The SP-100 is an electrical generating nuclear power system for space operation. This paper describes the nuclear reactor control systems and the methods used to assure reliable performance for the 10-year design life. Reliable performance is achieved by redundancy and by selecting highly reliable components and design features. Reliability is quantified by analysis using established reliability data. Areas lacking reliability data are identified for development testing. A specific development test description is provided as an example to demonstrate how this process is meeting the system reliability goals

Gilchrist, A. Richard

Gleason, Thomas

Schuster, Gary

English

NASA Technical Reports Server

N94- 29651
SP-100 Control Drive Assembly Development
Thomas Gleason*, A. Richard Gilchrist*, Gary Schuster*
ABSTRACT
The SP-100 is an electrical generating nuclear power system for space
operation. This paper describes the nuclear reactor control systems and the
methods used to assure reliable performance for the 10-year design life. Reliable
performance is achieved by redundancy and by selecting highly reliable
components and design features. Reliability is quantified by analysis using
established reliability data. Areas lacking reliability data are identified for
development testing. A specific development test description is provided as an
example to demonstrate how this process is meeting the system reliability goals.
INTRODUCTION
The nuclear system control and shutdown functions are provided by the
moving reflector segments surrounding the reactor core and by the in-core safety
rods. The moving reflector drive system is capable of precise reactor control as
well as reactor shutdown while the safety rod drive system provides fail-safe
reactor shutdown capabilities. Figure 1 shows the overall reactor system
arrangement and the relative locations of the two reactor control systems. Figures
2 and 3 show components tested and typical results obtained. Table 1 gives the
nominal design environments of the control systems.
DESIGN DESCRIPTION
Reflector Control System
Reactor power control is achieved by moving neutron reflecting material
axially relative to the reactor core. Electromechanical drive units located aft of the
radiation shielding provide the power to move the reflector sections. A ball nut
and screw assembly converts the rotary motion of the drive motor to linear motion
needed to move the reflectors. A brake assembly locks the reflector in position
when it is not being driven. Figure 1 includes a figure of the single reflector drive
assembly. The drive system is capable of moving the reflector segments in small
increments and has precise positioning capabilities. Figure 2 shows some of the
test results for the reflector components tested.
*General Electric Company, San Jose, CA
367
https://ntrs.nasa.gov/search.jsp?R=19940025148 2020-06-16T14:35:46+00:00Z
Safety Rod
Reactor shutdown capabilities are provided by the incore safety rods for
launch and for severe accidents. Figure 1 includes a schematic of a safety rod
and its drive system. Figure 3 shows some of the test results and key features for
the safety rod components. Each rod contains sufficient neutron poison to
prevent operation. When the poison is located within the core region, the safety
rod is mechanically locked in position to prevent inadvertent withdrawal.
Electromechanical drive assemblies located aft of the reactor shielding provide the
unlock and the drive functions needed to move the poison out of the reactor core
region. Electromagnetic clutches and brakes connected to the drive motor are
used to select the desired drive and/or unlock function. A spring motor device
stores energy to move the safety rod to the shutdown position on command or
automatically at predetermined reactor conditions.
Design Challen.qes
The mechanical designs of the two systems share many common features
all of which share the challenge of reliable operation in the harsh environment of
high temperature, vacuum and radiation. Both include stepper motors, clutches,
brakes, bearings, sliding surfaces, and springs, all of which must remain
functional and maintenance free for 10 years. Material selections are limited for
this extreme environment. Changes in material properties and strength
characteristics, dimensional stability, self-welding, diffusion and evaporation must
be considered for each material selected and for each material couple.
Moving and sliding elements near the reactor core region are challenged by
the extremely high temperatures. Refractory metals are needed for the structural
materials with carbide coatings on the critical contacting surfaces. Radiation
induced material swelling is a major concern for items located near the reactor
core.
RELIABILITY CONSIDERATIONS
Overview
Reliability of the reflector control and safety rod drive assemblies for the
overall 1C-year mission depends upon the presence of initial operational reliability,
operational reliability throughout the mission, and an end-of-life significantly
beyond the mission time.
Initially, the reflectors and safety rods must be capable of moving to the
368
operational positions and remaining there with high reliability. This depends on
the drive assemblies 1) being capable of performing their functions subsequent to
fabrication and assembly--confirmed by acceptance testing, 2) surviving the
launch--assured by design and structural analysis according to prescribed NASA
methodology, validated by component and subassembly testing, and 3) reliably
deploying and going into initial operation, given that they survived launch--assured
by qualification testing, low component failure rates and the short time lapse
before initial operation.
For the duration of the mission between start-up and end-of-mission,
redundancy and low component failure rates assure the reliability of the reflector
and safety rod functions to hold, scram, and shutdown. For most functions, the
loss of a reflector or a safety rod can be tolerated.
Satisfactory margins between the mission time and the mean lifetime of the
reflector control and safety rod drive assembly critical components assures that
wear will not be a source of mission failure. Identification of failure modes and
associated failure mechanisms in Failure Modes and Effects Analysis provides the
basis for failure mechanism analysis, a process whereby identified failure
mechanisms are examined for adequacy of data to support lifetime validation. If
sufficient data are available, design margin studies are performed to provide an
assessment of the margin of lifetime beyond the mission time. If sufficient data
are lacking, specific needs are identified and factored into development programs
planned or underway.
Assurance of Initial Operation Reliability
Prior to the SP-100 System Design Review held in 1988, structural analyses
were done for the reflector control and safety rod drive assemblies. As part of
these analyses, the structural response of the drive assemblies to the dynamic
loading and vibration of launch were evaluated. Satisfactory margins were
demonstrated for both types of drive assemblies. Since the System Design
Review, the designs of both drives have changed; the changes to the safety rod
drive assembly are small enough that the analysis results still hold true. The
reflector control drive assembly changes are significant and, therefore, the
structural analysis will be repeated. In the interim, preliminary analysis indicates
satisfactory margin is available.
Mission Reliability Enhancement Throu.qh Redundancy
In the reflector control drive assemblies, redundancy has been built into the
coils of the motor, brake, and position sensors, allowing reflector failures to be
tolerated while meeting mission requirements.
369
In the safety rod drive assemblies, redundancy has been built into the
safety rod drive motors, clutches, and brakes, supporting particularly the
withdrawal function necessary for initial operation. To enhance the operational
reliability throughout the mission, dual coils are built into the scram clutches so
that inadvertent scram is less likely due to coil wire failure or a controller false
signal. Multiple safety rod drive assemblies assure that the scram function will be
reliable over the mission time, since, for most anticipated events requiring safety
rod action, insertion of one safety rod is sufficient to safely shutdown the reactor.
Lifetime Reliability Throuqh Failure Mechanism Control and Desi.qn Marqins
Understanding of life limiting failure mechanisms through failure mechanism
analysis (FMA) and the validation that lifetime margins exist through design
margin studies (DMS), provide assurance that component end-of-life is sufficiently
beyond the mission time. To date, 23 FMA's and four DMS's have been
completed for the reflector control and safety rod drive assemblies.
Of the completed DMS's, one addressed the reflector and safety rod
cladding failure due to embrittlement from oxygen released from the beryllium
oxide (BeO) during irradiation. The conclusion of this study was that over the
mission time the oxygen released in the lattice by the beryllium transmutation can
be easily accommodated within the BeO lattice itself. As such, no oxygen release
by diffusion from the BeO will occur. The potential failure mechanism identified
and evaluated is not operative over the 10-year mission and cannot lead to safety
rod or reflector element cladding failure.
Another completed DMS addressed the reflector and safety rod cladding
failure due to BeO volumetric bulk expansion from irradiation induced swelling or
microcracking. End-of-life was defined to correspond with take-up of the gap that
exists at beginning-of-life between the BeO and the cladding. This is a
conservative limit in that actual failure would correspond to considerable
expansion beyond take-up of the gap, producing cladding distortion and,
ultimately, jamming of the safety rod or reflector. Detailed designs will provide
gaps that assure the desired lifetime goal are met.
A third DMS addressed the crack/ship/spalling/wear failure mechanisms of
the alumina coating used to protect reflector drive assembly tantalum-tungsten
alloy components within self-aligning bearings. The conclusion was that the
coating, on the basis of tests performed in the early 1970's in connection with the
advanced zirconium hydride reactor control system drives, had a reasonable
margin on lifetime. This result will be reviewed for consistency with the modified
application of the self-aligning bearing as we change the design from the hinged
to the sliding reflector concept.
370
The fourth DMS addressed one of several failure mechanisms that could
lead to self-welding of the safety rod to the molybdenum alloy thimble liner -
specifically, the liner material fusing to itself in the event of liner material pickup
onto the hardfaced safety rod slider bearing. Three generic material transport
processes were identified as applicable to this failure mechanism.
Sufficient data were available to show that two of these processes had very
large margins to failure over the mission lifetime; data to close out the third will
come from experiments planned in the development program.
Twelve planned DMS's will address life-limiting failure mechanisms affecting
coils, insulation and mechanical components. Included are several DMS's which
explore other failure mechanisms that could lead to self-welding of the safety rod
to the thimble liner. One of these is self-welding of the actual safety rod slider
bearing carbide coating to the molybdenum alloy liner of the thimble. Another is
the loss of the carbide coating in that, should loss of coating occur, the PWC-11
substrate material of the slider bearing would then be exposed to the
molybdenum alloy, resulting in a couple more prone to self-welding. The causes
that drive or enable these mechanisms to process have been identified as thermal
aging, thermal cycling, and radiation. The need to adequately understand these
failure mechanisms and the respective lifetime margins are the basis for the
development programs described below. This program is a typical example of the
ongoing process development and material evaluation resulting from the failure
mechanism analysis.
SAFETY ROD MATERIAL SELECTION AND TESTING
Theoretical considerations and practical experience suggest a refractory
metal carbide paired with Mo-41Re alloy is a promising material couple for use in
the control rod/thimble system. This combination should not self-weld.
Furthermore, the chemical stability of these carbides and their refractory nature
make them likely to withstand the reactor core environment. Testing is required
under prototypic conditions to validate material selections and applications.
371
Table 1. Design Environment
Reflector
Reflector Drive
Safety Rod
Safety Rod Drive
NOMINAL
TEMPERATURE, K
1320
811
1600
811
NOMINAL
PRESSURE, TORR
10 .9
10 g
10.9
10 .9
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE
May 1993
4. TITLE AND SUBTITLE
27th Aerospace Mechanisms Symposium
6. AUTHOR(S)
Ron Mancini, Compiler
7. PERFORMING ORGANIZATION NAMEtS) AND ADDRESS(ES)
Ames Research Center
Moffett Field, CA 94035-1000
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)
National Aeronautics and Space Administration,
Washington, DC 20546-0001; California Institute of Technology,
Pasadena, CA 91109; and Lockheed Missiles and Space Company, Inc.,
Sunnyvale, CA 94088
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Point of Contact:
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Conference Publication
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REPORT NUMBER
A-93053
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AGENCY REPORT NUMBER
NASA CP-3205
Ron Mancini, Ames Research Center, MS 213-1, Moffett Field CA 94035-1000;
(415) 604-6319
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12b. DISTRIBUTION CODE
The proceedings of the 27th Aerospace Mechanisms Symposium, which was held at theAmes Research
Center, Moffett Field, California, on May 12, 13, and 14, 1993, are reported. Technological areas covered
include actuators, aerospace mechanism applications for ground support equipment, lubricants, latches,
connectors, robotic mechanisms, and other mechanisms for large space structures.
14. SUBJECT TERMS
Pyrotechnics, End effector, Band drives, Tribology, Array deployment, Actuators,
Latches, Linkages, Antennas, Pointing mechanisms, Sampling
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SP-100 control drive assembly development

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