Nuclear Engineering Department Massachusetts Institute of Technology M artian S urface R eactor Group Martian Surface Reactor Group December 3, 2004

Nuclear Engineering Department

Massachusetts Institute of Technology

Martian Surface Reactor Group

Martian Surface Reactor GroupDecember 3, 2004

MSR Group, 12/3/2004Slide 2



Motivation for Mars

• “We need to see and examine and touch for ourselves”

• “…and create a

new generation of

innovators and pioneers.”




Motivation for MSR

10/15/2004 Slide 1Competition Sensitive – Do not distribute outside of NASA / Draper / MIT team

Bill [email protected]

NASA Concept Exploration and Refinement Study

Surface Power

Lunar Surface Power Options

Fission Reactor

Fission Reactor + Solar

Radioisotope + Solar

Solar

Chemical

Duration of use

Ele

ctric

Po

we

r L

eve

l (kW

e)

Martian Surface Power Options

- Solar power becomes much less feasible

- Mars further from Sun(45% less power)

- Day/night cycle- Dust storms- Too-short Lifetime for

Martian missions

- Nuclear Power dominates curve for Martian missions.




Nuclear Physics/Engineering 101




Nuclear Physics/Engineering 101




Goals

• Litmus Test– Works on Moon and

Mars– 100 kWe – 5 EFPY – Obeys Environmental

Regulations– Safe

• Extent-To-Which Test– Small Mass and Size– Controllable – Launchable/Accident

Safe– High Reliability and

Limited Maintenance

– Scalability




MSR Components

• Core

– Nuclear Components, Heat

• Power Conversion Unit

– Electricity, Heat Exchange

• Radiator

– Waste Heat Rejection

• Shielding

– Radiation Protection




MSR Mission

• Nuclear Power for the Martian Surface

– Test on Lunar Surface

• Design characteristics of MSR

– Safe and Reliable

– Light and Compact

– Launchable and Accident Resistant

– Environmentally Friendly




Proposed Mission Architecture

Habitat Reactor




CORE




Core - Design Concept

• Design criteria:

– 100 kWe reactor on one rocket

– 5 EFPY

– Low mass

– Safely Launchable

– Maintenance Free and Reliable




Core - Design Choices

• Fast Spectrum, High Temp• Ceramic Fuel – Uranium

Nitride, 33.1 w/o enriched

• Lithium Heatpipe Coolant• Tantalum Burnable Poison• Hafnium Core Vessel• External Control By Drums

• Zr3Si2 Reflector material

• TaB2 Control material

• Fuel Pin Elements in tricusp configuration




Heatpipe Fuel Pin

Tricusp Material

Core - Pin Geometry

• Fuel pins are the same size as the heat pipes and arranged in tricusp design.




Core – Design Advantages

• UN fuel, Ta poison, Re Clad/Structure high melting point, heat transfer, neutronics performance, and limited corrosion

• Heat pipes no pumps, excellent heat transfer, reduce system mass.

• Li working fluid operates at high temperatures necessary for power conversion unit (1800 K)




Core – Reactivity Control

• Reflector controls neutron leakage• Control drums add little mass to the system and offer high

reliability due to few moving parts

Radial Reflector

Control Drum

Reflector and Core Top-Down View

Reflector

Core

Fuel Pin

Fuel

Reflector

Zr3Si2 ReflectorTotal Mass:

2654kg

42 cm

88 cm

10 cm

10 cmReflector




Core - Smear Composition

Material Purpose Volume Fraction7Li Coolant 0.07275946915N Fuel Compound 0.353381879

NatNb Heatpipe 0.076032901181Ta Poison 0.037550786NatRe Cladding/Structure 0.110216571235U Fissile Fuel 0.116593812238U Fertile Fuel 0.233464583




Core - Power PeakingPeaking Factor, F(r)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

-22 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22

Core Radius (cm)

F(r

)

31.1InDrumsRPPR 24.1OutDrumsRPPF




Operation over Lifetime

keff over Core Lifetime

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

0 1 2 3 4 5 6

Years at Full Power

keff

BOL keff: 0.975 – 1.027

EOL keff: 0.989 – 1.044

effk = 0.052

effk = 0.055




Launch Accident Analysis

• Worst Case Scenarios:– Oceanic splashdown

assuming• Non-deformed core • All heat pipes

breached and flooded

– Dispersion of all 235U inventory in atmosphere




Launch Accident Results

Reflectors Stowed Reflectors Detached

Water Keff=0.97081±0.00092 Keff=0.95343±0.00109

Wet Sand Keff=0.97387±0.00095 Keff=0.96458±0.00099

•Total dispersion of 235U inventory (104 kg) will increase natural background radiation by 0.024%•Inadvertent criticality will not occur in any conceivable splashdown scenario




Core Summary

•UN fuel, Re clad/structure , Ta poison, Hf vessel, Zr3Si2 reflector

•Low burn, 5+ EFPY at 1 MWth, ~100 kWe,

•Autonomous control by rotating drums over entire lifetime

•Flat temperature profile at 1800K

•Subcritical for worst-case accident scenario




Future Work

• Investigate further the feasibility of plate fuel element design

• Optimize tricusp core configuration

• Examine long-term effects of high radiation environment on chosen materials




PCU




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




PCU – Design Choices

• 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




PCU – Heat Extraction from Core

• Heat Pipes from Core:

– 127 heat pipes

– 1 meter long

– 1 cm diameter

– Rh wall & wick

– 400 mesh wick

– 0.5mm annulus for liquid

– Li working fluid is Pressurized to boil @ 1800K




PCU – Heat Pipes (2)

• Possible Limits to Flow– Entrainment– Sonic Limit– Boiling– Freezing– Capillary

• Capillary force limits flow:

Qmax

llL K A

w

21re

lg l ( )sin

l

ll




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




PCU – Thermionics Design




PCU - Thermionics Issues & Solutions

– Creep @ high temp

• Set spacing at 5 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

134134133 ),( BaCsnCs




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• 83.5:1 Transformers increase voltage to 10,000V

– 1,385kg total for conversion/transmission system




PCU – Heat Exchanger to Radiator

• Heat Pipe Heat Exchanger




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 – 4%

• Maximum cooling loss due to one failure – 0.79%




PCU – Future Work

• Improving Thermionic Efficiency

• Material studies in high radiation environment

• Scalability to 200kWe and up

• Using ISRU as thermal heat sink




Radiator




Objective

• Design a radiator to dissipate excess heat from a nuclear power plant located on the surface of the Moon or Mars.

44s TTσεAQ




Goals

• Small mass and area

• Minimal control requirement

• Maximum accident safety

• High reliability




Decision Methodology

• Past Concepts

– SP-100– SAFE-400– SNAP-2– SNAP-10A– Helium-fed– Liquid Droplet– Liquid Sheet




Component Design

• Heat pipes– Carbon-Carbon shell– Nb-1Zr wick– Potassium fluid

• Panel– Carbon-Carbon

composite– SiC coating

Fin

Heat Pipe

3 mm

Thermal Radiation

5 mm

Wick Vapor Channel




Structural Design

• Dimensions– Height 3 m

– Diameter 4.8 m

– Area 39 m2

• Mass– Panel 200 kg

– Heat pipes 60 kg

– Supports 46 kg

Core & Shield

2.0 m

0.5 m 0.5 m

0.60 m

2.4 m

2.54 m

Radiator Panel

Heat Pipe

1.45 m




Support

• Supports– Titanium– 8 radial beams– 3 circular strips

Core

Radiator Panel

1.5 m

1.25 m

Support Anchor 6.54°

Strips




Analysis

• Models– Isothermal– Linear Condenser

Radiating Area needed to Reject 900 kW

0

10

20

30

40

50

60

70

80

700 800 900 1000 1100 1200 1300 1400

Radiator Temperature (K)

Rad

iato

r A

rea

(m2 )

• Comparative Area– Moon 39 m2

– Mars 38.6 m2




Future

• Analysis of transients

• Model heat pipe operation

• Conditions at landing site

• Manufacturing



Martian Surface Reactor Group

Shielding




Shielding - Design Concept

• Dose rate on Moon & Mars is ~14 times higher than on Earth

• Goal: – Reduce dose rate to

between 0.6 - 5.7 mrem/hr

• Neutrons and gamma rays emitted, requiring two different modes of attenuation




Shielding - Constraints

• Weight limited by landing module (~2 MT)• Temperature limited by material properties (1800K)

Courtesy of Jet P

ropulsion Laboratory




Shielding - Design Choices

Neutron shielding Gamma shieldingboron carbide (B4C) shell (yellow) Tungsten (W) shell (gray)

40 cm 12 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




Shielding - Dose w/o shielding

• Near core, dose rates can be very high• Most important component is gamma radiation




Shielding - Neutrons

Material Reason for Rejection

Z > 5 Inefficient neutron slowing; too heavy

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




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 26.8*103

Boral (B4CAl) 20.3 1.24*105 323.2

Borated Graphite (BC)

22.7 4.28*104 111.1

Boron Carbide (B4C)

21.1 3.57*104 92.9




Shielding - Neutrons (3)

• One disadvantage: boron will be consumedDose vs. Distance at t=0 and t=5 years

0

2

4

6

8

10

12

14

16

18

20

5 10 15 20 25 50 75 100 125 150 175 200 225 250

meters

mre

m/h

r

Dose after 5years ofoperationDose at 0 yearsof operation




Shielding - Gammas

Material Reason for Rejection

Z < 72 Too small density and/or mass attenuation coefficient

Z > 83 Unstable nuclei

Pb, Bi Possible, but have low melting points

Re, Ir Difficult to obtain in large quantities

Os Reactive

Hf, Ta Smaller mass attenuation coefficients than alternatives




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




Shielding - Gammas (3)

• Depending on mission parameters, exclusion zone can be implemented




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– @ 200 kW(e) mass is

2.19 metric tons– @ 50 kW(e), mass is

1.78 metric tons




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




Shielding - Alternative Designs

• 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




Shielding -Alternative Designs (2)

-The Moon and Mars have very similar attenuation coefficients for surface material

-Would require moving 32 metric tons of rock before reactor is started




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.




MSR Assembly Sketch

• Breathtaking Figures Goes Here




MSR Mass

Component Mass (kg)Core 4310PCU 1385Radiator 306Shielding 1975Total 7976

Bottom Line: ~80g/We




MSR Cost

• Rhenium Procurement and Manufacture

• Nitrogen-15 Enrichment

• Halfnium Procurement




Mass Reduction

• Move reactor 2km away from people

– Gain 500kg from extra transmission lines

– Loose almost 2MT of shielding

• Use ISRU plant as a thermal sink

– Gain mass of heat pipes to transport heat to ISRU (depends on the distance of reactor from plant)

– Loose 360kg of Radiator Mass

Bottom Line: ~60g/We




MRS Design Advantages

• Robustness

• Safety

• Redundancy

• Scalability




MSR Mission Plan

• Build and Launch

– Prove Technology on Earth

• Earth Orbit Testing

– Post Launch Diagnostics

• Lunar / Martian Landing and Testing

– Post Landing Diagnostics

• Startup

• Shutdown




MSR GroupExpanding 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

Documents

Nuclear Engineering Department Massachusetts Institute of Technology M artian S urface R eactor Group Martian Surface Reactor Group December 3, 2004