived byproducts which decay away in a century. The thorium fuel cycle is
ill-suited to bomb makers, so the risks of nuclear war in space, using the
technologies presented here, are far less than other means of space warfare.
Rather, this new approach has the potential to expand the human economic
sphere, open up new frontiers for science and industry, and to help improve
life back home on earth.
Acknowledgements
Helpful discussions
were provided by AJ Kragt, Sawyer Powell and Penghui Heng.
To Know More About Insights in Mining Science
& Technology click on: https://juniperpublishers.com/imst/index.php
Comments
Post a Comment