Thulium-170 heat source

An isotopic heat source is formed using stacks of thin individual layers of a refractory isotopic fuel, preferably thulium oxide, alternating with layers of a low atomic weight diluent, preferably graphite. The graphite serves several functions: to act as a moderator during neutron irradiation, to minimize bremsstrahlung radiation, and to facilitate heat transfer. The fuel stacks are inserted into a heat block, which is encased in a sealed, insulated and shielded structural container. Heat pipes are inserted in the heat block and contain a working fluid. The heat pipe working fluid transfers heat from the heat block to a heat exchanger for power conversion. Single phase gas pressure controls the flow of the working fluid for maximum heat exchange and to provide passive cooling.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a neutron activated heat source, in particular, an 
isotopic heat source using the isotope thulium-170. 
2. Description of Related Art 
Isotopic heat sources use the release of energy from a radioactive isotope. 
The isotope is created either as a result of fission or by irradiating a 
target material with neutrons in a nuclear reactor. In neutron 
irradiation, the target atomic nuclei capture irradiating neutrons and are 
converted into a neutron activated isotope. The target material is chosen 
to provide the energy release rate and decay characteristics of interest 
in the activated target. This energy release can be absorbed as heat and 
exploited for many uses, such as for a power conversion system. 
Typically, reactor target materials are formed into thin flat disks. During 
irradiation, neutrons are highly absorbed at the target surface, resulting 
in fewer neutrons available for absorption in the center of the target. 
The reduction in neutrons, called flux depression, results in lower 
activation in the target center compared to the target surface. Thin 
targets provide a more efficient use of target material by reducing flux 
depression. 
Targets may contain a material that acts as a moderator during irradiation. 
Neutrons that pass through the target atoms unabsorbed can collide with 
moderator atoms, slow down, and become more susceptible to capture by 
other target nuclei. Moderators thereby increase the efficiency of the 
production of the activated isotope. An ideal amount of moderation causes 
the neutron energy distribution to peak in the energy region of high 
cross-section for the target material. 
Isotopic heat sources are useful when combined with a power conversion 
system because the energy release is reliable, and the power output 
diminishes in a known manner as the isotope decays. The heat sources have 
greater energy density, by several orders of magnitude, than chemical 
batteries. Depending on the half-life of the isotope, the heat sources can 
be used for months or years, rather than having a life of hours or weeks 
that is typical of a chemical battery. The sources are compact and 
portable, which is especially useful for exploration or surveillance in 
remote areas such as Antarctica, in space, or underwater. 
Presently, isotopic heat sources are available that use isotopes such as 
strontium-90, cobalt-60, and plutonium-238. These isotopes are 
environmentally hazardous because they are easily dispersed, and their 
half-lives are on the order of years. 
Thulium-170 has also been considered as a fuel for heat sources. Targets 
with stable thulium-169 are irradiated and converted into thulium-170 (and 
thulium-171, etc.). Thulium-169 has a high neutron cross-section, lowering 
the irradiation time (and cost) needed to produce thulium-170. Thulium is 
advantageous as a fuel because of its refractory properties; that is, 
thulium is very stable at high temperatures and has a high melting point 
(heat of fusion). Thulium-170 is a better heat source from an 
environmental standpoint because of its relatively short half-life (129 
days), its chemical stability, and refractory nature. 
Several isotopic heat sources using thulium-170 have been developed. The 
thulium fuel has been in the form of thulium hydride, thulium metal, 
thulium oxide, and a mixture of thulium oxide and thulium metal. The 
thulium fuel is usually encapsulated or encased in a material with a high 
melting point and low neutron cross-section. These materials are usually 
metals or high atomic weight (high Z) materials, such as molybdenum, 
tantalum, tungsten, zirconium, steel, nickel, or platinum-rhodium alloy. 
The casings provide containment of the target material before and after 
irradiation. 
Using high Z material to encapsulate targets presents several problems: the 
heat source weight is increased, pre-fabrication of the capsules is 
needed, and high Z materials produce more bremsstrahlung radiation after 
target irradiation than low Z materials. Accordingly, a more useful heat 
source would comprise a refractory fuel with a short half-life and a 
diluent of low atomic weight (low Z) material. The low Z material would 
reduce the weight of the heat source. The low Z material would also 
produce less bremsstrahlung radiation than a high Z material, requiring 
less shielding. The reduction in shielding and source weight is 
advantageous in creating portable power sources. Individual thulium fuel 
parts would not be encapsulated, minimizing pre-fabrication time and 
expense. Suitable containment would be provided by an outer vessel 
containing all of the thulium fuel parts. 
SUMMARY OF THE INVENTION 
The present invention provides a heat source fuel stack that is internally 
moderated during irradiation and requires minimal shielding due to minimal 
production of bremsstrahlung radiation. The fuel stack needs little or no 
post-activation handling, which saves time and prevents prolonged 
radiation exposure. The invention also provides a heat source apparatus 
for efficient heat removal. 
The fuel stacks comprise an isotopic fuel and a low atomic weight diluent. 
The fuel, preferably thulium oxide, is refractory and produces an isotope 
during neutron irradiation with a relatively short half-life. The diluent 
is refractory and heat conductive, preferably graphite. A stack of thulium 
oxide fuel and graphite disks is irradiated in a reactor in a conventional 
manner to form a fuel stack. 
In the described embodiment, the heat source apparatus comprises heat pipes 
for heat removal, a heat block, holes in the heat block for inserting 
irradiated fuel stacks and heat pipes, a structural container, insulation, 
and radiation shielding. The irradiated fuel stacks and heat pipes are 
mounted in the heat block. The heat block, preferably made of graphite, is 
encased in a sealed structural container that is surrounded by layers of 
insulation and shielding. The heat pipes extend beyond the container and 
shielding and contain a heat pipe working fluid. The working fluid 
transfers heat from the heat source to a heat exchanger. A single phase 
gas restricts the flow of the heat pipe working fluid at an established 
interface. 
The low atomic weight diluent in the fuel stack has several advantages. In 
the preferred embodiment, graphite dilutes the thulium oxide fuel and acts 
as a moderator, increasing the efficiency of thulium-170 production. 
Graphite and other low Z materials do not produce as much bremsstrahlung 
radiation as high Z materials; therefore, the fuel stacks require less 
shielding, reducing the weight of the heat source. Graphite is also an 
excellent heat conductor, increasing heat removal efficiency. 
In the preferred embodiment, the heat source apparatus provides two passive 
mechanisms for containment and heat dissipation in the case of source 
overheating. In the first mechanism, the heat pipes are oversized in 
length to permit passive cooling. A heat pipe working fluid circulates in 
the heat pipes between the heat source and the heat exchanger. Beyond the 
heat exchanger, the heat pipe contains a single phase gas. The interface 
between the working fluid and the single phase gas is preferably located 
at the heat exchanger. If the heat source temperature increases, the 
working fluid vapor pressure increases and moves the working fluid-gas 
interface away from the heat source so the heat pipes have more surface 
area for cooling. As a second mechanism for providing containment and 
cooling, the insulation layer is designed to fail at a temperature below 
the failure temperature of the inner container and its contents. 
The present invention has many potential uses. The heat source coupled with 
a power conversion system provides a reliable, refuelable and relatively 
long-lasting power source. This type of power system could be used for 
autonomous or remotely controlled vehicles. These power sources are 
particularly useful for exploration or surveillance in remote environments 
such as space or underwater.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The preferred embodiment of the invention, shown schematically in FIG. 1, 
comprises an isotopic heat source 10. For purposes of illustration, one 
fuel stack 12 is shown adjacent to one heat pipe 14 which extends from the 
heat source 10 to a heat exchanger 16. The heat pipe 14 contains a working 
fluid 18 that transfers heat from the heat source 10 to the heat exchanger 
16. The working fluid 18 flows along an inner surface 20 of the heat Pipe 
14 which comprises means for capillary action. The heat pipe working fluid 
18 can be restricted by the pressure of a single phase gas 22, the source 
of which is a gas reservoir 24. 
FIG. 2 is a vertical cross-section of a preferred embodiment of the heat 
source 10. FIG. 3 is another view of the embodiment shown in FIG. 2 along 
line 3--3. FIG. 2 illustrates a plurality of fuel stacks 12. The fuel 
stacks 12 comprise a refractory fuel 26 and diluent 28. The fuel 26 is 
neutron activated to form a relatively short-lived isotope that produces 
heat. The preferred embodiment for the fuel 26 is thulium-169 in the form 
of thulium oxide (Tm.sub.2 O.sub.3). The diluent 28 is a refractory, heat 
conductive, and low atomic weight material. The preferred embodiment for 
the diluent 28 is graphite. 
In the preferred embodiment, the fuel stacks 12 are formed of a plurality 
of thin individual layers of thulium fuel 26 and graphite 28. The thulium 
layers 26 and graphite layers 28 are stacked in an alternating pattern. 
The fuel stacks 12 are irradiated in a conventional manner with thermal 
neutrons, converting thulium-169 to thulium-170 (and thulium-171, etc.). 
After irradiation, one or more of the fuel stacks 12 are mounted in one or 
more holes 32 in a heat block 34, preferably made of graphite. In the 
preferred embodiment, the fuel stacks 12 are cylindrical and fit snugly 
into the heat block 34. A plurality of heat pipes 14 for heat removal are 
arranged in a plurality of holes 36 in the heat block 34. In the preferred 
embodiment, the heat pipes 14 are enclosed at both ends and may be 
oversized in length, extending beyond the heat exchanger 16 to provide 
additional heat rejection area. 
The heat block 34 is surrounded by a sealed structural container 38, which 
is surrounded by an insulation layer 40. The heat block 34 is also encased 
in at least one layer of radiation shielding 42,44, made from a suitable 
structural material such as iron or tantalum. In the preferred embodiment, 
an inner layer of the shielding 42 surrounds the insulation layer 40 and 
an outer layer of the shielding 44 surrounds the inner layer of the 
shielding 42. Free convection space fills the cavity 46 defined by the two 
layers of the shielding 42,44. 
Holes 48 defined by the outer layer of the shielding 44 are located along 
the inside perimeter of the outer layer of the shielding 44. The holes 48 
are present at both the top 50 and bottom 52 ends of the heat source 
apparatus 10. 
In the preferred embodiment, the neutron activated fuel 26 is thulium in 
the form of thulium oxide. However, thulium in the form of thulium hydride 
or thulium carbide, as well as an altogether different radionuclide, might 
be used. 
In the preferred embodiment, the diluent 28 is graphite. Alternative 
embodiments for the low atomic weight diluent 28 are possible, including: 
zirconium hydride (hydrogen), beryllium oxide (beryllium), boron, lithium, 
and beryllium. 
Graphite is advantageous as a diluent 28 for several reasons. Graphite is 
highly refractory, which allows the heat source 10 to operate at high 
temperatures. Graphite and thulium oxide do not react appreciably at high 
temperatures. Also, graphite is readily available and inexpensive. 
Diluting thulium layers 26 with intervening graphite layers 28 may enhance 
the production of thulium-170 in the irradiation reactor and reduce the 
shielding needed around the fuel stack 12. The production of thulium-170 
is increased because graphite acts as a moderator during irradiation. 
Shielding of the fuel stack 12 is reduced because graphite, being a low 
atomic weight material, produces less bremsstrahlung radiation than high 
atomic weight materials. Graphite also stops the beta particles and 
secondary electrons produced in radioactive decay. 
In the preferred embodiment, the fuel stack 12 comprises alternating layers 
of fuel 26 and diluent 28. The layers of thulium fuel 26 and graphite 
diluent 28 may be thin, flat, circular individual disks or wafers. The 
layers of thulium fuel 26 do not exceed one centimeter thickness in order 
to reduce flux depression. The thulium fuel layers 26 are placed with 
alternating layers of diluent 28 to form the fuel stack 12. In an 
alternate embodiment, the thulium fuel 26 can be flame sprayed or plated 
on graphite disks 28. Thulium oxide powder and graphite powder could also 
be mixed and heated to form a sintered body. 
After the fuel stacks 12 are irradiated, the stacks 12 may be placed 
directly into the heat block 34, eliminating post-activation handling. 
Alternatively, graphite layers 28, possibly of another thickness, may be 
substituted or inserted in the fuel stacks 12 to further minimize 
bremsstrahlung radiation. Excess graphite layers 28, of course, could be 
removed. 
The fuel stacks 12 are designed to maximize the opportunity for salvaging 
and recycling thulium fuel 26 and graphite diluent 28 from expended fuel 
stacks 12. The heat source 10 is designed to permit refueling for long 
term use. 
The heat pipes 14 provide means for heat removal. The heat pipes 14 contain 
a heat pipe working fluid 18, such as sodium, which is chosen according to 
the desired heat block 34 temperature. The working fluid 18 transfers heat 
from the heat source 10 to the heat exchanger 16. The heat pipes 14 are 
oversized in length to carry the working fluid 18 to the heat exchanger 16 
and to permit passive cooling. 
In the preferred embodiment, the working fluid 18 transfers heat by 
repeated cycles of vaporization and condensation. The working fluid 18 
vaporizes in the region of the fuel stack 12. The vapor expands and 
travels through the heat pipe 14 to the heat exchanger 16. The vapor 
cools, releases heat and condenses onto an inner surface 20 of the walls 
of the heat pipe 14 in the region of the heat exchanger 16. The inner 
surface 20 has means to allow capillary action. The condensed working 
fluid 18 flows back to the heat source 10 region by the capillary action 
means on the inner surface 20 to begin another cycle of vaporization and 
condensation. This heat transfer system can operate in a zero gravity 
environment or in a modest gravity field in any orientation. 
During the operation of the heat source 10 with the heat exchanger 16, the 
flow of the heat pipe working fluid 18 is restricted at an easily 
controlled interface by a single phase gas 22. The single phase gas 22, 
such as argon, is supplied from a sealed reservoir 24 attached to a heat 
pipe 14. The pressure of the single phase gas 22 restricts the flow of the 
working fluid 18 to direct heat to the heat exchanger 16 for maximum 
efficiency. Therefore, if the heat block 34 overheats, the vapor pressure 
of the working fluid 18 increases, causing displacement of the single 
phase gas 22, thereby expanding the heat rejection surface of the heat 
pipes 14 and permitting passive cooling. Conversely, if the pressure of 
the single phase gas 22 is increased, the working fluid 18 is displaced 
and the surface area of the heat pipes 14 for heat rejection is reduced 
(shortened). 
In an alternative embodiment, the heat pipes 14 need not extend linearly, 
but may be designed to fold back around toward the heat source 10 to 
reduce space requirements. Additionally, the number and arrangement of the 
heat pipes 14 and fuel stacks 12 are variable, depending on the power 
density and efficiency of heat removal required. 
The structural container 38, the insulation layer 40 and the radiation 
shielding 42,44 may be made of a variety of materials, depending on the 
particular use requirements. One embodiment for the structural container 
38 is an x-ray absorbing material such as tantalum. The preferred 
embodiment for the insulation layer 40 is a material designed to fail at a 
high temperature that is below the failure temperature of the structural 
container 38. In the case of heat block 34 overheating, the insulation 
layer 40 would melt away, allowing thermal radiation to occur from the 
structural container 38 to the layer of inner shielding 42, thus providing 
containment and heat dissipation. Aerogel is one example of such an 
insulation material. 
The free convection space 46 between the layers of the shielding 42,44 
provides yet another opportunity for passive cooling of the heat source 10 
in the event of heat block 34 overheating. 
The description of the invention presented above is not intended to 
encompass all variations of the system but has attempted to present 
illustrative alternatives. The scope of the invention is intended to be 
limited only by the appended claims.