Jeanne Dion
ME 388R.2
5/06/2005
Stirling Radioisotope Generators:
An Efficient Alternative to Power Future Space Missions
By Jeanne Dion
The University of Texas at Austin
ME 388R Nuclear Power Engineering
May 6th, 2005
Abstract: The Stirling Radioisotope Generator (SRG) is being developed as a high
efficient source of electrical power for future NASA missions. Using the heat from the
radioactive decay of Plutonium Dioxide fuel, the SRG employs a dynamic Stirling Cycle
to generate 110 W of electrical power at impressive system thermal efficiencies
exceeding 20%. SRGs provide substantial improvement of electricity production and
inherently better fuel utilization than its predecessor, Radioisotope Thermoelectric
Generators (RTGs). Each Free-Piston Stirling Convertor, designed by the Stirling
Technology Company (STC), generates no friction between moving parts and are
maintenance free. This paper presents a comprehensive view of the SRG system
including its history, motivation for development, and comparison to the existing RTGs.
Major system components and subsystems are discussed along with corresponding
reliability considerations.
Table of Contents
Introduction
Background
History
Development
Comparison to Current Technology
Stirling Cycle
Stirling Thermodynamics
Free-Piston Stirling Convertors
SRG Convertors Components
Flexure Bearings and Clearance Seals
Heater Head
Linear Alternator
Advanced Controllers
SRG System
Step 2 GPHS Modules
Advanced Vibration Reduction System
SRG Reliability
Uncertainties
Conclusion
Nomenclature
Figures
References
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Introduction
Stirling Radioisotope Generators are potential highly efficient power system that
could replace Radioisotope Thermoelectric Generators in future NASA missions. The
Since the 1990’s, the Department of Energy (DOE), in support of Project Prometheus,
has funded the development and evaluation of Stirling Radioisotope Generators.
Research and development is being conducted under contract with the DOE at the NASA
Glenn Research Center (GRC), Lockheed Martin (LM), and the Stirling Technology
Company (STC). The SRG is a modular design intended for use on a variety of NASA
missions, particularly for the exploration of deep space and neighboring planetary terrain.
Initial plans project the first mission to use an SRG to launch around 2010 on an
unmanned mission to Mars (DOE 2005).
Radioisotope power systems are needed for missions where solar power is
impossible. Since 1961, over 40 Radioisotope Thermoelectric Generators (RTG) have
been used to provide all or partial electrical power for spacecraft [DOE 2002]. The
current thermoelectric power systems offer high reliability due to the absence of moving
parts. Although current technology using thermocouples to convert heat from radioactive
decay into electricity are proven and reliable, they demonstrate low thermal efficiencies
of around 7%. With system thermal efficiencies exceeding 20%, the Stirling
Radioisotope Generator (SRG) is being considered as a highly efficient alternative to
power future NASA space missions. Perhaps the most important feature of the SRG is
the use of flexure bearings and clearance seals that allow for highly efficient and frictionfree operation. The SRG’s ability to perform over 3 times more efficiently over long
missions makes its the potential replacement of RTGs very likely in the future. The SRG
system design utilizes similar modular Step 2 General Purpose Hear Source (GPHS) units
as the current traditional RTGs. Free-Piston Stirling Convertors use the heat source to
drive a dynamic Stirling cycle. SRGs provide substantial improvement of electricity
production and inherently better fuel utilization than RTGs. Although it is being
developed as modular heat source capable of powering a variety of missions, the SRG’s
first mission is projected to be on a Mars rover [Thieme 2002]. If successful, the SRG
will be the first dynamic power conversion system used for space applications and will
represent a huge technical achievement.
This paper presents a review of the current SRG power system design beginning
with the background of SRG power systems highlighting the motivation of its
development. A brief description of current radioisotope space generators will be given
to establish a comparative datum to the SRG system. For a comprehensive understanding
of the SRG system, an explanation of the Stirling thermodynamic cycle and its
implementation as a Free-Piston Stirling Convertor is provided. Next, a summary of the
STC’s Stirling convertor and its components is presented including descriptions of the
flexure bearings, clearance seals, heater head, linear alternator, Step 2 GPHS modules,
and advanced controllers. A reliability assessment will emphasis and discuss potential
functional and structural failures of the SRG. Lastly the future of the SRG will be
presented within concluding remarks.
Background
History. The current work to adapt a Free-Piston Stirling Convertor for spacecraft
power systems is founded on previous work done at the NASA Glenn Research Center
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(GRC) to develop a Stirling automobile engine in the mid 1970s [Schreiber 2002]. The
Automobile Stirling Engine Project, funded by the DOE, laid the foundation for the
development of the SRG. The established history of Stirling power convertors has
utilized GRC’s expertise in the following technical areas: materials, structures, structural
dynamics, magnets, cycle analysis, and computational dynamics [Schreiber 2002].
Each SRG system consists of two GPHS modules and two Free-Piston Stirling
Convertors. A cross sectional view of half an SRG system is shown in Figure 1. At the
beginning of mission (BOM), the GPHS modules each provide 250 W of thermal power
from the radioactive decay of plutonium-238 fuel pellets.
Development. Lockheed Martin (LM) is the SRG’s System Integration
Contractor. Plans are underway to complete the 110 We Stirling Radioisotope Generator
(SRG110) engineering unit in 2006 [Thieme, 2004]. The DOE contracted the Stirling
Technology Company of Kennewick, WA to provide the Stirling convertors for the SRG
system. STC developed Technology Demonstration Convertors (TDC), which are
designed to provide 55 We each when operated at 650 C and 120 C between the hot and
cold ends, respectively [Schreiber 2002]. Each SRG system will include two Stirling
convertors which are based from the TDC design; see Figure 2. The TDC demonstrates
the long life and supports the SRG concept design. STC is currently working with GRC
and LM to adapt the TDCs into a flight convertor (FP) [Thieme, 2004]. The new FP
redesign will allow for hermetic sealing and reduce weight [Qui 2002].
Table X. Comparison of TDC and FP Convertors
A major technical challenge exists in minimizing the unwanted system vibrations.
With a TDC dynamic system, this can be challenging since the physical dynamics are
dependent of complex thermodynamic analysis.
In actual power systems, multiple SRG units may be included to increase
modularity and redundancy. Previous vibration testing only addressed system dynamics
of a single SRG unit and has not discussed multiple SRG configurations.
Comparison to Current Power Systems. Static power systems, like the
Radioisotope Thermoelectric Generator (RTG), have been used successfully for over
three decades to provide electricity on spacecraft such as Pioneer, Viking, Voyager,
Galileo, and Ulysses [DOE 2004a]. Eighteen General Purpose Heat Source (GPHS)
modules, as shown in Figure 3, each contain 600 g of Plutonium-238 Dioxide (PuO2) fuel
and provide 4500 W of net thermal power at the beginning of mission (BOM). Silicon
germanium thermocouples directly convert heat from the radioactive decay of PuO2 into
electricity [Space-1 2002]. Because there are no moving parts, RTG systems have an
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inherently long life and high reliability. The RTG, with its impeccable safety record, has
proven to be a reliable and maintenance-free source of electric power for long space
missions where solar power systems are not practical. Without radioisotope power
systems, deep space exploration and planetary surface missions would not be feasible.
The solid-state thermoelectric direct conversion in RTGs exhibit low thermal
efficiencies of about 7% [DOE 1996]. Since the price of the PuO2 fuel is the dominate
cost factor in the RTG, reducing the fuel inventory is desirable. This decrease in
radioisotope inventory can be achieved with higher thermal efficiencies [Schreiber 2002].
With a system efficiency exceeding 20%, the SRG is an attractive alternative to RTG
systems. The SRG system is a modular design that uses new and improved GPHS
modules, similar to the ones used in past RTGs, and thermodynamically independent
Free-Piston Stirling Convertors. The SRG engineering unit is estimated to be comparable
in size and weight to the RTG. The most impressive advantage SRGs exhibit is their
ability to convert thermal energy dynamically into electricity with frictionless, noncontacting Free-Piston configuration; see Figure.4 The use of flexure bearings and
clearance seals allow the Stirling convertor to seal the working fluid space without the
use of lubrication.
Radioisotope
Generatro
RTG
SRG
Table XX. Comparison of RTG to SRG
Mission Life
Thermal
#GPHS
(yrs)
Efficiency
modules
5
7
18
14
25
2
Power Output
(We)
315
110
Stirling Cycle
Stirling Thermodynamics. In 1816, a Scottish minister named Robert Stirling
patented an “air engine” [American 2002]. This “air engine” only needed a temperature
differential to output work efficiently. Using the expansion and compression of gas, the
“air engine” operates on a Stirling Cycle with an external heat source and heat sink. With
different gases as the working fluid, Reverend Stirling’s “air engine” became known as
the Stirling Engine. Stirling Cycles are a closed system that uses external combustion. In
1873, it was discovered that, with a refrigerant and exchanger, the process may be
reversed to make a refrigerant process and take work as an input [Morrison 1999].
As a working fluid’s temperature increases and decreases so does the pressure. If
a volume of gas could be reheated and cooled quickly, the resulting pressure differential
can be used to perform work. Since rapidly cooling a contained volume of fluid is not
practical, a means for displacing the fluid between a hot and cold end is necessary to
sustain a pressure wave. Such is the underlying principle that drives the displacer-type
Stirling Engines.
The displacer piston shown in Figure 5 shuttles the fluid back and forth to the hot
and cold ends respectively and creates a pressure wave. A displacer-type Stirling Engine
consists of a displace piston, working fluid space, power piston, bounce space as shown
in Figure 5. The displacer piston does not compress the gas; a sufficiently large gap
exists between the inner diameter of the housing and the outer diameter of the displacer
piston to allow the fluid to flow freely from the hot and cold ends [Tuttle 1998]. Thus,
the sole purpose of the displacer is, as its name indicates, to displace the fluid (stages 2 to
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3 and 4 to 1 in Figure 6 show the displacer moving air at a constant volume). Isothermal
compression occurs from stages 1 to 2, as most of the fluid is in the cold end; see Figure
6. The resulting low pressure at stage 2 causes the displacer to drop, thus shuttling the
fluid to the hot end. As the working fluid heats up the pressure rises in the constant
volume heat addition process from stages 2 to 3. The high pressure forces the power
piston out and produces a work output from 3 to 4 during isothermal expansion. A
constant volume heat rejection occurs from stages 4 to 1.
Free-Piston Stirling Convertor. The displacer and power piston respond to the
pressure differential in the working space as mass-spring-damper systems [STC 2002].
Both the power and displacer pistons are mounted with flexure bearings, which act as
springs allowing each to move in the axial direction. The system is dominated by the
power piston’s mass. When properly tuned as a dynamic mass-spring-damper system, a
free-piston configuration will resonate on its own at the systems natural frequency
[Shelter 2002].
SRG Convertor Components
This section contains a description of important components found in an SRG Stirling
Convertor.
Flexure Bearings and Clearance Seals. A major advantage of the SRG is its
ability to operate maintenance-free over the life of the mission. STC’s convertor design is
marketed as a long-lasting, reliable dynamic system that generates no wear in the absence
of contacting parts. The proven high reliability of the convertor is made possible by using
flexure bearings and clearance seals.
Previously life-limiting sliding seals found in most Stirling Kinematic Engines are
replaced with integrated flexure bearings and clearance seals. Figure 7 shows two axially
mounted flexure bearings [STC 2002b]. The flexures support the power piston or the
displacer piston by allowing them to move axially and constraining them with radial
stiffness. Table 1 shows some advantages of flexure bearings.
Table 1. Advantages of Flexure Bearings [STC 2002]
1
No friction between moving parts
2
Low manufacturing costs
3
Highly accurate modeling techniques
4
Mechanically simple and highly predictable
5
Proven long operating life
Fabricated from sheet metal at very precise tolerances, spiral kerfs allow for
elastic deformation; see Figure 8. The center of the flexure bearing is clamped
concentrically to the piston at the center and clamped to the Stirling machine along the
circumference [White 1996]. A mounted flexure flexing upward and downward is
pictured in Figure9a and 9b respectively [STC 2002]. The spiral kerfs allow for free
axial movement while their thick arms resist radial movement.
Between the piston and the housing, there is a clearance seal (an extremely small
gap less than 0.025 mm) [White 1996]. This clearance seal prevents dynamic leaking of
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the convertor pressure vessel. Thus, the convertor can be hermetically sealed to form an
airtight system.
Heater Head. The heater head conducts thermal energy from the GPHS module
and delivers it to the Stirling power cycle. At elevated temperatures the grain size of the
Inconel 718 material increases. This makes the creep the dominate failure mode over the
14 year mission life [Shah 2004]. Probabilistic and deterministic Heater life assessment
has been conducted at GRC [Thieme, 2005].
Higher thermal efficiencies can be achieved if the SRG’s hot end temperature at
the heater head could be raised. Mass optimizations indicate that reducing the cooling
systems mass by raising the cold end temperature is ideal; this objective can also be
achieved by subsequently increasing the hot end temperature at the heater head. In
Figure10 the same efficiency can be obtained for varying rejection, temperature if the hot
end temperature is increased [White 2001]. The TDC convertor IN718 heater head is
designed to operate at a maximum temperature of 650 C. Hot end temperatures of 850 C
can be obtained using advanced superalloys and 1200 C with refractory metals and
ceramics.
Linear Alternator The linear alternator converts the reciprocating motion of the
power piston shaft into electrical power without any mechanical linkages. Since many
magnetic materials’s brittleness renders them inapplicable for the SRG Stirling convertor,
extensive investigations at GRC are being conducted on rare earth permanent magnets
(REPM) [White 1996].
Among the GRC tests are magnet characterization tests before and after shortterm and long-term magnet aging test [Geng 2005]. Magnet aging is measure by
comparing pre-aging and post-aging magnet characterization data. The magnet aging
fixture subjects up to 10 magnet samples to a uniform demagnetizing field.
These magnets are not dynamically loaded by the power piston. The SRG cooling
system cools the cold end of the convertor and the moving magnets.
Advanced Controllers. Even though SRG controllers may be designed to be
“application specific,” many features are the same. The controller should be compact,
light, robust, highly efficient, and have few parts [Thieme 2004 ]. The Advanced
Controller also rectifies the AC power and filters the output [Shah 2003].
The SRG controller can be programmed to maintain constant voltage output or
constant heater temperature as the operating conditions vary. Over the life of the
mission, the radioisotope heat will mitigate with time. The onboard electronic controller
ensures the SRG operates at steady state.
To maintain constant voltage output, the amplitude of the power piston stroke
must be maintained. As the heat input diminishes, the pressure in the working space is
reduced. This causes less power input into the power piston and subsequently a drop in
output voltage. The controller senses the reduced voltage and lessens the load (ie.
reduces the damping) of the linear alternator. The controller lessens the damping and
allows the power piston to regain stroke amplitude. With power piston amplitude
restored, the voltage output is constant as the system returns to steady-state operation
[Shalten 2002].
For some cases as the heat input temperature reduces, it is beneficial to maintain
the heater head temperature. In this case, the controller senses the reduced heater head
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temperature and responds by increasing the linear alternator’s load. This increased
damping mitigates the power piston stroke and reduces the voltage output. This drop in
power output decreases the amount of heat rejected from the working space. Since less
heat is removed, the working space heats up and returns the heater head to the desired
temperature. Steady-state operation is achieved at constant heater head temperature with
reduced power output [Shalten 2002].
SRG System
General Purpose Heat Modules. As previously mentioned, the GPHS modules
are existing units that have been used for several decades in the RTG systems. Over the
years, several modifications have been implemented to improve the robustness of the
“heritage” GPHS modules. The new modules are called “Step 2 GPHS” [Pantano 2005].
Some of these changes have affected the thermal gradient from the fuel to the interface of
the SRG heater head; see Figure 11. Comprehensive thermal analysis has been
conducted and is being used to guide the SRG development.
Each GPHS module is required to produce thermal energy over the life of the
mission and to minimize the release of hazardous radioisotopes in a variety of accident
scenarios. Figure 12 shows an exploded view of a Step 2 GPHS module [Pantano 2005].
The iridium cladding of the PuO2 fuel pellets provide containment during operation and
accident scenarios. Iridium is very strong and ductile allowing it to deform rather than
cracking upon impact loads [Pantano 2005]. Graphite Impact Shells (GIS) are designed to
absorb large amounts of energy upon impact and are made of Fine Weave Pierced Fabric
(FWPF) graphite. A Carbon Bonded Carbon Fiber (CBCF) disks, bonded to the
aeroshell in Figure 11, are layered outside each GIS provide thermal insulation during
“high temperature reentry transients” [Pantano 2005].
Adaptive Vibration Reduction System. The Adaptive Reduction System
(AVRS) will mitigate system vibrations when both convertors are operating as expected.
Since the dynamically opposed convertors within an SRG system are thermodynamically
independent of each other, one can fail without affecting the other’s power production
[Shah 2003]. In the case of one convertor failure, the vibration cancellation benefit will
be lost and may lead to a complete system failure if not mitigated. In the case of one
convertor failure, the AVRS will measure the vibration level via a load cell or an
accelerometer then apply a compensating signal back through the balance motor from an
amplifier [Thieme 2000]. Initial AVRS testing show that unbalanced vibrations under
normal operation will be reduced by a factor of 500.
Reliability.
Using new technology that greatly differs technically from its predecessor RTG
system, reliability assessment of the SRG is very import in ensuring its efficacy.
Reliability of the SRG system is the estimated probability that all components will
operate with designed intent under a variety of operating and accident scenarios [Bents
1989]. Reliability is in this context is related to but not synonymous with safety, which
will be discussed later.
Traditionally deterministic methods are the standard approach to reliability
assessment. Supplementing statistical methods with detailed physical-based models of
the components and knowledge of the interactions within an integrated system can
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greatly improve the reliability estimation [Shah 2003]. Virtual experiments and computer
simulations can be used to predict component and subsystem behavior under different
conditions.
SRG system objectives are to complete the following 3 goals throughout the 14
year mission life:
1. operate efficiently
2. perform reliably
3. operate free of maintenance
For mission lives exceeding 14 years, the reliability assessment should cover the
entire duration of the mission for each of the major components or subsystem [Shah,
2003]:
1. GPHS modules
2. Stirling Power Convertor
3. Mechanical Structure
4. Radiator
5. Control Electronics
6. other hardware (fasteners etc.)
Also, the assessment should span periods covering assembly, pre-launch handling, assent,
space flight, and potential descent into a planetary surface [Shah 2003].
Within an individual Stirling convertor subsystem, important factors that determine
its reliability are shown in Table X below.
Table X. Summary of Failure Modes and Possible Effects of SRG Convertor
Components [Shah 2004].
Convertor Component
Possible Failure
Mechanism
Creep
Structural
heater head
regenerator
Fibers Shedding
displacer
Axial misalignment
flexures
clearance seals
Cyclic Fatigue
Axial misalignment
cooler system
Loss of coolant
linear alternator
Effect
Decreased
efficncy, Power
loss
Decreased
efficiency
Cyclic flexure
failure
Friction loss,
decreased
efficiency
Decreased
efficiency
Decreased
power
variable
Induced Vibration
Demagnetization
Structural
Controller and electronics
variable
fasteners
functional
structural
The heater head and linear alternator magnets, as mentioned previously, have
received particularly intense attention for long life assessment. Specifically, the heater
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head is the only component subject to failure driven creep from the high temperatures it
is exposed to [Shah 2003].
Uncertainties. Each of the mentioned subsystems and components must be assessed
individually. Obtaining accurate estimates of the individual components is crucial to
determining the system reliability. Since the Stirling convertor design is void of all
mechanical wear between moving parts, it may not be possible for individual components
to fail during the mission. However, although components and subsystems may not fail
individually, their interfaces and interaction are likely to dominate the overall reliability
of the system [Shah 2003]. Since an actual power system will use multiple SRG units,
any unnecessary “error” in the design margins and factor of safety information (if
available at all) will greatly hinder the accuracy of the reliability estimation.
Structural, electrical, electro-mechanical, electronics, and thermal uncertainties are
the most important sources of uncertainty within an SRG system. For the convertor, the
heater head and flexures are the dominating uncertainty variables [Shah 2003].
Several SRG units may be used within a given power application some of which are
included for redundancy if another one failed. Discrepancies in manufacturing may
occur for the Stirling convertors and the SRG despite the fact that will be made to the
same specifications. For this reason, cause of failure amongst the SRG systems is likely
to be different for each individual assembly. These failures are expected to be caused by
process related events possibly in conjunction with unintended human interactions.
Given these considerations, SRG redundancy is expected to be effective measure to
increase system reliability; see Figure 13 [Shah 2003].
Conclusion
The DOE is evaluating the SRG as a possible replacement of RTG to provide
electrical power for a variety future NASA missions with and without atmospheres.
Rather than designing unique power systems for different missions, a modular design
approach will likely reduce cost, reduce design time, improve reliability, and increase
redundancy. SRGs are a competitive power system because they show promise as safe,
reliable, highly efficient, long-lived, and impressively maintenance-free due to the use of
flexure bearings and clearance seals. Tests are being conducted to optimize heater head
fabrication parameters and reduce weight. The STC is working closely with the GRC to
produce and test the FP convertor, which is based off the original TDC proof of concept
convertor.
Although to date investigations show progress and promise for the SRG110, the
plans to pursue generation 2 and 3, see Figure 14, designs have been canceled due to a
lack of financial support from NASA [E.M.B. 2005]. Plans are underway to complete the
SRG110 engineering unit by 2006. Given continued success, the first mission to use an
SRG110 power unit will be a Mars Rover in 2010.
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Nomenclature:
BOM- Beginning of Mission
DOE- Department of Energy
FP-Flight Prototype
GPHS- General Purpose Heat Source
GRC- Glenn Research Center
LM- Lockheed Martin
RTG- Radioisotope Thermoelectric Generator
REPM- Rare Earth Permanent Magnets
SRG- Stirling Radioisotope Generator
SRG110- Stirling Radioisotope Generator Engineering Unit
STC- Stirling Technology Company
TDC- Technology Demonstration Convertor
Figures:
Figure 1. Break-away view of half an SRG system [Shah 2003]
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Figure 2. Dynamically Opposed Stirling Convertor Configuration
Figure 3. Radioisotope Thermoelectric Generator [DOE 2004]
Figure 4. Major Components of a Free-Piston Stirling Convertor [Qui 2002]
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Figure 5. Displacer Type Stirling Engine, adapted from Shalten. [2002]
Figure 6. P-V Diagram (left) and T-S Diagram (right) for and Ideal Stirling Cycle [Tuttle
1998].
Figure 7. Two Flexure Bearings [STC 2002].
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Figure 8. A Variety of STC’s Flexure Bearings [STC 2002]
Figure 9a. Mounted Flexure Bearing Flexed Upward [STC 2002]
Figure 9b. Mounted Flexure Bearing Flexed Downward [STC 2002]
Figure 10. FP Heater Head (left) Compared to TDC Heater Head (right)
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Figure11. Efficiency Curves for 55 We TDC [White 2001]
Figure 12. Exploded View of Step 2 GPHS Module [Pantano 2005].
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Figure 13. Redundant SRG Configuration
.
Figure14. NASA Development Plans
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http://www.stirlingengine.com/faq/one?scope=public&faq_id=1#4 (American 2002)
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White, Maurice A. and Colenbrander, Kevin, “Generators That Won’t Wear Out”
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