Unformatted text preview: Chapter 2 SPACE POWER REACTORS
© M. Ragheb
9/11/2007 2.1 INTRODUCTION
Fission reactors are expected to play a critical role in upcoming human planetary
missions. The primary sources of electrical power for the Apollo spacecraft were fuel
cells, but nuclear power was utilized during these missions to the moon to operate surface
A lunar base is contemplated by NASA’s Constellation (Cx) Program spacecraft
which consist of the Aries I and Aries V launch vehicles, the Orion crew capsule, the
Earth Departure Stage and the Lunar Surface Access Module. These spacecraft will be
capable of performing a variety of tasks, from Space Station resupply to lunar landings
with the goal of full operational capability no later than 2014 and returning USA
astronauts to the moon by 2020.
Radioisotopic Thermo electric Generators (RTGs) convert thermal power from
238 the alpha decay of Pu to electrical power by way of solid state thermoelectric elements. Fig. 1: Astronaut Harrisson H. Schmitt on the Apollo 17 lunar mission, 1972, left.
Solar array and thermoelectric generator power systems on the moon, right. NASA
RTGs have also been used on the surface of Mars on the two Viking landers. The
two Mars Exploration Rovers relied on radioactive heater units for internal thermal
control keeping the electronics and charged batteries from freezing during the Martian nights. Electrical power on the surface of Mars was generated by solar panels in spite of
atmospheric dust conditions that limit the amount of solar radiation that reaches the
For power requirements in space, USA missions have relied almost solely on fuel
cells, RTGs, and solar cells for energy. The single exception is the SNAP-10A 45 kWth
fission reactor, launched in 1965. Russia has utilized fission reactors on more than 30
satellite surveillance missions. These power sources offer distinct advantages for
extended missions on the moon or Mars. RTGs become prohibitively massive at high
electrical powers. Fig. 2: Spirit Rover used on Mars during assembly and testing. NASA Photograph.
The Cassini spacecraft mission to Saturn and its moons like Titan, carries three
238 RTGs and 32.8 kg of Pu fuel that provide a total electrical power of 870 We. Highefficiency thin film silicon solar cell arrays can produce 676 We/kg and triple-junction
InGaAs solar cell arrays can produce 360 We/kg at geosynchronous orbit . Fuel cells
on the Space Shuttle produced electricity at 130 We/kg at a continuous output of 7kWe.
For an estimated power budget of 100 kWe for surface missions, electricity produced
exclusively by these technologies becomes impractical. However, a combination of these
technologies and nuclear fission reactors with Stirling engines may provide a more
practical solution to electrical power needs and thermal control for surface exploration. 2.2 NUCLEAR TECHNOLOGY FOR PLANETARY ENGINEERING The largest application of nuclear technology in space would be to terra forming
Mars, and make it possible for the kind of life that exists on Earth. Currently the Mars
atmosphere is much thinner than Earth’s and is composed primarily of carbon dioxide as
shown in Table 1.
Earth and Mars are very different worlds. Mars is inhospitable and harsh for life.
Without topographical variations water on Earth can cover the whole surface to a depth
of 3 kms. Mars does not contain liquid water, even though there are indications that it is
present as permafrost.
Table 4: Comparison of the Martian and Terrestrial atmospheres, volume percent.
78.08 O 0.13 20.95 Ar
HO 1.60 0.93
<1-4 CO 95.32 0.035 2
2 2 2 10-100 O (Ozone, ppbv)
3 o Surface temperature, C
Surface pressure, mbars -53 +15 6.36 1,013 Fig. 3: Mars photographed by the Hubble Space Telescope showing frozen fields of
carbon dioxide at the poles.
The average temperature of the Earth is about 15 degrees Celsius, whereas Mars
is at a frigid –53 degrees Celsius temperature. The thin atmosphere of Mars has a surface
pressure of 6.36 mbars, compared with Earth’s atmosphere of 1,013 mbars. Mars’ atmosphere is primarily carbon dioxide with a small amount of nitrogen and trace
amounts of oxygen, whereas the atmosphere of Earth is primarily nitrogen and oxygen
with small amounts of argon, water vapor and carbon dioxide. Earth is full of plant and
animal life forms both microscopic and macroscopic, whereas Mars is devoid of life on
its surface. It is possible that some forms of life exist under its surface in localized areas
of volcanic heat generation.
Nature may have already started the process of terra forming Mars. The
photograph of Fig. 3 displays vast fields of frozen carbon dioxide at its poles. These
fields are eroding suggesting that the atmosphere of Mars is getting denser and that the
climate of Mars could undergo a greenhouse effect leading to its warming. This warming
could release vast amounts of frozen water from its permafrost.
The process can be encouraged by humans by exploding thermonuclear devices at
the poles releasing more carbon dioxide to its atmosphere, followed by the insertion of
bacterial life from Earth that would use solar photosynthesis to generate oxygen in the
Mars’ atmosphere eventually leading to the spread of life on the now dead planet.
The generation of oxygen would result as a byproduct from the photosynthesis
process acting on H O and CO in the presence of light and chlorophyll. In this process
2 2 carbohydrates are produced and used by the plant organism as food.
nH 2O + mCO2 + hν → Cm ( H 2O ) + mO2 The relevant
(1) Ozone as O is photo chemically produced from O . The evolution of ozone will
3 2 be crucial to sustaining life on Mars like on Earth. It is important for absorbing the
biologically lethal solar ultraviolet radiation with wavelength 0f 200-300 nm. Life on
Earth is surmised to have started initially in the oceans, whose water provided shielding
against it. Once the ozone layer formed, living organisms were able to propagate to the
land surface about 600 million years ago. The presence of life on land initiated a
complex cycling of nitrogen, carbon, hydrogen and oxygen elements and compounds
between the atmosphere and the biosphere. 2.3 MARS MISSION
On Mars, nuclear power would be needed. Because of dust storms and high wind
speeds, a Mars colony would have to be sheltered underground or carved into hillsides,
and need a reliable power supply for heat, transportation, food production, water supply,
communications and other life supporting measures. The environment on Mars is very
harsh. Temperatures average at below 273 degrees K, and are at 148 degrees K at the
polar regions. The climate is dry and hostile, threatening the astronauts at every turn.
Providing energy, particularly heating for the astronauts cannot depend on solar energy
or on radioisotope generators, and needs a nuclear reactor source. A mission composed
of 4 astronauts would need a power supply of about 140 kW(e). Most radioisotope
238 generators have used Plutonium , and assuming a dynamic conversion system's
efficiency of 30 percent, the thermal energy needed for the astronauts is: 140 x (100/30) = 466.66 kWth.
238 One needs about 1.8 kg of Pu per kWth produced. Thus one needs: 1.8 x 466.66 = 840 kgs of Pu 238 . This amount is beyond any possible existing supply, and suggests that such a
mission, for reliability reasons, would require at least two nuclear reactors producing a
thermal power of 0.5 MW(th) each, for a total of 1 MW(th) of power. During the
Martian day, three solar power systems at 10 kW(e) each, may supplements their needs. 2.4 HEAT PIPE OPERATED MARS EXPLORATION REACTOR
A Heat Pipe Operated Mars Exploration Reactor (HOMER) providing between 50
and 250 KWe has been proposed for life support, operations, in-situ propellant
production, scientific experiments, high-intensity lamps for plant growth and other
activities on a Mars mission. This is crucial, since a solar array providing the same
power on Mars would require a surface area of several football fields. In addition, day
and night, geographical sunlight issues, seasonal variations and dust storm environments
would not affect a fission reactor system. Figure 4 shows the core design of such a
design producing 125 kWth of power. The rotating drums around the circumference
achieve power level control. These consist of a neutron absorbing side and a neutron
scattering and reflecting side, allowing power control without the need for terrestrial used
control rods. Moving parts are also eliminated by the use of heat pipes transferring heat
for rejection by radiation to space without the use of pumps and moving parts. Fig. 4: Cross Section of heat pipe space reactor of 125 Kw(th) power, showing the
peripheral control drums
The core contains stainless steel clad uranium dioxide fuel. The fuel pins are
structurally and thermally bonded to a sodium heat pipe. Heat is conducted from the fuel
pins to the heat pipes which carry the heat to the power conversion system.
The core design is compatible with different types of power conversion cycles:
thermoelectric, thermionic, Brayton, Striling, Rankine or Alkaline Metal Thermal to
Electric Converter (AMTEC) using high pressure Na vapor.
It will take at least a decade of research and development, with an expense of at
least $50 billion to prepare for a Mars mission. NASA has been lately trying a strategy of
“faster, cheaper, better,” in its exploration of Mars, leading to about a 2 out of 3 as a
success rate. With manned space mission, a higher degree of reliability will be needed. 2.5 LUNAR BASE
The USA has targeted placing a base on the moon by 2020 as a precursor to
launching a manned mission to Mars. Humans have been to the moon already in the
1970s as part of the Apollo program.
After the climactic triumph of the Apollo moon missions, the public lost interest
in continued human exploration of the Moon. The USA President Nixon administration
cut deeply into NASA's forcing it to focus on robotic missions to more distant, more
mysterious worlds like Mars and Jupiter and sent only two small orbiting spacecraft to
the Moon. Clementine, a joint effort with the Department of Defense, found signs of frozen water at the lunar south pole in 1994. In 1998 and 1999, the Lunar Prospector
mission, which found even stronger evidence of ice and mapped out the Moon's
gravitational and magnetic fields.
A reason to return and establish a permanent base on the moon would be to assist
a mission to Mars. Because the Moon's gravity is 1/6 of Earth's, gathering raw materials
there such as metal for the spacecraft to water for the astronauts to drink would be much
cheaper than hauling them up from Earth. So the cost and difficulty of traveling to Mars
would be reduced. A moon base would also serve as a proving ground for new
technologies developed for a Mars mission.
The presently accepted theory about the formation of the moon is that about 4.45
billion years ago, a planetary body the size of Mars slammed into the infant Earth, tossing
a blob of material into space that became the moon. With only 1/8 the Earth's mass, the
Moon long ago cooled to the core, leaving it geologically dead. It is also too small to
gravitationally hold on to an atmosphere.
One can also suggest another theory that it was Mars itself that collided with
Earth with the latter ending up in the collision with most of the water on both planets.
On Earth, plate tectonics has destroyed almost all of the surface rocks from its
first billion years. On the moon, those rocks are still on the surface. The youngest rocks
on the moon are as old as some of the oldest rocks found on Earth at 3.2 billion years.
The craters on the moon also preserve a record of the early bombardment of meteors.
The Apollo astronauts brought back 843 pounds of rocks from the moon. The
similar mix of oxygen atoms in the rocks of the moon and Earth showed the two had a
common ancestry instead of the moon's forming elsewhere and then being captured by
the Earth's gravity. The chemical composition also showed there had never been
significant amounts of water in most areas, except possibly the polar regions. These
rocks came from just the six Apollo landing sites, leaving the rest of the surface, the size
of Africa, unexplored. The top layer of crushed rock and dust, known as the regolith has
not yet been explored and it holds information accumulated over billions of years.
As astronomers try to look farther into the universe, they need a large telescope
that can stay focused on a single patch of sky for weeks or months. A near absolute zero
temperatures and an airless environment are needed to prevent blurring. A nearby moon
base would allow easy repairs and upgrades. A large infrared telescope is proposed to be
constructed in a deep crater at the Moon’s South Pole. The mirror of such a telescope
might consist of a round dish, 20 yards wide, with a reflecting liquid such as mercury that
spun at a rate of two revolutions a minute. The centrifugal force, coupled with the
moon's gravitational force, would push the liquid toward the outer edges of the dish to
form a perfectly curved surface for gathering star light. Not only will a lunar telescope
be more sensitive than the Hubble Space Telescope, but it should be able detect galaxies
and stars far fainter than will be seen by Hubble's planned replacement. It may even pick
up light from the very first stars of the universe half a billion years after the postulated
A lunar base would provide a Noah’s ark protecting a copy of life beyond
possible sudden and unexpected extinction from volcanic activity, viral infections or
comets and asteroids impacts. Another reason to build a base on the moon is to mine it
3 for the He fusion fuel (Fig. 5). This could be used as fuel for space travel nuclear
rockets, as well as be shipped to Earth to provide with deuterium from the ocean’s water, a virtually inexhaustible supply of aneutronic fusion energy. 3 Fig. 5: Mining machines could roam the moon harvesting He as rocket fuel and
energy source on earth.
3 The surface of the moon is notorious for a significant presence of the He isotope
adsorbed on its surface from the solar wind. Data from lunar samples suggest that the
3 moon contains more than a million tons of He . As estimated by Gerald Kulcinski from
3 the University of Wisconsin at Madison, for every ton of excavated He nine thousand
tons of life supporting compounds such as water, oxygen, nitrogen will be mined, as well
as six thousand tons of hydrogen that could be used with oxygen to produce electrical
power and water in fuel cells. 2.6 HEAT PIPE REACTORS HOMER-15 AND HOMER-25 DESIGNS
The Heat pipe Operated Mars Exploration Reactor (HOMER-15) is a nuclear
fission reactor concept for future lunar and Martian surface missions. The reactor core
contains uranium nitride fuel pellets contained in stainless steel fuel pins that produce a
total of 15 kWth of thermal power. Sodium filled heat pipes transfer the thermal energy
to a Stirling engine that produces 3 kWe of electrical power for an overall thermal
conversion efficiency of: ηth = Pe
= = = 20 percent
Pth 15 5 A 25 kWe HOMER-25 version uses uranium dioxide fuel and transfers heat to six
Stirling engines through potassium filled heat pipes. Fig. 6: Heat pipe reactor HOMER-15 configuration.
The HOMER-15 reactor is a modular reactor design with an arrangement of fuel
pins, heatpipes, and neutron reflectors. The mass of the reactor is shown in Table 2.
There are a total of 19 heatpipes and 102 fuel pins in the core design, including 13 six-pin
modules and 6 four-pin modules. Six-pin modules are located near the center and fourpin modules are located near the outside of the reactor core.
Four-pin modules, arranged around a heat pipe as shown in Fig. 7, experience
higher thermal stress because of their asymmetrical arrangement. They are located near
the exterior where the temperature is lowest. Fuel pin modules are arranged in a
hexagonal core shape with a beryllium oxide neutron reflector pin located at each corner.
The core measures 18.1 cm from one edge of the hexagon to the opposite edge.
Table 2: Mass summary for HOMER-15.
4.7 Fig. 7: Cross section of four pin and six pin heat pipe modules 2.7 HEAT PIPES AND FUEL PINS CONFIGURATION
Heat pipes are bonded to a stainless steel tricusps that run along the length of a
fuel pin as shown. As a safety feature, the hollow tricusps are filled with boron carbide
(B C) as a thermal neutron absorber. This allows the reactor to remain subcritical in the
4 event of an accident in which the core is flooded with water such as from a leak in the
Fuel pins are constructed with SS-316 with an outside diameter of 1.59 cm and
thickness of 0.635 mm. Each fuel pin contains a 36 cm stack of fuel pellets contained
within a stainless steel sleeve.
A 4 cm stack of BeO pellets surrounds the fuel pellets at the end of each pin.
Including an end cap on each fuel pin, the total length of each fuel pin is approximately
44 cm. The fuel pellets are made of 97 weight percent enriched uranium nitride.
Uranium nitride (UN)is a high density, high thermal conductivity fuel. This results in a
smaller core size with lower total mass and lower operating fuel temperatures. The
stability of UN fuel is lower than uranium dioxide (UO ). Fabricability of UN is more
2 difficult than UO and the most recently produced UN fuel in the USA was during the
2 late 1980’s for the 100 kWe Space Reactor SP-100 radiation experiments. In the event of
a cladding failure of a UN pin, there would be little effect on the thermal or neutronic
characteristics of the core. In the Martian atmosphere, which is primarily carbon dioxide,
this would result in surface carbonization, but UC and UN have similar densities and
The HOMER-25 is a larger scale version of the HOMER-15 reactor. The mass of
HOMER-25 subsystems are listed in Table 3 and key reactor parameters are listed in
The reactor core consists of 156 fuel pins and 61 heat pipes, arranged as shown in
Fig. 4. The fuel pins and heat pipes are not bonded together in individual modules as in
the HOMER-15 reactor. Instead, they are arranged in a hexagonal monolith lattice as shown in Fig. 8.
Heat pipes are located at the four corners of the hexagonal core instead of
beryllium oxide reflector pins. Heat pipes in the HOMER-25 reactor use potassium
instead of sodium as a working fluid. Boron carbide wire is located in the region
between the fuel pins and heat pipes in the lattice as a thermal neutron absorber.
Table 3: Technical Characteristics for the HOMER-25 design.
931.7 0 Peak fuel temperature [ K] 0 914.9 Average fuel temperature [ K]
Peak fuel burnup 0.27 %
2 Average fast flux (>100 keV) [n/(cm /s)] 2 7.04×10 12
12 Average moderated flux (<100keV) [n/(cm /s)] 1.37×10
2 Average total flux in fuel [n/(cm /s)] 8.41×10 Peak fast fluence (>100 keV) [n/cm ] 1.64×10
880 2 0 Average heatpipe temperature [ K]
0 Boiler saturation temperature [ K] 847.8 Stirling temperature, cold end [ K]
Net electrical power [kWe]
Stirling output power [kWe]
Reactor thermal power [kWth]
Rejected power through radiator [kWth] 21 860 Stirling temperature, hot end [ K] 12 414.4 0 0 Radiator temperature [ K] 25
400 Required radiator area [m ] 75.8 0 2 Table 4: Mass summary for HOMER-25
Mass [kg] Mass [kg]
Reactor (fuel, HPs in-core, monolith, reflector)
Internals and controls
Heatpipes above core and boiler
Reactor module subtotal
Power management and distribution
37.5 Power conversion system subtotal
Secondary heat transfer
Secondary heat subtotal
Total mass 485.0
2133.1 Fig. 8: Fuel pins and heat pipes configuration in the HOMER-25 design.
Uranium dioxide has more design heritage than uranium nitride. UO with
2 Zircaloy cladding is used in commercial reactors and has been studied with 316 stainless
steel cladding in liquid metal fast breeding reactor experiments. The Russian space
program used UO clad in molybdenum, but information about this is not publicly
2 available. A domestic study of UO molybdenum fuel would be required to validate its
2 0 use. UN/SS and UO /SS are limited to clad temperatures less than approximately 973 K
2 due to loss of creep strength in the fuel cladding. 2.8 STIRLING ENGINES
Stirling engines are desirable for space power applications since they operate at
the highest efficiency of any heat engine. This decreases fuel burnup, radiation levels,
and the amount of heat that must be rejected by the reactor. Heat pipes from the
HOMER-15 or HOMER-25 reactor are connected to a heat exchanger that transfers heat
to the heater head of the Stirling engine. The exit temperature for a stainless steel reactor
0 0 is 900 K and the average temperature of the heater head is 850 K. This thermal energy
is converted to mechanical energy by pistons inside the Stirling engine, which is then converted to electricity by a generator. The pistons are supported by flexure bearings in a
highpressure helium working fluid.
The baseline design for HOMER-15 is a single 3 kWe Stirling engine. An
alternate configuration of three 1 kWe engines, or for redundancy in case of failure, three
1.5 kWe engines could be used. In the HOMER-25 design, six Stirling engines are
configured to produce 25 kWe. Only four of the six engines operate at one time, leaving
two for redundancy in care of failure. This is a necessary feature since current Stirling
engines technology is not suitably reliable at high temperatures.
In a single Stirling engine configuration, the unbalanced load from the single
piston will cause vibration in the system. This effect can be offset by attaching a counter
moving mass balance at the end of the piston. In multiple Stirling engine configurations,
the engines can be arranged opposite to each other to cancel forces and angular momenta
from the moving pistons.
The heat exchanger serves an important structural element. The heat pipes,
reactor core, control drums, and radial reflector are suspended from the heat exchanger
and the Stirling engine is supported above it. In this arrangement, the heat pipes are free
to expand during warm up to operating conditions. 2.9 HEAT PIPES DESIGN
The heat pipes are constructed of 316 stainless steel with the same diameter as the
fuel pins, but a thickness of 0.889 mm. At one end, the evaporator section of the heat
pipes in the core uses an annular/wick structure, as shown in Fig. 9. Fig. 9: Evaporator region of heat pipe.
The wick is composed of stainless steel and the heat transfer fluid is sodium or
potassium or reactor designs use a sodium-potassium eutectic which is liquid at room
temperature. The heat pipes pass 40 cm through the axial reactor shield. In this region,
the heat pipes are insulated with a SS-316 vacuum thermos structure. The length of the
heat pipes in the condenser section or sodium boiler is 20 cm. Including small axial gaps
at the shield core interface and the shield-boiler interface, the total length of each heat
pipe is approximately 107 cm.
0 Stainless steel 316 is compatible with sodium up to temperatures of 1,050 K if the oxygen content in the sodium is maintained below ~10 ppm. 2.10 MATERIAL CHOICES
The primary material for reactor components316 stainless steel, chosen as a low
cost and off the-shelf material. SS-316 also has material advantages when exposed to the
Martian atmosphere, which is predominately carbon dioxide. Stainless steel is carburized
by CO , increasing its emissivity. The CO atmosphere also provides a good thermal
2 2 conduction path in fuel pin and heat pipe connections. However, carburization tends to
make stainless steel more brittle. The use of stainless steel structures makes it
unnecessary to hermetically seal the reactor core, but dust buildup may cause problems
with the internal components.
Ideally, power output could be increased by decreasing the diameter of the heat
pipes and increasing the number of fuel pins per module, but this would increase the
internal operating conditions of the reactor core and stainless steel is limited to use at
relatively low temperatures compared to temperatures in the reactor core. Stainless steel
0 can not be used as a structural material above 873 K due to thermal creep. 0 The maximum allowable stress or 2/3 rupture stress for 316 stainless steel at 923 K is
approximately 35 MPa for a ten year operational lifetime. 2.11 SAFETY CONSIDERATIONS
In the case of a heat pipe failure, the operating temperatures in the fuel cladding
0 would reach 1,067 K, leading to a significant shortening of the reactor lifetime. An
important design improvement would be to reduce the maximum temperature after the
0 failure of a heat pipe to under 970 K to minimize thermal creep. Although stainless steel
has several advantages in a CO atmosphere, it is incompatible with CO for long term
2 2 0 exposure at temperatures above 923 K. 2.12 REACTOR CONTROL
The core reactivity is controlled by six stainless steel clad BeO cylindrical drums
arranged symmetrically about the core in the radial reflectors as shown in Fig. 4. Each
drum contains a 1 cm thick B C absorber section along a 120 degrees arc.
4 The drums can be rotated so that either the B C face or the beryllium face of the
4 drum is oriented towards the reactor core. Each drum is independent and can be operated
in sequence to provide the necessary reactivity conditions in the core.
The radial neutron reflector is composed of six beryllium oxide (BeO) sections
clad in stainless steel.
A severe reduction in the strength and thermal conductivity of BeO in the
20 reflectors and drums occurs at fluences above 2×10
micro cracking. Fluences of 1.2×10 21 2 2 n/cm due to radiation-induced n/cm in the HOMER-15 reactor produce more 0 severe micro cracking below 600 K. Cracks in the BeO reflectors would allow neutrons
to leak to the outside, affecting the neutron properties inside the core. There is no known
21 2 solution to avoid micro cracking in BeO at fluences above ~1×10 n/cm
0 and temperatures below 900 K. Replacing the solid slabs of BeO with fine-grained BeO
encased in stainless steel would prevent thermal cracking but the design lifetime of fine
grained BeO is uncertain.
0 21 Below 770 K the impact of neutron irradiation fluences up to 1×10 2 n/cm on
0 the properties of BeO is small. Significant swelling occurs at temperatures above 900 K,
requiring extra volume within the stainless steel cladding of the reflector. The proposed
0 radial reflector operating temperature of 873 K is a high temperature for BeO.
High temperature He embrittlement in BeO from the (n,α) reaction with Be is a
21 concern for neutron fluences greater than 1×10 2 0 n/cm at temperatures above 800 K.
0 The strength of irradiated BeO decreases rapidly at temperatures above 873 K in BeO
21 2 0 irradiated to greater than 1×10 n/cm . Operation of the radial reflector below ~823 K
should be suitable for the lifetime of the HOMER-15 reactor. 2.13 NEUTRON SHIELDING
Lithium hydride (LiH) has been used previously in neutron shielding technology
in the ANP and SNAP space nuclear reactor programs. Of candidate shielding materials,
LiH has the best neutron attenuation per unit mass due to its low density (0.775 g/ml) and
high hydrogen content of 12.68 in weight percent.
Hydrogen is primarily responsible for neutron moderation in LiH. LiH has poor
thermodynamic properties: low thermal conductivity and high coefficient of thermal
expansion. LiH is difficult to fabricate and the only fabrication capability in the USA is
the Y12 National Security Complex in Oak Ridge, Tennessee.
0 At temperatures above 700 K, hydrogen dissociation is pronounced. In hot spots,
a streaming path for higher energy neutrons can be created, leading to a loss of shielding
effectiveness. The upper operating temperature can be increased by enclosing the LiH in
0 a thin walled stainless steel pressure vessel. For temperatures above 800 K, the
dissociation pressure and hydrogen permeation rate through stainless steel are
0 unacceptably high. At temperatures below 600 K, radiation-induced swelling and
cracking is significant. In the SP-100 reactor program LiH shield material was limited to
0 600 to 700 K for the final design.
Other possible candidates for shielding include lithium magnesium alloy, ZrH,
TiH, and hydrogenated graphite foam.
Water shielding would be the least expensive method of shielding since there is
extensive terrestrial experience with stainless steel and water. The effects of irradiation
on water shielding system would be easy and inexpensive to test on Earth. One concern
with water shielding is that the vapor pressure of water rises considerably as shielding
0 temperature approaches 400 K, requiring a sturdier pressure vessel and accommodation for extra volume for water vapor. At higher temperatures, the mass of the stainless steel
0 pressure vessel could become prohibitively large. To keep the temperature under 400 K,
a potential solution is to include heat pipes bonded to the shield and connected to a
In a study by Poston, et al, , the amount of thermal power that would be
rejected from the radial reflector into the water shield is about ~2 percent of the total
reactor thermal power. An important consideration in radiator design is that a large
radiator will cause greater neutron scattering, increasing the radiation dose to humans.
The major drawback of using water shielding is mass. Future missions would benefit
from the discovery of indigenous water to supply resources for shielding and other
human requirements. 2.14 REACTOR SITING
The reactor will need to be located at an appropriate distance from human
habitation and equipment to provide another level of safety. Radiation dose drops
approximately by the square of the distance away from the reactor, however, increasing
the distance from the reactor increases the mass of the cabling required to transmit
electricity to where it is needed. An optimization between shielding mass and cabling
mass would need to be determined.
Locating the reactor at a distance could also take advantage of the lunar
topography. Any regolith or structures between the reactor and human habitation is
beneficial for blocking radiation. The reactor could be located in a crater or on the
opposite side of a ridge to provide extra safety.
To protect humans during setup, the location of the reactor could be done by
moving the reactor with a vehicle or, if the reactor is launched separately from human
habitation modules, a small rover on the reactor lander could transport cabling from the
reactor to the habitation. A rover carrying cables would be less massive than a rover for
the full reactor. A disadvantage to locating the reactor away from humans is that it
becomes infeasible to transport reactor waste heat to provide habitation heat.
In locations where the topography does not have any useful shielding features,
burying the reactor in a hole is an option for shielding. The depth of the hole is relative to
the distance that the reactor can be located from humans. Poston found that to keep the
human dose limit under 5 rem per year the required hole depth, i.e., depth of the bottom
of the reactor, is approximately 1 m for a reactor 160 m away and 2 m for a reactor 40 m
Scattered neutrons irradiating the Stirling engine become a problem as the depth
of the hole increases. At approximately 1.5 m, the fast neutron fluence in the Stirling
alternators is minimized. For a deeper or shallower hole, the fluence increases, requiring
that the Stirling engines be surrounded by thicker shielding or located at a further
distance from the reactor. Either case adds system mass, so there is an optimal
arrangement for shielding mass, Stirling engine configuration, and depth at which the
reactor is buried. An added benefit to burying the reactor is shielding from
One important point that is not covered in regolith shielding discussion is the
mass of equipment needed for burying the reactor. There are two approaches to burying the reactor: manual digging and robotic digging. The gloves on space suits used on the
moon in the Apollo program were not comfortable for suitable for extended manual
labor. On Apollo 17 Astronauts Eugene Cernan and Harrison Schmitt experienced
blistered knuckles and fatigued muscles from using field geology tools on their three day
stay on the moon. Manually digging would require the astronauts to dig in the radiation
environment in the vicinity of the reactor or to dig a hole first and have the reactor moved
automatically into the hole.
An alternative to manually digging is to have a machine dig the hole for the
reactor. Though using lunar regolith would decrease shielding mass, but the mass of a
machine for burying the reactor would add extra launch mass. Both of these options are
less preferable than finding suitable indigenous topography that can be used. Machines
used for burying the reactor could also be utilized for protecting human habitats from the
ambient radiation environment. Mars and the moon do not have magnetic fields to
protect from solar or cosmic radiation, and the moon lacks an atmosphere that would
provide some protection. For protection, humans will be required to live in environments
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