1. Martian Surface Reactor Group December 3, 2004
Good Morning, my name is Mary Presley, I am part of the nuclear engineering design project team, and I am going to introduce the MSR – the Martian Surface Reactor.

2. OVERVIEW The Need Mission Statement Project Organization Goals
Project Description
Description and Analysis
of the MSR Systems
Core
Power Conversion Unit (PCU)
Radiator
Shielding
Total Mass Analysis
Conclusion
Here is a quick overview of what we are going to talk about today:
I start with the mission statement which describes the ultimate goal of this design project.
Then I’ll go into how our group was organized and what supplemental goals we have set for ourselves, and finally I will give a brief description of the overall design before handing it off to my group mates who will give a more detailed description and analysis of the four main systems of our reactor: the Core (shawn galliger), the PCU (Mike Short), the Radiator (David Carpenter), and the Shielding (Erik Johnson); then we will wrap up the presentation with a total mass analysis and system discussion given by Michael Stawicki.
Feel free to ask clarifying questions at the end of each section, but we ask that you please hold all other questions until the end of the presentation.

3. Motivation for Mars
“We need to see and examine and touch for ourselves”
“…and create a
new generation of
innovators and pioneers.”
As Americans, we have undertaken space travel because the desire to explore and understand is part of our character. Probes, landers and other vehicles of this kind continue to prove their worth, sending spectacular images and vast amounts of data back to Earth. Yet the human thirst for knowledge ultimately cannot be satisfied by even the most vivid pictures, or the most detailed measurements. We need to see and examine and touch for ourselves. And only human beings are capable of adapting to the inevitable uncertainties posed by space travel.
The fascination generated by further exploration will inspire our young people to study math, and science, and engineering and create a new generation of innovators and pioneers

4. Motivation for MSR So why nuclear?
Need 100kWe minimum for manned exploration and science
If ISRU is implemented then an additional 100kWe will be needed
Solar power not viable
Chemical power too massive
Nuclear power only viable alternative

5. MSR Mission Statement Nuclear Power for the Martian Surface
Test on Lunar Surface
Design Criteria
100kWe
5 EFPY
Works on the Moon and Mars
Our mission was to design a nuclear reactor for Mars that will produce 100kWe for 5 effective full power years. Realizing that all major technologies to be used on Mars must first be tested on the moon, the MSR must also work on the moon.

6. Organizational Chart
The first thing we did as a design team was divide ourselves into four systems groups, each group with a task manager. We also appointed a project management pair to coordinate the activities of the entire design team. Integration, iteration and optimization over the entire reactor (rather than optimization over just the individual systems) are important steps that tend to get lost when designing a system this complicated. Regular task manager meetings, keeping an open and informal dialog between groups, and having an overarching project management have helped us effectively integrate, iterate and optimize the MSR. You will hear more about this throughout the presentation.
Before splitting off into our four respective groups, as a design team we brainstormed a list of goals which we wanted our MSR to fulfill.

7. Decision Goals Litmus Test Works on Moon and Mars 100 kWe 5 EFPY
Obeys Environmental Regulations
Extent-To-Which Test
Small Mass and Size
Controllable
Launchable/Accident Safe
High Reliability and Limited Maintenance
Other
Scalability
Uses Proven Technology
These goals were (list goals, and mention that by launchable we assumed a size of 5m x 5m cylinder). As you see, these goals are broken up into sections. These categories define the decision methodology which each system customized and applied to major decisions. The litmus test category are the goals that each technology chosen had to meet in order to be considered further. The Extent-to-which test provided a comparative measure for which to judge possible technologies by – the chosen technology was the one that best met all of these goals. It is important to mention here that three of the four goals by which we judged our options speak to specific areas of safety (list the three). Finally, scalability and “uses proven technology” are listed under other because, while they were not part of the formal decision methodology, they were considered when creating our MSR design.
Taking all these criteria and translating it into a robust, light and reliable reactor we get…

8. MSR System Overview Core
Fast, UN, Zr3Si2 reflector w/ drums, Hf vessel
Power Conversion Unit
Cs Thermionics, Heat Pipes, D-to-A
Radiator
Heat Pipe/Fin, 950K
Shielding
2mrem/hr, Neutron/Gamma
Total Mass ~8MT
… this stunning picture! Okay, this is actually just a quick concept sketch of our reactor system…we’re saving the really good pictures for the rest of the presentation…
The Core makes up 52% of the reactor by mass.: read specs and fast reactor = small, non-critical upon launch accidents, Li heat pipe connection (at 1800K)
PCU: read specs and highlight, lots of redundancy, no single point failures
Radiator: read specs and high temperature radiation = very light, lots of heat pipe = redundancy
Shielding: goal of 2 mrem/hr, two layer shadow shield to protect crew/instrumentation, inner neutron-attenuating layer of boron carbide, outer gamma-attenuating layer of tungsten.
The entire reactor weighs on the order of 8 metric tons. <pause> and now you have an idea of what the total system looks like, we can jump to the individual systems, starting with the Core (Shawn Gallagher)

9. CORE

10. Core – Challenges Fit on one rocket Autonomous control for 5 EFPY
High reliability
Safe in worst-case accident scenario
Provide 1.2 MWth

11. Core - Design Choices Overview
Give a quick run trough of the decisions made and why, you’ll discuss the more important ones in the next slides

12. Core - Pin Geometry Li heatpipe Re structure UN fuel
Fuel pins are the same size as the heat pipes and arranged in tricusp design.
Highest centerline fuel pin temperature at steady state is 1890K.
Li heatpipe
Re structure
UN fuel

13. Core – Design Advantages
UN fuel, Ta absorber, Re Clad/Structure performance at high temperatures, heat transfer, effect on neutron economy, and limited corrosion
Heat pipes no pumps required, excellent heat transfer, small system mass
Li working fluid operates at high temperatures necessary for power conversion unit

14. Core – Dimensions and Control
Small core, total mass ~4.3 MT
Top-Down View
Isometric View
Control Drum
Reflector
Core
Fuel Pin
Fuel
42 cm
88 cm
10 cm
Neutron Absorber

15. Core – Isotopic Composition

16. Core - Power Peaking

17. Operation over Lifetime
BOL keff: – 1.027
= 0.052
EOL keff: – 1.044
= 0.055

18. Launch Accident Analysis
Worst Case Scenario
Oceanic splashdown assuming
Non-deformed core
All heat pipes breached and flooded
Problem: Large gap between BOL keff and accident scenario

19. Accident Scenario: NatHf Vessel
Introduce absorbing vessel
High thermal cross-section
Low fast cross-section
Core (24 cm)
Vessel Segment (1 cm)

20. Launch Accident Results
Reflector Position
Stowed
Detached
Water
Keff=0.970
Keff=0.953
Wet Sand
Keff=0.974
Keff=0.965
Inadvertent criticality will not occur in conceivable splashdown scenarios

21. Accident Scenario Design Constraint
NASA requires reactor to be subcritical in all launch accident scenarios
Improvements
Higher enrichment
Flatten keff vs. lifetime plots
Less mass
Design freedom increases if this constraint is eased
This is not to criticize NASA’s safety policy which is certainly understandable, but rather to point out that there is room for improvement should this constraint be eased

22. Core Summary
Core has UN fuel, Re clad/tricusp, Ta absorber, Hf vessel, Zr3Si2 reflector and TaB2 control material
Relatively flat fuel pin temperature profile
5+ EFPY at 1.2 MWth, ~100 kWe,
Autonomous control by rotating drums over entire lifetime
Subcritical for worst-case accident scenario

23. PCU

24. PCU – Mission Statement
Goals:
Remove thermal energy from the core
Produce at least 100kWe
Deliver remaining thermal energy to the radiator
Convert electricity to a transmittable form
Components:
Heat Removal from Core
Power Conversion/Transmission System
Heat Exchanger/Interface with Radiator
Mike, when you go through this slide, make sure to include some kind of segway from the Core…say something like “the core produces 1MWth, now what?”

25. PCU – Design Choices Reactor Heat Transfer from Core Heat Pipes
Power Conversion System
Cesium Thermionics
Power Transmission
DC-to-AC conversion
22 AWG Cu wire transmission bus
Heat Exchanger to Radiator
Annular Heat Pipes
Reactor
When you go through this slide, talk about the good aspects of the system and how this make the MSR so robust, rather than just reading the slide or trying to explain what these things are, since you do that in later slides.

26. PCU – Heat Extraction from Core
How Heat Pipes Work
Isothermal heat transfer
Capillary action
Self-contained system
Heat Pipes from Core:
127 heat pipes
1 meter long
1 cm diameter
Niobium wall & wick
Pressurized Li working fluid, 1800K

27. PCU – Heat Pipes (2) Possible Limits to Flow Entrainment Sonic Limit
Boiling
Freezing
Capillary
Capillary force limits flow:
Define the important variables in this equation, and give the number for the capillary limit and what that number means compared to what flux we need, give the margin percent.

28. PCU - Thermionics Thermionic Power Conversion Unit Mass: 240 kg
Efficiency: 10%+
1.2MWt -> 125kWe
Power density:
10W/cm2
Surface area
per heat pipe:
100 cm2

29. PCU - Thermionics Issues & Solutions
Creep at high temp
Set spacing at 0.13 mm
Used ceramic spacers
Cs -> Ba conversion due to fast neutron flux
0.01% conversion expected over lifetime
Collector back current
TE = 1800K, TC = 950K

30. PCU – Thermionics Design

31. PCU – Power Transmission
D-to-A converter:
25 x 5kVA units
360kg total
Small
Transmission Lines:
AC transmission
25 x 22 AWG Cu wire bus
500kg/km total
Transformers increase voltage to 10,000V
~1.4MT total for conversion/transmission system

32. PCU – Heat Exchanger to Radiator
Heat Pipe Heat Exchanger

33. PCU – Failure Analysis Very robust system
Large design margins in all components
Failure of multiple parts still allows for ~90% power generation & full heat extraction from core
No possibility of single-point failure
Each component has at least 25 separate, redundant pieces
Maximum power loss due to one failure: 3%
Maximum cooling loss due to one failure: 1%

34. Radiator

35. Objective
Dissipate excess heat from a power plant located on the surface of the Moon or Mars.
2 Goals: Reject 900 kW and keep cold side of PCU at 950 K.
3 Ways to transfer thermal energy: convection, conduction, and radiation.
This is a difficult task because these are two very different environments, and both lack the qualities which make heat rejection simple on Earth (atmosphere and abundant liquid water).
Radiators are used on satellites, shuttle, and ISS. This radiation is in the infrared to low visible spectrum.
The radiative energy transfer equation here tells all…
T to the 4th dependence means high temperature is key, but as is low background
To minimize area (size and mass) for a given power maximize temp and emissivity.

36. Environment Moon 1/6 Earth gravity No atmosphere 1360 W/m2 solar flux
Mars
1/3 Earth gravity
1% atmospheric pressure
590 W/m2 solar flux
-The Moon has sky temp of about 0 K due to lack of atmosphere.
-Mars has sky temp of about 300 K. But still insignificant due to T^4

37. Radiator – Design Choices
Evolved from previous designs for space fission systems:
SNAP-2/10A
SAFE-400
SP-100
Radiates thermal energy into space via finned heat pipes
SNAP-10A was flown by NASA, used pumped NaK.
Settled on SP-100 and SAFE-400 based concept: finned heat pipes, Carbon-Carbon.
Chose this because of redundancy, low weight, few or no moving parts, no controls.

38. Concept Choices Heat pipes Continuous panel Carbon-Carbon composites
One-sided operation
Heat pipes are annular around PCU then curve up to panel 4cm OD annular, 2cm OD pipe.
Avg. heat pipe length of 6 m.

39. Component Design Heat pipes Panel Carbon-Carbon shell Nb-1Zr wick
Potassium fluid
Panel
Carbon-Carbon composite
SiC coating
Why these materials?
-High Thermal conductivity
-High Emissivity
-Corrosion
-Melting and Boiling points
-High Specific heat

40. Component Design (2) Supports Titanium beams 8 radial beams
1 spreader bar per radial beam
3 rectangular strips form circles inside the cone
Heat pipes not load bearing.

41. Structural Design Dimensions Mass Conical shell around the core
Height 3.34 m
Diameter 4.8 m
Area 41.5 m2
Mass
Panel 360 kg
Heat pipes 155 kg
Supports 50 kg
Total 565 kg
Like an Onion!
39 m^2 required
Added area gives 5% emissivity loss tolerance.

42. Analysis: Models Models Variables Isothermal Linear Condenser Power
Sky temperature
Emissivity
Panel width
940 K
25 m2
Isothermal model basically uses eq on first slide to predict area, T is uniform so pure T^4.
Isothermal prediction of area: 25 m^2

43. Analysis (2): Temperature
Condenser model
Calculate sensible and latent heat loss
Length dependent on flow rate
Temperature (K)
Length (m)
950
940 1 2
945 K
940 K
Condenser model takes into account:
-sensible heat loss
-isothermal condensation
-temperature coupling to radiation rate
15 m^2 sensible heat loss region (1/3)
26 m^2 isothermal condensing

44. Analysis (3): Dust Dust Buildup
SiO2 dust has small effect on Carbon-Carbon emissivity
5% emissivity loss predicted for 5 yrs
Emissivity loss increases required area by 2 m2
Shows base area requirement is 39.5 for Moon and 39 for Mars.

45. Shielding

46. Shielding - Design Concept
Natural dose rate on Moon & Mars is ~14 times higher than on Earth
Goal:
Reduce dose rate due to reactor to between mrem/hr
2mrem/hr
ALARA
Neutrons and gamma rays emitted, requiring two different modes of attenuation

47. Shielding - Constraints
Weight limited by landing module (~2 MT)
Temperature limited by material properties (1800K)
Courtesy of Jet Propulsion Laboratory
Weight constraint due to lander capability
Temperature constraint due to reactor operating temperature
Want to plan for thermal contact with 1800 degree core even if not touching by design

48. Shielding - Design Choices
Neutron shielding Gamma shielding
boron carbide (B4C) shell (yellow) Tungsten (W) shell (gray)
40 cm cm
Total mass is 1.97 MT
Separate reactor from habitat
Dose rate decreases as
1/r2 for r >> 50cm
- For r ~ 50 cm, dose rate decreases as 1/r
These are the two choices by design team;
Explain layering, two pieces; reason for choices later.

49. Shielding - Dose w/o shielding
Near core, dose rates can be very high
Most important components are gamma and neutron radiation
Dose (mrem/hr) 20 10
This is why we have to shield at all.
Closer to core reaches 232 mrem/hr 2
Distance (m)
500
1000
2000

50. Shielding - Neutrons
Boron Carbide (B4C) was chosen as the neutron shielding material after ruling out several options
Material
Reason for Rejection
H2O
Too heavy Li
Too reactive
LiH
Melting point of 953 K B
Brittle; would not tolerate launch well
B4CAl
Possible; heavy, needs comparison w/ other options
Go through each material.

51. Shielding - Neutrons (3)
Boron burnup is a resolvable issue
Explain boron burnup.

52. Shielding - Gammas
Tungsten (W) was chosen as the gamma shielding material after ruing out several options
Material
Reason for Rejection
Z < 72
Too small density and/or mass attenuation coefficient
Z > 83
Unstable nuclei
Pb, Bi
Not as good as tungsten, and have low melting points
Re, Ir
Difficult to obtain in large quantities Os
Reactive
Hf, Ta
Smaller mass attenuation coefficients than alternatives
Re, Ir, Os are variation on the same theme.
Pb, Bi, W will be considered next.

53. Shielding - Design
Two pieces, each covering 40º of reactor radial surface
Two layers: 40 cm B4C (yellow) on inside, 12 cm W (gray) outside
Scalable
at 200 kW(e) mass is 2.19 metric tons
at 50 kW(e), mass is 1.78 metric tons
Describe geometry
Explanation for layers
Explanation for fraction of surface cover
Scalability:
@ 200 kW(e) use 43.2 cm B4C and 12.9 cm W (mass = 2.19 metric tons)
@ 50 kW(e) use 36.8 B4C and 11.2 cm W (mass = 1.78 metric tons)

54. Shielding - Design (2)
For mission parameters, pieces of shield will move
Moves once to align shield with habitat
May move again to protect crew who need to enter otherwise unshielded zones

55. Shielding - Design (3) core
Using a shadow shield requires implementation
of exclusion zones:
Unshielded Side:
32 rem/hr - 14 m
2.0 mrem/hr m
0.6 mrem/hr m
Shielded Side:
32 rem/hr - inside shield
400 mrem/hr – at shield
boundary
2.0 mrem/hr - 11 m
0.6 mrem/hr - 20 m
core

56. Shielding – Future Extensions
Three layer shield
W (thick layer) inside, B4C middle, W (thin layer) outside
Thin W layer will stop secondary radiation in B4C shield
Putting thick W layer inside reduces overall mass

57. Shielding – Future Extensions (2)
Distance (m) 40 10 60 80
100
Thickness (cm)
The Moon and Mars have very similar attenuation coefficients for surface material
Would require moving 32 metric tons of rock before reactor is started

58. Summary

59. MSR Assembly Sketches

60. MSR Assembly Sketches (2)

61. MSR Mass
Bottom Line: ~82g/We

62. Mass Reduction and Power Gain
Move reactor 2km away from people
Gain 500kg from extra transmission lines
Lose almost 2MT of shielding
Use ISRU plant as a thermal sink
Gain potentially 900kWth
Gain mass of heat pipes to transport heat to ISRU (depends on the distance of reactor from plant)
Lose 565kg of Radiator Mass
Bottom Line: ~67 g/We

63. MSR Mission Plan Build and Launch Prove Technology on Earth
Earth Orbit Testing
Ensure the system will function for ≥ 5EFPY
Lunar / Martian Landing and Testing
Post Landing Diagnostics
Startup
Shutdown

64. MSR Group Expanding Frontiers with Nuclear Technology
“The fascination generated by further exploration will inspire…and create a new generation of innovators and pioneers.”
~President George W. Bush

65. Additional Slides

66. Future Work – Core
Investigate further the feasibility of plate fuel element design
Optimize tricusp core configuration
Examine long-term effects of high radiation environment on chosen materials

67. MSR Fuel Pin Sketch

68. MSR Core Top View
This slide can be used to explain how the core might be ejected in case of a launch accident. The hole on the top of the radiator allow sufficient room to pull just the core and vessel, no reflector, out if needed.

69. 180Hf Cross Section

70. NatHf Cross Section

71. Radiological Hazard Mitigation
Hazard During
Mitigation Strategy
Pre-Launch and Launch
Operational Procedures, Appropriate facilities and equipment
Orbital Diagnostic Testing
Use of Nuclear Safe Orbit
Reentry Dispersal of Inventory
Inventory Reactivity Contribution Was Determined to be Negligible
Splashdown Inadvertent Criticality
Hafnium Core Vessel
Radiological Hazards need not limit utilization of nuclear power in space!

72. PCU – Future Work Improving Thermionic Efficiency
Material studies in high radiation environment
Scalability to 200kWe and up
Using ISRU as thermal heat sink

73. PCU – Decision Methodology
Brayton
Sterling
Thermionics
Small Mass and Size (Cost)
Actual PCU 2 1 3
Outlet Temperature
Peripheral Systems (i.e. Heat Exchangers, A to D converter)
Launchable/Accident Safe
Robust to forces of launch
Fits in rocket
Controllable
High Reliability and Limited Maintenance
Moving Parts
Radiation Resistant
Single Point Failure
Proven System
Inlet Temperature
Total
23.77
26.55
28.51

74. Solid-State vs. Dynamic PCU
Stirling Assumptions:
At 100kWe, operating at 10% efficiency
4MT net gain in PCU
Heat exchanger
4x 800kg engines
1MT gain in radiator
Scales with gain in efficiency, also gains radiator mass.

75. Solid State v. Dynamic (2)
Thermionics
Sterling
100 kWe
8.2
13.2
200 kWe
14.4
14.2
300 kWe 21
16.2
400 kWe
28.8
20.2
1 MWe
70.8
54.2

76. Radiator Future Work Analysis of transients Model heat pipe operation
Conditions at landing site
Manufacturing
Transients to model:
-startup with thawing
-day/night heating
-reactor power changes
Heat Pipe operation:
Flow rate
2nd sensible transfer side
Power limitations
Conditions:
Sun
Air
Soil
Mountains/Walls
Manufacture:
Better cost
CCC possibilities (thermal conductivity, emissivity, strength, corrosion)

77. Analysis (3): Location Environment Comparative Area
Moon closer to sun, but has no atmosphere
Comparative Area
Moon 39.5 m2
Mars 39 m2
Turns out sky temperature neglidgeable, so really function of power (extra kilowatt)

78. Shielding - Future Work
Shielding using extraterrestrial surface material:
On moon, select craters that are navigable and of appropriate size
Incorporate precision landing capability
On Mars, specify a burial technique as craters are less prevalent
Specify geometry dependent upon mission parameters
Shielding modularity, adaptability, etc.

79. Radiation Interactions with Matter
Charged Particles (α, β)
Easily attenuated
Will not get past core reflector
Neutrons
Most biologically hazardous
Interacts with target nuclei
Low Z material needed
Gamma Rays (Photons)
High Energy (2MeV)
Hardest to attenuate
Interacts with orbital electrons and nuclei
High Z materials needed
Here is where you can give a brief intro into the wonderful world of shielding by talking about what kind of radiation our reactor emits, and the which we are worried about: gamma = hardest to attenuate, neutron=most biologically damaging. Also, make sure you define attenuation here and low Z (low atomic mass).

80. Shielding - Dose Modeling
Dose distance dependence is modeled as:
r0/r for r < 2 r0
A* (r0/r) + B * (r0/r)2 for 2 r0 < r < 10 r0
(r0/r)2 for r > 10 r0
where r0 = core radius,
A = -1/8 and B = 9/4

81. Shielding - Neutrons (2)
Material
Maximum thickness for 1 MT shield (cm)
Dose Rate at shielding edge (mrem/hr)
Dose rate at 10 m (mrem/hr)
Unshielded
N/A
2.40*107
10.29
Boral (B4CAl)
20.3
1.24*105
0.1242
Borated Graphite (BC)
22.7
4.28*104
0.0427
Boron Carbide (B4C)
21.1
3.57*104
0.0357
Emphasize lower right number and how it is smaller than the rest

82. Maximum thickness for 3 MT shield (cm) Dose rate at 90 cm (mrem/hr)
Shielding - Gammas (2)
Material
Maximum thickness for 3 MT shield (cm)
Dose rate at 90 cm (mrem/hr)
Unshielded
N/A
2.51*106 Pb
17.1
594.66 Bi
19.4
599.88 W
10.6
480.00
Emphasize how W is much more effective than other materials

83. MSR Cost Rhenium Procurement and Manufacture Nitrogen-15 Enrichment
Hafnium Procurement

84. Mass Comparison MSR (100kWe) SNAP-10A (650We) SP-100 SAFE-400
Specific Mass (g/We) 82
670 54 25
Mass breakdown for SAFE: Core (97% enriched)= 500kg
Shielding (4cm W, Li) = 400
Radiator (300kWth at 500K) = 800
PCU (including heat exchanger 72kg X2) = 800kg