A Dusty Plasma Based Fission Fragment Nuclear ReactorRocket

A Dusty Plasma Based Fission Fragment Nuclear Reactor/Rocket Rodney Clark, Robert Sheldon, Robert Werka National Space Science & Technology Center Grassmere Dynamics, LLC NASA/MSFC NETS 2013 Feb 27, 2013

The Rocket Equation Vexhaust= Isp * g [d/dt(MV) = 0] ∆V = Isp * g * log( final mass / initial mass) Material Isp Limitation Solid fuel LH 2/LOX Nuclear Thermal Gas Core Nuclear MHD Ion 200 -250 350 -450 825 -925 ~2, 000 < 5, 000 < 10, 000 fuel-starved fuel starved energy-starved Fission Fragment ~1, 000 fuel-starved Matter-Antimatter Photons ~10, 000 fuel-starved 30, 000 - both-starved

Ideal Rockets

JPL Nuclear-Electric Concept Shielding, Fuel Shield shadow terminator Reactor Power Lines, Coolant tubes Cooling Fins Ion Thrusters Instruments

Nuclear-Electric to Mars One way trip time=922 Days! Thrust is powerlimited. Why can't we get more thrust? Not enough electricity.

The Problem of Energy Conversion 185 Kwe/1 MWth = 18% efficiency @1150 K! Must reject 82% energy as low-grade heat.

Carnot-cycle Engine Nuclear Reactor tied to a Gas-Brayton cycle generator Efficiency is no better than Carnot ~ 22% at T = 1050 K Higher efficiency requires advanced materials for higher T Reactor can run at higher power, but waste heat rejection=radiator mass.

Heat: The hidden killer The problem with space nuclear propulsion is NOT raw power, but how to eliminate waste heat. The more efficiently we can generate thrust, the less waste heat produced. Fission fragments can escape <1 micron U 235 dust without heating the grains much. The dust radiates heat very effectively, permitting high power levels.

Hot Fibers, Cool Dust 1µm Cs 137 U 235 Sr 90

Max Radiated Power in a DPFFRE with 1µ-dusty UN fuel (3080 K MP) Kg Fuel

What is a dusty plasma? Charged dust + plasma = a “plum pudding” Coulomb crystal, or as Cooper-pairs in BCS theory. Note surface tension & crystalline interaction. Iowa Auburn MSFC Dusty plasmas are held together by electrical charges in a neutralizing fluid (plasma), as in the Iowa groups experimental work in dusty plasmas. The plasma in turn may be influenced by magnetic fields, as in the Auburn groups current-stabilized levitation; or static magnetic fields (MSFC).

UN Fuel processing Dusty UN fuel is readily available, remanufactured during the Prometheus era.

Schematic of DPFFRE Fission fuel ~1µ dust, Dust radiatively cools Moderator reflects & moderates neutrons, C-C heat shield w/mirror finish Superconducting magnets direct FF, Electric power from hot wall coolant, Reactor hole allows FF to escape.

Re-entrant Nuclear Reactor Low density fuel requires that neutrons be repeatedly reflected through the engine—reentrant. Neutrons are thermal—moderated. Moderator must have low neutron capture cross-section—D, Be, C 13. 300 x Moderator blanket thick enough to contain the neutrons— ~1 meter. Moderator blanket must have minimal “holes” that leak neutrons (FF escape problem!) Spherical geometry is always best.

The Promise of DPFFRE Since the dusty fuel does not need to maintain mechanical integrity, it doesn't limit the age of the reactor. It is also replaced frequently depending on burn rate, since charge state of dust depends on fissioning fuel, spent fuel will be expelled. Solves lifetime limit.

Direct Conversion of FF to Power This is a tested concept for converting FF to electricity without the need for a thermal cycle and Carnot efficiencies. FF efficiency >95% possible

Fission Surface Power This is a 15 k. We lunar surface installation, or about 100 k. Wth. What is the electric power generated by DPFFR with the same radiators, and without any moving parts or high-temperature alloys? 480 k. We. That's 30 times better.

st 1 Gen FFRE Design “Pancake” dust design requires mirroring B-field Neutronically thick in 2 -D, FF thin in 1 -D Relatively compact, can fit on a single SLS launch

Callisto Manned Mission Concept

Callisto Mission Specifics

Mission Performance, Trajectories

The Next Step: Lighting the Afterburner By injecting gas into the Fission Fragment Beam, we can have any ISP we want, from 500 s to 2 Ms at constant power > 1 GWt. We call this “the afterburner” and makes all lower Isp rockets niche players.

Mf/Mi Comparison Missions mission rocket LH 2/LO X Xe Ion FF Grav Lens 10 yr 1. 2 e 4 72 1. 029 Oort Centaur Cld i 550 yr 30 yr e(2222) e(1066 6) 2. 69 e 43 2. 9 e 208 1. 95 24

Conclusions Dusty Plasma Fission Fragment Reactors are capable of generating 10's to 100's of GWth because they radiatively self-cool. The FF can be used as exhaust with ISP > 500, 000 which enables missions throughout the solar system. The FF can be used to generate direct electrical power, which can increase surface power installations by a factor of 30 for the same size w/o moving parts. The greatest mass of a DPFFRE is the moderator, followed by the magnet assembly. Current estimates are around 200 T dry mass= 2 SLS launches.

Extra Slides

Astronautics 1990: NJPE Russian 1990 patent on a nuclear jet engine. Design has some advantages over the dust pancake, and we are evaluating the geometry. Tends to be much bigger than the pancake, so it may be better for surface power systems. http: //www. sciteclibrary. ru/eng/catalog/pages/432. htm

IR to electricity conversion http: //dx. doi. org/10. 1103/Phys. Rev. Lett. 110. 074801 Triple-photon conversion of IR laser pulse to electrons in typical FEL (free electron lasers). Efficiency only ~2%, but could rival SS thermoelectric devices.

Mission to the Gravitational Lens at 550 AU Assume we accelerate half-way, decelerate the other half. (Not the most intelligent approach, but good for comparing technologies) so T_trip = 10 years. 2 2 Acceleration = 550 AU / (5 yr) = V / 5 yr=. 0027 m/s So V = 425, 000 m/s Isp (m/s/10) Mrocket / Mpayload 1, 500, 000 1. 029 1, 000 1. 04 500, 000 1. 09 MORAL of Story: 100, 000 1. 5 V ~ V_exhaust 10, 000 70. 6 450 1. 2 e 41

Fuel Fibers Fuel coated micron-thick fibers, emit >50% of fission fragments away from fiber. Fragments can be directed out of the system as propellant. Since 90% of energy is in fission fragments, then <55% energy is wasted as heat. Still, fibers get hot. Carbon fiber

Chapline’s Fission Fragment Rocket Magnetic yoke Moderator & magnet coils U 235 coated micron-thick spoke-fibers rotating fast Fission-fragment exhaust

Tsvetkov & Hart, Texas A&M

Enabled Missions

Dust Clouds Since we need a total amount of U 235 to achieve criticality, how do we collect enough dust grains without heating them? Organization.

Fragment Confinement

Toroidal Multipole Magnetic Trap

Dust suspension FAQs Can the dust be suspended while the rocket is accelerating? Yes, 1 g is typically no problem for labs. Will B-field change the dusty-plasma dynamics? Yes, but not much.

Terrella Lab ( NSSTC)

Levitated Dusty Plasma w/Magnets

The Dust Trap • Arc discharge on 3μ Si. O 2 dust grains charges them negative. Probable charge state on dust is – 10, 000 e/grain. • They are trapped in a positive space-charge region adjacent to ring current. The RC is formed by -400 V DC glow discharge on NIB magnet, streaming electrons ionize the air, maintain the RC. Phasespace mismatch of streaming electrons and trapped ions produces the space charge. Highly anisotropic B -field contributes as well.

Langmuir Probe mapping

Discharging Dust Won’t negatively charged dust discharge from thermionic emission? And won’t 100 nm dust have huge corona discharge current? Yes, but not as much as one might think.

Discharge vs Dust Size

Photoelectrons vs. size

Nuclear Pollution? Since radioactive fission fragments are emitted from the rocket, how dangerous is this for the Earth? From the two missions analyzed, we calculated how long each rocket is withing 10 Re of the earth, and how much fuel is burned during this time. 550 AU mission = 720 g U 235 = 3 moles 0. 5 Lightyr mission=3. 7 kg U 235 = 15 moles We modelled the transport through the radiation belts, ionosphere & stratosphere and decay lifetimes of 60 decay products. Short-halflife products decay before reaching the surface of earth. Long-halflife products produce almost no radioactivity. We list radioactive products that make it to Earth from 10 moles U 235, both by number and curies.

Modelled Pollution from 10 moles U 235/P 239 By moles (total radioactivity ~10% of U 235) Rb 87 Sr 90 Cs 135 Cs 137. 3 Nd 144 . 1. 2. 3 = 1 u. Cu =1800 Cu = 4 m. Cu =3600 Cu. 05 =. 01 n. Cu By Curies fast diff Sr 90 Ru 108* Cs 137 Ce 144 Pm 147* 1800 204 3600 1900 2300 slow diffusion 1800 110 3600 770 930 Cosmic Ray production C 14 = 266 Cu/yr

Next Steps 1. Understand the equilibrium charge state of radioactive dust (generalize the dusty plasma equations) 2. Demonstrate the multipole confinement of fission fragments, using alpha-decay as a proxy 3. Push dusty plasma confinement to high vacuum 4. Test the bimodal, electric power generation with alpha-emitters. (Nuclear batteries) 5. Neutronics for Li. H, cooling ducts, etc.

Conclusions An interstellar probe can be accomplished with a nuclear fission-fragment-fusion rocket as well as 550 AU gravitational lens or 1 Lyr Oort Cloud missions. We chose these missions to illustrate how close the fission fragment rocket comes to the stuff of science fiction but using the materials found already at hand. TRL~2 For example, 550 AU is very promising. At 350 MW, the rocket is still 1/10 of Nerva power, and could accomplish an even shorter mission than 10 yr (or bigger payload than 1 ton. ) Nor is pollution a real problem. Therefore high- V missions are enabled by a promising high-efficiency nuclear technology. Total mass < 120 T, could be launched by a single SLS.