NTP 101 Short Course

An Introductory Overview of Nuclear Science and Space Nuclear Power and Propulsion Systems ENERGY COMPARISON

 Chemical: combustion, reaction  Natural: solar (PV, thermal), EM tethers  Nuclear: radioactive decay, fission  Advanced nuclear: fusion, antimatter

Process Maturity Reaction Reaction Specific Specific Energy Energy Cost (eV) (J/kg) ($/kg)

6 -1 0 Combustion Proven CH4 + 2O2 → CO2 + 2H2O41010 –10 (CH4) -1 10 (O2) 7 0 Fuel Cell Proven 2H2 + O2 → 2H2O 10.2 10 10 (H2) -1 10 (O2) 6 12 6 Radioisotope Proven → 5.59 x 10 10 10 6 13 4 Fission Proven → 2 + γ 195 x 10 10 10 (XF > 93%) 6 14 3 Fusion Research → 17.57 x 10 10 10 (D) 6 6 3 → 18.35 x 10 10 ( He) Antimatter Research p + p- → γ 1.05 x 109 1016 1012 NUCLEAR = MISSION ENABLING

L = 46.9 m OD = 8.4 m Fission 341 mL of 235U = 50x MLOX = 630 MT

MH2 = 106 MT

Significantly extends mission capability by overcoming current technology limitations:  Power Reliable, robust, long-duration, power dense  Logistics (consumables) – Food, water, oxygen Time dependant  Human Factors factors mitigated by – μg atrophy rapid propulsion decreasing transit – Psychological isolation Shielding decreases – Radiation dose crew dose RADIOSIOTOPE DECAY

U-234  Heat produced from natural alpha (α) decay 5.6 MeV

Pu-238  (He-4)

238Pu Decay

 Service life activity (A) inversely proportional to

isotope half-life (t1/2)

/

Radioisotope Half-life Comparison RADIOISOTOPE POWER SYSTEMS

 General Purpose Heat Source (GPHS) Nuclide wt % 238 Fuel PuO2 Pu 83.6 239 Power 250 Wth (BOM) Pu 14.0 Mass 1.44 kg 240Pu 2.0

238 241 PuO2 Pellet. Size 9.3x9.7x5.3 cm Pu 0.4 Courtesy Department 242Pu 0.1 of Energy.

GPHS Stack. Courtesy Department of Energy.

GPHS-Radioisotope Thermoelectric Generator Multi-Mission RTG (MMRTG). Advanced Stirling Radioisotope Generator (RTG). Courtesy Department of Energy. Courtesy Department of Energy (ASRG). Courtesy Lockheed Martin

Power (We) 290 120 120 Efficiency (%) 6.6 6 24 GPHS Modules 16 8 2 Mass (kg) 54.4 43 27.1 Conversion SiGe Thermoelectrics SiGe Thermoelectrics Stirling Convertors Galileo x 2, Cassini x 3 Curiosity x 1 Mission Development Ulysses x 1, New Horizons x 1 2016 Mars Rover x 1 Why?

NTP = Mission Enabling

Isp = 880-900 seconds Leverage existing engine experience

“To extend and sustain human activities beyond LEO, rapid crew transit is required.”

- NASA STMD Technology Roadmap NUCLEAR THERMAL PROPULSION

 Conventional rockets utilize propellant & oxidizer

 NTP reactor uses nuclear fission to heat LH2 propellant U-235

 Neutron Neutrons ( 2.2) 180 MeV*

Fissile Nucleus  (U-235) Product Nuclei U-235 (KE 168 MeV)

NTP system diagram. Courtesy.

 Steady state full power density: MW/L

ROVER/NERVA (1955-1973): KIWI, NRX, PEWEE, NF, PHOEBUS MISSION DESIGN & THRUST CLASS

. Human requirements drive design

− 3 engine cluster, total 75-105 klbf

. Engine Thrust Class − NTP T/W not linear with engine size

. Total burn time − Lower with higher thrust engine − Impact engine duty cycle and reliability

NTP Thrust-to-Weight vs. Thrust. Courtesy CSNR. . NTP cost not linear with thrust − Majority of cost in fuel development − Smaller engine size = negligible cost Engine Burns Total Burn Total Burn impact Thrust (no.) time 2033 time 2033 − Subscale flight demos cannot be used (klb ) (min) (min) to meet human rating requirements 25 4 101 92 − A second engine will have to be designed and drive up costs 35 4 73 59 Courtesy L. Koss PARALLEL DEVELOPMENT PLAN

DU manufacture, characterization, DU property LEU coupon LEU coupon LEU fuel element LEU fuel element test (coupon & measurement manufacture irradiation manufacture irradiation

FUEL fuel element)

Power balance Power Balance Initial Power using low-fidelity using high-fidelity Full- Balance Full- empirical data empirical data scale non- scale nuclear nuclear Component, design, Component, design, Component, design, cold flow engine analysis, manufacture analysis, manufacture analysis, manufacture system test Low fidelity separate High fidelity separate High fidelity combined test ENGINE SYSTEM ENGINE effects tests effects tests effects tests

Prototypic Geometry Initial Zero Power Critical ZPC vs. Temperature ZPC PHYSICS REACTOR REACTOR

TRL TRL TRL TRL 3 4 5 6 ENGINE COMPONENTS

. Engine Systems − Reactor Neutronic Analysis − Power Balance . Reactor − Fuel − Tie-tubes − Reflector Rings & Control Drums − Injectifold − Bottom plate Neutronic Model Parametric Model − Internal Shield . Thrust Chamber Assembly − Pressure Vessel − Regen Nozzle & Nozzle Extension . Turbopump . Propulsion Stage − Lines, Ducts, Valves − Thrust Vector Control − Controller Integrated Component Models − Distance truss 155.7 kN (35 lbf) x 3 FUEL DEVELOPMENT

W coated ZrO2 (SEM, 2500x) Spark Plasma Sinter Sol-Gel dUO2 Powders (SEM, 700x)

CVD System

Chemical Etch System

61 Channel Hex HIP Can HIP Furnace Grinder Fuel Sample FUEL DEVELOPMENT

Sample heated to 2900 K

Compact Fuel Element Environmental Tester Nuclear Thermal Rocket Element Environmental Simulator (NTREES)

University & DOE Reactor Facilities Radiation Interactions THE ATOM

 The Atomic Model () – Atomic number (Z): number of protons in the nucleus. – Neutron number (N): number of neutrons in the nucleus. – Mass number (A=Z+N): protons & neutrons in the nucleus.  Isotope – Nuclei with the same Z but different A – Hydrogen (), Deuterium (), Tritium () Table of Nuclides IONIZING RADIATION

 Ionizing: energy to eject electrons (~10 eV)  Radiation Types – Charged particles: electrons, protons, heavy part. Sheet of – Uncharged particles: x-rays, γ-rays, neutrons Paper  Alpha (α) – He nucleus ejected from the atom nucleus.

– Range: cm/MeV (air), µm/MeV (solids). – Internal hazard Aluminum foil  Beta (β) or plastic – Electron ejected from the atom nucleus. – Range: m/MeV (air), mm/MeV (solids). ↑Z, ↑ρ – External hazard material  (lead) Heavy Charged Particles – Particles with A > 1. – External hazard  Gamma-rays (γ) – Photon emitted from the atom nucleus. Hydrogenous – Range: m/MeV (air), cm/MeV (solids) material – External hazard (H2O)  Neutron – Originate from the nucleus Radiation Types & Shielding Material – External hazard RADIOACTIVE DECAY

 Radioactivity – Unstable nuclei decay to reach the lowest energy (ground) state. – Each radioisotope decays at a unique exponential rate.  Units Decay Scheme – 1 Becquerel (Bq) = 1 disintegration/second – 1 Curie (Ci) = 3.7 x 1010 disintegrations/second

 Half-Life (/ – The time required for a quantity of radioactive material to be reduced by half of the original activity

/ –

/ – 1 Parent-decay, daughter-production plot NEUTRON INTERACTIONS

 Fission – Overcome binding energy to induce a nucleus to split . Fissile: fission with slow (thermal) neutron – 233U, 235U, 239Pu, 241Pu, 237Np  Fissionable: fission with fast neutrons – 232Th, 238U, 240Pu (and fissile 235U)  Microscopic Cross Section (σ) 235 U Fission Cross Section. Courtesy Duderstdat & Hamilton – Probability a reaction will occur – Dependent on incident neutron energy, nucleus, temperature – Units: cm2 or barns (1 b = 10-24 cm2) – Fission, scatter, radiative capture  Macroscopic Cross Section (Σ) – ∑ σ(atoms/cm) – N = Atomic Density (atoms/cm3)

235U & 238U fission cross section comparison. Doppler Courtesy Duderstadt & Hamilton. Broadening Radiation Protection DOSE COMPARISONS

Common Natural Radionuclides Nuclide Source Reaction Exposure 3H (Tritium) Cosmogenic 14N(n,T)12C Internal 16O(n,T)14N 14CCosmogenic14N(n,p)14C Internal

40 87 K, Rb Primordial long t1/2 Internal 222Rn, 220Rn Primordial Decay 238U & 232Th External

Occupational: 0.3% Scenario Dose Dose Fallout: 0.3% Nuclear Power: 0.1% (Rem) (mSv) Miscellaneous: 0.1% Average U.S. annual whole-body dose 3.6 Average radiation dose in the United States 0.36 NRC annual occupational total effective 5 50 whole-body dose limit

Normal Activity Doses No observable effects < 25 250 Activity Dose Possible temporary blood effects ~25 ~250 Commercial Air 0.238-0.66 mRem/hr NRC annual total occupational skin dose 50 500 Flight limit Head x-ray 78 mrem bone marrow Onset of radiation sickness - acute exposure 50-100 500-1000 1500 mrem skin NCRP astronaut career total whole-body 100 1000 Abdominal x-ray 535 mrem bone marrow dose limit 1700 mrem skin Death (L50/60) – acute exposure 300-450 3000-4500 Tobacco use 8 mrem/yr Death (100%) 1000 10,000 ALARA

 As Low As Reasonably Achievable (ALARA) – Cumulative dose kept ALARA considering technology, cost, etc.

Principle Operating Rule Method Time Dose rate proportional ↓ Time = • Train to perform tasks quickly. to time spent in • Perform tasks outside the radiation. ↓ dose rate radiation field when possible. • Never loiter. Distance Dose rate inversely ↑ Distance = • Keep away from sources. proportional to the • Use long-handled tools. distance squared from ↓ dose rate • Use remote manipulators. a source (↑Lx2 = ↓Dx4). Shielding Appropriate shielding ↑ Shielding = • Keep sources in shielded between you & containers (glove boxes, hot Remote manipulators radiation source ↓ dose rate cells, pigs, etc.) within a hot cell. decreases intensity. • Use portable shielding & PPE. RADIATION SHIELDING

 Shield has the potential to dominate reactor mass  γ-ray attenuation – ↑Z materials: W, Pb, dU

B4C  Neutron attenuation neutron I (t) shield – ↓Z materials: H, Li, B, H2O, LiH

∑ W -ray t shield

 Distance Truss Shadow Shield Concept – Rx-vehicle separation 2 – 1/r law (↑ Lboom = ↓ Mshield)

Distance boom and integrated vehicle radiation mitigation concept. Courtesy J. Caffrey Nuclear Materials ENRICHMENT & SNC Categories

Isotope M (kg)  Fuel enrichment (XF) 235 U (X > 20%) > 5 – (%) F Category I 233U> 2Strategic 239 Specialized facilities  Natural Uranium (natU) Pu > 2 Guards & Guns 235 233 239 High Cost – 0.711% U+2.5( U+ Pu) > 2

Isotope M (kg)  Depleted Uranium (dU) Category II 235U (X > 20%) > 1 – 0.2% F Mod. Strategic 233U> 0.5  Low Enriched Uranium (LEU) 239Pu > 0.5 – <20% 235U+2(233U+239Pu) > 1

– Light Water Reactor (LWR) 3-5% 235 U (10%< XF <20%) > 10 – NTP target 19.75% for affordability Isotope M (kg)

 High Enriched Uranium (HEU) 235 Category III U (XF > 20%) > 0.015 – >20% Low strategic 233U > 0.015 – Significant safeguards and expense University Reactor 239Pu > 0.015 Lower Cost – Most space reactor concepts: ~93% 235U+233U+239Pu > 0.015

235 U (10%< XF <20%) 1< M < 10 235 U (0.711%< XF <10%) > 10 Reactor Physics NEUTRON LIFE CYCLE

 Stable fission chain reaction – Neutron production, leakage and absorption rates balanced – Reactor power proportional to neutron population

Neutron Multiplication Factor : : k > 1 Super-critical k = 1 Critical k < 1 Sub-critical

= Neutrons released per fission 6-Factor Formula

η

ηthermal n production factor

resonance escape probability: p = fraction of fast n that escape resonance absorption & slow to thermal energies

& ε = fast fission factor Typical Thermal Reactor (235U) = 1.04 f = thermal utilization factor = 1.00 η = 1.65 p = 0.87 fast non-leakage probability ε = 1.02 f = 0.71 = 0.97 thermal non-leakage probability = 0.99 REACTOR CORE COMPOSITION

η

Reactor geometry Locked in Ratio of fuel to with fuel moderator density

 Fuel – Enrichment, density, shape: 235U, 19.75%  Moderator

– Low mass number material to slow neutrons: graphite, Be, H2 – Fuel-to-moderator ratio:  Coolant

– Remove heat, moderate, supplement reactor control with H2 flow rate  Structural materials – Maintain core geometry: support plates, spacer grids, tie tubes – Low neutron absorption cross section materials: Zircaloy, Al, Inconel, SS, 184W  Control elements – Absorbs neutrons to control multiplication

– High neutron absorption cross section materials: B, B4C, Cd, Gd, Hf  Reflector – Scatters leaking neutrons back into the core – Low neutron absorption cross section materials: Be, BeO, graphite  Pressure Vessel – Core and cooling containment: Al, Inconel, SS REACTOR GEOMETRY

 Reflected core = ↓ critical size (mass)

 Insulation to prevent heat loss is analogous to reflectors to R

prevent neutron leakage from ac the reactor core. c (r)

235U based Fuel Core

BeO BeO Axial Radial Bare vs. reflected critical assemblies. will not change since dependent on fuel composition and not geometry. Reflector Reflector r Reflected reactor geometry core cross-section REACTOR KINETICS & CONTROL

 Short-term transients (s, m, hr): start-up, maneuvering, shut-down  Long-term transients (d, w, m): fuel BU from BOL to EOL  Neutron population control = reactor control – Control drums: vary neutron population with absorber (vary f) – Coolant flow: harden spectrum with lower coolant flow rate (vary f)

Rotating Control Drum:

B4C Absorber Be Reflector

Control Drives @ 0°: Control Drives @ 180°: Subcritical Critical or Super Critical  Reactivity – The measure of neutron multiplication deviation from unity 1 – Units (relative to β): If →$1of reactivity (235U reactor: $0.4 is 0.4 $0.4)  ρ depends on neutron flux (φ) and thus Rx power

– ∆ ∆∆∆ ∆Σ,∆ ∆∆ DELAYED NEUTRONS

 Prompt Neutrons U-235 – Born at fission  – 98-99% of neutron population Neutron Neutrons ( 2.2) 180 MeV* – Short lifetime (l = 10-4 s) Fissile Nucleus  – Difficult to control (T = 0.1 s) (U-235) Product Nuclei U-235 (KE 168 MeV) Fission Chain Reaction Process  Delayed Neutrons – Born from fission product radioactive decay – 1-2% of neutron population 235U thermal fission delayed neutrons. Courtesy of Duderstadt & Hamilton.

– Longer lifetime (l = 0.6 – 80 s) Group t1/2 Decay Energy Yield Fraction (s) Constant (keV) (n/fission) Βi -1 – Controllable (T = 10 s) λi (s ) – β = delayed neutron fraction 1 55.72 0.0124 250 0.00052 0.000215 2 22.72 0.0305 560 0.00346 0.001424 3 6.22 0.111 405 0.00310 0.001274 βU235 0.0065 4 2.30 0.301 450 0.00624 0.002568 βPu239 0.002 5 0.610 1.14 - 0.00182 0.000748 6 0.230 3.01 - 0.00066 0.000273 0.0158 0.0065 REACTOR PERIOD

 Reactor Period – Time required for Rx power to change by “e” (2.718) – – As →1, T → ∞ (time independent neutron population) – T inversely proportional to ρ: ↑ρ=↓T, ↓ρ=↑T (desirable)  Example: – 1.001 – 10 , 10 – 0.1 – → – For 1 → 22,000prompt neutrons/sec

 Avoid reactor prompt critical & supercritical Reactivity-Period Chart. Courtesy of Dugan. – Prompt critical (ρ > β) Super critical (1 ) – Delayed neutrons not needed to sustain reaction – T will be very short (this should never occur) REACTIVITY INSERTION

Λ  Reactor Power Changes λ – Prompt jump/drop: Initially behaves as if there are Prompt no delayed neutrons. Term Delayed Term – Delayed neutron hold back effect: As delayed Λ neutron population increases there is a transition. decay constant – Stable Period: As T → asymptotic the Rx behaves

as if there are no prompt neutrons and delayed -ρ Step +ρ Step neutrons control the Rx. Insertion Insertion

Prompt Jump Delayed neutron Prompt hold back Drop

Reactor power level following a step reactivity insertion. Courtesy Duderstadt & Hamilton. 135Xe variation with reactor power level. Image courtesy of Duderstadt & Hamilton. TEMPERATURE REACTIVITY FEEDBACK

 Temperature Reactivity Feedback Coefficient ∆ – ∆ – 1. Nuclear: ∆ ∆ (Doppler broadening) = resonance absorption

– 2. Density: ∆ ∆∆∆ ∆ ∆

 Fuel & Moderator Feedback Coefficients ∆ – , ∆ ∆ – , ∆

 Stability

– Positive : ↑T = ↑ρ = ↑P = ↑T = unstable reactor (avoid) – Negative : ↑T = ↓ρ = ↓P = ↓T = stable reactor (design requirement) NEUTRON POISONS

 Fission product decay

– Produces daughter nuclides with large – Proportional to reactor power and operating time – 135Xe and 149Sm burnout at start up induce a positive reactivity insertion

fission → 1.7 19.2 6.58 9.17 2·10

fission → 2 54

∆ Poison σ b t/ hr ρ Peak buildup

6 2.7 x 10 9.2 2.5 3.0 10-11 hrs. Decays 4 4.1 x 10 53 0.4 0.6 10 days. No decay

135Xe buildup and decay following shutdown. Courtesy Dugan. 149Sm buildup following shutdown. Courtesy Dugan. DECAY HEAT

 Heat generation following shut-down – Result of fission product decay – 6-7% of reactor power generated immediately after shutdown. – Active cooling is required to keep core temperature within limits.

Reactor decay heat vs. time.  Failure to cool the reactor following shut-down – Fuel damage – Structural damage – Partial/full melt-down CONFUSED? OPERATIONAL ANALOGY

Airplane Reactor Altitude (A) Power (P) -

Airspeed (V) Period (T) Regulate and limit vehicle energy state Regulate and limit reactor energy state

(Vstall, Vne) (30 s ramp rate, -80 s shut down)

Vertical Velocity Indicator (VVI) Neutron Multiplication Factor (k) Rate of altitude change Rate of criticality change

Throttle Setting (% max RPM) Control Drum Setting (degree) Adjust fuel flow to vary Adjust reactivity to vary combustion rate (engine power) reaction rate (reactor power)

Multi-Engine Multi-Drum Individual power worth Individual reactivity worth Breaking Distance Shut-Down Margin (SDM) Runway length to achieve full-stop How far from critical

Dynamic Stability Reactivity Temp. Feedback () Positive or negative Positive or negative

Excess Power Excess Reactivity (ρex) Overcome induced/parasitic drag, adjust A Overcome BU,, Xe poison, adjust P level RECOMMENDED READINGS

Atoms, Radiation, and Space Nuclear Power by Joseph Radiation Protection, 3rd Ed. Angelo and David Buden by James E. Turner

To The End Of The Solar The Rickover Effect by System by James Dewar Theodore Rockwell REFERENCES

 J. Angelo & D. Buden, “Space Nuclear Power,” Orbit Book Co, Malabar, FL (1985).  J. Turner, “Atoms, Radiation, and Radiation Protection,” John Wiley & Sons, Inc, New York, NY (1995).  R. Faw & J. Shultis, “Radiological Assessment: Sources and Doses,” American Nuclear Society, IL (1999).  G. Knoll, “Raditiation Detection & Measurement,” John Wiley & Sons Inc, NY (2000).  W. Callister, “Materials Science and Engineering an Introduction,” John Wiley & Sons, Inc, New York, NY (1999).  R. Bolt & J. Carroll, “Radiation Effects on Organic Materials,” Academic Press, NY (1963).  G. Was, “Fundamentals of Radiation Materials Science,” Springer, NY (2007).  B. Ma, “Nuclear Reactor Materials & Applications,” Van Nostrand Reinhold Co, NY (1983).  M. Kangilaski, “Radiation Effects Design Handbook: Structural Alloys,” Batelle Memorial Inst., OH (1971).  C. Hanks & D. Hamman, “Radiation Effects Design Handbook: Electrical Insulating Materials and Capacitors,” Batelle Memorial Inst., OH (1971).  R. Chochran & N. Tsoulfandis, “The Nuclear Fuel Cycle,” American Nuclear Society, IL (1999).  D. Orlander, “Fundamental Aspects of Nuclear Reactor Fuel Elements,” Energy Research and Development Administration, TN (1976).  J. Duderstadt & L. Hamilton, “Nuclear Reactor Analysis,” John Wiley & Sons Inc, NY (1976).  S. Glasstone & M. Edlund, “Nuclear Reactor Theory,” Van Norstrand Co. Inc., NY (1952).  E. Dugan, “Nuclear Reactor Analysis,” Univ. of Florida course notes (2007).  W. Vernetson, “Nuclear Reactor Operations,” Univ. of Florida course notes (2007).  NRC SNM Categories http://www.nrc.gov/security/domestic/mca/snm.html, (2013).  NRC Radiation Areas http://www.nrc.gov/about-nrc/radiation/protects-you/hppos/hppos066.html, (2013).